Institute of Transportation Studies ◊ University of California, Davis
One Shields Avenue ◊ Davis, California 95616
PHONE: ( 530) 752- 6548 ◊ FAX: ( 530) 752- 6572
WEB: http:// its. ucdavis. edu/
Year 2006 UCD— ITS— RR— 06- 11
Assessment of Tire Technologies and Practices for Potential
Waste and Energy Use Reductions
Nicholas Lutsey
Justin Regnier
Andrew Burke
Marc Melaina
Joel Bremson
Michael Keteltas
Assessment of Tire Technologies and
Practices for Potential Waste and Energy
Use Reductions
May 15, 2006
Prepared under contract IWM- 03079 by:
Nicholas Lutsey
Justin Regnier
Andrew Burke
Marc Melaina
Joel Bremson
Michael Keteltas
Institute of Transportation Studies
University of California, Davis
One Shields Ave.
Davis, CA 95616
ii
Disclaimer: This report to the Board was produced under contract by Institute for Transportation Studies, University
of California Davis. The statements and conclusions contained in this report are those of the contractor and not
necessarily those of the California Integrated Waste Management Board, its employees, or the State of California
and should not be cited or quoted as official Board policy or direction.
The State makes no warranty, expressed or implied, and assumes no liability for the information contained in the
succeeding text. Any mention of commercial products or processes shall not be construed as an endorsement of
such products or processes.
S T A T E O F C A L I F O R N I A
Arnold Schwarzenegger
Governor
Allan C. Lloyd, Ph. D.
Secretary, California Environmental Protection Agency
•
INTEGRATED WASTE MANAGEMENT BOARD
Rosario Marin
Board Chair
Cheryl Peace
Board Member
Carl Washington
Board Member
( Vacant Position)
Board Member
Rosalie Mulé
Board Member
( Vacant Position)
Board Member
•
Mark Leary
Executive Director
For additional copies of this publication, contact:
Integrated Waste Management Board
Public Affairs Office, Publications Clearinghouse ( MS– 6)
1001 I Street
P. O. Box 4025
Sacramento, CA 95812- 4025
www. ciwmb. ca. gov/ Publications/
( 800) CA WASTE ( California only) or ( 916) 341- 6306
Publication # xxx- xx- xxx
Copies of this document originally provided by CIWMB were printed on recycled paper
containing 100 percent postconsumer fiber.
Copyright © 2006 by the California Integrated Waste Management Board. All rights reserved. This publication, or
parts thereof, may not be reproduced in any form without permission.
The statements and conclusions of this report are those of the contractor and not necessarily those of the California
Integrated Waste Management Board, its employees, or the State of California. The State makes no warranty,
expressed or implied, and assumes no liability for the information contained in the succeeding text. Any mention of
commercial products or processes shall not be construed as an endorsement of such products or processes.
Prepared as part of contract number IWM- 03079
The California Integrated Waste Management Board ( CIWMB) does not discriminate on the basis of disability in
access to its programs. CIWMB publications are available in accessible formats upon request by calling the Public
Affairs Office at ( 916) 341- 6300. Persons with hearing impairments can reach the CIWMB through the California
Relay Service, 1- 800- 735- 2929.
Join Governor Schwarzenegger to Keep California Rolling.
Every Californian can help to reduce energy and fuel consumption. For a list of simple ways you can reduce demand
and cut your energy and fuel costs, Flex Your Power and visit www. fypower. com.
iii
Table of Contents
Table of Contents................................................................................................................... iii
Acknowledgements............................................................................................................... iv
Executive Summary ................................................................................................................ v
Chapter 1. Introduction............................................................................................................ 1
Background ............................................................................................................................... ...... 1
Research Overview and Objectives................................................................................................. 2
Chapter 2. Literature Review .................................................................................................. 4
Background on Tire Service Life...................................................................................................... 4
Tire Maintenance Practices ............................................................................................................. 6
Tire Pressure Monitoring Technologies ........................................................................................... 9
Lower Rolling Resistance Tires ..................................................................................................... 11
Nitrogen Inflation Systems ............................................................................................................. 12
Trade- Offs in Tire Attributes........................................................................................................... 14
Chapter 3. Data Collection .................................................................................................... 18
Fleet Personnel Interviews............................................................................................................. 18
Driver Survey ............................................................................................................................... . 26
Chapter 4. Technology Implementation ................................................................................ 31
Nitrogen Inflation System............................................................................................................... 31
Tire Pressure Data Acquisition Systems........................................................................................ 33
Statistical Methodology for Comparing Pressure Loss .................................................................. 34
Chapter 5. Analysis of Tire Practices .................................................................................... 39
Private Vehicle Users..................................................................................................................... 39
Fleet Tire Management.................................................................................................................. 43
Chapter 6. Life Cycle Energy Analysis for Tires.................................................................... 48
Overview of Tire Life Cycle ............................................................................................................ 48
Tire Life Cycle Phases ................................................................................................................... 50
Variations in the Tire Life Cycle Energy Balance........................................................................... 59
Chapter 7. Best Practices for Vehicle Fleets......................................................................... 64
Maintenance Practices................................................................................................................... 64
Procurement Guide........................................................................................................................ 65
Chapter 8. Conclusions and Recommendations................................................................... 69
Conclusions.................................................................................................................... ............... 69
Recommendations ......................................................................................................................... 72
Abbreviations ........................................................................................................................ 73
Lists of Tables and Figures................................................................................................... 74
References..................................................................................................................... ...... 76
Appendix A. Fleet Interview Questionnaire ........................................................................... 80
Appendix B. Driver Survey .................................................................................................... 81
Appendix C. Equipment Specifications ................................................................................. 84
iv
Acknowledgements
The authors would like to thank the California Integrated Waste Management Board for its
generous support of this research. Special thanks go to Mitch Delmage, Calvin Young, and Linda
Dickenson for their leadership and guidance on the project. We would also like to express our
appreciation for the support from the employees of the Department of General Services’
employees at the Sacramento state garage, especially from Richard Battersby, Marco Cuellar,
Bryon Rush, Michael Pegos, Richard Oliver, Sheila Furr, and James Ramel for their dedication
and patience throughout the project. Also, thanks goes to the University of California, Davis
vehicle fleet employees for their input in the study. We are thankful for the technical support of
John Bolegoh of SmartTire and Don Lawe of Vehicle Monitor Corporation in the development of
the data acquisition system. Undergraduate researcher Ivan Gomez’s time and efforts were very
important in the timely completion or our research. We are grateful to LaBou Cafe & Bakery for
their support in providing survey incentives. Finally, we gratefully acknowledge the final copy-editing
by Jamie Knapp.
v
Executive Summary
Tire purchasing and disposal impose considerable cost and waste burdens on private vehicle
owners and fleet managers. This research investigates tire maintenance management practices
and tire- related vehicle technologies that have the potential to relieve some of these burdens. We
investigate behavior, attitudes, and practices of fleet personnel and individual drivers as they
relate to tire attributes and technologies. Based on this research, we analyze and recommend
critical practices that could improve tire purchasing, tire management, average tire life within
existing vehicles in vehicle fleets. We evaluate the tire wear and energy use of various tire
technologies and improved fleet tire management and find several fleet practices that offer
substantial potential improvements in tire- related energy and waste consequences. Advancements
in three particular areas – tire pressure monitoring, nitrogen as a tire inflation medium, and the
selection of tires with lower rolling resistance – are commercially available and promising in
terms of their potential benefits. Additionally, to demonstrate and empirically test the potential
impact of nitrogen as an inflation medium for tires, we deploy several technologies on fleet
vehicles, including data acquisition systems for retrieval of information from fleet vehicles and
nitrogen inflation equipment at the California Department of General Services vehicle fleet
facility. We develop the accompanying experimental design for testing the impact of nitrogen
inflation on these fleet vehicles. This experiment is created in such a way that the fleet personnel
can undertake the experimental testing and statistically evaluate the impact of nitrogen inflation
on their vehicle fleet. From our findings, we develop best practices recommendations, which are
meant to serve as a guide for improving tire practices in vehicle fleets.
1
Chapter 1. Introduction
Background
Although tire technologies have advanced considerably in the last several decades in terms of
durability, safety, and fuel economy, the use of tires still results in considerable cost,
environmental, and waste management consequences. The manufacturing of tires results in the
use of energy and natural resources. The rolling resistance of tires is an important component of
vehicle efficiency, thus impacting the fuel consumption and emissions of vehicles. Due to the
durability of tires, the disposal of tires poses a substantial waste management issue.
The federal government has several programs designed to minimize the adverse impacts of tire
use. Tires are a key consideration in the safety and fuel economy regulation of vehicles, as set by
the U. S. Department of Transportation’s National Highway Traffic and Safety Administration
( NHTSA). For example, automakers tend to place relatively low rolling resistance tires on new
passenger cars and light trucks to aid compliance with Corporate Average Fuel Economy ( CAFE)
regulations. A recent NHTSA regulation mandates devices on vehicles to aid in the monitoring
of tire pressure to ensure driver awareness of tire under- inflation; the measure is aimed at
improving vehicle safety but is also expected to yield fuel economy benefits ( NHTSA, 2005).
Also, by request of Congress, a National Academy of Sciences National Research Council ( NRC)
study recently assessed potential improvements in replacement tires, considering factors of rolling
resistance, tread wear, and traction, and investigated potential testing procedures and consumer
information campaigns for tires ( NRC, 2006).
The State of California is actively engaged in reducing the negative consequences of tires used on
public and private vehicles through numerous state agency programs. A 2003 California law
tasks the California Energy Commission with developing a tire efficiency program to promote
fuel- efficient tire purchasing and improved tire maintenance practices ( CEC, 2006). The CEC
and the California Air Resources Board ( CARB) assessed lower rolling resistance tires to be a
cost- effective method of reducing petroleum usage ( CEC and CARB, 2003). CARB regulates
pollutants involved in tire manufacturing and disposal practices. Additionally, CARB has
identified low rolling resistance tires as one method to reduce greenhouse gases in its proposed
new regulation of vehicle greenhouse gas emissions ( CARB, 2004). Beyond the “ in use” issues
addressed by CEC and CARB, numerous programs and projects undertaken by the California
Integrated Waste Management Board ( CIWMB) are aimed at reducing the waste, landfilling, and
related consequences at the end of tires’ useful life ( CIWMB, 2005).
Various tire management practices and tire technologies have the potential to defray the costs,
environmental burdens, and waste issues that result from tire use. More durable tires last longer;
purchasing such tires means less frequent tire purchases and fewer tires disposed. Improved tire
inspection and maintenance practices ( e. g., tire rotation, tire inflation) improve tire longevity, tire
safety, and vehicle fuel economy. Tire technologies, such as tire pressure monitoring systems
( TPMSs) increase driver awareness of tire maintenance needs. More efficient, lower rolling
resistance tires reduce vehicle fuel use and emissions. Increasingly tire waste is being diverted
from landfills to other uses, including tire- based aggregate for road building, use as fuel for
electricity or cement production, and other end- of- life management practices.
This research focuses on ways to improve vehicle tire procurement and maintenance to increase
tire longevity and decrease tire- related energy use. Researchers collected original data for this
2
project by coordinating with fleet personnel and customers ( i. e. drivers) and tracking vehicles of a
large government fleet. The large quantity of tires purchased, managed, and discarded by
government fleets represents a substantial expenditure. Government fleets also offer an
opportunity to test and implement improved tire practices and new technologies. Although this
research specifically investigates vehicle fleets, it addresses maintenance practices and
technologies that are applicable to both fleets and private vehicles.
Research Overview and Objectives
The project is primarily concerned with the practices that tire purchasers, vehicle maintenance
personnel, and vehicle users can undertake to reduce tire waste and lessen the environmental
consequences of tire use. Table 1 provides an overview of the key aspects of this investigation of
tire- related technologies and practices. The first part, “ Literature Review,” discusses the relevant
background on tire- related practices and technologies for this assessment in Chapter 2. The two
following sections – assessments of fleet personnel and individual driver practices – are presented
in Chapter 3, “ Data Collection.” Chapter 4 details installation and demonstration of tire
technology on fleet vehicles. The synthesis and analysis of these parts comprises the final
sections: Tire practice analysis ( Chapter 5), Life cycle assessment ( Chapter 6), and the
development of a “ Best Practices” guide for vehicle fleets ( Chapter 7).
TABLE 1. Research Overview
Research Parts Task Description, Key Aspects
Introduction
( Chapter 1)
• Introduce key elements of research project
• Describe motivation for research on tire waste, tire longevity, and tire- related
energy use
Literature review
( Chapter 2)
• Review available knowledge on tire practices ( inspection, maintenance, etc.)
and guidelines for proper tire usage and practices
• Assess current knowledge on various tire technologies, including “ smart tire”
devices, such as self- inflating tires and low- pressure alert systems, nitrogen
inflation, and low rolling resistance tires
Data Collection
( Chapter 3)
Fleet personnel practices
• Explore general fleet personnel practices with respect to tire purchase, use,
behavior, and maintenance
• Assess fleet personnel perceptions and willingness to implement different tire
inflation practices or purchase novel tire technologies
Private vehicle user practices
• Explore general vehicle user practices with respect to tire purchase, use,
behavior, and maintenance
• Assess vehicle user perceptions and willingness to implement different tire
inflation practices or purchase novel tire technologies
Technology Demonstration
( Chapter 4)
• Install and demonstrate novel tire technologies to conventional tires
• Examine how actions designed to increase tire longevity may impact vehicle
fuel use, emissions, and safety
Analysis of Tire Practices
( Chapter 5)
• Analyze tire service life impact of various modifications in tire maintenance
and management
Life Cycle Assessment
( Chapter 6)
• Analyze the life- cycle energy associated with various tire- related processes
and practices
“ Best Practices” Guide
( Chapter 7)
• Recommend practices and technologies for vehicle fleets
• Develop guide for vehicle fleets
3
A large number of potential technological innovations and best practices can be identified across
the life cycle of a tire ( i. e., material extraction, manufacturing, transport, use and disposal). The
present study focuses on the use phase of a tire. A detailed assessment of tire design options,
manufacturing methods, material recycling and other end- of- life practices is outside the scope of
this study. However, a life cycle framework is used to place into context the potential
improvements offered by the innovative tire technologies and practices addressed in this study.
These innovations and practices include the following: tire pressure monitoring systems, nitrogen
inflation systems, low rolling resistance tires, improved vehicle user tire maintenance, and
improved fleet tire management practices.
, The results of this research focus generally on tire maintenance and monitoring technologies and
tire practices; however, there is one primary limitation. The original data collection for this study
is based primarily on the tire- related practices and technologies deployed by a single government
fleet. As such, the research, analysis, and conclusions of this study are in some cases more
pertinent to fleet tire practices ( purchasing, maintenance, management, and disposal) than to
private vehicle user practices. However, the use of fleets as units of analysis is nonetheless
justified for several key advantages, including economies of scale for technology implementation,
centralized hub of many vehicles, access to many vehicle users for surveying, and consistency in
vehicle inspection and maintenance practices on vehicles being analyzed.
The objectives of the demonstration portion of this study are to bridge existing gaps in the
research knowledge on tire practices to demonstrate and evaluate the current state of nitrogen tire
inflation technology. With the current dearth of general information on fleet tire management
practices, this work is geared toward collecting such information and targeting areas for
improvement. Our findings on operating and maintenance practices are most likely to affect
fleets. As such, one of the key results of this work is the creation of a “ Best Practices” manual
that offers guidance on proper tire purchasing, inspection, and maintenance practices. Assessing
nitrogen technology with real- world, on- road data is likely to have implications first and foremost
for vehicle fleet operators who purchase, use, and maintain a large number of tires, and are
therefore significantly affected by tire- related costs and waste. Beyond aiding in fleet operations,
the formulation of guidelines regarding proper tire practices is also expected to offer direction for
government information programs for private vehicle users to support public waste, fuel use, and
emission reduction objectives.
4
Chapter 2. Literature Review
Tire attributes are subject to myriad government regulations and customer demands. NHTSA
regulations mandate tire tread testing and specification labeling on tire sidewalls. Fuel economy
regulations ensure low rolling resistance on new vehicle tires. The 2000 federal Transportation
Recall Enhancement, Accountability and Documentation ( TREAD) Act and the subsequent
NHTSA rulemaking updated and instituted new tests for tires and mandated tire pressure
monitoring systems on new light- duty vehicles. Beyond government requirements, consumer-demanded
attributes for tires include cost, ride comfort, noise, fuel efficiency, longevity, traction,
air retention, and speed rating – and some of these characteristics have complex and competing
trade- offs associated with each other ( Lamb and Pyanowski, 2002).
This chapter summarizes available information on tire maintenance practices technologies and
discusses the relevant trends in tire characteristics. Technologies investigated and summarized
include tire pressure monitoring devices, low rolling resistance tires, and nitrogen inflation. It is
important to emphasize that tire characteristics like tread wear, rolling resistance, and traction are
by no means mutually exclusive; as a result, the trade- offs of characteristics and their mutual
dependencies are discussed in the final section of this chapter.
Background on Tire Service Life
This section introduces the key aspects of tire life from tire purchase to replacement. Data are
presented on tire life and tire replacement to provide context for the upcoming sections that assess
new tire technologies that could impact these factors. In addition, this section introduces the key
aspects and variables for this report’s life cycle analysis of alternative tire technologies.
There are two primary markets, original equipment ( OE) manufacturer and replacement
equipment tires. OE tires are purchased in high- volume, long- term contracts for new passenger
vehicle models. Replacement tires are purchased by individual consumers and fleet owners as
needed. In both markets, many tire manufacturers offer many models with differing attributes
( performance, wear, cost, etc.). OE tire sales represent about one- quarter of the passenger tire
market. This market demands more lower rolling resistance tires to enable new vehicles to
comply with fuel economy and emissions certification requirements. The replacement tire market,
which comprises about three- quarters of passenger tire sales, generally demands tires with longer
life more so than lower rolling resistance ( Ecos, 2002). In both markets, the trade- offs in tire
attributes are complex and subject to competition between tire suppliers to innovate with new tire
composition and design to balance consumer demands for tire safety, durability, handling,
comfort, and fuel economy.
Tire life has showed marked improvements over the past two decades due primarily to technology
shifts. As shown in Figure 1, average tire life has improved from less than 30,000 miles per tire
in 1981 to more than 40,000 miles today ( RMA, 2002). The dominant tire technology factors
attributable to the increase in tire life are composition and design shifts over the last two decades.
The principal early factor increasing tire life was the shift from bias- ply to radial- ply tires. Since
then, tire longevity has increased largely due to innovations in tire composition, such as
improvements via the time- and equipment- intensive method of mixing silica and silicone
butadiene rubber compounds to give the best material properties of each ( Joshi et al, 2003).
5
While there has been substantial research in tire chemistry, the choice of monomers for elastomer
synthesis has been economically limited to butadiene, styrene, and isoprene. As a result, the
emphasis has been on improving the chemistry of the butadiene by adding neodymium ( Nd) or
bromine ( Br) to improve the polymerization of the tire compounds. Additional work has been
done in improving the process and removing the costly mixing steps ( Quirk, 2003).
0
5
10
15
20
25
30
35
40
45
50
1980
Average Miles Per Tire ( 1000)
1985 1990 1995 2000
Year
FIGURE 1. Average Tire Life, 1980- 2001 ( from RMA, 2002)
Although tire longevity has improved over the past decades, data on tires, average tire mileage,
and tire disposal are not well characterized in comprehensive, publicly available data. Different
subsets of tires in the vehicle fleet ( e. g., OE versus replacement, different vehicle types, different
driving styles) could have average lifetime mileage values that are different from the reported
( i. e., from RMA, 2002 data) average in ways that are not well characterized by existing public
data. For example, a CEC ( 2003) report found that OE low rolling resistance ( LRR) tires average
only 77% of the lifetime mileage of replacement tires. Additionally, the wide variety of
proprietary tire designs and compounds makes these figures difficult to apply to any particular
model of tire. Other prominent factors beyond tire technology that influence average tire life
include consumer choice in tire purchasing and vehicle user tire maintenance practices, although
the extent to which these factors have changed over the past two decades is not well known.
A key part in understanding tire life – and differentiating between the tire practices and tire
technologies that impact tire life – is determining why tires are ultimately replaced and discarded
on vehicles. A survey of available tires on the market reveals limited tire warranties that range
from 30,000 to 80,000 miles. Many of the most popular tire brands have warranties 50,000 to
60,000 miles. This warranty is generally contingent upon the customer documenting that they
have properly maintained the tires, including periodic rotation of the tires.
As indicated by Figure 1 above, actual average tire mileage is substantially below the ideal tire
life warranty mileage. The primary reason for this is that the majority of tires are not replaced
due to “ normal wear.” New tires generally start with approximately 9/ 32 inches of tread ( actual
tread depths generally range from 8/ 32 to 12/ 32 inches), and, for safety reasons, end when at the
minimum tire tread depth of 2/ 32 inch. ( In many states this is a legal minimum.) Due to varying
driving conditions and the differing inflation and maintenance practices of vehicle users, most
tires do not last until a “ normal wear” replacement. Michelin data on tire replacement indicate
that 10% to 30% of tires could be retired from the vehicle fleet due to sustained long- term wear,
6
while 40% to 60% tires are replaced for “ abnormal wear” reasons, 5% to 10% are replaced with
“ nothing observed,” and the remaining 20% are replaced for other reasons ( including road hazard
puncture, oxidation, and separation) ( Weissman et al, 2003). Factors related to, and potential
improvements to, tire maintenance practices are examined in the following section.
Tire Maintenance Practices
Key to assessing potential improvements in tire maintenance practices is quantifying current
inspection, inflation, and maintenance practices by vehicle owners and operators. Although data
on the subject is sparse, there currently does appear to be room to significantly improve vehicle
users’ tire maintenance practices. This section summarizes what is known about current tire-related
vehicle practices, with emphasis on potential areas of improvement for tire longevity.
Vehicle users’ knowledge and practice of proper tire pressure monitoring and maintenance is
thought to be generally poor. Proper tire inflation is prescribed by the vehicle manufacturer and
is displayed on the vehicle “ placard,” or sticker in the vehicle driver- side doorframe. This placard
pressure varies by vehicle, but generally values range from 25 to 40 pounds per square inch ( psi).
NHTSA ( 2005) found that placard values average 30 psi for passenger cars and 35 psi for light
trucks. These placard pressure values are the pressures that tires should be set at, as measured
“ cold,” or after the vehicle has been at rest for some time.
However, several studies have indicated that tires are consistently at pressures quite different
from the recommended placard pressure. One study suggests that vehicle users could use the
maximum tire pressure cited on the tire sidewall, which is generally about 40 psi, to set their tire
pressure, instead of the lower and correct placard value ( CSUS, 2003). More likely, however, is
persistent under- inflation of vehicle tires. A survey by NHTSA ( Thiriez and Bondy, 2001) found
that approximately 25% to 30% of light- duty vehicles have at least one tire that was under-inflated
by at least 25% below placard. This study found the average under- inflation for
passenger cars to be 6.8 psi ( or 23% of 30 psi) and for light trucks to be 8.7 psi ( 25% of 35 psi).
Moreover, these percentages have the potential to understate the magnitude of the under- inflation
problem. Because the pressure testing of many of the vehicles is likely to have been when
vehicles had just been driving, the reading will be tainted by the tires not being “ cold.” Even if a
vehicle operator attempts to set the inflation of the tires to the placard pressure, they could
ultimately be several psi too high. For example, based on Tooke ( 2003), if the internal tire
temperature is 20° F above the “ cold” placard temperature of 65° F, the tire pressure would be set
2 to 3 psi too low. Additional potential inflation- setting errors could result from the ambient air
temperature not being 65° F. Further inaccuracy is introduced by instrumentation errors. Gas
station tire pressure gauges are prone to over- reporting, with about one- third of station gauges
reporting at 4 psi or more greater than reference pressure ( NHTSA, 2001), and handheld “ pen-type”
are prone to inaccuracy.
7
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120
Contained Air Temperature ( F)
Inflation Pressure ( psi)
" Placard" pressure:
26 psi @ 65 ° F
FIGURE 2. Deviation from Set Pressure with Temperature ( from Tooke, 2003)
Two other tire maintenance practices of importance are tire rotation and wheel alignment.
Although there are not extensive data regarding vehicle operators’ general practices, the
substantial percentages of tires replaced due to “ abnormal” or uneven wear attest to a general
deficiency in following regimented rotation schedules and alignment checks. Rotation is
necessary due to the uneven wear characteristics of each wheel position on the vehicle. For
example, front- wheel drive vehicles which place braking, steering, and driving forces on the front
axle tires, result in a much faster wear rate for the front axle tires. While large tire misalignments
are likely to be noticed by drivers and corrected, smaller misalignments can go undetected and
cause significant accelerated and uneven wear.
Tire practices have a significant and quantifiable impact on tread life and, ultimately, tire
replacement. One degree of misalignment is estimated to double the rate of tire wear ( Trimbach
and Engehausen, 2003). Trimbach and Engehausen ( 2003) also reveal a considerable increase in
the tire tread wear rate as inflation decreases below the placard pressure, as shown in Figure 3. In
turn this tire wear increase results in a reduction in tire life. Tire manufacturer Goodyear reported
a linear relationship, where, for every one psi below vehicle placard pressure a loss of 1.78%
reduction of tire tread life would result ( NHTSA, 2002). Estimating from this relationship and
average under- inflation levels ( 6.8 psi for cars and 8.7 psi for light trucks), average tire life for
U. S. tires is reduced approximately 12% to 15%. This under- inflation- related reduction in tire
service life is examined more thoroughly in Chapter 5.
8
FIGURE 3. Influence of Inflation Pressure on Rate of Wear ( from Trimbach and
Engehausen, 2003)
Table 2 shows the reasons tires are replaced and the estimated percentages represented by those
replacements ( from Weissman et al, 2003). The most common replacement reason – for about
half of tires – is “ abnormal wear,” which includes tire unevenness due to improper tire inflation,
rotation, and alignment practices, but could also include a braking incident causing tire flat spots.
For example, under- inflated tires will wear more on the outsides of each tire rather than in the
middle, and over- inflated tires will wear more quickly in the middle. More infrequently, about
10% to 20% of tires are replacements for tires that have no visible defect or failure. Most likely
these tires are retired because one or more tire in its set is discarded for abnormal wear or other
reasons, and the tires are discarded as a set to maintain vehicle balance ( regardless of the
remaining tread on remaining “ good” tires). As many as 10% of tires are replaced due to road
hazards, punctures, and traffic accidents. Poor driving conditions and aggressive driving behavior
contribute to this category of discarded tires; to some extent the susceptibility to any puncture
incident increases with improper tire inflation levels. Oxidation and separation, accounting for
10% of tires discarded, are the result of chemical degradation processes involved with the aging
and overheating of tires.
9
TABLE 2. Tire Replacement Reasons
Tire
Replacement
Reason
Estimated
Percentage
of Tiresa
Description / Comments Practices for Improved Tire
Service Life
Normal wear 10- 30%
Tire tread depth wears down over
normal driving conditions from
original depth of approximately 9/ 32
inch to the minimum depth of 2/ 32
inch
Proper inspection and inflation
Nothing
observed 5- 10%
Tire with tread remaining that is
removed in a set of tires because one
( or more) tires are worn, punctured
or otherwise need replacing
Proper inspection and inflation
Abnormal
wear 40- 60%
Unevenness in individual tire’s wear
due to wear asymmetry, relative flat
spot ( e. g. from braking incident)….
Proper inspection and inflation;
rotating tires; balancing tires; aligning
wheels; non- aggressive driving
Road hazard 10% Tire leak or puncture due to driving
conditions or traffic accidents
Proper inspection and inflation; non-aggressive
driving
Oxidation and
separation 10%
Tire materials degrade through
chemical oxidation, aging, and
overheating
Proper inspection and inflation;
reduced moisture and oxygen in tires
a based on Weissmann et al, 2003, based on Michelin data from 1992 to 1999
This section is used as a starting point in understanding the background and key factors affecting
maintenance practices. We assess the extent to which improvements in tire practices could result
in increased tire service life in Chapter 5 and reduced tire life cycle energy use in Chapter 6.
Tire Pressure Monitoring Technologies
In 2000, Congress passed the Transportation Recall Enhancement, Accountability, and
Documentation Act ( TREAD) in response to tire safety problems on light trucks. As part of the
implementation of that act, NHTSA issued a ruling in December 2001 that after November 2003,
all light- duty vehicles must have a Tire Pressure Monitoring Systems ( TPMS) with a dashboard
indicator light to warn drivers if their tire pressure was low. Multiple legal actions by industry
and consumers challenging the ruling and how it would be implemented delayed its
implementation until the 2008 model year.
The two TPMS types are direct and indirect. Direct TPMS uses a sensor within the wheel to
directly measure pressure and other parameters and relay them to a receiver. Indirect TPMS uses
wheel speed sensors and infers inflation levels from the difference in their rotational speed.
Indirect TPMS requires a large degree of integration with a particular vehicle and its braking
system. Because of this degree of integration, it is an approach that is generally used by OEMs
rather than being an aftermarket solution.
10
Most of the 2008 models will detect the presence of low pressure tires by an indirect method
integrated with the anti- lock braking system ( ABS) of the vehicle. Low pressure of a tire will be
inferred from significant differences in the rotation speeds of one or more tires resulting from
decreases in the effective radius of under- inflated tires. This indirect approach meets the
requirements of the NHTSA rule requiring the ability to detect a pressure that is 25%
( approximately 8 psi) or greater below the proper inflation pressure of the tire, but it does not give
a direct, quantitative measurement of inflation pressure. This approach relies on training the
TPMS to tire conditions that are assumed to be uniform. Unfortunately, the accuracy of this sort
of a system can be compromised by road conditions or uneven wear and has a poor ability to
detect discrepancies in tires on different axles. In general, ABS- based systems have trouble
detecting more than one tire with pressure loss, under inflation warning thresholds vary by axle
and the detectable pressure threshold varies between 10% to 40% of the cold inflation pressure
level. In the present study, only direct measurement of tire pressure is considered as this
approach can detect much smaller degrees of under- inflation with a much higher degree of
reliability
Direct TPMS come in a variety of configurations and features. The main differences are the
circuitry and the type of measurements that are taken. All of the direct TPMSs measure pressure
via a sensor that is mounted within the rim of each tire. The sensors then relay their
measurements wirelessly to a receiver in the passenger compartment. Some of these receivers
evaluated were standalone units, and some were incorporated to the vehicle’s computer system.
The circuitry for measurement comes in two forms: Printed Circuit Board ( PCB) and Application
Specific Integrated Circuit ( ASIC). The PCB form uses a sensor on a printed circuit board in
conjunction with a microprocessor and other components to form its circuitry. The ASIC form
incorporates the sensor and other electronics into one sealed package. The ASIC form is much
more rugged than the PCB, protecting circuitry and connections from the harsh conditions found
in the tire.
All of the TPMSs that were evaluated for this study measured pressure and temperature, and
some measured wheel speed as well. Both Yokohama and Nokian have TPMSs that are in
prototype phase and have speed sensing capability ( Hattori, 2004; Hakanen, 2003). The TPMSs
with speed sensing capability are being developed for integration with a vehicle’s anti- lock
braking system, rather than as an aftermarket system. Only TPMS without speed sensing were
available for the purposes of this study.
An additional form of direct TPMS that is under development is Passive Transponder TPMS.
This system utilizes transponders that are built into or attached to a tire’s interior. To date, there
have been no systems for passenger tires that have been able to endure the manufacturing process
and provide reliable performance. An advantage to this sort of a system would be savings on
installation costs. A possible disadvantage to this sort of system is the need to replace a tire if the
sensor malfunctions.
The need to monitor tire pressure can be eliminated by utilizing a self- inflating tire system.
While there are systems available for high pressure tires and large trucks, only one was found for
passenger vehicles. The “ Auto- Pump” system manufactured by Cycloid has been used on some
of the Jeep Grand Cherokee vehicles. This system utilizes a centrifugal pump mounted in the rim
of each wheel to maintain tire pressure. Attempts to find information on this system yielded
nothing more recent than 2002. It is unknown if this company is still in existence. This
technology is not being considered for the demonstration portion of this study.
11
Lower Rolling Resistance Tires
When tires interact with road pavement the results are traction, which moves the vehicle, and
rolling resistance, which is an energy loss consequence. Tire rolling resistance energy loss
accounts for a substantial road load force that a vehicle must overcome to move at a given speed
and acceleration. As a result, tire manufacturers have consistently sought to minimize tire rolling
resistance, subject to the other demanded tire attributes. Over the past several decades, tire
manufacturers have made innovations in tire design and composition to steadily improve the
average rolling resistance of light- duty vehicle tires without compromising other tire qualities
such as traction, safety, and drivability. This section briefly discusses trends in lower rolling
resistance in tires and the prospects for further improvements.
Driven primarily by vehicle manufacturers’ concerns for achieving federal fuel economy and
emissions standard targets, tires’ rolling resistance has improved significantly. . Original
equipment ( OE) tires on new vehicles experienced an average rolling resistance coefficient
decrease of approximately 50% in the last 20 years ( Cook, 2003). Figure 7, based on data from
LaClair ( 2002) and CEC ( 2003), approximates this improvement in new tire rolling resistance
coefficient over time. These data were consistent with new data on late model tires from NRC
( 2006), which also found that the rolling resistance coefficients of individual tire brands and
models varies quite significantly, generally from 0.0065 to 0.013.
0
0.005
0.01
0.015
0.02
0.025
1980 1985 1990 1995 2000
Model year
Rolling resistance coefficient
Light trucks
Passenger cars
FIGURE 4. Tire Rolling Resistance, 1980- 2000 ( Based on LaClair, 2002 and CEC, 2003)
Rolling resistance improvements have resulted from both tire construction changes and
introduction of novel tire compounds. The major early ( i. e. through the 1970s and 1980s) rolling
resistance changes were due to the switch from bias- ply to radial- ply tires, which reduced the
hysteresis1 losses ( Schuring, 1980). Changes in tread design and material compounds have more
1 The mechanical energy loss in the form of heat loss associated with the deformation and recovery of a material ( in
this case, of a pneumatic rubber tire) going through a cycle.
12
substantially influenced rolling resistance since then. Currently, tread compounds utilize
different polymers, reinforcing fillers, and anti- degradants to simultaneously improve rolling
resistance, tire wear, and other properties; however specific details on these compounds are
complex and generally proprietary. Carbon has become the dominant filler, but recent advances
have been made in silica filler with a silane coupling agent, and various oils and polymers ( NAS,
2006).
We use the analyses from two recent prominent reports on LRR improvements for this report’s
assessment. First is a report prepared for the CEC by TIAX Corp. in response to California 2001
Senate Bill 1170 that authorized the CEC to investigate opportunities to increase the purchase and
use of low rolling resistance tires on vehicles in California as a means of reducing fuel
consumption. The major conclusion of the report is that the use of low rolling resistance tires in
California could increase average fuel economy and reduce fuel consumption by 3%, with the
fuel savings benefits outweighing the additional tire cost increase of the technology. This would
require a reduction of 20% in the rolling resistance of replacement tires. The limited tire test data
available for this study indicated that the rolling resistance coefficient of most of the tires was in
the range 0.01 to 0.011. Hence a 20% reduction in rolling resistance would put the low rolling
resistance tires in the range .0083 to .0092.
A recent, comprehensive text that explores tire rolling resistance is the National Research
Council’s Tires and Passenger Fuel Economy report ( NRC, 2006), which estimates potential
rolling resistance reductions and their impact on fuel consumption. This study more cautiously
discusses the potential for 10% rolling resistance improvements. The study finds the relationship
between the tire rolling resistance coefficient and fuel economy is well understood and quantified.
Based on multiple data sources and methods, a 10 % reduction in average rolling resistance of
passenger vehicles will lead to a 1 to 2 % reduction in fuel consumption; more specifically the
lower boundary improvements ( or 0.70 to 1.28 %) result from urban driving cycles, while higher
gains ( 1.60 to 1.96 %) result from highway driving conditions ( NRC, 2006). Independent
analysis conducted by ITS- Davis using ADVISOR vehicle simulation software confirms these
estimated % improvements. Additionally these data are consistent with the work of the CEC
( 2003). Based on the boundaries from the two studies ( CEC, 2003; NRC, 2006) we assess, in
later portions of this work, impacts of up to 20% improvements in rolling resistance coefficients.
Because of the higher energy inputs in manufacturing, it is crucial to account for upstream cycle
energy inputs, and we do so in Chapter 6. Additionally, rolling resistance, as it relates to other
tire attributes, is discussed further in the “ Trade- Offs” section of this chapter.
Nitrogen Inflation Systems
Inflating tires with nitrogen has long been the standard practice in racing and aerospace industries
and is now receiving considerable attention and limited use in trucks and passenger vehicles.
Costco Wholesale Corp. has installed nitrogen inflation systems at its 400- plus U. S. and Canadian
vehicle service locations to enhance tire sales and improve member benefits ( Manges, 2004).
Many smaller outfits have already adopted the technology and some of the largest tire service
providers, including Pep Boys and Big O Tires, are currently test marketing nitrogen inflation
( Manges, 2005; Miller, 2004). One article states that, by 2004, thousands of nitrogen inflation
units had been installed, and dealers generally charge between $ 2.50 and $ 12 per tire ( Tire
Review, 2004). Many tire service providers are unsure and many consumers remain unaware and
suspicious of the technology ( Manges, 2004). To note, Costco offers nitrogen inflation at no
additional charge to members who purchase their tires at its retail centers.
13
The use of nitrogen in lieu of air, which is approximately 78% nitrogen and 21% oxygen, as an
inflation medium for automobile tires has several purported benefits. Most potential benefits are
based on several factors: increased pressure retention by tires due to nitrogen’s lower
permeability through tire layers than air, reduced oxidation in the tire’s rubber compounds, and
nitrogen’s lower water retention resulting in more consistent air pressure with changing
temperature ( Baldwin, et al, 2004). Numerous media reports and anecdotal accounts refer to
these fundamental benefits in discussing and offering rough estimations about improved tire life,
improved fuel economy, and improved overall maintenance costs.
Table 3 shows selected chemical property difference between, air, oxygen, and nitrogen gases.
These properties were taken from Lange’s Handbook of Chemistry ( Dean, 1992), and these are
the most pertinent properties to the present discussion of tire inflation. Note that except for the
permeability in rubbers, the properties of the three gases are not very different. This is not
unexpected, as air is comprised of 78% nitrogen by volume. The major difference in properties is
in permeability through rubber for nitrogen versus oxygen. The permeability of air is less
straightforward to determine, largely because it is highly dependent upon the amount of water
vapor ( even in small amounts) that is present within air. From these known properties it is
plausible that a switch from air to nitrogen could reduce tire pressure leakage due to nitrogen’s
improved permeability and to the reduction in water vapor in the medium. Furthermore, the
presence of any water vapor in tire increases the occurrence of oxidation. The failure of tires due
to oxidation- related effects account for approximately 10% of tire failures, as previously stated in
Table 2.
TABLE 3. Selected Properties of Air, Nitrogen, and Oxygen
Property Air Nitrogen Oxygen
Molecular weight 29 28 32
Composition 78% N2 , 21% O2 100% N2 100% O2
Molecular diameter ( nm) --- .315 .292
Specific heat ( kJ/ kg 0K ) 1.007 1.039 .919
Heat conductivity ( mW/ m 0K) 26.2 26.0 26.3
Gas permeability thru rubbers *--- 9.43 23.3
* permeability coefficient of water vapor is 2290
There is, at present, a lack of comprehensive data to verify or validate the potential benefits of
nitrogen as an inflation medium for passenger vehicle tires. The only available experimental
study on nitrogen inflation did show reduced tire rubber oxidation under increased stress and high
temperature oven- aged conditions ( 65 psi tire inflation, at 60 C for up to twelve weeks).
However, the same study did question the true real- world “ reduced leakage” benefit of nitrogen
inflation for passenger tires because in these tires much of the leakage is associated with losses
around the rim flange and at the valve itself rather than permeating through the tire rubber
( Baldwin et al, 2004).
With this information and limited data, we estimate the extent to which nitrogen inflation systems
for vehicles could prolong tire service life for average vehicles. The primary mechanism
explored is the improved ( i. e., lower) oxidation effects of nitrogen as an inflation medium, and
how this could reduce the number of some premature tire replacements. We also assess the
potential benefits of improved retention of air pressure, for its potential impacts on tire longevity.
We emphasize that it is difficult to estimate quantitatively the improvement in fuel economy, tire
14
wear, and mileage life that could result from the use of nitrogen inflation. If the use of nitrogen
would result in significantly less variability in tire pressure, then improvements could be
significant – possibly as large as the 25% claimed in some of the articles on the subject. This
would only be the case if the tires using air were under- inflated by 10- 15 psi, which is quite large.
The magnitudes of the improvements strongly depend on the attention given to tire maintenance
using air. As a result we employ ranges for potential effects.
Trade- Offs in Tire Attributes
Although tire technologies and attributes were discussed independently above, there are known
critical dependencies and trade- offs associated with many of the attributes of tires.
Figure 5 illustrates with the “ magic triangle,” how three critical tire factors – durability, traction,
and rolling resistance – all have to be simultaneously balanced in the development and
manufacturing of tires technologies. It is commonly held that many LRR tires of the past have
delivered sub- optimal performance on at least one leg of the “ magic triangle.” However, the
development of newer silica- filled, lower rolling resistance tires continues offer promising
improvements simultaneously in fuel consumption, traction, and tire wear life as compared to
conventional carbon black- filled tires.
FIGURE 5. The “ Magic Triangle” of Tire Design
The design and construction of the tire and the selection of materials used strongly affect the
rolling resistance simultaneously with other attributes of tires. The switch from bias- ply to steel
belt construction in the 1970s resulted in a reduction of at least 25% in rolling resistance at the
same time as a large increase in tire mileage life. More recently, the NRC ( 2006) study attempts
to correlate the rolling resistance of the tires with tire geometric, traction performance, and tread
wear ratings, and some general conclusions can be drawn from the correlations. It was found that
in general the rolling resistance of tires for large rim diameter ( 16”) were lower than those for
small rims ( 13- 14”) and that tires designated as performance tires ( better traction and higher
15
speeds) had relatively higher rolling resistance than the average. It was uncommon to find a tire
with a rolling resistance coefficient less than 0.009 that also had high performance ratings. The
wear grades of tires vary over a wide range from Uniform Tire Quality Grading ( UTQC) ratings
from 200 to greater than 600. The correlation of these UTQG ratings and the rolling resistance
coefficient ( RRC) were found to be uncertain in general, but the tires with the lowest rolling
resistance (< 0.008) have low- to- middle wear ratings ( UTQG 300- 500) in nearly all instances, as
shown in Figure 6.
FIGURE 6. Tire Data on RRC and Tread Wear ( NRC, 2006)
Tires of all sizes are available with a wide range of characteristics and prices. Information on tire
load, speed, traction, wear, and temperature characteristics can be inferred from the tire ratings
required by the U. S. Department of Transportation ( US DOT). In addition, for many tires, the
manufacturer lists a mileage warranty. Unfortunately, no information is presently available to the
tire purchaser concerning the RRC of the tires. Hence it is not possible for the consumer to
determine the trade- offs between rolling resistance and the other tire characteristics. This
deficiency in the information available to the consumer is now recognized. Some rolling
resistance data are now available in the technical as well as the popular automotive literature, but
most consumers are not aware of it. Some of that information is reviewed in this section and
what it means relative to purchase decisions is discussed.
Information on tire characteristics including price is readily available on the web for most of the
tire manufacturers. Many of the large tire dealers have websites that list the tires available by
size, characteristics, and price using the USDOT rating designations. If rolling resistance was
included in the ratings, it would be rather straightforward to make the traction, wear, rolling
resistance, and price trade- offs that would be appropriate. Researching the tire lists on the web, it
soon becomes apparent that any trade- offs must be done for a fixed tire size and manufacturer as
each manufacturer seems to have a “ price niche”. In general, the tire price increases as the size
( rim diameter and tread width) increases ( NRC, 2006). In addition, tires with higher speed and
traction ratings are more expensive. Except for tires with very low wear rating ( UTQG rating less
than 300) and high wear rating ( UTQG greater than 600), there does not seem to be a strong
correlation between wear rating and price. Other rating and marketing factors seem to be more
16
important in the mid- range of wear rating. The manufacturer’s mileage warranty for these tires is
40,000 to 60,000 miles. In most cases, tires in the low range of wear rating have no mileage
warranty indicated and tires in the high wear range are warranted for 70,000 to 90,000 miles.
These high mileage tires are usually significantly more expensive than the other tires.
The key issue for this report is the trade- off between rolling resistance, wear, and prices. This
question has been considered in some detail in the recent report of the NRC Tire Committee
( NRC, 2006), which concluded that there was no clear correlation between rolling resistance and
price when size and speed ratings were fixed. Table 6 does not address the question of the
influence of tire wear on the trade- off between rolling resistance and price. The same report also
discusses the trade- off between rolling resistance, tread wear and price, but does not reach any
firm conclusion.
There are approaches to lowering rolling resistance and price without compromising tire wear.
For example, some data indicate that some tires on the market exhibit low rolling resistance with
good tire life and traction properties ( Green Seal, 2003). This is especially true for 16” rim tires
for which low aspect ratio ( 55) tires are available. Such designs seem to favor low rolling
resistance. Since the wear rating of most tires sold are in the UTQG range of 400 to 600 and
those tires have mileage warranties generally between 40,000 and 60,000 miles, it seems likely
that a reduction of at least 10% in RRC can be achieved without reducing tire life. This would
reduce RRC from about 0.01 to 0.009 in replacement tires. Future developments on tread
compounding could lead to further reductions to 0.008 or 0.007 without compromises in mileage
life and significant increases in price. If tire labeling would include a rolling resistance
designation, then there would be competition between tire manufacturers in that area and
improvements in rolling resistance would likely follow. Rolling resistance labeling would also
promote the development of a standard test procedure and a large increase in the availability of
rolling resistance data.
In addition to the manufacturing trade- offs qualitatively addressed above, there are several
important real- world trade- offs between maintenance practices, rolling resistance, and tread wear.
It is well known that under- inflation of the tires and wheel miss- alignment result in higher rolling
resistance. It is generally accepted ( Kelly, 2002) that RRC varies as the inverse of the square root
of the tire pressure ( RRC = RRCo ( P/ Po)-. 5 ). The increase in rolling resistance with under-inflation
for a typical tire is shown in Table 4. The pressure shown is gage pressure, not absolute
pressure. As can be seen in Table 4, the rolling resistance increases by about 1.8 % for each psi
of under inflation. And, applying the above finding that a 10% reduction in RRC will result in a
1.5% increase in fuel economy for a 10% reduction in rolling resistance, we estimate the extent to
which under- inflation impacts fuel economy. An average 7 psi under- inflation ( 20% below a 35
psi placard level) would result in a 11% increase in rolling resistance, and a 1.6% decrease in
average fuel economy. The mis- alignment of the tires can also increase the rolling resistance.
According to Duleep, 2005, the effect of toe- in alignment is a 1% increase in rolling resistance
per 0.15 degrees; the effect of the slip angle is larger, at 5% increase for 0.5 degrees and 16% for
1.0 deg slip of the tires.
17
TABLE 4. Variation of Rolling Resistance Coefficient with Tire Pressure
Tire pressure
( psi)
Standardized
Pressure
( P/ Po)
Rolling Resistance
Coefficient
Standardized Rolling
Resistance Coefficient
( RRC/ RRC0)
35 1.0 .010 1.0
32 .914 .0105 1.05
28 .8 .0112 1.12
25 .714 .0118 1.18
21 .6 .0129 1.29
18
Chapter 3. Data Collection
This project’s data collection concerns multiple facets of tire maintenance and tire technology.
Original data was collected through interviews with vehicle personnel about tire- related
procurement, inspection, and maintenance practices and surveys of vehicle users. This chapter
details the methods and results from these sessions.
Fleet Personnel Interviews
This section summarizes the information gathered from a series of interviews of vehicle fleet
managers and maintenance personnel. Information was gathered from two different fleets,
dubbed “ A” and “ B,” with facilities in northern California. This summary is intended as a
general narrative of key features of fleet management, tire maintenance and monitoring
procedure, tire replacement practices, and fleet receptiveness to new technologies as they relate to
our overall project. Information gleaned from these interviews informs the later project steps of
deploying, monitoring, and assessing new tire technologies in vehicles, and aids in the
development of the “ best practices” guidelines for fleets. In most cases, Fleets A and B shared
similar practices. Where noteworthy differences between the two fleets’ policies and practices
exist, these distinctions are highlighted. Note that quotations may not be verbatim from the
interviews but rather are meant to emphasize a general point made by interviewees. Also note
that all numbers given here are estimates from the interviewees, and do not involve any data
gathering from log sheets or databases.
Method of Information Collection
A total of three interviews were administered – all in similar settings. The Fleet A interview was
conducted as an open discussion between three researchers and four managers ( in positions or
areas of maintenance, management, purchasing, and technician) in an office on the fleet’s work
site. For Fleet B, two separate interview sessions were conducted – one with seven maintenance
or shop personnel and the other with four managers. Both of the Fleet B interviews took place in
the break room at the vehicle fleet garage.
The survey questions asked in the interview are reproduced in Appendix A. Although the entire
survey was conducted verbally for all sessions, blank question sheets were given to the
respondents to guide them through the topics and give them a chance to jot down additional notes
throughout. Both the Fleet A and Fleet B manager interviews lasted about one hour. The Fleet B
maintenance personnel interview, with more participants and more input, lasted an hour and a
half. Researchers took notes throughout the interviews; audio recordings of the sessions
facilitated clarifications of the notes. The below sections summarize and reorganize the
information from the discussion- style interviews.
General Fleet Information
Fleet A is a smaller fleet, with approximately 700 to 800 vehicles affiliated with a university
campus. Of this fleet, about a tenth of the fleet are heavy- duty ( i. e., one- ton or larger) trucks,
19
including the campus buses and larger utility trucks. There are also nine police cruisers, seven
40- passenger buses, and about 120 mid- size and small sedans. The rest, approximately 500
vehicles, are half- ton and three- quarter- ton pickup trucks and vans.
Fleet B is a larger fleet of several thousand vehicles, although only a fraction of these are seen
with any regularity. ( For example, some this fleet’s vehicles are sent off to college campuses for
extended periods of time.) Approximately 1,000 of the vehicles see the maintenance garage in
any given month. Of these vehicles, about three- quarters are shorter- term daily- or weekly- use
vehicles, similar to a commercial rental vehicle fleet. These short- term vehicles, being seen more
often by the shop, are generally checked and maintained more routinely. Most of the other
vehicles that are routinely seen are longer- term, generally monthly, leases. These vehicles are
seen less often at the fleet garage, and generally are brought into the shop after the driver has
either accumulated a list of “ to- do” problem items. In some cases these long- term vehicles had
repair work done by outside vendors.
General Tire Maintenance Practices
There is no official guidance manual or formalized set of procedures for tire maintenance and
monitoring. Vehicle and tire manufacturer specifications offer the bare minimum requirements
for tire inflation and maintenance, and, in addition, fleets implement their own routines for
monitoring and maintaining tires. Fleets include tire upkeep in their preventative maintenance
program that includes tire monitoring, tire rotation, oil change, etc., on each vehicle for every six
months or 6,000 miles of vehicle use ( whichever comes first). At that time, tires are inspected for
inflation pressure, tread wear, and any other defects. In addition to these scheduled maintenance
procedures, tires are visually inspected each time they come into the shop for obvious problems
or defects. Special attention is given to the vehicle tires if the operator, when dropping off the
vehicle, notes any particular problems with driving, handling, or road noise that may be
associated with tires. The “ long- term” vehicles that are checked out for many months at a time
are not monitored by the fleet personnel; these vehicles may or may not be monitored by vehicle
operators or other mechanics elsewhere.
Tire Monitoring and Inflation
During vehicle servicing, tire pressure is generally checked with handheld, pen- sized ( non-digital)
tire pressure gauges. Several respondents questioned the accuracy of the devices and
mentioned they are not calibrated or checked for accuracy. Several stated that the inflation was
always checked cold ( generally in morning, before the vehicles are driven). One technician noted
that checking cold makes a large difference – about 10 pounds ( per square inch). This procedure
is not uniform, however; one manager said that often, immediately after vehicles were in service,
the tire pressure is checked. Although managers and mechanics alike acknowledged potential
inaccuracy of the handheld devices and had seen digital tire pressure monitoring devices,
purchasing these devices was described as low priority. Despite the higher cost of the digital
gauges, the managers made it clear that operating budget was not an issue. One manager noted
the study to be an opportune time to invest in digital tire pressure readers.
Fleet maintenance personnel offered numerous tire pressure inflation guidelines or “ rules of
thumb” that they follow. Setting “ to the manufacturer specs,” or “ at least at the manufacturer
specs” ( from specifications in the vehicle owner manual and/ or as dictated by the plate inside the
vehicle door) were stated several times. The recommended pressure varies by vehicle and
20
specifically the wheel size and type on the vehicle ( e. g. Chevrolet Impala wheels, steel vs.
aluminum have different specs; the steel wheels— and naturally the tires fitting them— are
narrower than the aluminum ones, therefore calling for a higher inflation pressure).
However, when prompted to state the advisable value or range of values at which tire pressure
should set, many mechanics offered different responses based on varied reasoning. For example,
one mechanic noted that many clients will complain about a rough ride if the tire pressure is set
too high. In contrast, another mechanic’s recommendation was to set pressure 5 pounds ( all
respondents referred to the unit of “ pounds” more often than “ psi” or “ pounds per square inch”)
over the manufacturer specifications, claiming a result of increased fuel economy and reduced
wear. Other individuals offered their own ideal numbers of 32, 35, and 36. The “ 36” response
was followed with the explanation that you never know when the shop will see some of the
vehicles again ( it could sometimes be many months without routine maintenance or monitoring
for the longer- term leased vehicles), so it was best to err on the high side. It was mentioned that,
according to a manufacturer, tire pressure can be inflated to 40+ psi for improved economy and
wear ( with a negative trade- off of perceived road harshness and uncomfortable driving). One
mechanic mentioned that gas- electric hybrid tires are supposed to be inflated much higher – up to
55 psi.
Data Tracking
There is no set mechanism to track or log the life cycle of tires while also noting replacement,
wear and tread depth, maintenance/ repair work, and discarding. There are data taken on each
vehicle’s history that would contain information on some of these factors. Information related to
tire history, such as tire purchase, tire repair work, and discarding of tires are kept in work orders,
but are not specifically logged or thought to be readily available in a database. The tire recycling
company could keep more reliable data on discarding. Even the data that is available in work
orders for tire maintenance could be somewhat suspect; work orders could convey whether tire
work was done, but with a lack of description of the nature of the work ( e. g. tire repair, patch,
inflation, alignment), or the comments could be inaccurate ( e. g., order could refer to “ left front,
but it’s really right rear” tire). A manager suggested that a new log could be made and kept in the
vehicles to keep better track of tire history. ( This could be similar to the “ Automobile
Maintenance” record- keeping book already kept in the glove box.)
Tire Replacement Practices
Fleet workers were asked numerous questions about tire replacement practices in order to
increase researchers’ understanding of what dictates the life cycle for a tire in the fleet. General
reasons for disposing tires ( or retiring them from the fleet) included low normal tread wear,
irregular tread wear ( e. g. flat spot), irreparable puncture or defect, and replacing tires with a set of
two or four to maintain overall vehicle balance ( despite some tires having useful tread life
remaining). Several fleet mechanics commented on the importance of vehicle balance and on
tires’ link to the computers of the anti- lock braking systems ( ABS), emphasizing that it was
necessary to keep tires that are very similar in sets.
The tires that are disposed of for the reason of normal tread wear achieve their full useful life
cycle. Whether this full useful life is 20,000 miles, 40,000 miles, or more, is highly variable
based on vehicle type, tire type, and driving behavior. Interviewees were reluctant to offer any
21
usual, average, or expected tire life mileage. Personnel also did not offer any rules or guidelines
on tread depth to indicate the time for discarding the tires.
Tires did not last their full useful life for several reasons. Uneven or irregular wear could cause a
“ secondary vibration,” where a relative flat spot in one or more tires could have resulted from an
abrupt braking incident. Such an incident would make for uneven driving and would prompt the
driver to take the vehicle in to the shop to fix the unevenness with replacement tires. Tire defects,
tire separation, or tire oxidation are rarely the cause of replacing tires. In the case of vehicles that
are likely to be checked out for monthly leases, where there is a low likelihood of seeing the
vehicle soon, mechanics could opt to replace tires a little earlier than normal, to be on the safe
side, assuming that many drivers would not be monitoring their tires.
Fleet workers were reluctant to estimate when and why tires were replaced; they offered very
rough estimates when pressed. Fleet B estimated about half ( answers “ about 50%,” “ 35% to
45%”) of their tires made it to the end of useful tire- wear life – where low tire tread depth is the
primary reason for disposal. Fleet A estimated approximately 80% to 90% of their tires lasted
until the end of useful tire- wear life. More irregular reasons like tire unevenness ( e. g. flat spot) or
tire defect could be responsible for about 10% to 20% of replacements, Fleet B estimated. The
majority of these irregular reasons were thought to be due to driving- related problems like where
and how the vehicles are driven ( as opposed to inherent tire manufacturing defects).
Based on the above numbers, the remaining tire replacements, perhaps lower than 10% ( Fleet A)
or as high as 30% to 40% ( Fleet B), are discarded when useful life remains but a member of that
tire’s set ( two or four) is being discarded. Note that these numbers are rough estimates, on which
no data has been collected. When one tire is discarded due to uneven wear, the decision on what
to do with the other “ still good” tires in the set differs to some extent depending on the mechanic.
In Fleet A, most of these “ still good” tires would be put aside, and would remain stored until
another similar tire ( same size/ type and with very near the same tread- depth) was in need of a
similar tire to make a pair. In Fleet B, guidelines for the “ still good” tires were offered: if the
tires still had less than 20,000 or 25,000 miles on them or at least half of the tread remaining, they
kept them; however, if the remaining tires had more than 20,000 or 25,000 miles on them ( or less
than half of the tread) the tires would go to the disposal tire pile. For example, if a car with
25,000 miles on each tire came in with an irreparable flat, the whole set of four would likely be
replaced. These guidelines for miles and tread depth appear to be based on appearance of the
tires rather than on actual measurements. Also, if tires were in storage too long, they would be
discarded ( because of concerns about drying/ cracking).
On the issue of reuse tires within the fleet’s vehicles, a worker from Fleet B thought that perhaps
5% ( but probably less) of tires taken off of one vehicle would ever be placed onto another vehicle
in the fleet. Related to this lack of reuse of tires, a worker showed one of the interviewers the
rack of about twenty “ still good” tires that were ready to be reused for combining to a similar tire
size and type with approximately the same tread- wear. The tires were unlabeled and unsorted ( by
size, type), and the fleet worker commented on how this inconvenience and lack of organization
limited the likelihood that workers would opt to reuse these tires; it was simply much easier to
grab a new tire ( or set of tires).
Overall cost trade- offs factor into the mechanics’ decisions on tire replacement and
tire/ vehicle repairs. Fleet mechanics said that they tried to look at the “ bigger picture.”
Related to the question of replacing tires, fleet personnel sometimes reacted differently to a
driver claim of unevenness on the road. Even if a minor alignment problem could be the
cause of uneven wear in the tires, sometimes a decision could be made to replace the tires ( a
22
set of two at about $ 60 for the set or all four for $ 130) instead of a more labor- intensive
alignment repair ( at about $ 150); however, sometimes the tires could be replaced with the
alignment correction if the problem was more substantial. As with the alignment issue, the
“ bigger picture” cost perspective was cited in the case of replacing two versus four tires.
When only two tires were ready to be replaced ( but the other two tires still had some life
remaining) all four could be replaced. Considering that any job is a minimum of one hour
labor, it would be best to replace all four ( instead of later, perhaps in a couple months, having
another job to replace the other two).
In response to a separate question about whether tires were replaced individually, in twos, or as a
full set of four, Fleet A and B responses were roughly consistent with one another ( although,
again, the percentages are only crude estimations). Mostly, tires are replaced in pairs or as a full
set of four. Again, percentage estimates were offered only when prompted by interviewers.
Perhaps 10% of replaced tires are replaced individually. Approximately 45% to 75% of tires are
replaced as a set of two. The remaining 15% to 45% of tires are replaced as a full set of four at a
time.
Mechanics pointed out that law enforcement officers demanded new tires more frequently than
any other drivers. Law enforcement vehicles had special driving needs ( more aggressive driving,
handling, safety in pursuit driving situations) that were likely to cause more instances of uneven
wear; as a result, law enforcement drivers would dictate when their tires get replaced. When law
enforcement drivers requested new tires, the tires would be replaced. If there was a nail in a tire,
instead of patching the tires, the tire ( or set of tires) would be replaced. In part, this relative lack
of desire to repair could be due to the different ( softer) rubber of these higher traction tires for
police squad cars. Fleet personnel suggested that sometimes these drivers “ just wanted new
tires.”
Tire Purchasing
For both Fleets A and B, the key determining factors for tire purchasing were government-discounted
pricing contracts and maintaining the status quo. State government discount contracts
with several tire companies ( e. g. Goodyear, and Bridgestone/ Firestone) offer substantial
discounts from retail prices, and one of the fleets may receive additional discounting ( beyond
government pricing) from a local retailer. One worker stated that their discounts could get the
fleet $ 200 retail- priced tires for just $ 50 per tire. When asked about which qualities of tires they
focus on in purchasing new tires, they responded with common themes: “ stick with the same,”
“ are they the same as the old ones?” and “ never go cheaper or worse in quality than the
previous.” For fleet managers to consider new or different brands of tires, the new brands would
have to be as good as or better than the OEM tires and the tires they had chosen in the past.
Exceptions to these tire choice criteria were rare, but would be made, for example, for tires on
hybrid vehicles, or for a new trend in vehicle tires. In the latter case, such as a trend from 16”
wheels to 17” wheels, exceptions would be made when a standard brand is somewhat slower in
deploying these tires to the market than other companies.
After being pushed to speak more generally about tire qualities ( i. e. outside of contract/ status quo
related reasons for choosing particular tires), Fleet B workers listed some criteria. They
mentioned that the life of tires ( e. g., rated at 60,000 miles or more is better) was important. A
worker involved with purchasing commented that they would want to avoid switching to any
other brands and models to reduce chances of mismatch problems ( tire type, rating, wear
qualities), which could result in drivability problems. Also, there was a comment on tread design
23
regarding new RSA ( a Goodyear model) tires were better than some weather tires. However,
Fleet B personnel did not bring up topics of safety or fuel economy.
Fleet A offered up some commentary about the criteria that weighed into their purchasing
decisions. Safety comes first, and tire longevity is also very important. This fleet’s reduced tire
pricing has been so good that they can just look to get the best tires in terms of safety and
longevity, without being all that concerned about the potential trade- offs that these tires could
have with respect to per- tire costs. The rolling resistance of the tires was not mentioned until we
brought it up. The fleet’s only experience with LRR tires was with electric vehicles. The fleet
managers mentioned that the suppliers/ sales agents from whom they bought tires were very
helpful and knowledgeable with respect to Goodyear tires.
Independent of the tire contracts, workers offered additional commentary on tire brands.
Several were partial toward the Goodyear Regatta II tire, citing that it was a high quality tire
at a great price ($ 33/ tire, after discount). On the General brand, one worker commented, “ we
hate them” and others agreed, citing issues with balancing, uneven wear, and separation.
About Bridgestone/ Firestone, there was mention of a separation issue, but generally fleet
personnel thought the tire was a quality value tire.
Fleet Personnel Commentary on Vehicle Drivers
Fleet workers expressed general, mild resentment toward drivers of fleet vehicles due to their lack
of care for the vehicle they are operating. Mechanics of Fleet B said that the vehicle users,
especially the long- term leasers, should check oil and tire pressure, but they do not. Another
commented that he just wished that the drivers would, “ Keep tire pressures up … there’s no way
to train these people.”
Some problems pertaining to the vehicles and their tires are reported to the fleet personnel
upon vehicle return. In a 3- month period, one pool attendant gets five or so tire complaints
( generally concerned about safety) out of dozens of vehicles. Drivers can be either
indifferent or negligent in reporting obvious issues when bringing in cars. Sometimes flat
tires are not discovered until the attendant retrieves the vehicle from the parking garage, when
such a problem presumably would have been noticed by the last driver. Many drivers will
run vehicles on flat or extremely under- inflated tires for many miles, which ruins the tires and
risks tires shredding or blowing out.
Despite the generally negative outlook on the drivers, there was one small silver lining: The
late model Chevrolet Impalas are equipped with tire inflation warning indicators, and
operators are quite quick to bring them in for maintenance when indicators directed them to
do so. A technician commented that noticing under- inflation visually could occur only in
very serious cases of under- inflation ( i. e., of 50% too low, or 15 psi to 20 psi too low), but
with the indicators, people were more likely to bring problem vehicles in, if and when the
light came on.
Purchasing Budget and Impact on Tires
There is not an itemized tire- specific budget. Tires are incorporated in the general maintenance
budget. Fleet B managers estimated that tires could be about one- third of the maintenance
budget, excluding the costs from work that is contracted out to outside vendors ( see next section).
24
Fleet A managers, when prompted, estimated that the tire procurement and maintenance work
was about 25% to 35% of their total expenditures.
When asked if budget constraints affect their decisions at all, the purchasing agent for Fleet B
said that the fleet got what it needed and there was no perceived restriction or limit on getting
high- quality ( safe and long- life) tires. The managers echoed this comment. Manufacturer
discounts provide large cost reductions that allow more flexibility in purchasing higher quality
tires. Likewise, Fleet A managers said they were not constrained by budget. They focused on the
great value they get with tires ( e. g., $ 200 high- speed v- rate tires for police cruisers for about $ 50).
Additionally, they said they experience very low failure rates, get great tire performance, and
have safe, long- lasting tires.
Throughout the interview, several commented that time can be a much more significant cost to
the agency than money. This sentiment was present in a statement, noted earlier, that it was
cheaper to replace tires than to fix an alignment. It can be cheaper to change a tire than to do
other maintenance work in some circumstances. For example, if a car with more than 100,000
miles on the odometer will be retired at 120,000 miles, they may simply change tires instead of
realigning because it is less expensive.
Work Contracted Out
Both of the fleets contracted out tire work on the heavier trucks in their fleet ( generally 1- ton
and greater trucks, those with tires 20 inches or larger), primarily due to equipment
limitations at the fleet garages and personnel safety reasons. They lack the racks, floor jacks,
tire machine, wheel balancer, and tire retreading (“ recaps”) equipment needed for heavy- duty
trucks tires. The key safety concern: “ Why do the harder stuff?” Also, the ceiling height,
space, and time limit the fleet personnel from working on larger trucks. They commented that
they “ Use our vendors as a safety net” or “ We bring in what our guys can handle in a day’s
work. The rest we send out to our vendors.”
Disposal of tires is also contracted out. Fleet A pays $ 0.90 per tire for disposal for a state-registered
waste hauler ( TriC Tire in Sacramento) to pick up the tires. The tires are shredded and
used in road asphalt. They previously paid $ 3 per tire for disposal. Fleet B, on the other hand,
receives $ 0.75 per tire for removal of their tires. A private contractor, who also removes old
batteries, picks up a load of tires every two weeks and probably resells some of the tires with
significant tread remaining.
Tire- Related Technologies
Fleet managers offered generally optimistic outlooks on tire technology, past and present. One
technician commented that the that tire technology has made “ huge leaps” in the past decades –
that they have had virtually no trouble with the Goodyear tires they have been using and that they
now have a very low probability of rollover when tires blow out ( because the tires stay on rim
after blow out now). Managers were very optimistic toward research like this UC Davis study,
feeling that it was a “ real world” type project that can be used for wider benefits. The following
section highlights response from fleet personnel and managers on specific technologies related to
tires.
25
The technology of note for this UC Davis tire project with which the fleets have experience is
tire pressure monitoring systems ( TPMSs). These inflation monitoring systems have
indicators built into the vehicle dashboard of the late model Chevrolet sedans, including
Impala models. They are not on the police package vehicles. The dashboards feature an
indicator light that illuminates in instances of significantly under- inflated tires or slow-leaking
punctures. Describing the indicators as “ large,” “ hard to ignore,” and “ conspicuous,”
personnel thought that these systems were “ useful” and “ helpful” in that they probably “ scare
the driver into bringing in the vehicle” to get the light to go off when tire pressure is low.
This in turn was probably good for safety because it could help avoid a blowout. Personnel
noted that the shop visually checks tires anyway when they come in, so the TPMSs are not
thought to greatly impact vehicles that come into the garage regularly, but the systems would
help more with longer- term leased vehicles that are not checked as frequently.
Mechanics commented that the TPMSs had to be reset after a driver had taken the systems in
to be checked out in response to an indicator light. When asked if the systems were always
accurate, mechanics could not recall any incidents otherwise. One mechanic mentioned a
circumstance of a customer learning how to reset the indicator with the radio panel to make
the light go away without taking the vehicle in.
Fleet A also had some experience with a valve stem cap technology for truck and bus tires.
The valve stem caps had lighted indicators on the tip that had certain colors to indicate tire
pressure under- or over- inflation. Personnel found the caps especially useful on the back
inside dual wheels, where the inside wheel is hard to get at and hard to gauge. These devices
were quite expensive when the fleet first got them ( approximately $ 12 apiece). Even though
the cost may be down to $ 20 per set of four now, they said they could not justify the price to
continue purchasing.
Fleet A was familiar with nitrogen inflation; they had read about it and had salespersons pitch
the idea. They are generally in favor of implementing it if the department is willing to invest
the funds. Cited reasons for using nitrogen included that it is cleaner, nitrogen sustains air
pressure in tires longer, there is no fluctuation of pressure with temperature with nitrogen in
the tires, and there is a reduction in the oxidation of the tires. If offered the opportunity to
switch over to nitrogen inflation, the head mechanic responded, “ Why not?”
Fleet B was similarly in favor of nitrogen inflation for their tires. They mentioned that the
system does not lose pressure with temperature, and nitrogen allows no moisture inside the
tires. Personnel cited that they “ do it at Costco.” They recognized that it must have some
benefits if they are using nitrogen inflation in NASCAR and in aircraft tires. On the other
hand, one fleet worker suggested that they change and monitor tires so much that maybe such
a technology at their garage may have less impact: “ The benefit’s not gonna be that great for
us. We replace tires too often.”
There is minimal experience with low rolling resistance tires. Hybrid- electric vehicles in the
fleet come with low rolling resistance tires. The replacement tires for these vehicles are
specially ordered, and the tires are inflated to higher pressures than the normal tires.
Another technology idea was offered by a Fleet B employee. A pool attendant who oversees
vehicles coming into the garage raised the idea of a vehicle scanner. The vehicles, previously
embedded with a bar- code, could be scanned when entering the garage. The scanner
computer could flag or indicate whether the tires on the vehicle were old and need changing
26
with a “ tire repair/ check recommended” prompt. This could supplement the ongoing visual
checks.
Impact on Personal Tire/ Vehicle Decisions
The fleet manager and personnel were asked about whether their on- the- job exposure to many
vehicles and tires impacted their personal tire practices in any way. Several interviewees
commented that it impacted what tire brands to purchase ( pro- Goodyear, anti- General). One
mechanic mentioned that he checks tires monthly for tire pressure for better mileage and
wear. Another checks and adjusts every 6 months, citing seasonal changes. Another said that
he checks tire pressure more often due to his experiences at fleet services.
When asked about the impact of managing Fleet A’s tires on their own personal lives, one
garage head thought mostly about under- inflation. When his daughter is setting out on a long
( 8+ hour) trip, he tells her to be sure to check the tire pressure due to his concern about
safety. He, however, does not check his own tires that often. He is worried about under-inflation
more than anything else, because of their susceptibility to blow- out, especially if the
tires get hot on a long trip. Another commented that since manufacturers went from bias- ply
to radials years ago, there is much less burden on vehicle users, and there may be some
rationale for less concern about tires now. The porosity has gotten so much better, and the
tires retain their air much better than before; as a result, he used to check tire pressure all the
time, but now not so much.
Driver Survey
The driver survey portion of this study concerns a collection of data on driver behavior,
preferences, and attitudes on topics of vehicle maintenance, tire attributes, tire purchasing, and
new tire technologies. Of interest for this study were the following areas:
• Existing air pressure knowledge and practices
• Prior incidents of low tire inflation in personal vehicles
• Prior incidents of low tire inflation in fleet vehicles
• Method for recognizing low tire inflation
• Operator action in the case of low tire inflation
• Practices in personal vehicles
• Willingness to implement technological measures in fleet/ personal vehicles
Survey Method
The survey population is the users of fleet vehicles at a northern California fleet vehicle garage.
The sampling frame is the vehicle users who passed through the vehicle dispatch office at the
garage to check out vehicles between March 30, 2006 and April 18, 2006. These drivers,
government employees who obtain rental vehicles for their work duties, were solicited to
participate in the survey by a graduate student and/ or the fleet’s operating dispatch employee.
The drivers were given informational letters on the purpose of the survey and were given a
chance to ask any questions about the research. The drivers were then asked to fill out the brief
27
10- to 15- minute surveys before checking out their vehicles. The driver surveys were self-administered.
The informational cover letter and survey are reproduced in Appendix B.
About 40 to 70 clients passed through the dispatch office per day during the surveying period. By
the third week of surveying, there were many repeat clients who had already taken the survey and
were ineligible to take it again. A total of 165 surveys were returned. The response rate, based on
the number of unique vehicle users passing through the office who turned in surveys, is estimated
to be approximately 50%.
As suggested by the above survey topics, the surveys were designed to capture a wide array of
variables to better understand vehicle operators’ working knowledge of tires and potential barriers
and opportunities for new tire- related technologies. Most questions related to their own vehicles,
and several questions on the survey were asked specifically about the drivers’ use of the state
fleet vehicles. The survey results are summarized below, with emphasis on capturing information
relevant to developing a “ best practices” manual for managing fleet and gauging drivers’ interest
in new tire- related technologies that are assessed in this research project.
Because of the narrow scope of this study, and its aim of aiding fleet tire- related practices, there
are several limitations in generalizing the results of this study to the wider population of vehicle
users. The survey’s population ( client of one vehicle fleet) is not necessarily representative of the
population at large; to the extent that the government employees who are checking out vehicles
are not representative of general vehicle users. Additionally, to some extent, the survey- taking
fleet clients during the March- April timing of the study could differ from those clients who check
out vehicles at other times of the year, and therefore not be representative of the yearly population
of drivers. The context of the study involving research on waste and energy saving tire
technologies could induce some social desirability bias, if survey respondents were compelled to
give the “ right answers.” For example, vehicle operators could feel compelled to overstate how
well they take care of their vehicles.
Survey Results
The survey respondents were overwhelmingly personally responsible for maintaining their own
vehicle’s tire pressure inflation and tread wear. Most survey respondents, 77%, reported that they
do check their own tire pressure on their personal vehicles, while 13% rely on someone else to do
so, and the remaining 10% did not have their tire inflation monitored. When drivers discovered
improper tire pressure, whether from an actual tire pressure measurement or visual observation,
87% personally restored their tire pressure to the correct level while 11% took the vehicle to an
auto garage to have someone else refill the tires. Likewise, 90% of vehicle operators reported to
check the tread wear of vehicles they own.
The frequency with which tire inflation was monitored by vehicle users varied greatly. A small
percentage, 10%, checked their pressure weekly or within a month. Most common frequencies
were approximately monthly monitoring with 36% of respondents ( including responses between
6 and 12 times per year), and seasonal monitoring with 33% of respondents ( including responses
from 2 to 5 times per year). Significantly, 11% of respondents reported that they relied on
appearance of the tires to dictate the frequency of tire pressure monitoring and inflation. The
remaining 11% used other various timing indicators ( e. g. when vehicle is serviced) to dictate how
often they had their tires checked. One driver reported reliance on the dashboard indicator
( presumably from a new late model TPMS) to inform on whether any low pressure tires.
28
Those surveyed generally rely on simple devices or visual inspection for monitoring of tire
inflation and tread wear. Of the persons who monitored their own inflation pressure, the vast
majority, 72% of vehicle operators, rely on “ pen- type” tire pressure gauges. Other tire pressure
measuring devices were less common: Digital gauges ( 11%) and dial- type gauges ( 11%). And
5% of respondents relied on “ visual inspection” to gauge their air pressure. To monitor tread
wear, 81% of respondents relied on “ visual inspection,” while another 10% used a coin ( e. g.
penny, dime, quarter) to approximately gauge remaining tread life. Much smaller percentages
( 1% and 7% for digital and ruler- type gauges, respectively) utilized actual measurement devices.
A sizeable number, 27% of respondents did not offer an answer for the correct tire pressure for
their vehicle. Respondents’ reported values for their vehicles’ correct tire inflation pressure were
consistent with average vehicle placard levels in the NHTSA ( 2005) nationwide survey.
Reported correct tire pressures for respondents’ personal vehicles are shown in Figure 6. The
mean reported tire pressure was 33 psi ( median 32 psi). The responses for drivers who monitored
their own vehicles tires had the following distribution for their vehicle’s correct tire pressure:
15% less than 32 psi, 43% from 32.0 to 33.9 psi, 28% from 34.0 to 35.9 psi, and 13% at 36 psi
and above.
0%
10%
20%
30%
40%
50%
Less than 26
26 to 27.9 psi
28 to 29.9 psi
30 to 31.9 psi
32 to 33.9 psi
34 to 35.9
36 to 37.9
38 psi or greater
Reported Correct Tire Pressure on Personal Vehicles
Percent of Responses
n = 121
mean = 33.3
median = 32.0
standard deviation = 4.2
FIGURE 7. Responses for Correct Vehicle Tire Inflation Pressure
Several survey questions inquired about tire procurement decisions and the relative importance of
various tire attributes. Using a five- point Likert- type scale, tire attributes were rated from 1 =
“ Not Important” to 5 = “ Very Important.” The results from this question are ordered and reported
in Figure 8. Safety was the highest- rated tire attribute, with an average score of 4.6 on a 5.0- point
scale, and with 92% of respondents reporting it as a 4.0 or 5.0. “ Expected tread life” registered
the second highest score, followed by wet- whether performance. Each of these three factors
scored higher, on average, than the factor for tires’ purchase price in importance in the tire
purchasing decision.
29
Tire Attribute “ Not Important” “ Very Important” Mean Score Standard
Deviation
4.60 0.74
4.28 0.94
4.17 0.96
4.11 0.91
3.98 1.06
3.80 1.12
3.74 1.14
3.57 1.18
2.91 1.30
2.50 1.27
0.0 1.0 2.0 3.0 4.0 5.0
Style/ Appearance
High- speed performance
Noise
Comfort/ Ride
Fuel economy
Handling
Price
Wet- weather performance
Expected tread life
Safety
FIGURE 8. Driver Tire Attribute Importance on Five- Point Scale
When asked more generally and qualitatively to define “ How do you decide which brand and
type of tires to buy?” a wide range of answers were offered. Many respondents were brand loyal
( e. g. always buy Michelin), while others were loyal to specific retailers ( e. g. always go to Pep
Boys). Also, many drivers’ first response was regarding the pricing or whether certain tires were
on sale. In purchasing tires for vehicles, drivers reportedly spent on average $ 100 per tire ( mean:
$ 108, median: $ 100, as shown in Table 5). A large number of respondents deferred to the advice
of shop mechanics or used consumer guides or customer reviews to guide their purchasing
decisions. A smaller but still substantial number of operators referenced tire ratings. Few
responses offered particular tire attributes or factors, like safety, handling, tread life ( as discussed
above), as the key factor on deciding on tire purchases.
TABLE 5. Average Price Paid for Last Tire Replacement
Average Price per Tire Number of Responses Percent of
Total
$ 50 or less 6 5.8%
$ 51 to $ 75 18 17.5%
$ 76 to $ 100 29 28.2%
$ 101 to $ 125 24 23.3%
$ 126 to $ 150 16 15.5%
$ 151 to $ 175 5 4.9%
$ 176 or greater 5 4.9%
103 100%
Drivers were asked about their interest in several emerging tire technologies: lower rolling
resistance, TPMS, self- inflating or “ run flat” tires, and nitrogen inflation. In some cases,
technologies were defined in simpler terms for the general survey audience ( e. g. “ more efficient”
instead of “ lower rolling resistance”), and their benefits were identified. For example, in regard
30
to nitrogen inflation, the phrase “ to hold pressure longer” was added. Additionally, respondents
were not offered “ I don’t know” as an option to force them to speculate on the concept of the
technology and its potential benefit.
Figure 7 shows results from this inquiry about vehicle users’ interest in new tire technologies. As
above for the tire attribute question, allowable responses were in a check box five- point Likert-type
scale. To note, all of the technologies drew, on average, positive responses, with scores
ranging from 3.4 to 4.2 on a five- point scale. Of the technologies, higher efficiency tires for fuel
savings drew the most interest, with a score of 4.19. Dashboard tire pressure indicators, akin to
TPMSs, for vehicle safety rated second, and self- inflating or “ run- flat” tires to aid drivers in
emergencies rated third. Respondents showed the least interest in “ nitrogen inflation” with a
score of 3.4 out of 5.0; however this technology’s highest standard deviation suggests that it is the
technology with the most disagreement or perceived uncertainty about its purported benefits.
Tire Technology “ Not Important” “ Very Important” Mean
Score
Standard
Deviation
4.19 0.95
4.03 1.14
3.80 1.11
3.40 1.24
0.0 1.0 2.0 3.0 4.0 5.0
Nitrogen inflation to hold pressure longer
Self- inflating or “ run flat” tires for emergencies
A gauge in your dashboard that tells when tires are under-inflated
for safety reasons
Tires that are more efficient and save you fuel
FIGURE 9. Interest in Emerging Tire Technologies on Five- Point Scale
31
Chapter 4. Technology Implementation
This section describes the implementation of new tire technology at the Department of General
Services vehicle fleet in Sacramento and the preparation and development of an experimental
design by which to test the new technologies. A main conclusion of this study’s literature review,
fleet interviews, and driver surveys is the critical nature of maintaining proper tire pressure in
vehicle’s tires toward improving tire mileage life, reducing tire rolling resistance, and thus
improving fuel economy. Tire pressure technologies, such as tire pressure monitoring systems
( TPMSs) and nitrogen inflation systems, appear to offer significant potential in this area of
maintaining proper tire pressure. TPMSs offer the potential to track real- time records of tire
pressure and temperature, and the systems have the ability to introduce an interaction between the
vehicle and driver to alert the driver of the status of the tires. One variant of these technologies is
already installed on a limited number of the fleet vehicles; we, however, install higher- precision,
“ direct type” TPMS technology to monitor and quantify changes in tire pressure for the fleet
vehicles. Nitrogen as a tire- filling medium offers the potential to reduce leakage, and therefore
less maintenance and longer tire life. With these benefits not yet substantiated in the literature,
our experimental set- up intends to bridge this data gap.
This section summarizes the key installation features of the technologies and the methods that are
to be employed to carry out testing on the vehicle fleet. A brief overview of the nitrogen inflation
equipment is given. To facilitate diagnosing and testing of the tire pressure impacts of nitrogen
inflation technologies, data acquisition systems from which time- stamped tire pressure changes
can be monitored are installed on the vehicles. After a brief summary of this installation process,
a statistical testing methodology by which to experimentally determine the inflation retention
potential of nitrogen as an inflation medium for the fleet vehicles is presented. Further details
and system specifications can be found in Appendix C.
Nitrogen Inflation System
The nitrogen inflation system is comprised of several interconnected elements. The source of the
nitrogen used is the compressed air system of the facility. As illustrated in Figure 10, nitrogen is
isolated from compressed air by a semi- permeable membrane in the nitrogen generator. The
semi- permeable membrane allows nitrogen to pass to the storage tank and shunts all other
components of the compressed air to the permeate port. The nitrogen is then held at an elevated
pressure in the storage tank and dispensed with the inflator as needed for test vehicle pressure fill-ups.
The storage tank allows for nitrogen to be stored for periods of high demand. The inflator
allows for uniform and automatic setting of tire pressure. The inflation pressure can be adjusted
by the user to ensure that the correct placard pressure is always used.
32
FIGURE 10. Semi- Permeable Membrane of Nitrogen Generation Equipment ( Parker-
Hannifin, 2006)
There are several large manufacturers of nitrogen inflation systems, including Branick, Parker-
Hannifin, and Ingersoll- Rand, as well as several smaller manufacturers. Parker- Hannifin was
chosen as our supplier of nitrogen inflation equipment based on their large market share, proven
record of performance, and suitability to the conditions of the application. Further details about
this procurement decision are laid out in Appendix C.
FIGURE 11. Nitrogen Inflation Equipment ( Parker- Hannifin, 2006)
33
Once a vehicle has had its tires inflated with nitrogen, green valve stem caps are applied to
conspicuously notify fleet personnel that the tires are nitrogen- filled. In addition to the green
valve stem caps, the test vehicles have prominent stickers reading “ nitrogen inflation installed” on
the back ( non- reflective) side of the rear view mirror assembly.
Tire Pressure Data Acquisition Systems
The main components of the tire pressure ( and temperature) data acquisition systems for each
vehicle are the four strap- mounted, in- tire sensors, the receiver, and the data acquisition module
( DaQ). The sensors used in this study are of the direct type, sending gauge pressure and
temperature data to the receiver over a wireless signal ( as opposed to the indirect type, using the
relative wheel speeds across axles to infer differences in pressure via effective radii). With a
motion switch, the sensors only send data on timed intervals when the study vehicle is being
driven, thereby saving battery power and eliminating unnecessary data transfer. The receiver
passes data packets onto the DaQ, which stores them in memory.
The four sensors to be installed in each test vehicle were designated with their wheel positions
and respective colors ( pre- assigned by SmarTire – P1 green, P2 red, P3 blue, and P4 yellow –
with vehicle positions as shown below in Figure 12) and entered into a spreadsheet- based lookup
table, which kept track of all vehicles with their respective equipment. A test set- up consisting of
a DC power supply and IBM compatible computer was then prepared for each vehicle’s
equipment to ensure proper functionality and programming of the sensor identification numbers
( IDs). As the receivers relay wireless signals indiscriminately, it is necessary to input each
vehicle’s sensor IDs into the DaQ to prevent it from taking data from other cars in the area. The
sensor ID is printed on a barcode on each sensor. Programming the sensor IDs into the
equipment, via the barcodes, was done before installation of sensors into the wheels
FIGURE 12. Key Components of Tire Pressure Monitoring and Data Acquisition Systems
( SmarTire, 2005)
34
Once each bundle of equipment – comprised of four strap- mounted sensors, installation hardware,
a receiver, and a DaQ programmed with the IDs of the four sensors – has been tested, it is ready
for installation on a study vehicle. The sensors are installed in the vehicle rims, and the
instrument cluster is installed inconspicuously under the passenger seat. Power for the sensors is
provided by an internal battery, while power for the receiver and DaQ is provided by the vehicle
electrical system. The components that are connected to the vehicle electrical sensor utilize a
fused, switched positive lead. This allows the vehicle electrical system to be protected from
excessive current draws that would result in electrical damage or a discharged battery.
With the equipment installed, the DaQ accumulates time- stamped tire pressure and temperature
data. When the data is collected from a vehicle, the same procedure as the test setup is used.
Unlike the test setup, however, the equipment is now bundled under the passenger seat of the
vehicle, making a laptop computer imperative. A sample of offloaded data is illustrated in Table
6; the raw data, exported by the DaQ in simple tab- delimited, plain text format, is now ready for
statistical analysis.
TABLE 6. Sample Data Output
Packet
Line # Pressure Temp Sensor
Voltage Life Units Sensor
ID Timestamp Converts
to Units
1 273.68 kPa 01129558 Wed Apr 12
12: 48: 42 2006 39.695 psig
2 36 ° C 01129558 Wed Apr 12
12: 48: 42 2006 96.8 ° F
3 2.85 Volts 01129558 Wed Apr 12
12: 48: 42 2006 — —
4 0 — 01129558 Wed Apr 12
12: 48: 42 2006 — —
Statistical Methodology for Comparing Pressure Loss
This subsection covers the statistical methodology for comparing the pressure loss characteristics
of nitrogen- inflated tires against air- inflated tires. The test fleet is a set of Chevrolet Cavaliers
from the fleet’s daily trip vehicle fleet. The vehicles have between 60,000 and 100,000 miles on
their odometers. Discussed in this section are the development of two hypothesis tests of interest:
( 1) a comparison of pressure loss per car per time and ( 2) a comparison of tire position pressure
loss with regards to the type of inflation. As will be described, the tests are constructed in this
way to account for the sensitivity that pressure loss might have to either the car itself, or the tire
position.
Table 7 shows an example of the data format after downloading initial data points from the test
vehicles. Each row is uniquely identified by the combination of observation date, a vehicle
identification number, and tire sensor identification ( ID). As discussed above, the position
variable represents the location of the wheel on the car. The gas column contains a string
representing the inflation gas used in the tire to denote whether the tire is filled with air or
nitrogen ( presented as N2). The pressure column is the average psi for that tire on that day. Note
this averaging distinction from the above Table 6 ( for initial individual data points): the
measurements are averaged because, even though they are temperature standardized, multiple
factors cause small variances over the day. The standard deviation column provides information
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on the measurement variance for the particular tire, and the final column provides the number of
measurements that were used to calculate the average.
TABLE 7. Sample Tire Pressure Data
Date Vehicle TireID Tire position Gas
Average
pressure
( psi)
Pressure
standard
deviation
Data points
4/ 12/ 2006 243132 607069 1 air 38.5478 0.1435 6
4/ 12/ 2006 243132 616162 2 air 40.2946 0.1024 3
4/ 12/ 2006 243132 1774362 3 air 40.4228 0.528 9
4/ 12/ 2006 243132 1774370 4 air 39.1973 0.5266 7
4/ 12/ 2006 81238 60364 1 N2 40.3690 0.5261 10
4/ 12/ 2006 81238 60366 2 N2 40.2946 0.1024 3
4/ 12/ 2006 81238 60368 3 N2 39.1449 0.5096 8
4/ 12/ 2006 81238 60370 4 N2 38.5478 0.1435 6
The cars in this experiment are part of a rental fleet. Because the cars will be driven by different
people on different roads in different climates we must consider that the pressure changes across
the tires in a car might be correlated. In order to compare car dependent pressure losses we
develop a scoring system. The following scoring system is one which looks at the sum total of air
pressure loss per car over the test period. The measure, the “ S score,” quantifies the accumulated
gas flowing out of the tire, without regard to the origin of the gas.
Figure 13 graphically illustrates with example data the reduction of tire pressure and refilling of
air over a given time period. The dots represent pressure readings over 100 successive days for a
single tire. On day 51, the tire was inflated back to 32 psi. It can be seen that the dots do not
decrease uniformly over the time period; there is a general downward trend, but sometimes the
pressure reading increases between days. This is explained by the fact that the standard deviation
of the pressure readings for a given day, 0.5 psi, substantially exceeds the typical psi pressure loss
per day by an order of magnitude ( e. g. a tire that loses 4 psi over 6 months has a pressure loss rate
of ~ 0.02 psi/ day).
FIGURE 13. Illustration of the Tire Pressure Data
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We are interested in measuring only the total loss of pressure over the time period. The loss for
the first 50 days is equal to the sum of the losses over that time period. This distance is
represented by the line marked ‘ d1’ on the chart. Likewise, for the second 50 days the
accumulated loss is represented by the line marked ‘ d2.’ By adding the lengths of those two lines
we get the total pressure loss for the tire over the 100- day period.
The “ S score” is the measure of the accumulated pressure loss for all of the tires on a car over the
observation period. The equation for the S score is:
S p p I p p i i j i j
n j i j i 1, j ( )
4
1 , , 1,
( )
+
= + > = ΣΣ − ×
where:
n = the car identification number ( 1 – 49)
i = test day number
j = tire number ( 1- 4)
Let pij = the average pressure of tire j, on day i
This score represents the total loss of air pressure in psi for car n over the test period. The I
operator is the indicator function; in this application we only include the difference value if it is
less than the prior value. This allows for us to account for the refilling or replacement of the tire.
With this definition of the S score, we state the first hypothesis:
Hypothesis 1: The accumulated pressure losses per car, the S scores, for nitrogen-and
air- filled tires are the same.
We will pair hypothesis 1 with the alternative hypothesis that the accumulated pressure loss for
nitrogen inflated tires is less than that of air inflated tires. In statistical terms, this is called a one-sided
test; we are not concerned that nitrogen will perform worse than air. The S score will be
additionally useful for detecting outliers. Vehicles with extraordinarily high S scores should be
examined for abnormal usage.
The second dependency we must account for is that of tire position. Tire position may be a factor
in pressure. For example, a front tire on a front- wheel drive car could be more stressed than a rear
tire, or tires on one side of the vehicle may be more stressed than the other side. Here, we state
our second hypothesis:
Hypothesis 2: The accumulated pressure loss, by tire position, for air- filled tires is
equal to the accumulated pressure loss for nitrogen- filled tires.
As with hypothesis 1, we will pair hypothesis 2 with the alternative that the accumulated pressure
loss by tire position is less for nitrogen than for air. Again, this is a one- sided test, for we are
assuming that nitrogen will perform at least as well as air as an inflation gas. In actuality,
hypothesis 2 involves four separate tests, i. e. nitrogen in tire 1 vs. air in tire 1, nitrogen in tire 2
vs. air in tire 2 and so on.
The tests will be processed and analyzed by a computer program. Because of the processing and
analysis requirements, the statistical tests are made to be simple, robust and conservative. The
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two- sample “ t- test” meets all of these requirements and is a sufficient tool to statistically examine
the two stated hypotheses. A rigorous explanation of the t- test method is beyond the scope of the
paper, but we provide a brief explanation here.
Two samples of data are compared in a two- sample t- test. The samples are randomly selected
subsets of numeric observations ( e. g., height, weight, cost, etc.) from a larger population. Each
sample has three important properties: a size, a mean, and a standard deviation. The sample size
is the number of observations contained in the sample. The mean is the sum of all the
observations divided by the sample size. The standard deviation is a numeric description of the
variability of the sample around the mean.
Using these three properties along with a statistical tool called the t distribution we can develop a
confidence interval at a given percentage level for the true mean of a population. For example, we
consider a sample of 20 tires that lose pressure at an average rate of 1.0 psi/ month with a standard
deviation of 0.2 psi. A 95% confidence interval for the true pressure loss per month is equal to,
sample mean ± ( t value ) x ( standard deviation / square root of sample size)
For the given example, the results is –
1.0 ± 2.08 * ( 0.2 / 4.47) = [. 91, 1.09]
Thus we would expect that, 95% of the time, the true population of values for tire pressure loss
mean would fall between the values 0.91 and 1.09, given our sample.
For a two- sample t- test, we have two samples and thus two intervals. If the intervals overlap, such
as [. 91, 1.09] and [ 1.05, 1.15], we say that they are not significantly different. If they do not
overlap, such as [. 91, 1.09] and [. 74, .86], we say they are significantly different.
This point on determining statistically significant difference from a two- sample t- test is depicted
in Figure 14. In the hypothetical illustration, to “ means not equal.” The horizontal axes in the
plots represent the numeric values of the mean. The vertical axes provide a measure of the
probability of a mean being a given value for the distributions. In the “ means not equal” plot, the
true means likely falls in the range from 2 to 4 for the lower value and from 6 to 8 for the higher
value. Since these two ranges do not overlap, the test determines that the means are significantly
different. For the “ means equal,” the lower mean most likely falls in the range from 2 to 4 and the
higher mean in the range from 3 to 5. The distribution curves substantially overlap, as well as the
confidence intervals for the mean value. Thus we would conclude that the sample means were
not significantly different.
38
“ Statistically significant difference” “ Not statistically significant difference”
FIGURE 14. Illustration of Significant Difference Between Two Samples
A key assumption of the statistical t- test is that the samples be normally distributed. Determining
that a sample is normally distributed, however, is difficult when dealing with small sample sizes,
as we are here for this experimental design. However, the t- test is robust with regards to this
assumption ( Neter et al, 1996). The test works properly with sample sizes of 10 or less provided
the distributions are not skewed. Our samples contain either 24 or 25 observations. Given that the
data will be the summations of small values over a long period of time it seems reasonable to
expect the data will not be skewed. Skewed data is likely when observing phenomena in which
the forces that shape low values differ from those that shape high values. For example, income
distributions are often skewed because there is an absolute lower bound of zero ( no income) with
no similar cap on high incomes. Based on our initial acquisition of sample test data, we have no
reason to expect the data will not be appropriate for a t- test.
The data will be processed and analyzed using the open source statistical package called “ R” ( see
Leisch, 2006 for further details). All pressures will be temperature standardized to 25° C and
recorded in gauge PSI. The operator will not need to know how to use R; all that will be required
is that he set the working data directory and then run a script.
Processed output files providing the full temperature standardized data set and the daily averaged
temperature standardized data set will be made available in a character delimited file that can be
examined with a statistical software package or Excel, should that be desired.
The hypothesis test results will be printed to an output file. Sample means and the confidence
intervals will be printed for each test along with a statement declaring whether the hypothesis was
accepted or rejected. This computation will be invisible to the operator, who will receive the test
data and an indication of whether the populations are statistically different or the same. It is
suggested for any automated statistical analysis, including this one, that it be reviewed by a
qualified analyst prior to accepting it as a valid decision- making tool.
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Chapter 5. Analysis of Tire Practices
This section seeks to integrate information gleaned from the literature review, the driver surveys,
and the fleet interviews on tire- related maintenance practices. We analyze the cycle of tires in a
fleet of vehicles from the perspective of private vehicle users, and then specifically apply the
same tire flow model to fleet management, with an emphasis on formulating recommendations on
how fleets can better manage their vehicles’ tires. The primary aim here is to assess potential
improvement to current practices for use in the life cycle research of Chapter 6 and for the
development of a “ Best Practices” manual for fleets regarding tire practices.
Private Vehicle Users
Based on information collected from our literature review, interviews, and surveying, it became
clear that the known data on tire management is sparse, and, therefore, we developed a set of
assumptions and simple tire flow model to assess changes to tire practices. A generalized
illustration of tire flow – from new installed tires to the discarding of tires – is shown in Figure
.15. The paths’ categories are based on those given in Weissman et al ( 2003) for replacement
reasons, using a dataset from Michelin tires introduced above in Table 2.
New tire
“ Other conditions”
( road hazard, puncture,
oxidation, separation
Disposal
Tire replaced due
to tread wear
“ Abnormal” or
uneven wear
Tire removed in a set
( but tread remaining)
( less than 2/ 32- inch tread)
Path F:
Path B:
Path C:
Path E: Path D:
Reuse “ Nothing observed”
FIGURE 15. General Schematic of Fleet Tire Management Paths
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Path A in Figure 15, “ normal” or gradual tire wear, is the ideal scenario, where tires make it to
their expected, rated service life ( e. g., 50,000, 60,000, 75,000 miles) which differs by tire brand
and model. Keeping tires in the fleet on Path A requires following proper tire maintenance
practices: tire pressure inflation, tire rotation, and wheel alignment. Categories for premature tire
failure, Paths B, C, and D, include tires with “ Abnormal wear,” “ Nothing observed,” and “ Other
conditions.” The aim of this section is to determine and prioritize practices to minimize the
number of tires that prematurely lea