NBS TECHNICAL NOTE 528

Tire Use Survey

The Physical Condition,

Use, and Performance of

Passenger Car Tires in the

United States of America
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NBS TECHNICAL NOTE 528

ISSUED MAY 1970

Nat. Bur. Stand. (U.S.), Tech. Note 528, 59 pages (May 1970)

CODEN: NBTNA

Tire Use Survey

The Physical Condition, Use, and Performance of

Passenger Car Tires in the United States of America

J. L. Harvey and F. C. Brenner

Office of Vehicle Systems Research

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

""»&.« o» **

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402

(Order by SD Catalog No. CI 3.46:528). Price 60 cents.
11

FOREWORD

There have been few systematic attempts to determine the way

in which people maintain and use their tires; indeed, we believe this study reviews most of the available data.

Because of this limited experience the art of deriving meaningful tire data through survey has not been fully developed.

As a consequence, it must be pointed out that some of the

conclusions and interpretations derived from the data in this

study are suggested rather than proved.

As one of many examples illustrating the limitations of the

survey material, the Texas Highway Department study shows that

50 percent of the tires on the rear axles of cars involved

in wet weather accidents in certain areas of that state had

bald or thin tires, as compared with only 25 percent of such

tires on front axles. In order to conclude with certainty

that it is safer to place the least worn pair of tires on the rear axle, we should ideally have comparable data on the incidence of bald and thin tires on front and rear axles of cars

not involved in accidents in the same area.

In the absence of these data, the authors were forced to use

the only tread depth data available on a general population;

namely, the Davidson Laboratory summer survey, and to conclude

that there is strong evidence that it is safest to have one's

least worn tires on the rear axle. One can reason that loss

of rear wheel traction on curves, and rear wheel lockup in

panic stops are major contributors to accidents.

The authors frequently comment in the report on the inadequacy of available data. It is their hope that these comments

will be beneficial to those planning surveys of tire condition

and use in the future.
by

on

I . ABSTRACT

This review summarizes the results of two surveys of the physical condition and use of

passenger car tires in the continental U.S.A., conducted under contract for the Office

of Vehicle Systems Research (OVSR) of the National Bureau of Standards (NBS) during

the period June 1967 through May 1968.

Data and results are compared with those in four related reports recently published

the Traffic Institute of Northwestern University. The latter reports cover studies

the use, condition, and performance of passenger car tires on the Illinois Tollway

during the period September 1966 through August 1967. Pertinent information from various other sources is also included.

Based on the compiled evidence, some conclusions are drawn concerning the physical

condition, use, and abuse of passenger car tires in service during these years, particularly with regard to tread depth, inflation pressures, and degree of overloading.

The various estimates of tire disablement rates are compared, and contributions of

tires to motor vehicle accidents explored.

The authors make several recommendations directed toward upgrading the level of quality of passenger car tires in service, and reducing their abuse, with the hope and

expectation that service performance will be improved, and contributions of tires to highway accidents reduced.

Key words: Accident; review; survey; tread wear; tire disablement; tire inflation;

tire loading; tire pressure gages; tire use.

II. HIGHLIGHTS

A greater percentage of severely worn tires were found on vehicles in accidents

than in the general tire population.

Vehicles in accidents have a larger number of severely worn tires on their rear

axles than on the front suggesting that it is safer to have newer or less worn

tires on the rear axle (see Foreword)

.

There is a considerable body of evidence indicating that the risk of tire disablement and of loss of vehicle control rises sharply for tires having less than about

2/32-inch of tread depth remaining.

Air towers at service stations are generally not accurate. A motorist relying on

a tower gage will have only about a 20 percent chance of getting within ± 1 psi of

the inflation pressure he wants. A single point calibration of the tower gage

could decrease the error substantially.

More than one out of four cars has at least one seriously underinf lated tire (4

psi or more below the recommended normal inflation pressure for the vehicle).

About 2 percent of cars are overloaded by 10 percent or more; about 6 percent are overloaded to some extent. Most overloading occurs on station wagons and light

trucks, and on passenger cars during vacation trips.

On the average most cars are loaded to about 80 percent of their allowable load

(at the existing equivalent cold tire inflation pressure)

.

III. SUMMARY

Based on this review of available information on the physical condition of passenger

car tires in service within the continental U.S.A. and of their use and performance

primarily on throughways and interstate highways, we believe that the following con- clusions may be drawn. We do not contend that the list is complete, nor do we lay

claim to complete originality; such was not the purpose of this review. Indeed, the

majority of the following conclusions are at most restatements of those drawn by the

authors of the various papers reviewed. In many instances, we have been able to test
IV

their conclusions in the light of other information; that was one of the purposes of

this review. Some of the following are conclusions drawn by the authors of this

paper; for these they must and do accept full responsibility.

A. Sampling

1. Selection of sampling points and sampling periods has exceedingly important

effects on the vehicle population sample and on the tire condition and use data

obtained

.

2. Use of service stations as sampling sites, while most convenient for the sampling

team and causing the least interference with traffic, virtually excludes fleet and

farm vehicles, and introduces a definite bias toward more heavily driven vehicles,

particularly those on long trips away from home [6]

.

B. Effects of Car Size, Age, and Style (Section 3.1)

1 . Car Size

Although the tires on small cars (less than 2000 lb) have less average tread depth

remaining than those on larger cars [2], differences in tire disablement rates are not

statistically significant (see table 12).

2. Vehicle Age

Vehicle age is a highly significant factor in the rate of tire disablements, other

factors being equal. Baker and Mcllraith report the following tire disablement

indexes by vehicle age [2]:

Cars less than 2 years old 1.00

Cars 2 to 7 years old 1.73

Cars more than 7 years old 2.44

This significance is not surprising in view of their findings, confirmed by Automotive

Crash Injury Research's North Dakota study [19], that average tread depth remaining

decreases steadily with vehicle age. It is probable that a study of the data acquired

in the three tire condition and use surveys would show that the major portion of bald

and thin tires are to be found in older cars (see section III.F.3).

3. Vehicle Style

Rear tires on station wagons and front tires on light trucks appear to be the most

frequently overloaded classes of passenger tire service. About 10 percent of rear

tires on light trucks were also found to be overloaded in two surveys by the Davidson

Laboratory of Stevens Institute of Technology. In the latter studies it was further

observed that vehicles on vacation trips had overloaded tires more frequently than

those being used for any other trip purpose. Rear tires were the more frequent

offenders (see section 3.3.b(l).

C. Tire Types, Sizes, and Sources (Section 3.2)

1. Retreads

Some evidence was found in the work covered by this review indicating that retreaded

tires are more prone to failure (see section 4.1.b(4)).

2. Two-Ply vs Four-Ply Tires

Based either on driver recollection or on the assumption that all 2-ply, 4-ply rated

tires were original equipment, these were found to constitute 32, 35, and 43.5 percent

of the sample populations in the respective physical condition and use surveys.
D. Tire Loading (Section 3.3.b)

1 . Average Load

There is general agreement among the physical condition and use surveys that passenger

car tires in service are, on the average, loaded to around 80 percent of allowable

load at their equivalent cold pressures (Pc) . About 6 percent are overloaded to some

extent; roughly one-third of these or about 2 percent are overloaded by more than 10 percent of rated load at Pc or maximum Pc (see table 4)

.

2. Loading Tires for Test

Although some few tires in service do carry extreme overloads , this is frequently due

to serious underinf lation. It therefore appears unreasonable to test a tire at more

than 10 percent overload plus a reasonable factor of safety--perhaps not to exceed

20 percent overload.

3. Small Diameter Tires

Evidence has been presented that 13-inch diameter tires on nominally six passenger

cars (all manufactured prior to issuance of MVSS-110) are more frequently overloaded

in service than the larger tire sizes, by a frequency of about 2 1/2 to 1 (see

section 3. 3 .b (3) ) .

4. Need for Driver Education

MVSS-110, which became effective on January 1, 1968, will do much to alleviate tire

overloading in the future. This standard specifies requirements for selection of

tires by the vehicle manufacturer in terms of normal loading and maximum load conditions for a given vehicle. Nevertheless, car owners still must be educated; they must

recognize that higher cold inflation pressures are required for maximum load conditions, and for high speed driving, and that failure to maintain proper inflation

pressures will seriously shorten tread life and increase the risk of tire failure

through overheating and overstressing.

E. Tire Inflation

Car owners need to have the importance of proper tire inflation impressed upon them.

It is quite apparent from the work reviewed here that many car owners do not keep their

tires properly inflated.

1. Underpressured Tires

Surveys indicate that a substanital number of tires in service are underinf lated.

Lepisto [7] reports that 11 1/2 percent of the "knowns" in the Southwest Research

Institute (SWRI) tire population sample had corrected ambient pressures below the

minimum inflation pressure for which a loading for the specific tire size is given in

MVSS-109 (see section 3.3.b). Drs . Olson and Bauer in a General Motors survey [11]

of inflation pressures in the tires of 405 cars in their company parking lots provide

data indicating that some 10.9 percent of the tires had cold inflation pressures 4 psi or more below the vehicle manufacturer's recommended inflation pressure. A

recent Uniroyal survey [12] has indicated that 28.4 percent of 1904 cars had one or more tires at 20 psi or less.

2. Overpressured Tires

Although perhaps less hazardous from a safety standpoint, seriously overinflated tires

can also lead to vehicle handling problems particularly if only one or two of the

tires on the vehicle are in such condition. Lepisto [7] reported 3.2 percent of the

"knowns" in the SWRI survey as having Pc values above maximum Pc ; Baker and Mcllraith

[2] reported finding 1.9 percent of such tires in their second Illinois Tollway study.

3. Air Tower and Hand Pressure Gages (Sections 3.4 and 3.5)

The car owner often does not have adequate means available to him to maintain his

tires at correct inflation levels. Surveys indicate that many air towers at service

stations are out of order, indicating poor maintenance; and that their pressure
VI

readings are frequently substantially incorrect. Moreover, hand tire gages of the

type that the careful car owner might purchase for his own use are too often so inaccurate as to do more harm than good (see section 3.4.c).

F. Tread Depths, Tread Wear Patterns, and Tire Defects

(Sections 3.6, 3.7, and 3.8)

1 . Tire Position

Existing data on tire position (front or rear) vs tread depth are somewhat conflicting

(see table 8). There is strong evidence that a significant number of car owners place least worn tires on the front wheels, whereas available accident experience

indicates that the least worn tires should be placed on the rear (see sections 4.1.b,

4 . 2 .b , and c)

.

2. Uneven Wear

A sizeable fraction (10 to 20 percent) of tires in service show sufficient variation

in groove depth to suggest uneven wear as the result of either overinf lation , underinflation, out of balance, or other mechanical problem (see section 3.7).

3. Vehicle Age

Tread depth remaining on tires decreases with vehicle age (see section 3.1). In their

study of the condition and use of tires on the Illinois Tollway [2], Baker and

Mcllraith report finding the following average tread depths remaining:

Cars less than 2 years old 7.55 32nd of an inch

Cars 2 to 6 years old 7.30 32nd of an inch

Cars more than 6 years old 5.52 32nd of an inch

4. Bald Tires*

(a) There is evidence of significantly greater hazards of both tire failure and

accidents with bald tires. The evidence is sufficient to justify further surveys

leading, perhaps, to actions by appropriate authorities limiting the use of such tires

(b) Baker and Mcllraith have shown [3] that the likelihood of a tire disablement

increases rapidly with decreasing tread depth. They have calculated that the risk of

disablement for a tire having 2/32-inch of tread depth remaining is 'about 10 times

that for a new tire (see section 4.1.b).

(c) Based on a sampling of the general population of such tires in the Eastern U.S.A.

[6], it appears that about one-eighth of passenger car tires in service are bald

(see table 8) .

(d) Fortunately, there is also definite evidence that most drivers recognize that

bald tires are unsafe for sustained high speed driving. Only 2 to 4 percent of bald

tires were found in samplings of throughway and interstate highway traffic. It is

possible that many drivers anticipating a long trip away from home at sustained high

speeds replace marginal tires rather than risk a tire failure.

(e) A vehicle with one bald tire very likely has a second, and perhaps more. In their third study [3] , Baker and Mcllraith reported that "when the disabled tires were

bald, 58 percent of the cars had one or more [other] tires in the same condition."

In their second study [2] they pointed out that "approximately 2 out of 3 (415 of 607) cars with one bad tire also had a second" (see sections 3.7 and 4.1.b).

Bald tires are those having less than 2/32-inch tread depth remaining.
Vll

5. Warranty Failure Experience

On the basis of evidence collected in the SWRI survey and the two DL surveys (summer

and winter), it appears that warranty claims are made against 2.5 to 3 percent of

new tires

.

G. Tire Disablements (Section 4.1)

1. Tire Failures

Tire failure (including punctures) comprise 40 to 60 percent of all vehicle disablements (excluding "out of gas") on throughways and expressways (see table 10; also

sections 4.1.b and 4.2). Experience indicates that about two-thirds of all drivers

(mostly all male) change their own tires.

2. Effect of Temperature

According to Baker and Mcllraith [1] , the rate of tire failures rises quite rapidly

with ambient temperature, other factors being equal (see section 4.1. a). They

estimated the average year round rate of tire disablements on the Illinois Tollway to

be about 50 per million vehicle miles (minimum) . That for the general passenger car

tire population in the Eastern U.S.A., based on driver recollections was reported [5] to be more than double this, reflecting the generally poorer condition of tires and

the much higher incidence of bald and thin tires among the general population as compared to throughway (Tollway) traffic.

3. Rate of Tire Disablements

If one assumes a disablement rate of 50 per mvm as a reasonable estimate of actual

experience, and an average tread life expectancy for new tires of 20,000 miles, one

may calculate that one new tire in four experiences some form of disablement prior to discard for tread wearout.

H. Tire Disablements and Accidents (Section 4.2)

1. Need for Data

There is an urgent need for reliable data on the exact causes of motor vehicle

accidents. Information presently available is scanty; moreover, much that is

available is highly subjective and, therefore, suspect.

Needed are studies by competent and well-equipped accident investigation teams having

both the opportunity and the authority to work, and a procedural rigor approaching

that encountered in investigation of aircraft crashes. We understand that at least

two such approaches to this task are in progress at this time [16, 21].

2. Bald and Thin Tires in Accidents

The incidence of bald and thin tires on accident vehicles in at least two of the four

accident studies reviewed here is substantially higher than those found in samplings

of comparable motor vehicle traffic populations. Moreover, by far the greater number

of such tires are found on the rear wheels of the accident vehicles (see sections

4.2.c(l) $ (2)).

3. Driver Age and Experience

Driver age and experience appear to be extremely significant factors in the ability

to avoid an accident in the event of a sudden tire failure. In their study of

accidents following tire disablement on the Illinois Tollway [4], Baker and Mcllraith

found that younger women and girls are especially prone to an accident in the event

of tire failure (see section 4.2.a(3)). In tire disablements involving male drivers,

those under twenty appear more than twice as prone to an accident as older men.
Vlll

4. Power Steering

There is evidence that power steering is of some value in enabling a driver to maintain control of the vehicle in the event of a sudden tire failure (see section

4.2.a(3)) .

5. Disablements and Accidents

Estimates of tire disablement per se as a major contributing cause of accidents

involving four-wheel, four-tire vehicles lie in the range of 0.6 to 1.6 percent of

such accidents in non-congested areas (see sections 4. 2. a § c) . However, such data do

not take into account those accidents in which loss of road traction, or insufficient

road traction, was the major contributing factor leading to the accident. It is

evident, for example, from the Texas Highway Department's study (see section 4.2.c(2))

that poor tires are major factors in wet weather accident.

IV. RECOMMENDATIONS

A. Maintenance of Adequate Inflation Pressure

As pointed out in III.E.l above, a substantial number of cars are operating with one

or more tires at inflation pressures of 20 psi or less. This situation causes the

tire to flex excessively in the sidewall region, enhancing the probability of a tire

disablement

.

The extensive development of high speed roads in and around cities makes it increasingly likely that these underinf lated tires will be used at high speeds, thereby

developing high temperatures which increase the danger of tire disablement. Needless

to say, a tire failure in heavy traffic at the high speeds where such failure is more probable is more likely to produce severe bodily injury and greater property damage

than at lower speeds.

In addition, underinf lation reduces tread wear; affects the handling properties of

the vehicle, thereby reducing its ability to respond safely; and reduces the feeling

of stability which is a factor in riding comfort.

Higher than normal loads on vehicles without compensating higher inflation pressures

also causes the tires to flex excessively, increasing the probability of tire

failure. However, most such cases of tire overload would be corrected if the

inflation pressure were increased within the permissible range (see table 3)

.

Since the evidence suggests that maintenance of proper inflation pressure in tires is

of major importance in safeguarding the motorists' safety, pocketbook, and comfort;

and since it is evident that the tires on many cars are not properly inflated, we recommend the following:

1. That a massive education program be undertaken to teach the public the importance

of proper tire inflation, and how to achieve it.

2. That measures be adopted to ensure that every service station provides and

maintains a source of compressed air equipped with a pressure gage regularly

calibrated to within the accuracy of ± 1 psi recommended by recognized trade

associations in the tire industry. Tower function and calibration of the pressure

gage could, for example, be checked by the local weights and measures inspector

on his regular rounds to inspect and certify the gas pumps.

3. That the gas and oil companies in cooperation with tire industry associations

conduct campaigns to encourage service station personnel to foster better maintenance of tire inflation pressures in customers' vehicles. A major part of the

campaign would be the training of service station attendents in the proper

maintenance of tires, with special emphasis on proper inflation.

4. That manufacturers of hand tire pressure gages test, certify, and guarantee this

same accuracy (± 1 psi) on each and every gage offered for sale.
We feel that if the above recommendations are implemented, the major responsibility

for tire care and maintenance will rest on the driver- -and that is where it belongs.

B. Prohibition of Use of Dangerous Tires

There is evidence of significantly greater hazards, not only of tire failure but of

an accident, with bald tires. The evidence is sufficient to justify further studies

leading, perhaps, to action by appropriate authorities limiting the use of such tires.

Indeed it may be found desirable to establish 2/32-inch as the minimum average tread

depth for acceptance in periodic inspections of cars in use. Tires with deep cuts

and sidewall cracks should also be rejected.

Such cuts or sidewall cracks were found in over 4 percent of all tires examined in one survey [2]; in another study [3], 11.1 percent of disabled tires examined had either

cracks or blisters, or both.

Other tire defects (such as fabric breaks) indicative of incipient or early failure

should also be considered as adequate grounds for rejection.

C. Driver Education

Driver education courses with practical or simulated practical experience in handling

sudden tire disablements, skidding, and other sudden emergencies, may be of value in

reducing accident rates among inexperienced drivers.
-x

- CONTENTS

Page

FOREWORD ii

I. ABSTRACT iii

II. HIGHLIGHTS iii

III. SUMMARY iii

A. Sampling iv

B. Effects of Car Size, Age, and Style iv

C. Tire Types, Sizes, and Sources iv

D. Tire Loading v

E. Tire Inflation v

F. Tread Depths, Tread Wear Patterns, and Tire Defects ... vi

G. Tire Disablements vii

H. Tire Disablements and Accidents vii

IV. RECOMMENDATIONS viii

A. Maintenance of Adequate Inflation Pressure viii

B. Prohibition of Use of Dangerous Tires ix

C. Driver Education ix

* * *

1. INTRODUCTION 1

1.1 Scope and Objective 1

1.2 Background 1

2. A BRIEF REVIEW OF THE AVAILABLE STUDIES 2

2.1 Davidson Laboratory Studies 2

2.2 Southwest Research Institute Study 3

2.3 Studies by the Traffic Institute of

Northwestern University 5

2.4 Data Collection 6

2.5 Instrumentation 6

2.6 Ambient (Cold) Tire Inflation Pressure Correction 7
-xiPage

3. RESULTS AND CONCLUSIONS, PHYSICAL CONDITION AND USE STUDIES. . . 10

3.1 Car Styles, Sizes, and Age Distributions 12

3.2 Tire Types, Sizes, and Sources 13

3.3 Tire Loading and Tire Inflation 14

3.4 Air Tower Pressure Gage Accuracies 21

3.5 Hand Automobile Tire Pressure Gages 23

3.6 Tread Depths 24

3.7 Tread Wear Patterns 26

3.8 Tire Defects 26

4. TIRE DISABLEMENTS AND VEHICLE ACCIDENTS 29

4.1 Tire Disablements Not Followed by Accidents 29

4.2 Tire Disablements Followed by or Found in Accidents .... 35

5. ACKNOWLEDGMENTS 41

6. REFERENCES 42

APPENDIX A. Davidson Laboratory Site Data Sheet

APPENDIX B. Davidson Laboratory Vehicle and Tire Inspection Form

APPENDIX C. Southwest Research Institute Vehicle and Tire Inspection Form

APPENDIX D. Traffic Institute Tire Use and Condition Data Collection Form

TABLES

Page

1. Average computed pressure rise (psi) and standard deviation

of examined tires due to increase in operating temperature

above ambient 9

2. Psi rise as related to period driven 9

3. A comparison of tire load findings in the tire use and

condition studies 17

4. Overloaded tires, percent of total 18

5. Tire load distribution by tire size and style 19

6. Number and percent of cars classified by number of

underinf lated tires at or below specified pressures 21

7. Measured pressure deviations from tower indicated pressures

(number of stations) 24

8. Tread depths, physical condition and use surveys 26

9. Summary of tire defects found in condition and use surveys ... 27

10. Frequency of tire disablements on the Illinois Tollway . , . , . 30
-XI 1

- Page

11. Distributions (percent) by size and age of cars surveyed .... 31

12. Tire disablement indexes by vehicle size 31

13. Tire disablement indexes by vehicle age 31

14. Size and age of cars 33

15. Age and sex risk indexes following tire disablements based on Tire Study No. 1 37

16. Bald and thin tires in wet weather accidents relative to those

found in condition and use surveys 39

17. Tires and brakes as causes of accidents on the New York State

Thruway 40

FIGURES

1. Changes in tire pressure with steady 60 mph driving 8

2. Change in inflation pressure, bias ply tire 10

3. Distribution of pressure error measured on 405 cars 20

4. Inflation pressure deviations between air tower and test gage. . 23

5. Tire disablement rate related to tire wear 35
-1-

TIRE USE SURVEY --

THE PHYSICAL CONDITION, USE, AND PERFORMANCE OF PASSENGER CAR TIRES

IN THE UNITED STATES OF AMERICA*

J. L. Harvey and F. C. Brenner

1. INTRODUCTION

1.1 Scope and Objective

This report summarizes the results of two surveys of the physical condition and use of

passenger car tires in the continental U.S.A. These surveys were conducted for the

Office of Vehicle Systems Research (OVSR)** of the National Bureau of Standards (NBS)

during the period from June 1967 through May 1968.

The data and results obtained in the two surveys are compared with those of four

related studies recently published by the Traffic Institute (TI) of Northwestern

University. Other data obtained in the course of the OVSR's own studies on passenger

car tires are also included, as well as pertinent information received from various

sources

.

1 . 2 Background

The National Traffic and Motor Vehicle Safety Act of 1966 (P.L. 89-563) provides

for a ". . .a coordinated national safety program and establishment of safety

standards for motor vehicles in interstate commerce. . ." Title II, "Tire Safety,"

of this Act provides that the Secretary of Transportation ". . . shall require that

tires [for motor vehicles] ... be permanently and conspicuously labeled with such

safety information as he determines to be necessary to carry out the purposes of the

Act."

Fulfillment of this responsibility placed on the Secretary by the Congress requires

a knowledge of the general physical condition of tires in the hands of the motoring

public, and of there use and performance in service.

In accordance with an agreement between the Departments of Commerce and of Transportation, the Office of Vehicle Systems Research of the National Bureau of Standards was assigned a responsibility for developing a technical background and expertise in three

areas of auto safety, and for making recommendations to the National Highway Safety

Bureau of the Department of Transportation (DOT) for motor vehicle safety standards in

these three areas. One of these is tires.

A review of the literature on tires in 1966 disclosed little or no information on the

general physical condition, use, and possible abuse of passenger car tires in service,

or on the extent to which poor tire condition and actual tire failures may contribute

to motor vehicle accidents. Recognizing the need for such basic information on the

subject, the OVSR contracted in early 1967 for two surveys to be made of the physical

condition and use of passenger car tires in service on roads and highways throughout

the continental U.S.A. The Davidson Laboratory (DL) of the Stevens Institute of

Technology, Hoboken, New Jersey, was commissioned to survey passenger cars' tires in

that portion of the U.S.A. east of the Mississippi. The area west of the Mississippi,

excluding Alaska and Hawaii, was assigned to the Southwest Research Institute (SWRI)

of San Antonio, Texas.

*This work was carried out at the National Bureau of Standards under the sponsorship

of the Department of Transportation, National Highway Safety Bureau (FH- 11-6090) . The

opinions, findings, and conclusions expressed in this publication are those of the

authors and not necessarily those of the National Highway Safety Bureau.

**For brevity, initials in parentheses following an organization shall indicate its

abbreviation for the rest of the report.
This paper summarizes the results of the two surveys; the data are also compared with

those of similar or related studies that have recently appeared in the literature.

Various data obtained in the course of studies by the OVSR and by others and pertinent

to the discussions at hand are likewise included.

Most important among the other tire condition and use surveys covered herein is a series of studies conducted by the Traffic Institute of the Transportation Center at Northwestern University on the Illinois Tollway in 1966 and 1967. The results of

these studies were published in 1968 as four reports [1, 2, 3, 4]*. This work was financially supported by the Rubber Manufacturers Association (RMA)

.

A report [5] summarizing the four TI studies was presented by J. Stannard Baker at the

48th Annual Meeting of the Highway Research Board. This abbreviated report emphasizes

"results rather than methodology."

2. A BRIEF REVIEW OF THE AVAILABLE STUDIES

To make valid comparisons and draw reliable conclusions from related but not parallel

studies in any broad and relatively unexplored area, one must first understand and

evaluate the intents of the various observers. Their approaches, sample selection,

instrumentation, methods of analysis, and other pertinent factors must be carefully

examined and compared, one to another. In an attempt to provide the reader with

sufficient background to permit him to exercise his own value judgments concerning

the validity of the comparisons made and conclusions drawn in this review, we shall

here briefly describe the available main surveys on the physical condition and use of

passenger car tires in service. We shall attempt to summarize adequately their

purposes, approaches to sampling, methods of sampling, instrumentation and measurement

techniques, etc. However, it is neither possible nor desirable to explore fully every

facet of these aspects in this review. Those readers desirous of more detailed

information are urged to examine the basic documents.

2.1 Davidson Laboratory (DL) Studies [6]

The principal portion of the DL studies by I . R. Ehrlich and M. P. Jurkat summarizes

the summer survey and conclusions drawn. A second covers a much more restricted

winter survey, plus a comparison of the vehicle population stopping at service

stations with the population passing by. A third section describes the survey teams,

their assigned areas for sampling, equipment provided for field measurements, their

preliminary training, a copy of the team leader's manual, and samples of the data

sheets and handouts furnished for use in the field. Included also are brief reports

by the leaders of three of the seven survey teams, describing some of their problems

and experiences in carrying out the field surveys. The final section presents the

procedure developed by Kamm and Parsons for converting field measurements on the tires

to an equivalent cold inflation pressure, for use in determining inflation condition,

manufacturer's load rating, and the ratio of actual measured load to allowable load

carrying capacity (this subject is discussed further in section 2.6).

As previously stated, the DL was commissioned by OVSR "to conduct a survey of the

actual conditions of passenger car tire usage on the highways of the United States

east of the Mississippi River." The DL was further instructed to select a sample

population that was insofar as possible representative of the general population of

such tires in service in its assigned area. After consideration of the various possibilities for obtaining such a representative

sampling, the DL concluded that while somewhat imperfect, service stations as sampling

sites offered the most practical means for obtaining a reasonably representative

sampling of the general population of passenger car tires, while providing the

minimum of inconvenience to the motorist and to the normal flow of traffic. It was recognized that use of gasoline stations probably introduces sample bias by the

following:

a. Omitting many fleet-owned and farm-based vehicles (usually fueled and serviced

through their own facilities)

.

*Figures in brackets indicate literature references at the end of this paper.
b. Placing greater emphasis on more frequently used vehicles (see introduction to section 3)

.

It was further recognized that to avoid the introduction of extraneous biases (local

vs long trip, commuter vs pleasure driver, resident vs resort vacationer, etc.) , sampling sites as well as sampling periods must be carefully selected and distributed.

The DL postulated that the population density of an area was an important indicator of

individual driving habits, and chose counties as population density units for site

selection, using a selected distribution of counties by population density roughly

representative of the Eastern U.S.A. Counties within each of 10 arbitrarily chosen

population density groups were randomly selected. In general, three service stations

were chosen within each selected county, on the following basis:

a. A station selected at random from the telephone book in the county seat.

b. A service station on the largest highway passing through the county.

c. A station which local inquiry indicated as having a high stopping volume.

d. However, if the county contained a resort area, a fourth station within that area was also selected and used for sampling.

Both surveys were taken in 1967. The summer survey was made during the period July

through September, the winter survey in late December. In the summer survey seven three-man teams, comprising primarily engineering students on summer vacation,

surveyed 60 counties distributed throughout the Eastern U.S.A. Samplings were taken

in all but seven of the 27 states (Delaware, Florida, Louisiana, Maine, New Hampshire,

Vermont, and Wisconsin were omitted). The summer survey population comprises a total

of 4502 vehicles sampled at 137 different sites.

The winter survey was much more limited. This task was performed by a single roving

three-man team covering sampling sites in Maryland, Pennsylvania, New York, and

Massachusetts. A total of 199 vehicles was measured at seven selected service

stations. In these surveys, as in all other studies discussed in this review,

attention was restricted to four-wheel, four-tired vehicles having passenger car tires

as standard equipment; i.e., only passenger cars, station wagons, and some light

trucks* were sampled and inspected.

Appendix A is a sample site. data sheet, Appendix B is a sample copy of the three-page

vehicle and tire inspection form used in the field.

2.2 Southwest Research Institute Study (SWRI) [7]

The SWRI study had the following three primary goals:

(1) Determination of the physical condition and usage of passenger car tires on U.S.

highways in the Western States.

(2) Analysis of the nature and frequency of manufacturing defects in new or slightly

used tires (as reported by motorists)

.

(3) Examination of tire tread rubber and other sections discarded along main highways .

In contrast to the Davidson Laboratory, which was directed to select a population

sample representative of the general (total) population of passenger car tires in

service in its designated survey area, the SWRI was instructed to restrict its

attention to the long distance traveler. It was thought that a comparison of this

group with the general population of Eastern motorists would lead to some interesting

and informative conclusions with respect to the general physical condition, age,

remaining tread depth, and relative loading of tires by the two populations.

*SWRI only excluded light trucks
This difference in intent of the two surveys is an important one, and does reveal

some important differences in tire condition and use. Additional information has been

obtained by selecting out of the DL summer survey population a portion (approximately

4 percent) that is also representative of high speed through traffic, and comparing

these data with that of the total DL population and with the other two related

surveys [2 , 7]

.

a. Sampling Sites

The desired deliberate bias of the SWRI sample somewhat simplified the selection of

sampling sites, as such a population is relatively easy to obtain on turnpikes and on

the Federal Interstate Highway System. Sampling sites were distributed roughly in

accordance with population concentrations in basic areas (geographic divisions) of the

Western U.S.A. as follows:

Southwestern States - 3 sites

Rocky Mountain States - 2 sites

Pacific Coast States - 4 sites

Upper Plains States - 1 site

Middle West States - 1 s ite

TOTAL 11 sites

The following specific areas were then selected for the survey:

Texarkana, Texas

Van Horn, Texas

Needles, California

Grand Junction, Colorado

Wendover, Utah

Fresno, California

Burns , Oregon

Murdo, South Dakota

Emporia, Kansas

Rolla, Missouri

Either two or four service stations were chosen at each sampling point in the above

10 specific areas. On turnpikes (limited- access highways) only two service stations

were used: at other locations (state highways or Federal Interstate Highways) four

service stations were selected, with due consideration to the proximity of other gas

stations and their number in relation to the local population. Every effort was made

to select sampling sites located about 2 to 4 hours travel time from large population

centers. About 40 service stations in all were selected and used for sampling.

b. Sampling Period, Sampling Times, and Survey Teams

Sampling days were distributed throughout the year on the following basis:

Month Sampling Days

July '67 4 August '67 4 September '67 3 November '67 2 January '68 2 March '68 2 May '68 3 TOTAL 20

On sampling days at each of the selected sites sampling was performed only during the

period from 10 am to 4 pm, thus covering the period of highest traffic volume while

excluding the major portion of any localized commuter traffic.

Survey teams were recruited locally in each of the 10 selected specific areas. Resident high school and college science teachers were favored as team leaders, with

interested students as members. Team leaders were depended upon for selection,

training, and supervision during the survey. (Special training for the team leaders,

if any, has not been disclosed in the final report.) Samples of the data sheets and

questionnaires used by the SWRI teams in their survey are presented in Appendix C.
The SWRI survey is summarized in a final report [7] by Paul R. Lepisto dated

September 30, 1968.

The report contains considerable information from the motorist (such as the source of

purchase of new and retread replacement tires) which may be of interest to those

concerned with the distribution and marketing of passenger car tires.

2.3 Studies by the Traffic Institute of Northwestern University [1, 2, 3, 4]

The Traffic Institute (TI) of the Transportation Center at Northwestern University,

Evanston, Illinois, has recently published a set of four papers [1, 2, 3, 4] by

J. Stannard Baker and G. Delan Mcllraith on the condition and use of passenger car

tires in service. The four papers cover a series of surveys conducted on the

Illinois Tollway, a network of approximately 190 miles of divided, limited- access highway around and radiating from Chicago. These surveys, which began in September

1966 and extended over a period of about one year, were conducted in cooperation with

the Illinois Toll Highway Commission and the Illinois State Police. As stated

earlier, the work was financially supported by the Rubber Manufacturers Association

(RMA)

.

The first paper discusses a set of three surveys concerned with the frequency of tire

disablements, as reported by motorists traveling on the tollway. The second is a use and condition study comparable to those conducted by the DL and SWRI for the

OVSR. The third and fourth are concerned with tire disablements and their relation

to actual motor vehicle accidents.

We shall explore the background for the second report in some detail here, and reserve

the other three for discussion later on in this review (section 4.)

a. Sampling Sites

Field data for Tire Study No. 2 were collected by members of the Tollway Battalion

of the Illinois State Police, under the supervision of Robert Witt of the Field

division of the Traffic Institute, at all five service areas of the Tollway during

October, November, and December of 1966. Tires of 1746 four-wheel, four-tired

vehicles were examined; as in the other surveys, additional information was recorded

on the car, driver, and nature of the trip (see Appendix D)

.

Baker and Mcllraith state that their objective in Tire Study No. 2 was "to obtain

data on ordinary tire use and condition [on the Illinois Tollway] to compare with

similar data obtained in connection with tire disablements; that is, to serve as a "control' to judge what factors contribute to tire disablement."

b. Sampling Times and Survey Teams

As in the DL survey, survey crews consisted of three men, one interviewing the driver

while the other two made measurements. Sampling was scheduled from 7 am to 11 pm

including weekends. Cars traveling in both directions were surveyed at different

times at all five service areas on the Tollway.

c. Evaluation of Sample Population

It is apparent that the population sample of passenger car tires in this TI study,

representative as it is of the general population of motorists using the Illinois

Tollway, should be comparable to that of the SWRI survey. It is, however, probably

less purely representative of vehicles being used on long trips at sustained high

speeds. As evidenced by the third survey in TI Study No. 1, there is considerable

local short-haul traffic on certain portions of the Tollway.

Baker and Mcllraith state that the "data obtained probably represent tires in better

than usual condition because people who pay tolls probably have good tires and

motorists on long trips are less likely than others to be using old tires." (See

section 3.6 for further evidence of the latter.)
In the third survey referenced above, we shall find that older cars with poorer tires

comprised a significantly larger portion of the population than in the other two

surveys of long-haul traffic reported in TI Study No. 1. It would appear that the

long trip at sustained high speed is the more important factor leading to an upgrading

of tire quality on tollways and on the interstate highway system. Again, an economic

factor may be important. In addition to the desire to avoid the hazards and delay

of a tire failure, many motorists faced with a long trip probably choose to replace

marginal tires at their regular dealer, before starting out.

2.4 Data Collection

In all three surveys, the teams collected data on the vehicle and obtained information

by questioning the driver, in addition to measuring and examining the vehicle's four

tires. The Traffic Institute survey sheet (Appendix D) represents perhaps the

minimum information necessary for a meaningful survey of tire condition and use. In both the DL and SWRI surveys, additional information was sought from the driver,

principally with regard to the sources and ages of the vehicle tires, whether he had

experienced any tire defects or failures in the previous 2 years, and the extent of care normally given the tires on the vehicle. Such information is highly subjective

and frequently quite inaccurate. As pointed out by Ehrlich and Jurkat [6], "many of

the items. . . were pure guesses of the driver, especially those given by a non-owner."

Psychological factors also may have influenced the responses elicited (by the form,

tone of voice, and sequencing of some of the questions asked). Some examples of these

effects are discussed section III of [6].

We are concerned that in none of the surveys was a systematic record kept of the

number of motorists refusing to participate and the suspected reasons therefore. One

suspects that most refusals are made by motorists unwilling to be bothered or to spare

any time. On the other hand, how many refused to participate because they were aware

of and embarassed by the poor condition of their tires?* Only a few such motorists

could significantly bias some of the surveys reported here.

2.5 Instrumentation

Survey teams in each of the studies were provided with dial-type tire pressure gages,

tread depth gages, and tire load scales. Teams in the DL study were in addition

provided with Alnor Pyrocon Model 4000a pyrometers for measurement of tire sidewall

temperatures

.

In our opinion, none of the reports provides sufficient information on the precision

and accuracy of the instrumentation used or the field measurements made with the

equipment. Nor is there complete assurance that adequate calibration checks were made

in the field by all of the survey teams. Indeed, the DL reports lead one to doubt that

calibration of the instruments was adequately maintained in the field at all times

during their surveys; one team leader mentions [6] resetting the needle on the tire

pressure gage "to zero as it had been the day before." The TI and SWRI reports state

that all pressure gages were periodically compared with Marsh master gages.

In view of the range of loads to be measured (500-2000 lb per wheel) , the capacity

(20,000 lb) of the General Electrodynamics Corporation MD-400 loading scale used in

the DL survey seems inordinately high. However, if comparable to the Black and

Decker Loadometer used by the Illinois State Police in the TI survey and by OVSR in some recent work, it can be read to ± 5 lb and is capable of accuracy to ± 10 lb

within the range used. The SWRI, on the other hand, used a hydraulic tire load

measuring device of their own design. In view of the precision available in the

scales used in the TI study, this SWRI "Ground Load Indicator" appears to have an unacceptably high measurement error. Lepisto states that "the minimum accuracy of

the instrument was within 10 percent of the true weight."

*The DL had some data on refusals encountered. M. P. Jurkat reported the following

experience at three of the sampling sites in their summer survey [6]: 1 out of 20,

5 out of 41, and 13 out of 100.
-7-

2.6 Ambient (Cold) Tire Inflation Pressure Correction

Tire design, construction and, most important, inflation pressure, have significant

effects on the steering, handling, and ride characteristics of passenger vehicles.

Car and tire manufacturers devote considerable attention to proper matching of tires

to the vehicle, and tire inflation pressures are always carefully specified by the

vehicle manufacturer. Too often, however, such recommendations are unknown to garage

and service station personnel and are likewise blithely ignored by car owners. We

shall discuss the available information on tire inflation pressure and the problems of

maintaining correct pressures later (section 3.3).

Tire inflation pressure not only affects vehicle ride and handling characteristics,

but also determines the load which a given size and type of tire may safely carry.

Both the tire load rating pressure and the vehicle manufacturer's recommended tire

inflation pressures are given as ambient (or cold) pressures. Now it is well-known

(among technically oriented individuals at least) that tires heat up in use and

inflation pressures rise. The RMA's "Consumer Guide to Tire Care and Safety" [8]

states that "it is normal for inflation pressure to increase up to 6 pounds or more

above the cold pressure levels shown in the table."

Tire pressures measured in field surveys must necessarily be taken "hot." Detailed

information on the extent of this pressure rise, and on the effects of tire size,

design or construction, number of plies, ambient temperature, road conditions (wet,

dry, snow, icy), vehicle speed, travel time, cooling rates on standing, and possibly

other factors, is not available in the literature. Each group of investigators

therefore conducted enough experiments to satisfy themselves as to the extent of

correction needed to obtain the equivalent cold inflation pressue (Pc) from the hot

tire pressure measured in the field.

a. Traffic Institute Studies [1, 2, 3, 4]

Baker and Mcllraith assumed that all their cars would have traveled "some distance at steady speeds of about 60 mph . " The TI staff therefore conducted a series of 100-mile

trips on the Tollway at a steady 60 mph, using the project's Chevrolet station wagon

equipped with standard 4-ply, 8.25-14 tires. Runs were made starting with the tires

at ambient (cold) pressures of 15, 19, 24, 28, and 32 psi. The car was stopped, after

5, 10, 20, 40, and 80 minutes at 60 mph, and tire pressures quickly measured. At the

end of each 100-mile trip, pressure cooling curves were taken by measuring the tire

pressures at approximately the same intervals. The results of these experiments are shown in figure 1. Allowances were made in correcting the hot pressures on cars which had traveled only

short distances before examination. Baker and Mcllraith state [2] that "for the IS percent with less than 20 miles since last long stop, less correction for hot pressure

was required." The authors further state that "the refinement of making tests on various sizes of tires with various tread depth was not considered worthwhile for the

purpose although it would have been an interesting experiment."

b. Davidson Laboratory Surveys [6]

In the DL survey measured hot tire inflation pressure was transformed to the

equivalent ambient (cold) pressure (Pc) using a correction based on simultaneous

measurement of the tire sidewall temperature by an Alnor Pyrocon Model 4000a contact

pyrometer. Kamm and Parsons discuss the correction procedure in a report dated

February 1968 (Section IV, [6]). In a laboratory test program a tire was equipped

with various thermocouples, two of which were placed in the tire air chamber. The

instrumented tire was then mounted on the rear wheel of a test vehicle and driven on

a chassis dynamometer. The authors report finding a linear correlation of chamber

temperature vs tire sidewall temperature for the instrumented tire.

Using the instrumented tire and the Alnor pyrometer in road tests, a linear

correlation between tire sidewall temperature minus ambient and chamber temperature

minus ambient was developed for a range of ambient temperature (70° to 100°F) . A

number of other tire sizes, with varying tread depths, were road tested also, and from

these data a secondary correction was developed as a function of tread depth and tire cross section.
CHANGE IN TIRE AIR PRESSURE

- RUNNING AT 60mph

I I I I I 1 I I

STANDING

_L_1 I I I i 20 40 60 80 100 20 TIME (min)

40 60 80 100

Figure 1. Changes in tire pressure with steady 60 mph driving,

p. 14)

(Ref. 2, Exhibit 7

To obtain the Pc of a tire, the sidewall temperature as measured in the field was first corrected for the cross -section and tread depth effects; the final pressure

correction was made by applying the ideal gas law and assuming a constant volume

change to ambient temperature:

where

(Pc + 14.7) (Ph + 14.7) I|

Pc = equivalent ambient (cold) tire inflation presssure, psi gage. (The Tire

and Rim Association tables of allowable load are given in terms of gage

pressure .

)

Ph measured hot inflation pressure, psi gage

Tc = ambient temperature, K Th = corrected hot tire chamber temperature, K Kamm and Parsons do not present any data showing comparisons of actual hot tire

chamber temperatures vs values calculated from the measured sidewall temperatures, nor

do they present comparisons of calculated equivalent cold pressures vs actual measured

values obtained by allowing test tires to cool. In the final report (Section I, [6]),

Ehrlich and Jurkat present a table showing the average computed pressure rise of tires

surveyed due to increase in operating temperature above ambient as a function of tire

load ratios. It is reproduced below as table 1.
-9-

Table 1. Average computed pressure rise (psi) and standard deviation of examined

tires due to increase in operating temperature above ambient

(From Ref. 6, Section I, R-1336, Table XV, p. 20)

LOAD RATIO

GE but LTa Tires

- 1.0 4848

1.0 1.1 282

1.1 1.2 84

1.2 1.3 8 1.3 1.4 6 1.4 1.5

1.5 - 2 Undet ermined 3097

FRONT TIRE REAR TIRE BOTH TIRES

Average Average Average

Inflation

SDb

0.97

No. of Inflation

SDb

1.04

No. of Inflation

SDb Rise Tires

4528

Rise Tires

9376

Rise

1.4 1.5 1.5 1.00

1.5 0.98 176 2.1 1.30 458 1.7 1.15

1.5 1.00 57 2.4 1.24 141 1.9 1.18

1.3 0.88 33 2.8 1.22 41 2.5 1.30

1.7 0.91 4 1.3 0.94 10 1.5 0.90

- - 2 1.8 1.99 2 1.8 1.99

2.3 0.15 2 3636

0.9 1.07 4 6733

1.6 1.03

Equal to or Greater Than (GE) but Less Than (LT) ) Footnotes added by Harvey

Standard deviation

With exception o pressure rises a the 5-6 psi foun

generally accept

contained a cons

5 miles) , thus 1 inflation pressu

at approximately

traffic (maximum

should develop a

f those t re of the

d by both

ed as the

iderable

eading to res, the

100 perc

speed 30

somewhat

ires overl

order of

the TI an norm. It numb e r of

the small

OVSR found

ent rated

mph) and

greater p

oaded 20 p 1.5 to 1.9

d SWRI for

may be ar short trip

average p that the

load incre

2.9 psi at ressure ri

) and Brenner

ercent or more, th

psi . This is sub

extended running

gued that the DL s traffic (60.8 per ressure rise . In inflation pressure

ased 1.7 psi in 2 the end of 5 mile

se under the same

e average computed

stantially less than

at highway speeds and ummer survey population

cent were less than

a study of tire

of a radial ply tire

miles of travel in city

s . A bias ply tire

circumstances [9].

Southwest Research Institute Survey [7]

The method used by the SWRI for calculating Pc from the actual measured hot tire

inflation pressure was based on that used by the Traffic Institute.

The latter 's experimental results were confirmed by SWRI in a series of test runs on three sizes of tires (6.50-13, 7.75-14, and 8.45-15), using a fleet of 24 cars. All

tires were 4-ply rated, but whether actually 4-ply is not stated in their report;* all were "inflated to 24 psi cold."

The SWRI obtained results quite similar to those reported by Baker and Mcllraith (see

figure 1) , but noted a one psi greater rise in the rear (driving) tires than in the

front (driven) tires. Assuming that the TI's curves were based on the overall mean

pressure rises, the SWRI allowed for this observed difference by subtracting 0.5 psi

from the TI's correction on a front tire, and adding 0.5 psi for a rear tire. The

following correction factors were thus calculated for times of 10, 20, 30, and 40 or more minutes of driving (at sustained highway speed) since the last stop:

Table 2. Psi rise as related to period driven (From [7], Exhibit 3, p. 12)

Minutes

10

2030

40

100

Mean Correction

psi Rise Front Tires Re ar Tires

3.5 3.0 4.0

4.5 4.0 5.0

4.9 4.4 5.4

5.2 4.7 5.7

5.4 4.9 5.9

Rounded Correction

Front Tires Rear Tires

*It may be assumed, with negligible error, that all OE tires were 2-ply, 4-ply rated,

and all bias replacement tires 4-ply in both construction and rating.
lOci. NBS/OVSR Tire Inflation Pressure Studies [9]

OVSR has measured the change in pressure

tire with different initial cold inflati

monitored by means of a rotary pressure

test car and connected to a precision pr

driven at a steady 60 mph on an intersta

cool until a steady pressure was obtaine

temperature of 60°F (15.5°C) are reporte

reported by SWRI and TI . Within the ran rule of thumb, the steady state inflatio

Note that the tires cool quite rapidly o decrease quite rapidly during the first assures the reader that adequate conside

whether all cars were checked promptly o' normally consumed in making the four hot

in a 2-ply, 4-ply rated 7.35-14

on pressures. The pressure was union installed on the right rea essure gage in the vehicle. The

te highway for 50 miles and then

d. The results corrected to an

d in figure 2 and agree in gener

ge of cold inflation pressures s n pressure at 60 mph rises about

n standing; measured hot inflati

10 minutes. Unfortunately, none ration was given to this factor,

r after standing a bit, and how

inflation pressure measurements

bias ply

continuous ly

r wheel of the

vehicle was allowed to ambient

al with those

tudied, as a 6.5 ps i

.

on pressures

of the reports

One wonders

much time was

46i

ALL PRESSURES ARE CORRECTED TO

AN AMBIENT TEMPERATURE OF 60°F

5 10 15 20 25 30 35 40 45 50

10 20 30 40 50 60 70 80 90 100 110 TIME (min)

Figure 2. Change in inflation pressure, bias ply tire.

3. RESULTS AND CONCLUSIONS, PHYSICAL CONDITION AND

USE STUDIES

As we have seen, it was the in

to select and examine a sample

population of passenger car ti

opinion that the DL achieved i consideration for economics an sampling of the general popula

the DL have themselves demonst

service stations as sampling s vehicles," it may be argued th

and that for this reason any t biased toward such vehicles.

tent of the Davidson Laboratory in their summer survey

that would be fully representative of the general

res in use in the Eastern half of the U.S.A. It is our

ts purpose in sample selection quite well. With due

d practicality, we doubt that a more representative

tion of passenger car tires could be obtained. While

rated in their station-road comparison [6] that use of

ites places "greater emphasis on more frequently used

at such vehicles wear out proportionally more tires,

ire condition and use study perhaps should be somewhat

However, the size of the sample in this DL summer survey
-11-

(4502 vehicles examined at a total of 137 different gasoline stations) was dictated

solely by an availability of time and manpower. We have no real measure of its

adequacy in representing the general population of passenger car tires in the Eastern

United States

.

In the small winter survey conducted by the DL in the last 2 weeks of December 1967,

"a heavy emphasis" (72 out of a total of 199 vehicles examined) was placed on turnpike

driving. Ehrlich and Jurkat point out that this bias "was somewhat deliberate, since

the summer survey showed a concentration of tire abuses by people on vacation."

Again, sample size was dictated by the limitations of time and manpower. The sampling

is so small, and so limited geographically, that it can probably do no more than

provide an indication of- the true winter picture in similar areas

.

The SWRI survey, as we have observed earlier, was directed entirely toward the high

speed, long distance, through traveler portion of the vehicle population. The much

simpler task of obtaining this population sample was accomplished by appropriate

selection of sampling sites on or near interstate highway systems, and by selecting

sampling times deliberately to exclude most of any local commuter traffic. The SWRI

survey covered some 11,385 vehicles (passenger vehicles only; no trucks) for a total

of 45,540 tires examined.

The TI Study No. 2 as we have noted earlier, was made on a population considered to

be representative of general traffic on the Illinois Tollway. This population is

similar to that of the SWRI survey, but contains a larger fraction of commuter and

other short haul traffic. To obtain some feel for this, we have taken exhibit 10

(Distance Driven Since Last Hour Long Stop) from the TI Study No. 2 and table 2 (Duration of Driving Since Last Stop) from the SWRI final report [7] , made some simplifying assumptions and rearranged the data as follows:

Minutes Percent of Sample

Since Last

Stop

0-20

20-49

50-119

120-199

200-300

300 or more

SWRI a TI#2 b

12.4 14.9

12.5 26.2

29.3 42.7

34.3 8.6

5.7 5.3

5.8 2.3

100.0 100.0

Assumes last stop was for one hour or more Assumes average speed of 60 mph

In the TI Study No. 2, a total of 1746 vehicles were inspected, equivalent to 6984

passenger car tires examined. Baker and Mcllraith state in their report as follows:

As the survey progressed, data collected were examined. Size of

sample was considered adequate when additional batches of data had

almost no effect on the averages and ratios established for data

items of particular interest.

While perhaps not statistically sound, such an empirical approach certainly presents

a very strong practical appeal.

As the actual sampling and examination of vehicles in all of these surveys were performed at gasoline stations or service areas, it may be assumed that all are more

or less equally biased toward the more frequently used vehicles in the general

population.

Keeping always in mind the different aims and sample populations of these various tire

use and condition studies, as well as their relative successes in achieving their

selected goals, we shall attempt to analyze and compare their various findings. Hopefully, we shall produce a more meaningful picture of the physical condition and use

of passenger car tires in the United States than any of the single studies alone can provide. Using such information as we have on the incidence of tire disabilities and

of tire failures, we may then explore the relative importance of tires as contributors

to motor vehicle accidents.
-12-

3.1 Car Styles, Sizes, and Age Distributions

The make, model, body style, and year were recorded in all three of the tire use surveys. The TI and DL studies included some light trucks in their sampling though

neither explicitly defines the classification; presumably it covers any four-wheel,

four-tire vehicle designed primarily for carrying materials, equipment, and supplies

but having passenger car tires as standard equipment. However, the SWRI restricted

its attention solely to passenger cars and station wagons.

In the first three TI reports, the vehicles surveyed are divided into three classes

according to size as follows

:

Standard- -more than 3000 lb;

Compact--2000 to 3000 lb;

Small--less than 2000 lb.

The vehicles were likewise divided into three categories by age as follows: 12TI Study No.

3 4 Survey

Period Apr, May, Jul 67 Oct-Nov 66 9/1/66-8/31/67 9/1/66-8/31/67

New 66-67 65-66 65-66

Medium 60-65 60-64 60-64

Old 59 or older 59 or older 59 or older

TI Study No. 1, to be discussed in greater detail in section 4.1. a, indicates that the

likelihood of tire disablement increases with vehicle age. We shall see that this is

attributable to the presence of older and more worn tires. Mcllraith and Baker comment

[1] that "compared to cars less than 2 years old, those 2 to 7 years old had 1.7 times

as many [tire] disablements per million vehicle miles; those more than 7 years old had

2.4 times as many. This difference is believed actually to be due much less to

vehicle age than [to] tire age."

In their second study [2], Baker and Mcllraith reported finding average groove depths

(in 1/32-inch) by car size as follows:

Standards 7.45

Compacts 7.36

Small 6.76

They attribute the lower average groove depth on the small cars at least partially to

the possibility of less groove depth to begin with on the smaller tires for these

cars. Despite the lower average groove depth, their Study No. 1 indicated that car size is not a significant factor in the frequency of tire disablements; nor was car size found to be significant in their second study of tire disablements [3] . Both

findings lend credence to their explanation for the lower groove depth.

Data comparing the distributions of bald* and thin** tires with vehicle age would be

of interest, but unfortunately none of the three TI tire use and condition surveys

present such data.

In section 3.3.b we shall discuss some observations of tire loading as a function of

vehicle type.

*Bald : 2/32-inch or less of tread depth remaining.

**Thin; tread greater than 2/32-inch but less than or equal to 4/32-inch.
-13-

3.2 Tire Types, Sizes, and Sources

All three surveys attempted to obtain considerable information on the types, sizes,

sources, and construction of the passenger car tires on the vehicle (see Appendixes B,

C, and D) . Much desirable information, such as the type of fiber in the cords and even the actual number of plies, was not always available on a reliable basis. Frequently sidewall information was abraded away to such an extent as to be illegible.

Some of the problems encountered in obtaining these data are discussed by one of the

DL survey team leaders in his final report [6]. Indeed the TI , apparently recognizing

the improbabilities of obtaining reliable data, made no attempt to ascertain tire

source or fiber type.

Comparing the questionnaires used in the three surveys, we find the following:

_TI DL SWRI

1. Tire Make, Model, Size

2. Ply and Ply Rating

3. Cord Type

4. Source (OE, Replacement, Recap)

5. Tube?

6. Snowtire?

No indication of possible difference between ply rating and

actual plies

Nylon or rayon only

Recap? (Yes or No)

The SWRI in its survey sought to obtain considerable additional information from the

driver of the vehicle, such as the precise sources of purchase of the tires and their

ages in months, his usual highway driving speed, and questions concerning this normal

care of the tires. Analyses of the data obtained in response to these inquiries, for

such value as they may have, are to be found in the SWRI final report. In particular,

comparisons of tire purchase sources are made with those reported in an earlier

economic study by the Federal Trade Commission on the manufacture and distribution of

automotive tires [10].

a. Original Equipment and Retreads

Excluding unknowns (5 1/2 percent of total) , the SWRI reports finding [7] about 32 percent original equipment tires, vs DL ' s finding of slightly over 35 percent in its summer survey [6] and 25 percent in its small winter survey. Again excluding

unknowns, the following percentages of recaps were reported.

Yes Yes Yes

Yes Yes a No b Yes

c Yes Yes

Yes No No

No Yes No

Survey R sfe rence Percent Retrea j a ds Percent Unknown

DL- -Summer 6 Table VI 6.1 8.3

Winter 6 Table XXII 0.0 3.2

SWRI 7 Table 5 5.7 8.1

TI Study 2 2 Exhibit 14 0.26 29.1

Excluding unknowns

Of total sample

The number of recaps found in these surveys is perhaps considerably less than one might expect, in view of the extent of retreading done annually in the U.S.A. Howj ever, one must remember that much of the retread work is done on truck and bus tires,

and for taxi cabs and other fleet cars, none of which were included in any of these

surveys. Moreover, doubtful casings would be classed as unknowns.

b. Tire Styles, Sizes, and Plies

Of the three surveys, only the SWRI has provided a tabulation of the tires examined

in its study according to style and size. Of a total of 45,540 tires, only about one
14-

43,,5 55 .4.8

.3

percent were radials. Not unexpectedly, the majority (56.37 percent) fell into the

14-inch diameter, standard low section category.

Of the three tire condition and use surveys, only the TI has reported a distribution

by ply count. Ply counts were available on 4949 (70.8 percent) of the 6984 tires

examined in their survey; the TI reported the following ply count distribution for

these tires [2] . Percent

2-ply

4-ply

6-ply

8-ply

100.0

All the 2-ply tires were 4-ply rated and are presumably OE . Tube type tires (that is,

tires actually designed for inner tubes and excluding tubeless tires in which tubes were being used) were reported as a rather surprising 6.2 percent of the total.

c. Regular vs Snow Tires

In a classification of tires as either "regular" or "snow tires," the DL reported

finding [6] exactly 4 percent snow tires in its summer survey. Here again, a significant fraction (11.6 percent) of the total sample was unknown (not typed). In their

winter survey, of a total of 796 tires examined on the 199 vehicles in the sample, the

DL found 301 snow tires (39 percent) with only 24 tires not classified either way.

While it is undoubtedly true that when their tread is worn down past the point of

usefulness as snow tires, they are worn out as regular tires, the authors wonder if it

would not be wise to separate out snow tires in any tire condition and use study, and

examine their tread depth distribution separately (particularly during the winter) . In the annex to their main report, Ehrlich and Jurkat note the definite skewing toward

deeper tread depth occasioned by the presence of the large proportion of rear snow

tires in the winter survey. The authors comment: "Apparently, many car owners check

the tread of their snow tires more closely than that of their regular ones."

It is viewed as unfortunate that no record of snow tires vs regular tires was made in

either of the other two condition and use surveys reviewed here,

3.3 Tire Loading and Tire Inflation

As we have noted earlier, tire loadings and inflation pressures are critical factors

in determining the steering and handling characteristics, tire traction, and response

of a motor vehicle on the road. Moreover, proper inflation of the vehicle tires,

according to the load being carried and the distribution of that load, is of critical

importance in terms of tread wear and tire life. We therefore want to know how well

car owners are aware of these factors, how they load their tires in service, how well

they maintain correct inflation pressures- -and otherwise care for their tires. We also

need information on the general quality of facilities and service available to the

motoring public for maintaining the correct inflation pressure in automobile tires. We

shall explore these factors here using such information as we have found available to date .

a, Classifying a Tire Population

In all three tire condition and use studies, the authors consider the effects of

proper tire inflation on tire load distribution, and provide comparisons with the

distribution as actually found in the sampling. Much of this information is given in

table 3 following.

Baker and Mcllraith present an interesting discussion of the terms "underinf lation,"

"overinf lation," and "overloading" in their Tire Study 2. In reviewing the three

studies, it was our strong feeling that a general agreement 6n the use of these terms

would have been extremely helpful. Such agreement, accompanied by analyses of the

sample populations both as actually found and as they would have been had recommended

tire inflation practices been fully utilized, would have added much more to our rather

limited knowledge of the use of passenger car tires, we beieve. We offer the follow<-

ing thoughts for consideration.
-15-

Overloading . A tire carrying more than the allowable (rated) load for its

equivalent cold inflation pressure (Pc) is certainly overloaded under existing conditions of use. Such a condition is unsafe; it shortens the life

of a tire and may lead to sudden failure. However, as Baker and Mcllraith

point out, unless the actual loading exceeds the allowable load for the

tire's maximum permissible inflation pressure, we may just as correctly

describe such a tire as underinflated

.

Overinflat ion . A tire having a Pc value in excess of its maximum permissible (cold) inflation pressure (32 psi for a 4-ply tire) is unquestionably

overinf lated and is in an unsafe condition. We might also use the term

"overpressured" to designate such tires.

Underinflat ion . A tire may be described as underinf lated if it has a Pc

less than that required for the actual load on the tire, as stated earlier.

However, the term "underinflated" should be limited to those tires carrying

less than their maximum load ratings.* Any tire loaded beyond its maximum

load rating must be recognized as "overloaded" under any inflation condition.

Underinflation also constitutes an unsafe operating condition. Seriously

underinflated tires heat up rapidly sometimes to higher than normal operating temperatures, and may fail catastrophically at sustained high speeds.

Analysis of a Sample Population . With the above ideas in mind, a sampling of an existing population of tires TiT service with respect to loading and inflation pressures can be divided into five regions. See the diagram of tire loading vs cold

inflation pressure which illustrates the following thoughts.

Safe Operating Pressure Region . This is the operating range and includes

all combinations of actual loadings and inflation pressures not definitely

considered as unsafe. Except for the bounding lines on left and above,

tires in this region have a "pressure reserve" in the sense that they provide reserve load carrying capacity for the actual tire loading under normal

driving conditions. The boundary on the left is the Vehicle Design Pressure

(VDP) , the manufacturer's recommended normal inflation pressure; on the right

the Maximum Permissible Pressure, the maximum cold inflation pressure to which

the tire may be inflated.

Underinflated for Actual Load . This region includes all tires which are underinflated for the actual tire loading, but which condition can be

corrected, i.e., brought into the safe operating region merely by adding

air. The dividing line between these regions is the line representing the

minimum acceptable inflation pressure for the actual load on the tire and

for normal driving conditions.

Underpressured . Tires in this region are underpressured and probably also

overloaded. However, the condition is also correctable by adding sufficient

air to the tire to bring the cold inflation pressure into the operating

range

.

Overpressured . Tires in this region are overpressured, but the condition

can be corrected merely by letting air out of the tire.

Overloaded . Tires in this region are overloaded, and this condition cannot

be corrected except by reducing load or appropriately increasing tire size.

By appropriate theoretical adjustments of Pc to meet the actual measured loadings, the

sample population could then be reduced to the following two categories:

-- Properly inflated

-- Overloaded at max Pc

"Load rating at maximum permissible inflation pressure for the tire (MVSS-109)

.
-16-

MAX.

RATED.

LOAD

o

<O

UJ

LOAD- ^—

ALLOWABLE PRESSURES

VEHICLE DESIGN

PRESSURE

MAX. PERMISSIBLE

PRESSURE

COLD INFLATION PRESSURE

b. Tire Loading

Although the necessary basic information was acquired in each of the three tire condition and use studies, none of the reports presented an analysis as described above.

Moreover, the three presentations of tire loading vs inflation pressures are each so different that it is difficult to compare them on any common basis.

For example, in analyzing the tire load distribution among its survey population, the

SWRI excluded from consideration those tires for which the corrected Pc values (Baker

and Mcllraith method) were outside "the allowable limits as specified by the Federal

Motor Vehicle Safety Standard, Title 23; Part 225, Standard 109, as amended

February 1, 1968." In the SWRI report, Mr. Lepisto states:

When a tire exceeded the maximum allowable inflation pressure after adjustment, it was placed in the "Overinf lated" category. When a tire was below

the minimum allowable inflation pressure, it was placed in the "Underinflated"

category

.

In table 12 of his report we find that "Underinf lated" tires comprised 11.5 percent

of the known population, and "Overinf lated" tires 3.2 percent. For the standard low

section bias ply tires comprising the preponderant majority (89 percent) of the SWRI

survey sample population, the minimum inflation pressure for which a load is given in

MVSS-109 is 20 psi. Our reaction is that the SWRI reports finding a substantial

number of tires in its survey that are seriously underinflated and therefore a potential safety hazard.
-17

In their analyses of tire load distributions, neither the DL nor the TI setup such

categories. In calculating the load ratio for overpressured tires, each divided the

actual load by the rated load for the maximum permissible pressure. Neither of their

final reports makes any mention of seriously underinflated tires, nor is it stated how

either calculated the tire load ratios for such tires, if found.

Baker and Mcllraith in Exhibit 19 of TI's Tire Study 2 report finding 87.5 percent of

their sample "at or below design load pressure." They define design load on a wheel

as "the weight on that wheel of the car with passengers in every seat, a certain

amount of luggage, and any special accessories," and state that this pressure is

ordinarily 24 psi. We shall discuss the occurrence of serious underinflation further

in section 3 . 3 . c

.

It is with some misgivings, therefore, that we offer in table 3 tire loading data from

the three tire condition and use studies for comparison. The data in the table are based on expressing the actual measured load on the tire to allowable load based on

tire size, ply rating, and inflation pressure using either MVSS-109 (by SWRI), RMA

(by TI) , or Tire and Rim Association (T§RA) (by DL) tables. The data are based on all

the tires in each survey for which the necessary information was available except as stated in our footnotes.

Table 3. A comparison of tire load findings in the tire use and

condition studies

1. Av percent of rated load

at equivalent cold pressures

2. Percent overloaded at equiv.

cold pressure

3. Percent overloaded by more than

10 percent of rated load at equiv. cold pressure

4. Percent remaining overloaded 2.4 0.5 C 0.9

at max allowable inflation

pressure

TI #2

David

Summer

son Lab

Winter SWRI a

80.4 77.6 79.6 80 b

5.9 6.5 9.3 6.2

1.8 2.0 2.0 2.1

These data exclude tires for which the equivalent cold pressure was outside the

allowable limits as specified in MVSS-109 (as amended February 1, 1968).

Approximate modal value, [7], figure 1. Percent overloaded at 32 psi.

(1) Overloading by Vehicle Type and Trip Purpose . Though lacking ease of comparability, one with another, each of the studies presents aspects of the passenger car tire

loading picture that are in themselves interesting and enlightening. In the DL reports, for example, we find analyses of tire load distributions by vehicle type

(sedan, coupe, convertible, station wagon, light truck, or other) and also by stated

trip purpose [6, Tables X, XXV, XI, $ XXVI]. The former disclose that rear tires on station wagons and both front and rear tires on light trucks appear to be the principal

offenders with respect to overloading of tires. The latter tables indicate, not

unexpectedly, that rear tires are overloaded substantially more frequently on vacation

trips than for any other trip purpose. Pertinent data from the four tables are presented in table 4.

In the SWRI report, Mr. Lepisto presents similar but more limited data on load distributions for front and rear tires by vehicle body style (two door, four door, or station wagon) . These data have been recast to eliminate "Underinflated" and "Overinflated" categories. They are also presented in table 4.
Table 4. Overloaded tires, percent of total

No. of 100- 110 More than Total All

Survey Body Style Vehicles Percent

Front Rear

110 Percent

Front Rear

Overli

Front

Daded

Rear

Tires

Davidson Laboratory Summer Station Wagon 300 5.1 9.9 2.1 9.1 7.2 19.0

Light Trucks 75 10.5 7.7 4.6 4.2 15.1 11.9

Vacation 460 5.2 10.6

All Vehicles 2800 7.2 5.7 6.5

Winter Vacation 56 1.8 17.0

All Vehicles 125 10.1 8.3 9.3

Southwest Two Door 3.8 4.7 1.6 2.3 5.4 7.0 6.2

Research Insti- Four Door 3.6 3.9 1.2 2.0 4.8 5.9 5.4

tute Station Wagon 2.7 6.9 0.9 4.7 3.6 11.6 7.6

Traffic Insti- Station Wagon 58 1.4 3.6 3.2 8.4 5.8

tute Other Cars 356 2.1 1.3 7.9 6.1 6.0

These data confirm the DL findings that rear tires on station wagons are more frequently overloaded than either set on any passenger type vehicle (the SWRI survey,

contrary to the other two, excluded light trucks).

In the report on TI Study #2 Baker and Mcllraith present an even more limited breakdown by body type. They describe their sample population as follows:

Of the 1,746 four-tired vehicles surveyed, 250 (14.3 percent) were

station wagons; the remainder, with the exception of a few small

trucks, were ordinary passenger cars. Their Exhibit 25 provides a comparison of tire loadings for station wagons and for

passenger cars. Contrary to the other two surveys, the data indicate about the same overall extent of overloading of tires for station wagons as for cars (5.8 percent vs 6.0 percent): Rear tires of station wagons were found to be more frequently over- loaded than front tires (as in the other two surveys) but not to a much greater extent

than overloading of front tires on cars. Pertinent data excerpted from Exhibit 25 are also presented in table 4.

(2) Overloading, Front vs Rear Tires : We have seen in table 4 that the SWRI con- firmed the DL finding that rear tires on station wagons are more frequently overloaded

than either set on any other passenger vehicle. However, these SWRI data indicate

that rear tires are overall, the more frequently overloaded set. This finding is con- trary to those of both the TI and DL surveys. In Exhibit 22 of the TI Tire Study 2, we find that about 7.1 percent of front tires are reported as overloaded at their

equivalent cold pressures (Pc) vs about 4.8 percent of rear tires. The DL data summer survey [6] show 7.2 percent of all front tires overloaded to some degree vs only 5.7

percent for rear tires. It is true that the SWRI survey did not include any light

trucks, whereas these vehicles were included in the other two surveys. Nevertheless,

the presence of 153 light trucks out of a total of 4502 vehicles in the DL survey

(less than 3.4 percent) with 15.7 percent of their front tires overloaded to some degree vs 9.8 percent of their rear tires, does not appear sufficient to account for

the difference. (The separate contributions of trucks in the TI study is not

reported.

)

With regard to overloading of rear tires on station wagons, one is led to wonder how

much of the problem is genuinely overloading (i.e., actual tire loading in excess of

rated load at maximum permissible inflation pressure) and how much is actually underinflation. Most car manufacturers recommend a 4 psi higher inflation pressure for rear tires on a station wagon. A GM study [11] leads one to suspect that more station

wagons are inflated to "the same pressure all around" than not. Without presenting

any evidence, Baker and Mcllraith state in TI Study 2 that "wagons have more rear

tires underinflated whereas cars have more front tires underinflated.
-19-

(3) Tire Loading vs Tire Size . In the SWRI report [7] , Lepisto presents an analysis

of tire load distributions by tire sizes and construction styles; the data are summarized in their table 16. Again recasting the SWRI data to exclude the "Underinflated" and "Overinflated" categories for which load conditions were not reported,

we obtain the data shown in table 5. Table 5. Tire load distribution by tire size and style

Total

Size Style or less Percent 110 Percent Overloaded

13" Standard 83.2 9.1 7.7 16.8

15.2

100 Percent

Style or less

Standard 83.2

Radial 84.8

Standard 94.8

Series 70 93.5

Radial 92.9

Standard 94.4

Series 70 93.6

Radial 95.6

100-110 More than

Percent 110 Percent

9.1 7.7

10.2 5.0

3.8 1.4

5.2 1.3

1.1 6.0

3.8 1.8

6.4 0.0

3.5 0.9

14" Standard 94.8 3.8 1.4 5.2

6.5

7.1

! r," Starul.-ifcJ 1)4,.

I

/,.:-, i 5.6

6.4

4.4

The data indicate that 13-inch diameter tires, comprising some 6.5 percent of the SWRI

population, were more frequently overloaded than the larger sizes by a factor of about

2 1/2 to 1. These 13-inch tire sizes are found on nominally six passenger cars. It

is apparent that many of these vehicle/tire combinations do not provide a reserve

tire load capacity sufficient to handle the number of passengers and accompanying

impedimenta that the motoring public expects of a six passenger vehicle. Presumably

MVSS-110, which became effective on January 1, 1968, will do much to alleviate tire

overloading in the future. This standard specifies requirements for selection of

tires by the vehicle manufacturer in terms of both normal loading for the vehicle and

maximum load conditions. Nevertheless, car owners still must recognize that higher

cold inflation pressures are required for maximum load conditions and for high speed

driving, and that failure to maintain adequate inflation will seriously shorten tire

life and greatly increase the risk of tire failure through overheating and overstressing.

c. Underpressured Tires

We have mentioned earlier that the SWRI reported finding 11.5 percent of its sample

population having a Pc not only below VDP , but even below the minimum allowable

inflation pressure as specified by MVSS-109. There is no confirmatory information

available in the other two reports. However, Drs . Paul L. Olson and Herbert J. Bauer,

in reporting a survey of inflation pressures in the tires of 405 cars in company parking lots [11], provide data that sheds some additional light on the frequency of

underpressured tires in service. The histogram of their data is shown in figure 3.

It appears that some 120 tires out of the total of 1620 tires examined, or about 7.5

percent, had cold inflation pressures 5 psi or more below the car manufacturer's

recommended pressure. If we assume that this recommended pressure is 24 psi or less,

such tires are seriously underinf lated (underpressured); the SWRI's finding of 11.5

percent of such tires in their population sample does not appear unreasonable.

The Uniroyal Tire Company measured tire inflation pressures on 1904 parked cars during

the spring and summer of 1969. The study [12] did not include vehicles having VDP '

s

below 24 psi. One survey covered 25 cities, and three others were area studies. The

number of cars examined at each location is shown in the following list:

Location Cars

Atlanta 72 Baltimore 30

Boston 24

Buffalo 24

Charlotte 24 Chicago 48

Dallas 73

Allen Park 48
-20

Location Cars

Houston 24 Los Angeles 72 Memphis 48

Miami 24 Philadelphia 24 St . Louis 24 Salt Lake City 24 San Antonio 48

San Francisco 48

Cincinnati 24 Kansas City- 72 New York City 48

Phoenix 24 McKee ' s Rocks 24 Denver 26 Minneapolis 24 Seattle 24 Los Angeles Area 507

Minnesota Area 247

Texas Area 205

Total 1904

250 1-

§ 200 CD

P

c/> LU

o

oc

LlI

150

H- mn100 _

50-

J I I II 1 -10 -8-6-4-2 2 4 6 8 10 MAGNITUDE OF PRESSURE ERROR IN POUNDS

Figure 3. Distribution of pressure error measured on 405 cars

(Ref. 11, p. 5)
-21-

Each survey classified the data by number of cars having one, two, three, or four

tires at or below a given inflation pressure. All the data related to underinflated

tires at cold pressures of 20 psi or less. The data in each survey for cars with

three and four underinflated tires was combined and a Chi-square test made to determine if any of the several populations was significantly different at the 95 percent

confidence level; none was. The data were therefore combined and are shown in table

6. The table indicates that 541 of 1904 cars (28.4 percent) had one or more tires at

20 psi or lower (cold) inflation pressure. The last two columns on the right indicate

the total number and percent of cars with one or more tires at or below the specified

inflation pressure. The data are cumulative; that is, the cars and tires at a higher

inflation pressure include those below.

Table 6. Number and percent of cars classified by number of

underinflated tires at or below specified pressures

Total Number of Cars Examined = 1904

Tires per Car and Percent of Population Total

Inflation 1 Per- 2 Per- 3 Per- 4 Per- PerPressure Tire cent Tires cent Tires cent Tires cent Cars cent

20 316 16.6 126 6.6 65 3.4 34 1.8 541 28.4

18 170 8.9 48 2.5 21 1.1 7 0.4 246 12.9

16 79 4.1 16 0.8 3 0.2 1 0.1 94 5.2

14 40 2.1 4 0.2 2 0.1 1 0.1 47 2.5

12 20 1.1 3 0.2 23 1.2

With one or more underinflated tires

Based on the data in table 6, the total number of tires with 20 psi or less inflation

J pressure is 899* or 11.8 percent of the total tire population. This agrees with the

J approximate 10.9 percent that can be derived from the GM data [11], graphically shown

] in figure 3, and the 11.5 percent obtained by SWRI after estimating the cold inflation

pressures by the B-M method (see section 3.3.b).

The agreement lends credence to the B-M method and consequently to the estimates of

the frequency of tire overloading based on these estimated cold inflation pressures.

If it is assumed that underinflated tires occur 11.8 percent of the time with equal

probability in every tire position on a vehicle, we would expect 60.5 percent

(0.882^ x 100) of the cars to have no tires at 20 psi or under, and 32.4 percent

(4 x 0.882^ x 0.118) with only one tire so underpressured.

The Uniroyal data indicate that 71.6 percent of the cars have no tires at or below 20 psi and that only 16.6 percent have one such tire. It is apparent that the occurrence

of two (or more) underinflated tires on any given car in an actual population substantially exceeds that from a random distribution. It may be inferred that this

difference reflects a greater concern on the part of some drivers for maintaining

proper tire inflation pressure; and on the other hand, a greater carelessness on the

part of others

.

3.4 Air Tower Pressure Gage Accuracies

In the GM survey described above, Olson and Bauer report that "about 80 percent of the

vehicles checked had tires that were underinflated or overinflated by 2 psi or more,"

and that "the correct pressure differentials were found on less than 10 percent of

those cars on which differential pressure are specified for front and rear tires."

Later on they make the following comment:

Most serious from the standpoint of handling, however, were the variations

in air pressure on individual cars. About 60 percent of the cars checked

had one or more tires that were at least 5 psi above or below specifications.

(1 x 316) + (2 x 126) + (3 x 65) + (4 x 34) = 899
-22-

The authors conclude that "the results of surveys made by the GM Research Laboratories

indicate that little or no attention is paid to tire pressure by the majority of

motorists," and add that "the situation is further complicated by the fact that ser- vice station attendants are, in general, poorly informed in this area, and dispense a good deal of misinformation concerning tire pressure specifications."

Let us consider now the motorist who reads his car owner's manual and is concerned

about maintaining proper inflation in his tires. Relying on service station air

towers and attendant's hand tire gages, what are his chances of getting his tires

properly inflated? Assuming that he has a car manufactured after January 1, 1968,

MVSS-110 requires that the following information about his car be made available to him:

Designated seating capacity (total number and seat locations)

Vehicle capacity weight (i.e., rated cargo and luggage load plus 150 lb

times the vehicles designated seating capacity)

Recommended tire size

Recommended cold tire inflation pressure for maximum loading, and

(presumably) the recommended cold inflation pressure for normal driving

conditions (as defined by the manufacturer)

Although he does not have access to a highway load scale, the intelligent motorist who

is willing to take the trouble to do so with the above information available to him

should be able to estimate the proper cold inflation pressures for his tires fairly

closely. After all, for most common tires sizes, the difference in load carrying

capacity in going from 24 psi to 32 psi is only about 17 percent. But how accurately

will his tires be inflated to the pressures he requests?

Let us ignore the problem of tire temperature and consider only gage accuracies. In the GM survey, Olson and Bauer report that of service station gages checked at a dozen

gasoline stations, all but one were "accurate within 2 psi throughout the test range."

However, the RMA recommends a tolerance of ± 1 psi. Moreover, the authors themselves

do not seem to consider an error of ± 2 psi in tire inflation entirely acceptable.

The results of other surveys are distinctly less heartening.

a. NBS/OVSR Air Tower Study

The OVSR has recently reported the results of a study of the accuracy of air tower

pressure gages sampled in the Washington, D.C., metropolitan area [13]. Fifty service

stations were chosen at random from a population of some 200 in selected nearby

suburban areas. Using a test gage ± 0.15 psi accuracy) connected to the spare tire of

the survey vehicle', the air tower gage at each of the 50 stations was tested at settings of 20, 24, 28, 32, and 36 psi. The sequence of pressure settings was randomized for each tower. At each setting, the spare tire was pumped up until the

tower bell ceased ringing; the pressure was then read with the test gage. The latter

was calibrated against a master gage (accuracy ± 0.10 psi) at the beginning and end of

each day to assure reliability of the data. Histograms of the results for three tower

settings are presented in figure 4. Statistical analysis provided the following information:

1. A motorist using an air tower in the suburbs described has only a 20 percent

chance of obtaining an inflation pressure within ± 1 psi of the setting he selects on

the air tower.

2. The probabilities for larger pressure deviations are as follows:

Deviation from Probability

Tower Setting Percent

± 2 psi (or greater) 62

± 3 psi (or greater) 45

± 4 psi (or greater) 32
23-

IOi8 -

4-

J m 1 1 1 1

1

TOWER SETTING = 32 psi

n.n i i i i i i

i n.n

-6 -4 -2 2 4 6 8 10 12 14 16

TOWER SETTING = 28 psi

-20246 DEVIATIONS (psi) Figure 4. Inflation pressure deviations between air tower and test gage

(Ref . 13, p. 8)

.

The results also showed that air towers incorrect at one setting were in error on the

other four settings by about the same amount. It appears that a simple periodic

calibration to ± 1 psi at one tower setting would be sufficient to provide corresponding accuracy throughout the range normally used.

b. Davidson Laboratory Surveys [6]

The DL Survey teams checked air towers at the 137 sampling sites in its summer survey,

also at the seven Chicago area service stations used for the winter survey. Of the

latter, only one had a functioning air tower, and towers at the six others were either

frozen or broken. In the summer survey about 47 percent of the 137 service stations

either had no air supply or required an auxiliary tire gage for checking tire

inflation pressure. Similar to the OVSR survey, their data indicate that "if a tower

delivered a low reading at one pressure, the measurements were low throughout the

range examined." Their results are given in table 7.

3.5 Hand Automobile Tire Pressure Gages

The OVSR has recently conducted a study of the accuracy of hand tire pressure gages.

The gages tested can not be assumed to be a random sample of hand gages in service;

all gages tested were the property of NBS employees who submitted them voluntarily.

Each hand gage was tested at 24, 28, and 32 psi against a master gage accurate to 0.10 psi. The hand gage was read to the nearest 1 psi. The population consisted of

251 pencil and 23 dial types; the two populations were not analyzed separately.
-24-

Table 7. Measured pressure deviations from tower indicated pressures

(Number of Stations)

Measured Tire Tower Pressure Setting

Pressure

Deviations 20 psi 25 psi 30 psi 35 psi 40 psi

(psi) No. Percent No. Percent No. Percent No. Percent No. Percent

No Answer* 64 47.1 62 45.6 63 46.3 65 47.8 70 51.5

-8 1 .7 -7 1 .7 1 .7 -6 1 .7 -5 3 2.2 1 .7 1 .7 2 1.5

-4 2 1.5 3 2.2 4 2.9 3 2.2 5 3.7

-3 3 2.2 5 3.7 5 3.7 3 2.2 4 2.9

-2 5 3.7 8 5.9 7 5.1 5 3.7 6 4.4

-1 10 7.4 9 6.6 10 7.4 13 9.6 9 6.6

36 26.5 33 24.3 35 25.7 33 24.3 32 23.5

+ 1 10 7.4 9 6.6 6 4.4 5 3.7 4 2.9

+ 2 1 .7 2 1.5 3 2.2 4 2.9 2 1.5

+ 3 2 1.5 3 2.2 1 .7 1 .7 3 2.2

+ 4 + 5 Total Stations 136 100.2 136 100.0 136 99.8 135 99 .3 136 99.9

Av Error (psi) -.48 -.57 -.70 .64 - .85

Std Deviation 1.60 1.76 1.65 1.73 1 .76

*Including those stations with

a. No integral gage with air supply

b. Broken pressure control or indicator

c. No air supply

There was no significant variation in the distribution of differences between the

master gage and the hand gages at the three pressures. The results were as follows:

35 percent differed by less than ±0.5 psi

75 "

" " ""

± 1.5 " 90 "

" " "

"

± 2.5 " 97 "

" " " "

± 3.5 "

Consumers Research, Inc. [14] reported 6 of 12 brands of new gages tested in 1955 were

accurate to within 1 psi, whereas in 1962 only one of 13 brands was that accurate.

Consumers Union [15] in 1968 tested 35 pencil and 4 dial type gages. Six samples

of each of the 39 models were tested at 10°, 70°, and 110°F. Only 10 models were within 1 psi at all temperatures.

3.6 Tread Depths

In all three major studies, tread depths were measured using commercially available

gages designed for the purpose. Tread wear patterns were examined also.

Baker and Mcllraith [2] describe the method used for measuring tread depth in the

Traffic Institute surveys as follows:

Groove depth of each tire was measured at three points: inside, middle,

and outside. All measurements were made in 1/32 in. The minimum

remaining groove depth and the maximum variance of groove depth from

minimum were tabulated for each tire. . . . These two figures give a measurement of the wear on a tire and also of the uneveness of the wear.

The DL reports [6] do not describe how the tread depths were taken in their survey

except to state "The tread depth recorded was the minimum of approximately five

readings taken around the tire at each position." The Team Supervisor's Manual

provides a brief instruction on manipulation of the tread depth gage. However, it

appears from Table XIII (Tire Wear Patterns) in their final report that the DL survey
-25-

teams may also have measured inside, center, and outside grooves on each tire. In any event, the values given in Table VIII (Measured Tread Depth) are described as "Minimum Measured Tread Depth to Nearest 1/32-inch."

Lepisto, in the SWRI report [7] states that "Three tread depth measurements were

taken on each tire across the tread surface and the total was averaged." None of the

reports state, even approximately, what fraction of the tire circumference was gaged,

nor again is there much similiarity in the analyses and presentations of the data.

Baker and Mcllraith in their Tire Study No. 2 present a distribution by tire position

that is quite informative. The data show little difference by tire position either in

tread depth distribution or in average tread depth remaining: right front, 7.50; left

front, 7.66; right rear, 7.46; and left rear, 7.51 (all in 32nds of an inch). The

authors conclude that: "The small differences between the average groove depths of

the four tire positions suggest that wear is relatively even on all tires or that

people rotate their tires, or both. ..."

In the DL reports separate distributions are provided for front and rear tires. In addition, Ehrlich and Jurkat have broken out snow tires and report them separately,

providing tread depth distribution for (1) regular tires, (2) snow tires, (3) not

typed, and (4) all. Thirty-nine percent of the 772 typed tires examined in their

winter survey (late December in Maryland, Pennsylvania, New York, and Massachusetts)

were classified as snow tires vs only 4 percent of the 15,912 typed tires in the summer survey. The authors state that "In contrast to the summer survey, which showed

a relative uniform distribution of minimum tread depth, the winter survey is skewed

toward deeper depth by the presence of a large proportion of rear snow tires at 11/32 inches."

With respect to tread depth distributions at least, it would appear desirable to

treat snow tires separately, particularly during the winter months of any tire

condition and use survey in areas where snow tires are common.

Let us now examine and compare, as best we can, the tread depth findings in the three

condition and use studies. In doing so we shall take special note of the percentages

of tires in each survey having less than 2/32-inch of minimum tread depth remaining.

The Rubber Manufacturers Association's Tire Inspection Guide lists the following

condition as a cause for rejection of passenger car tires without tread wear indicators : Any tire worn to the point where less than 2/32nds of an inch of tread

design depth remains in any two adjacent tread grooves, exclusive of tire

bars, at three equally spaced intervals around the circumference of the

tire, or where any part of the ply or cords is exposed.

In section 4.2 of this report we shall learn that in 57 percent of tire failures in an investigation of some 60,000 accidents, the California State Highway Patrol reports

finding that tread depths were less than 2/32-inch [16]. Moreover, we shall there

have cause to return to examine bald and thin tires, wherein we shall define bald

tires as those having a minimum remaining tread depth less than 2/32-inch, and thin

tires as those having a minimum tread depth remaining that is equal to or greater

than 2/32-inch, but not more than 4/32-inch. We shall review in section 4.2 a report

by the Texas Highway Department [17] on the condition of the tires on some 240 wet

weather accident vehicles, disclosing that the incidence of bald and thin tires (as

defined above) on the rear wheels of these vehicles is about 40 percent higher than

that found in the general population as represented by the DL summer survey, and

about 3 times as large as the percentage found in samplings on interstate highways

(SWRI survey) or tollway systems (TI Study No. 2).

Comparative data on average minimum tread depths remaining and on the fractions of the

sample populations having minimum tread depths of 2/32-inch or less are presented in

table 8. Note the lower tread depths found in both the DL surveys, also that the

percentages rejectable by the RMA standards is about 3 times those of the SWRI and TI surveys. It is evident that many drivers using the turnpikes for long trips at high

speeds replace marginal tires rather than risk a possible tire failure. A breakout

of 183 vehicles from the total population of the DL summer survey, representing high

speed, throughway traffic only confirms this conclusion. We shall see further

evidence that tire quality in the general population is significantly poorer than that

among high speed through travelers when we consider TI Study No. 1 under section 4.1.
-26-

Table 8. Tread depths, physical condition and use surveys

TI STUDY NO. 2 Tread depth < 2/32", percent

Av min tread depth, 32nd-inch

SWRI SURVEY

Tread depth < 2/32", percent

Av min tread depth, 32nd-inch

a

DL SURVEYS

Summer

Tread depth < 2/32", percent

Av min tread depth, 32nd-inch

Thruway Vehicles Summer Survey

Tread depth < 2/32" , percent

Av min tread depth, 32nd-inch

Winter

Tread depth < 2/32", percent a Av min tread depth, 32nd-inch

Total Tire Position

Tires in

Sample LF RF LR RR All

6,984

3 5 3.7 5.2 4.6 4.2

7 7 7.5 7.5 7.5 7.5

45 ,053

1.7

7.2

Both Fronts Both Rears All

17,948

3 b

4 b 11. 8^

6.1 d

11 12.

3

C

6 5.8 C

728

2 .2 4.4 3.3

7 5 6.6 7.0

796

7.0*

7.6 r 185

9 U

5 e 12 .9 6.6

Percent of known population

Snow tires 0.6 percent of population

Snow tires 6.5 percent of population

Snow tires 3.5 percent of population

Q Snow tires 1.8 percent of population

Snow tires 73.9 percent of population

3.7 Tread Wear Patterns

Baker and Mcllraith examined the variance of tire groove depth in their population

sample of 6984 tires [2]. They report finding 3.6 percent as having differences of more than 4/32-inch groove depth (inside, middle, and outside). Somewhat surprisingly, there were more such tires on the rear wheels than on the front: LF, 1.9

percent; RF, 2.6 percent; LR 4.4 percent; and RR, 5.6 percent. Almost 10 percent of

the tires had more than 3/32-inch variance in groove depth, "enough to suggest

definitely uneven wear caused by overinflation, underinf lation , or improper mechanical

adjustment." The authors examined for other tire ailments as well; these will be

discussed in section 3. 8. a following.

Ehrlich and Jurkat present a tabulation of tread wear patterns in their main report

[6]. Of the 18,008 tires examined in their summer survey, they report 27.8 percent

of front tires and 17.8 percent of rear tires as showing some unusual tread wear

pattern.* Their tabluation shows 22.2 percent of front tires and 21.8 percent of rear tires as having differences in groove depth (inside, middle, and outside);

however, the minimum difference established as their criterion is not stated.

3.8 Tire Defects

a. Tires in Service

Baker and Mcllraith in their report on the use and condition of tires on the Illinois

Tollway present an interesting and informative discussion of various tire defects

found in their survey. As the authors describe their summary of tire ailments [2]

,

*Inside, outside, both sides, center, bald, waves, feathered, flat spot, or step wear,

as defined by MIL-STD-1224 , "Visual Inspection Guide for Pneumatic Tires

(Non-Aircraft) .
-27-

it "aims to point out conditions about which a car owner or driver should do something

or of which he should at least be aware."

In table 9 we have attempted to assemble comparable information on these conditions,

insofar as such information is available in the final reports of the three studies.

Table 9. Summary of tire defects found in condition and use surveys

Percent of Tires Examined

DL Surveys

TI No. 2 SWRI Summer Winter

Overloading

1. As Found 5.9 6.2 b 6.2 9.3

2. Not correctable by 2.4 0.9 0.5

inflation

Not enough tread left d 4.2 1.7 11.8 12.9

Sidewall cracks 4.2

£ Uneven wear 3.6 - 22.0

Overpressured

1. As found 1.9 3.2 5 2. Not correctable by

deflation 0.1 0.9

Bulges or blisters^ 0.17 - - -

Underpressured - 11.5

Excluding "unknowns" or "No Response"

Does not include the 11.5 percent of underpressured tires reported for which load

conditions were not given

Require larger tire or removal of some load

Minimum groove depth less than 3/32 -inch

One or more cracks in outside wall large enough to be probed by fingernail

More than 4/32 inch differences in depth remaining in different grooves

°Any lump or swelling on visible part of tire

ECP below permissible range for specific tire

(1) Percentages of Tires and Cars Examined . Excluding uneven wear, Baker and

Mcllraith reported finding 1022 of the total of 6984 tires examined, or 14.7 percent,

with one or more deficiencies serious enough to warrant correction. There were a total of 1117 ailments found, or 1.09 per defective tire. These 1022 tires were on

only 607, or 34.7 percent, of the 1746 vehicles examined. As the authors point out,

"approximately two out of three (415 out of 607) cars with one bad tire also had a second bad tire."

Ehrlich and Jurkat reported 77.2 percent of the 18,008 tires in their summer survey

as having none of the tire wear patterns for which they made observations (see section

3.7 above). They too found that some tires had more than one defect.

(2) Tread Rubber on Roadsides . The tread rubber strewn along roadsides is taken as evidence of tire failure. This kind of failure has been attributed to the attainment

of high temperatures in the tire caused by sustained high speed, overloading, high

pavement and ambient temperatures, either single or in combination. An attempt was

made by SWRI [7] and DL [18] to determine the source and frequency of this type of

failure by examining the rubber collected along the roadsides.
-28-

Both groups found that the amount of large truck tire rubber exceeded the passenger

car tire rubber. However, they were not able to draw conclusions about the frequency

of the event or otherwise interpret their findings in a useful way. We may explain

this if we compare the practices of commercial truckers with what is likely to be the

reaction of the driver of a passenger car or small truck when a tire throws tread

rubber

.

The long haul truckers do not stop to remove a tire if it is possible to continue.

Consider a case in which one tire of a dual pair has a blowout or throws a piece of

tread. The truck continues to roll (the cost of the tire is less than the cost of the

delay) , and the tire may be completely destroyed leaving a trail of rubber along the

road

.

If a passenger car tire experiences a tread separation, the driver will stop to find

the cause of the noise. In any event he will probably be forced to remove the failed

tire before continuing; little or no rubber will appear on the road from this source. Therefore, a single truck tire failure may produce a large number of pieces of rubber

whereas a single passenger car tire may produce none.

b. Failure Experience, New Tires

In both the SWRI and the DL summer survey, drivers were asked about tire problems or failures experienced in the previous 2 years. We must recognize the fallibility of

such information; nevertheless, it is of interest to examine and compare their data.

Of the 4502 drivers interviewed in the DL summer survey, 1195 answered "Yes" to the

question, "Have you had a tire problem in the last 2 years?" Of these, 366 said the

failure was covered by warranty with 213 "not sure;" 2194 drivers reported no failures, while 1113 did not know.

Of the 11,385 drivers interviewed in the SWRI survey, 1462 responded "Yes" to the

question, "Any defects in newly purchased tires within the last 2 years?" In the

SWRI report, Lepisto concludes that "Almost one-half (42.7 percent) of the original

equipment tires required some kind of warranty claim within the last 2 years."

These authors cannot agree with this conclusion. It appears to them that Lepisto has

assumed that all of the reported tire warranty failures (1462) were OE failures, for

with the "approximately 30 percent (3415) of the cars in the sample. . . equipped

with OE tires" (see page 41 of his report), we have

., .-. ,- X 100 = 42.8 percent of new cars having a tire warranty failure.

The above calculation also ignores the fact that each new car comes equipped with five new tires

.

Assuming that on the average about two new tires are purchased for each vehicle every

year (20,000 miles average tread wear per tire prior to discard; average of 10,000

miles driven per vehicle per year) , we can arrive at some estimates of warranty

failures on new tires as percentages of all new tires purchased (including OE) for

both the SWRI and DL (summer) surveys:

Tire Defects or Failures under Warranty as Percent of All New Tires Purchased

DL Summer Survey : Assuming that half of the "not sure" category were actually failures under warranty,

we have ~ 100 366 + ~T~

4502 x 2 x 2 = 2 - 6 P ercent

SWRI Survey : Assuming that each of the "yes" responses represented only one tire

defect or failure, we have

100 T-1—=-5-= = =- = 3.2 percent 11,385x2x2 ^
-29-

However, the above calculations ignore the fact that each car in a survey that was purchased new actually brought five new tires into the survey. The above calculations

therefore somewhat over estimate the percentages of warranty claims on new tires.

Using the reported data on OE tires in each survey, we can recalculate as follows:

PL Summe r Survey : 35.5 percent of tires surveyed reported as OE [6]

213

100

+ ~2~ ^_ = 100 472J = 2 4 percent 4502 (0.354 x 5 + 0.646 x 2 x 2)

1UU 4502 x 4.35 Z,H P erceni:

SWRI Survey : 31.8 percent of tires surveyed reported as OE [7]

1462 nA „ 1462

100 11,385 (0.318 x 5 + 0.682 x 2 x 2) 10 ° 11,385 x 4.32 3- ° P ercent

As the DL summer survey asked for all tire problems rather than just those on newly

purchased tires, as in the SWRI survey, we can make some further estimates. Again

we shall assume that half of the "not sure" category were actually under warranty:

Warranty claims as percent of all tire failures reported for the previous 2 year

period in the DL summer survey:

213 366 + ~-

100 11Q[. ——-—- = 40 percent

Tire failures in previous 2 years as percent of total tire positions:

1195 100 45Q2 -

4— = 6.6 percent

The DL summer survey questionnaire also asked each driver to estimate the number of

miles the car was driven each year. Using this information and the reported tire

failures, Ehrlich and Jurkat calculated a tire failure rate of 113 per million

vehicle miles. In section 4.1. a, we shall observe that this failure rate is almost

2 1/2 times that found in the Traffic Institute studies. The major part of this

difference can probably be ascribed to the difference in the two populations sampled.

4. TIRE DISABLEMENTS AND VEHICLE ACCIDENTS

Most of the information presently available on tire disablements and their relation

to motor vehicle accidents is that which was obtained during the series of studies

by the Traffic Institute of Northwestern University on the Illinois Tollway during

the period September 1966 through August 1967 [1, 3, 4]. As in the tire condition and

use studies, we shall briefly review each of the three reports by Baker and Mcllraith

in an attempt to provide the reader with sufficient background to exercise his own value judgments, then compare their findings with such other information as we have

available to us

As in the tire condition and use studies, many interesting details will necessarily

be omitted in this review. The reader who wants the full picture is urged to consult

the original reports.

4.1 Tire Disablements Not Followed by Accidents

a. Frequency of Tire Disablements

In their introduction to TI Study No. 1, Baker and Mcllraith present an excellent

discussion of the factors and problems involved in obtaining reliable data for a tire

disablement study. The authors point out that the principal factors affecting tire

disablement rates are the following:
-30-

--Vehicle speed

--Condition of tires, including inflation pressures and loading

--How the vehicle is driven

--Type and condition of highway

--Weather conditions, principally temperature.

With regard to sample size, the authors state that it was arbitrarily decided to continue a survey "until about 50 disablements had been recorded, but not for more

than 3 weeks."

(1) Number and Purposes of Surveys . Three tire disablement surveys were taken, all

by interviewing drivers at selected toll booths on the Illinois Tollway. As in all

of the studies included in this review, only four-wheel, four-tired vehicles equipped

with passenger car tires were considered.

The first survey was taken at South Beloit, the farthermost exit on the Tollway, in

April and early May 1967. The second, designed to evaluate the effect of temperature,

was also conducted at South Beloit, in July of the same year. It was recognized that

the second survey would probably include a higher percentage of long-distance,

through-traffic (vacationers), and we shall see evidence that this was so. Nevertheless, it was felt that the principal factor difference would be that of temperature.

The third survey was made in such fashion as to emphasize the effects of different

driver populations. Carried out at the same time as the second survey, and therefore

at the same ambient temperature, the third was taken at a toll booth near the intown

Kennedy Expressway, to include a maximum percentage of short-trip commuter traffic to

and from downtown Chicago, in contrast to the preponderance of long-distance travelers

at South Beloit.

(2) Data and Results . In the surveys, each driver was asked two questions:

(a) Where did you enter the Tollway? and (b) Since entering the Tollway, did you

experience a tire disablement or other vehicle trouble?

Drivers reporting a difficulty were questioned further. However, the interview was designed to delay such drivers no more than 30 seconds. All replies were accepted

without question. Results of the three surveys are presented in table 10.

Table 10. Frequency of tire disablements on the Illinois Tollway

Total Vehicle Disablements

Percent Av Miles Ambient Av Miles Tire

No. of Tire Tire Driven on Temp, °F per Tire Failures

Survey Period Drivers Failures Other Failures Thruwayb Range Av Failure per mvmc

35-63 48 34,322 29

60-75 69 22,287 45

60-75 69 14,112 71

Excluding "out of gas" category

Based on random sampling of about 6000 cars in each survey

Per million vehicle miles

Vehicle overheating comprised 30 percent of total in this survey vs less than

1 4/24-

5/11/67

32,034 56 33 63 60

2 7/10-

7/25/67

23,332 67 37 64 64

3 7/10-

7/28/67

29,792 38 53 42

d 18

percent in each of first two surveys
-31-

In their report Baker and Mcllraith demonstrate that for each of the surveys there is no reason to reject their preliminary hypothesis that the observed incidence of tire

disablements would follow a Poisson distribution. As in their tire condition and

use survey, cars surveyed were classified by size and by age (see section 3.1). Based

on a sampling of 3000 cars from each survey, distributions by vehicle size and age were as shown in table 11.

Table 11. Distributions (percent) by size and age of cars surveyed

(Based on the 3000 car samplings from each survey)

Vehicle

Age

Standar d Compact Small Totals

1

6.0

Survey

2

4.3

3

8.9

1

0.9

Surve

2

0.2

y

3

0.9

1

0.6

Survey

2 3

0.0 0.6

1

7.5

Si

2

4.5

irvey

3

Old 10.4

Medium 33.2 34.0 42.2 7.5 6.7 8.1 1.8 0.9 3.5 42.5 41.6 53.8

New 42.7 47.1 31.6 6.6 5.9 3.0 0.7 0.9 1.2 50.0 53.9 35.8

81.9 85.4 82.7 15.0 12.8 12.0 3.1 1.8 5.3 100.0 100.0 100.0

Av S3.

13.3 3.4

Comparing the distribution matrix by size and age for the vehicles experiencing tire

disablements with the corresponding matrix (table 11) based on the sampling of 3000

cars, Baker and Mcllraith found for each survey that there was no statistically

significant difference between the two matrices for either size or age using a Chisquare test at the 95 percent confidence level. Later in their report, the authors

combine the three pairs of distributions into one by weighting "to take account of

different vehicle mileages." Using the resultant pair of matrices for all vehicle

surveys and for all vehicles experiencing a tire disablement in Chi-square tests at the 95 percent confidence level, Baker and Mcllraith then found that although again

classification by vehicle size v/as not statistically significant, vehicle age very

definitely was (Chi-square value 19.5 vs the critical value of 5.99).

Using the distributions by vehicle size and age of the tire failures reported in each

survey and the corresponding survey distribution (based on the 3000 car samplings

from each survey population), the authors calculated disablement indexes. Establishing those for the standard size and for the new cars in each survey as 1.00,

reduced disablement indexes were calculated. Those by size are presented in table 12

and by age in table 13.

Table 12. Tire disablement indexes by vehicle size

Vehicle Size Survey 1 Survey 2 Survey 5 Standard (3,000 lb) 1.00 1.00 1.00

Compact (2,000 to 0.80 1.45 1.32

3,000 lb)

Small (2,000 lb) 0.53 -- a 1.79

No tire failures reported

Table 13. Tire disablement indexes by vehicle age

Vehicle Age Survey 1 Survey 2 Survey 3 Overall (combined)

New (1966-67) 1.00 1.00 1.00 1.00

Middle (1960-65) 1.38 1.89 1.98 1.73

Old (1959 or older) 1.76 3.96 2.94 2.44
-32-

Minor findings in the surveys included the following:

--Neither driver sexnor age appeared to be significant.

--About two-thirds of all drivers changed their own tires.

--Women constituted about 9 percent of the driver population surveyed.

--About 3 1/2 percent of the disabled vehicles had no spare.

In presenting the results of these three surveys, Baker and Mcllraith present some interesting conjectures regarding the affect of ambient temperature on the frequency

of tire disablement. To this we should like to add that there were 40 percent fewer

old cars in the second survey (more new cars on vacation trips in July) . The

temperature effect is therefore probably even more significant than the data show,

as it is apparent from this review that the tires on older cars are in poorer

condition and have less tread remaining.

The highest disablement rate occurred in the third survey, which emphasized commuter

traffic. In similar fashion, it may be argued that the higher disablement rate is

attributable principally to the much higher incidence of old and medium age cars in

the third survey. It is also true that vacationers and other long-distance, throughtravelers have tires in better condition than those of the general population (see

table 8) .

In closing their report Baker and Mcllraith use a combination of reasonable assumptions and estimates to arrive at an overall annual rate of about 46 tire disablements

per million vehicle miles (mvm) or one for every 22,000 miles of vehicle travel. The

authors emphasize that this figure is "only a rough estimate for four-tired vehicles

on limited access roads" comparable to the Illinois Tollway with respect to mean

annual temperature, road surface, quality of maintenance, and traffic mix. The

authors cite (but do not reference) a 1962 Eisenhower Surveillance Project, "a threeday continuous surveillance study of a 4.5 mile length of the Expressway in 1962."

The findings of this project were that: 1) of a total of 122 vehicle stoppages on the

4.5-mile length of Expressway, 64 (or 52.5 percent) were tire disablements, "almost

exactly the same as that obtained in this study;" 2) 69 percent of the drivers changed

their own tires; and 3) the estimated disablement rate was 42 per mvm. Assuming that 50 tire disablements per mvm is a reasonable estimate, and that average

tread wear (i.e., useful life) for a new tire is 20,000 miles, we may calculate* that

about one new tire out of every four experiences some form of disablement prior to wear out

.

b. Disablements Not Followed by Accidents

In the third of their series of four reports, Baker and Mcllraith consider a total of

407 special reports on tire disablements collected by state highway patrolmen on the

Illinois Tollway during the 12 month period September 1, 1966, through August 31,

1967. As stated by the authors, the purpose of the Tire Study No. 3 was "to discover

1) circumstances of tire disablements which were not followed by accidents to compare

with those which are followed by accidents [see their Tire Study No. 4], and

2) conditions of disabled tires to compare with tires in general use."

(1) Sample Size . Based on annual reports of the Illinois Tollway police battalion,

the 407 special reports on disabled tires constituted about 15 percent of the approximately 2800 annual cases of "Assistance Rendered, Tire Trouble." Based on the 60,000

annual tire disablements on the Tollway estimated in Tire Study No. 1, the special

reports amounted to about 0.75 percent of all tire disablements.

(2) Information and Measurements . The four page tire disablement report used is

presented by the authors in their report. Tread depth measurements were made on all

tires on the disabled vehicle, as were inflation pressures on the nondisabled tires.

Loads on the tires were estimated using the curb weight of the vehicle from published

10

6 x4—r = 9nn 200 new tires per mvm

2x10

50/200 = 1/4

L
-33-

data assuming standard accessories and a full fuel tank, plus the patrolman's

estimates of occupant and cargo weights. Weight distribution between front and rear

tires was also based on published data for each vehicle.

It was most unfortunate that as emphasized by the authors, "There was no question

listed on the report form to ascertain time between the car stopping and the tire

inflation pressure being taken by the patrolman." As a consequence, estimates of

actual equivalent cold pressures (Pc) become extremely doubtful.

(3) Vehicles and Drivers . A comparison of the 407 disabled vehicles in Tire Study 3 by size and by age with similar classifications for the population samples of the

Traffic Institute Studies 1 and 2 is presented in table 14.

Car Size

Car Age

Old

STANDARD COMPACT SMALL TOTALS

Study

l a Study2 Study

3 Studyl a Study2 Study

3 Study

l a Study2 Study3 Studyl a Study

2 Study3

6.4 3.6 10.8 0.7 0.3 1.0 0.4 0.1 0.2 7.5 4.0 12.0

Medium 36,4 23.0 44.5 7.4 2.5 6.9 2.1 0.5 1.0 45.9 26.0 52 .4

New 40.5 63.0 29.7 5.2 5.5 5.2 0.9 1.5 0.7 46.6 70.0 35.6

TOTALS 83.3 89.6 85.0 13.3 8.3 13.1 3.4 2.1 1.9 100.0 100.0 100.0

Average of the three surveys, each weighed equally

It is apparent that vehicle size has no significance in the disablements. Vehicle

age is another question. In both Studies 1 and 3, more old and medium age cars experienced tire disablements than were found in the Tollway physical condition and

use study. The distribution by vehicle age in Study 3 is roughly the same as that

found in the third survey (this one included the maximum of short-haul commuter

traffic and yielded the high failure rate) on frequency of tire disablements in Study

No. 1 (see section 4.1. a).

About 70 percent of the 407 vehicles had power steering, about the same as the

percentage (73 percent) found in Study No. 2. Not surprisingly, women drivers

constituted a much larger percentage than in the general population. However, neither

sex nor driver age were significant with respect to the occurrence of a tire

disablement

(4) Tire Types and Sources . Of the 407 disabled tires, 44.5 percent were 2-ply

construction vs 43.5 percent of the population in the TI's condition and use survey

[2]. Although 6- and 8-ply tires amounted to 1.1 percent of that population, none

was found among the disabled tires.

With respect to retreaded tires, only 0.26 percent of the population sample in the TI Study No. 2 for which definite information on retreading was available were retreads.

In contrast, 11 of the 407 disabled tires, or 2.71 percent, were retreads. Baker and

Mcllraith report that the difference is statistically significant "at the 0.01 level

of confidence."

(5) Inflation Pressures and Loading . Baker and Mcllraith assumed "in all cases that

the [tire inflation] pressure was taken 'hot,' that is, fairly soon after the vehicle

had come to rest." As only 10 minutes standing after stopping reduces the necessary

correction by about half (see figure 1) , this procedure may have resulted in Pc values

substantially below their true values. The authors acknowledged this.

A pressure distribution is given for the nondisabled tires, showing 95.3 percent at or below design pressure, 1.2 percent overpressured , and the remaining 3.5 percent

somewhere between design and permissible. As estimates, the resultant tire load

distributions and average loading (79 percent of rated load) for the accompanying non- disabled tires on the 407 vehicles are quite similar to those found in all four of the

condition and use surveys covered by this review (see table 3 and section 3. 8. a). One

may conclude that neither underpressured nor overloaded tires were especially
-34-

significant factors in these 407 tire disablements. Of the nondisabled tires, only

72 (6 percent) appeared to have been overloaded; of these, 52, or 72 percent, could

have been inflated sufficiently to carry the loading without exceeding the maximum

permissible pressure.

(6) Difficulties Reported by Drivers . Baker and Mcllraith explore the kinds of

difficulties reported by the drivers and their various reactions and responses to the

tire disablement. However, our reaction is that it is exceedingly doubtful that

uniformed highway patrolmen will receive frank, honest responses to questions of this

type from the average motorist. The authors themselves recognize this problem later

in the report, and point out that the motorist grossly underestimated vehicle speeds

and stopping distances. Moreover, "All but a very few [of these] tire disablements

occurred on virtually straight road."

We consider it significant that 38 of 326 drivers (11.6 percent) expressed concern

about possible loss of control, and that three reported actual loss of control (none

resulting in an accident)

.

(7) Physical Condition of the Tires . As indicated earlier, minimum groove depth

and maximum variance in groove depth were measured on all four tires; in addition,

all were examined carefully for cracks and blisters. The average minimum remaining

groove depth for the disabled tires was 4.44 32nds of an inch, and for the nondisabled,

4.89 32nds . These values are very much lower than the averages found in both the TI and SWRI physical condition and use surveys on limited access highways, and are even substantially lower than those found by the DL in samplings more representative of

the general population of passenger car tires in service (see table 8)

.

Of the disabled tires, 28.0 percent were resectable by the RMA criterion for remaining

tread depth (see section 3.6), as were 21.3 percent of the nondisabled. Compare these

figures with the 6.9 percent of such tires found in the TI's Tire Study No. 2 on the

Tollway, and the 4.8 percent found by SWRI in a similar condition and use survey of

long-distance travelers on high-speed, limited access highways. These percentages are even higher than the 17.7 percent with tread depths of 2/32-inch or less found by the

Davidson Laboratory in their two surveys more representative of the general population

of passenger car tires in use. Baker and Mcllraith report [3] that:

When the disabled tires were bald, 58 percent of the cars had

one or more [other] tires in the same condition. When the

disabled tires were not bald, only 24 percent of the other

tires on the same cars were bald.

It is also of interest to note that 18 drivers among the 407 claimed to have suffered

a previous tire disablement on the same trip.

(a) Cracks and Blisters . Cracks and/or blisters were present on 11.1 percent of the

disabled tires; the corresponding figure for the nondisabled tires was not given.

This value is about 2 1/2 times the 4.4 percent reported by Baker and Mcllraith in

their physical condition and use survey [2]

.

(b) Risk Indexes . In their evaluation of the accumulated data, Baker and Mcllraith

calculated a "'risk index'. . . obtained by dividing the percent of disabled tires

with a . . . [minimum] remaining groove depth by the percent of tires in use with

the same remaining groove depth." They then multiply these risk indexes by the

average number of disablements per mvm, to arrive at estimates of the number of

disablements per mvm to be expected for any given tread depth remaining. The very

striking results are tabulated in their report [3] and graphically represented in

figure 5. We believe it to be quite apparent from this and other information (see 4.2.c) that

bald tires (i.e., those having a minimum remaining tread depth of 2/32-inch or less)

should be either recapped or replaced. The likelihood of a disablement for such

tires appears to be about 10 times that for a new tire, according to TI Study 2.
-35-

MINIMUM ALLOWED

2/32 INCH

J l_g_L_J

-2 -4

NEW HALF-WORN TREAD

GROOVE DEPTH REMAINING

BALD

figure 5. Tire disablement rate related to tire wear (Ref. 3, Exhibit 29, p. 43).

(8) Tire Position Effects . Among the 407 disabled ti

tread depth remaining (LF, 3.40; RF, 4.24; LR, 4.99; RR

depth remaining, in 32nds of an inch). Among the nondi

slightly less tread depth remaining (LF, 4.99; RF, 4.95

for the disabled left front tires, however, differences

407 vehicles were slight. Despite this absence of any

tread depth remaining (disabled LF's excepted), 64 perc

rear tires and 38 percent of all tire failures occurred

again take note of this high incidence of rear tire fai

Texas Highway Department information on wet weather ace

res, the front tires had less

, 4.6 7 minimum average groove

sableds, rear tires had

; LR, 4.75; RR 4.71).. Except

among all the tires on the

substantial differences in

ent of the disablements were

on the right rear. We shall

lures when we consider the

idents in section 4.2.c.

4.2 Tire Disablements Followed by or Found in Accidents

In this section we shall consider tire disablements found in or followed by accidents.

We shall find the available information much more limited than that on any of the

other areas of passenger tire service experience already discussed; and this despite

an importance that, to our minds at least, outweighs all the rest.

We have two reports to consider, with additional pieces of information from various

other sources. As in the previous section on tire disablements, the major work

available is a Traffic Institute study [4] . The other report is a study of some 378

accident vehicles in rural areas of North Dakota [19].

a. Disablements Followed by Accidents on the Illinois Tollway

In their Tire Study No. 4, Baker and Mcllraith report on the incidence of tire

disablements followed by accidents on the Illinois Tollway. Data for this report were collected during the calendar year period September 1, 1966, through August 31, 1967.

This was the same period during which data for their third tire study was taken. For

the investigation, Tollway police were asked to fill out a special supplementary tire

report in addition to their regular accident report whenever a passenger car or light

truck was involved and had a "flat tire," regardless of how or why the flat had

occurred. The investigators also obtained the disabled tire and rim for detailed

examination whenever possible.
-36-

In the introduction to TI Study 4, the authors point out that "Tires may contribute

to accidents in two ways: first, by the extent to which their road traction may be

limited; and second, by their disablement, which may affect control of the vehicle."

Baker and Mcllraith correctly point out that their study was directed only toward the

second effect. As we shall see in section 4.2.c, the importance of the first factor

is much too great to be ignored. Unfortunately, the Texas Highway Department's wet

weather accident data is all that was available to us for this review.

(1) Efficiency of Data Collection . During the period, 1432 accidents involving a total of 2196 four-wheel, four- tire vehicles were reported on the Tollway. Supplementary reports of tire disablements were received on only 80 (3.6 percent) of these

vehicles; 39 of the disabled tires and rims were obtained. Regular police accident

reports indicated that an additional 32 supplementary tire reports should have been

turned in. Based on a telephone sample,* it was further estimated that "there may

have been as many as 123 additional vehicles with tire disablements following

accidents, making a possible total of 235 or 10.7 percent of all four-tired vehicles

in accidents" during the 1 year period. The estimated 123 additional vehicles

amounted to 6 percent of the 2051 vehicles for which specific tire disablement

information was not available.

(2) Incidence of Tire Disablements in Accidents . As stated above, Baker and

Mcllraith estimated that about 11 percent of the accident vehicles had a flat tire,

incurred either before or during the accident. This percentage compares favorably

with similar figures quoted in their Tire Study No. 1**. However, it is about 10

times higher than the figure reported by the State of California Highway Patrol for

a 6-month study of tire failures on similar accident vehicles (see section 4.2.c).

Based on a close study of all available information on each of the 112 vehicles

positively known to have had a disabled tire on examination after the accident (i.e.,

so stated in police reports) , Baker and Mcllraith classified the tire disablements as follows

Number Percent

Accident followed disablement 13 11.6

Questionable 20 17.8

Disablement followed accident 79 70.6

Total 112 100.0

Using a series of arguments, the authors conclude that among these the maximum number

of tire disablements causing accidents could only have been 17 (or 15 percent of 112).

Applying this percentage to the 123 possible undiscovered tire disablements, they

conclude that the minimum number of tire disablements resulting in accidents was 13,

and the maximum number 17, plus 15 percent of the 123, or a total of 36. This is

0.59-1.64 percent of the 2196 four-tired accident vehicles, or 0.91-2.58 percent of

the 1432 accidents involving such vehicles during the period. Using their own estimate of 60,000 tire disablements experienced annually on the Tollway by fourwheel, four-tired vehicles [1], they conclude further that between 0.022 and 0.060

percent of tire disablements on the Tollway result in accidents.

(3) Other Results and Conclusions . Among other interesting aspects of these accident

data, Baker and Mcllraith consider the effects of power steering, tire type and

condition, age and sex of driver, and type of tire failure as related factors in tire

disablements leading to accidents. We shall summmarize these very briefy.

(a) Power Steering . In their Tire Studies 2, 3, and 4, Baker and Mcllraith listed

power steering among the survey questions. In Studies 2 and 3, they found that 73

percent and 70 percent of the vehicle population samples were so equipped. Interestingly enough, of the 21 accidents definitely or possibly following a tire disablement

*Two of a selected 35 of the motorists living in the Chicago area reported

disablements that did not appear in the accident reports.

**Ohio Turnpike figures of 12, 11, and 10 percent for years 1964, 1965, and 1966,

respectively; 6.5, 6.5, and 6.4 percent reported by the New York Thruway Authority

for years 1966, 1967, and 1968. However, the latter figures imply that the tire

disablement was the cause of the accident (see section 4.2.c).
-37-

for which information was available, only 10 (48 percent) of the vehicles has power

steering. The authors state that "There is less than one chance in 20 that the

differences in percentages is due to chance." An analysis of accident data in the

1968 Annual Report of the New York State Thruway Authority [20] confirms this finding

(see section 4.2.c).

(b) Tire Type and Condition . The percentage of 2-ply tires was nearly the same in

disablements not followed by accidents as in those followed by accidents. Data on retreads and tube type tires was insufficient to draw any inferences.

(c) Age and Sex of Drivers . Baker and Mcllraith also analyze the data on age and sex

of the drivers in the 33 accidents definitely or possibly following tire disablements

for which information was available. Fourteen of these (42 percent) were women,

compared with the following percentages found in other studies:

Women Drivers

Study Percent of Sample

TI No. 1 10.

a TI No. 2 8.8, TI No. 3 29.

D DL Survey --c SWRI Survey --d

Percent of drivers experiencing tire disablement

More women than men wait for assistance in changing a tire

c Taken but not reported

Data not taken

The authors compare the driver age-sex distribution matrix [4] with that presented in

their Tire Study No. 1 for motorists experiencing a tire disablement without having

an accident. Dividing the percentage of drivers in each category for disablements

followed by accidents by the percentage in the corresponding category for disablements

not followed by accidents, they arrive at risk indexes for the various classes of

drivers. These are compared in table 15.

It appears that driver experience plays an important role in the ability to cope with

a tire disablement. Girls and younger women appear especially prone to accidents in

event of tire failure, probably owing principally to the more limited amount of

driving done by them.

Table 15. Age and sex risk indexes following tire disablements

based on Tire Study No. 1 [4, Exhibit 34]

Women Both

21.

a 5.17 a 5.4 a .95

1.30 .77

.70

All .657 3.89 a 1.00

Age Men

Less than 20 1.8

20 to 35 .49

35 to 50 .70

50 to 65 .78

More than 65

Significant at the 0.01 level [of confidence]

(d) Type of Tire Failure . Of 17 disablements followed by accidents, four drivers

(24 percent) reported a blowout vs 47 blowouts in 321 tire failures reported in the

TI's Tire Study No. 3. Baker and Mcllraith state that the difference is not

significant at the 5 percent level of confidence, but does suggest that as one might

reasonably expect, it is more difficult to avoid an accident in the event of a sudden and unexpected tire failure. However, they point out that in their Tire Study

No. 1, 52 percent of the tire disablements were classed as "sudden" by the motorist.
-38-

b. Condition of Tires After Automobile Accidents in North Dakota

Data on the condition of tires on 378 automobiles involved in rural accidents in North

Dakota that resulted in one or more injuries are analyzed in a report by J . K. Kihlberg and Arthur Stern of the Automotive Crash Injury Research (ACIR) Center of

Cornell Aeronautical Laboratories, Inc. [19]. The authors state that "the purpose of

the study has simply been to test a new method of data collection and to take a first

look at the potential usability of tire condition data."

(1) Data Collection . Data were collected for ACIR over a 13-month period beginning

in July 1964. Investigating police offficers measured tread depth, taking the

apparent minimum point at the center line of the tire. General tire condition, evenness of wear, and any damage were reported. Pressures were estimated by

inspection rather than by actual measurement. All four tires were so evaluated

whenever possible. Summer accidents numbered nearly twice as many as winter ones. Of

the 378 vehicles, 290 had four standard tires (regular tread), 58 had standards front, snow tires rear, and 30 had various other combinations. Of the 1470 tires on which

information was available, 91 percent were regular tread; all but three snow tires were on the rear wheels.

(2) Findings . Some results of this study considered pertinent to this review are summarized below:

(a) About 10 percent of the tires had less than 2/32-inch tread depth remaining (see

also table 8) .

(b) Vehicles with higher mileage had less tread depth remaining on the tires

(confirms other studies)

.

(c) No tire failure was reported as a contributing cause of an accident. (However,

investigating officers were not requested to evaluate the cause of the accident.)

(d) Uneven wear was reported on 3.5 percent of the tires, and twice as frequently for

front tires. (Compare 3.6 percent found in TI Study No. 2--but more rears than

fronts. See section 3.7.)

(e) No left side vs right side effects were apparent. (Compare 55 percent failures

on right side in TI Study No. 3--also not significant.)

(f) Some evidence found that a fair number of car owners seem to keep their best

tires on the front of the car. Of 282 vehicles with all standard tires, 189 had no significant difference in tread depth, front to rear. Of the remaining 93, 67 had

deeper tread on the front tires. Using a Chi-square test, Kihlberg and Stern found

the difference to be significant at the 0.5 percent level.

(g) Rear tires were generally more worn than front tires, as indicated by (f) above.

About 14 percent of all rear tires were bald (i.e., rejectable by RMA criterion) vs only about 6.5 percent of all front tires. Confirming the DL summer vs winter

surveys, Kihlberg and Stern found that "use of snow tires in the rear is strongly

associated with poor front tires." (See table 8 and section 4.2.c(l) and (2).)

c. Other Information

(1) California State Highway Patrol . In commenting on proposed Federal standards for

passenger car tires in December 1966, the California State Highway Patrol [16]

presented highlights of a 6-month study of tire failures in accidents undertaken by

them in 1965. The survey covered some 60,000 accidents involving four-wheel, fourtire vehicles (i.e., passenger cars, station wagons, and light trucks), and occurring

principally on freeways and in rural areas. The following information was reported:

(a) Five hundred forty-five tire failures were reported or 0.91 percent of the

60,000 fatal injury and property damage accidents investigated (whether these were

all tire disablements or only those leading to accidents is not stated)

.

(b) Seventy-four percent occurred on straight roads.

(c) Sixty percent occurred on freeways.
-39-

(d) Fifty-eight percent involved the vehicle running off the road.

(e) Sixteen percent involved another vehicle.

(f) Fourteen percent occurred on station wagons (about the same as registration

percentage)

(g) Seventy-two percent of the tire failures were rear tires, evenly divided between

left and right.

(h) Fifty-seven percent of the disabled tires had less than 2/32-inch of tread depth

remaining (i.e., were bald).

(i) Eighty percent were reported as blowouts.

We must recognize that the above data are based on accidents occurring only in

relatively noncongested areas. As such, the information probably represents the

minimum contribution of tire disablements to accidents. The California State Highway

Patrol stated that a repeat of the survey, in greater detail and with better control,

was planned.

(2) Texas Highway Department . The Texas Highway Department examined the tread depths

of all tires on some 240 four-wheel, four-tire vehicles involved in wet weather

accidents [17] . We think that a comparison of their findings with those of the three

tire condition and use studies is of considerable interest. We shall again define

bald tires as those having a minimum remaining tread depth less than 2/32-inch, and

"thin" tires as those having a minimum tread depth remaining equal to or greater than

2/32-inch but not greater than 4/32-inch. A comparison of these two classes for the

four studies is presented in table 16.

a b Table 16. Bald and thin tires in wet weather accidents relative to those found in condition and use surveys

Percent of Total Population Sample

LF RF LR RR All

Wet Weather Accidents

in Texas

TI Tire Study No. 2

DL Summer Survey

Total Sample

Thruway Vehicles

Only (183 cars)

SWRI Survey

Bald Tires

Thin Tires

Total

8 14

22

6

39

7 1926

22

21.8

27.7

49.5

2128

49

4

1

5

14.8

23.7

38.5

Bald Tires

Thin Tires

Total

3 10145

7

2

3 11147

29

5.2

13.6

18.8

4 14

19

6

73

4.2

12.5

16.8

Bald Tires

Thin Tires

Total

Both Fronts

11.3

19.1

30.4

Both Rears

12.3

23.4

35.7

All

11.8

21.2

33.0

Bald Tires

Thin Tires

Total

2.2

14.3

16.5

4.4

20.5

24.9

3.3

17.3

20 .6 Bald Tires

Thin Tires

Total

1.7 C 12.

C 14. 5 C

Minimum tread depth less than 2/32-inch

Minimum tread depth equal to or greater than 2/32-inch, but not more than 4/32-inch

Percent of all tires, excluding unknowns
-40-

These wet weather accident data show a much higher incidence of bald and thin tires

than either the TI or SWRI physical condition and use surveys. If we may assume that

the average condition of tires in the Western U.S.A. is not substantially different

from that of the general population in the Eastern half (as represented by the DL survey), the incidence of bald and thin tires in wet weather accidents is, overall,

higher than that in the general population, especially for rear tires.

Moreover, note the significantly higher percentages of bald and thin tires on the rear; wheels of the wet weather accident vehicles. We have observed in previous studies (Til

Study No. 2, DL survey, and North Dakota accident data) that all four tires on most

cars are about equally worn. Yet on these some 240 vehicles there were almost 3 times'

as many bald tires on the rear wheels as on the front, and about two- thirds more thin

tires. As also evidenced in the ACIR study [19] (see 4.2.b), it would appear that some car owners place their best tires on the front wheels, possibly expecting to

maintain better control in the event of a blowout or skid. Analyses of accident data,

on the other hand, indicate that a vehicle's best tires should be placed on the rear wheels

.

(3) New York State Thruway Authority 19th Annual Report, 1968 [20] . In view of

Baker and Mcllraith's brief citations of tire disablement and accident data taken from

various annual reports for the Ohio Turnpike and New York Thruway, we believe that a more detailed examination of the pertinent data in at least one of the more recent

annual reports is in order. We shall assume that the vehicle disablement and accident

experience on any one of these throughways is probably more or less representative of

all, and will take a close look at that reported by the New York State Thruway

Authority in its 19th Annual Report for the year 1968 [20] .

The report states that of 5031 accidents reported on the Thruway in 1968, "the major

vehicular cause was tire blowouts - 232, with an additional 92 mishaps attributed to worn or defective tires."

From the data on "Accident Analyses" in the report, we have constructed table 17.

Table 17. Tires and brakes as causes of accidents on the New York State Thruway

1968 1967

Total Accidents Reported 5031 4822

Blowouts Reported as Cause 232 222

Vehicles Having Power Steering, percent 54 41

Defective or Worn Tires as Cause 92 92

Total, All Tires Reported as Cause 324 314

All Tires as Cause of Accident, percent 6.4 6.5

Blowouts as Cause of Accident, percent 4.6 4.6

Defective Brakes Reported as Cause 119 70 Percent of All Accidents Reported 2.4 1.5

As Baker and Mcllraith have emphasized however [1], it is by no means certain that in

all of the above accidents was the tire disablement actually the cause and not a result of the accident.

(a) Blowouts and Power Steering . It is interesting to note that over the 2 year

period^ 48 percent of the accident vehicles for which a blowout was reported as the cause were equipped with power steering. This percentage is exactly that reported

by Baker and Mcllraith in their fourth study [4] for vehicles in accidents following

a tire disablement. If we can assume, as seems quite reasonable, that the New York

State Thruway traffic population had percentages of vehicles with power steering

similar to those found by Baker and Mcllraith on the Illinois Tollway (73 percent in

Study 2, 70 percent in Study 3), the difference is again significant at the 95 percent

level. We may therefore conclude with greater certainty that vehicles equipped with

power steering are less prone to accidents following a tire disablement, even a blowout

.

It is also of interest to observe that defective tires are reported about 3 times as often as defective brakes as the cause of accidents on the New State Thruway.
-41-

(b) Tires vs All Vehicle Disablements . Using the "Emergency Service" data of the

report, it is possible to estimate tire disablements as a percentage of all vehicle

disablements on the Thruway, for comparison with the findings of Baker and Mcllraith

for the Illinois Tollway:

1968 1967

Total Service Calls (excluding "Out of Fuel") 84,887 77,834

Tire Trouble 14,794 14,675

i Assuming that, as on the Illinois Tollway [1], roughly two-thirds of all drivers

change their own tires, we have the following:

All Tire Disablements (estimate) 44,000 44,000

All Vehicle Disablements (estimate) 114,000 107,000

Tire Trouble, percent 39 41

These percentages are considerably smaller than the Illinois Tollway experience

reported (see table 10).

(c) Other Information from New York State Thruway Authority . We were interested to learn that the Thruway authority has vehicle inspection teams operating at selected

:

! interchanges at varying times. In 1968 these teams checked over 31,000 vehicles for

condition of tires, lights, horn, and windshield wipers, and the use of seat belts.

This amounted to only 0.023 percent of the some 134,600,000 individual trips on the

] Thruway for the year. Of the vehicles checked, "a total of 869, or 2.80 percent [of

': those examined], was refused entry to the Thruway because of serious [safety] defects.

An additional 2,204, or 7.10 percent, were warned to have minor deficiencies corrected

promptly .

! Also reported by the New York State Police patrolling the Thruway system were 6345

arrests during the year for vehicle inspection violations, 4085 arrests for unsafe

tires, and 132 for inadequate brakes.

I 5 . ACKNOWLEDGMENTS

The authors are indebted to the Uniroyal Tire Company, General Motors Corporation, and

the Texas Highway Department for making available internal reports and for permission

to use the data. We are also grateful for permission to quote and reproduce data,

tables, and figures from the four tire studies by Mcllraith and Baker sponsored by

the Rubber Manufacturers Association, and from the Consumer Reports and Consumers

Bulletin magazines.

To Miss Elaine Beale and Mr. James Payne our thanks for their many helpful editorial

suggestions and careful preparation of the paper for publication.
-42-

6. REFERENCES

[ 1] Mcllraith, G. Declan, and Baker, J. Stannard, Frequency of tire disablements

among four-tired vehicles on the Illinois Tollway- -Tire Study 1. Traffic

Institute, Northwestern University, Evanston, 111., January 1968.

[ 2] Baker, J. Stannard, and Mcllraith, G. Declan, Use and condition of tires on four-tired vehicles on the Illinois Tollway- -Tire Study 2, Traffic Institute,

Northwestern University, Evanston, 111., June 1968.

[ 3] Baker, J. Stannard, and Mcllraith, G. Declan, Tire disablements not followed by

accidents among four-tired vehicles on the Illinois Tollway - -Tire Study 3,

Traffic Institute, Northwestern University, Evanston, 111., July 1968.

[ 4] Baker, J. Stannard, and Mcllraith, G. Declan, Tire disablements followed by

accidents among four-tired vehicles on the Illinois Tollway- -Tire Study 4,

Traffic Institute, Northwestern University, Evanston, 111., 1968.

[ 5] Baker, J. Stannard, and Mcllraith, G. Declan, Tire disablements and accidents on high-speed roads, Highway Research Record 272, 24-38 (1969).

[ 6] Passenger tire use survey (consolidated report), Davidson Laboratory, Stevens

Institute of Technology, Hoboken, N. J. 07030 (Report to the National Bureau

of Standards on NBS Contract CST-398), including the following reports:

I. Characteristics of tire usage in the Eastern United States,

a survey (final report), Ehrlich, I. R., and Jurkat, M. P.,

Davidson Laboratory Report R-1336, October 1968.

II. Characteristics of tire usage in the Eastern United States, a survey, station-road comparisons and winter survey, Ehrlich, I. R.,

and Jurkat. M. P., Annex to Report 1336, March 1969.

III. Tire use survey, interim report based upon summer observations,

Schaefer, A. R., Letter Report 1286, October 1967.

IV. A procedure for determining ambient tire pressures from field

measurements, Kamm, I. 0., and Parsons, F., Technical Note 784,

February 1968.

Consolidated report available from the CLEARINGHOUSE, Springfield. Va.

22151, Accession No. PB 189487: $3 hardcopy, 65<j: microfiche.

[ 7] Lepisto, P. R., Research report on the physical condition of passenger vehicle

tires in highway use (final report to the National Bureau of Standards on NBS Contract CST-399) , Southwest Research Institute, San Antonio. Texas

78228, September 30, 1968. Available from the CLEARINGHOUSE, Springfield,

Va. 22151, Accession No. PB 189271; $3 hardcopy, 65£ microfiche.

[ 8] Consumer guide to tire care and safety, Rubber Manufacturers Association, 444

Madison Avenue, New York, N. Y. 10022, April 1968.

[ 9] Private communication from J. Preston and B. G. Simson of the NBS Office of

Vehicle Systems Research to Dr. F. Cecil Brenner.

[10] Staff Report to the Federal Trade Commission on the Manufacture and Distribution

of Automotive Tires, March 1966, 117 pp.

[11] Olson, Paul L., Bauer, Herbert J., A survey of practices in automobile tire

inflation pressures, General Motors Corporation, Warren, Mich., Research

Publication GMR-433, December 6, 1964, 5 pp.

[12] Private communication, K. A. Reid, Uniroyal Tire Company, to F. C. Brenner,

dated September 16, 1969; and E. H. Wallace, Uniroyal, Inc., to F. C. Brenner, dated October 2, 1969, Subj : Uniroyal Low Inflation Pressure

Surveys

.
-43-

[13] Simson, B. G., and Radlinski, R. W. , The accuracy of air tower pressure gages in

suburban, Washington, D.C., National Bureau of Standards (U.S.), Tech. Note

512, 9 pp, (December 1969).

[14] Tire gauges, Consumers Bulletin 45_, No. 10, 22-24 (October 1962).

[15] Automobile tire gauges, Consumer Reports 3_3 (No. 2), 94-96 (1968).

[16] California State Highway Patrol Comments on Proposed Federal Motor Vehicle

Safety Standard No. 109, p. 74-75 (December 1966).

![17] Letter from J. C. Dingwall, Texas Highway Department (Austin, Texas), to Dr. F.

C. Brenner, NBS Office of Vehicle Systems Research, dated January 20, 1969.

|[18] Private communication from I. R. Ehrlich, Davidson Laboratory, Stevens Institute

of Technology, Hoboken, N. J., to Dr. F. C. Brenner, NBS Office of Vehicle

Systems Research, May 13, 1969.

[19] Kihlberg, J. K., and Stern, A., Condition of tires after automobile accidents in

North Dakota, Cornell Aeronautical Laboratory, Inc., Buffalo, N. Y. 14221,

CAL Report No. VJ-1823-R20, October 1968.

[20] New York State Thruway Authority 19th Annual Report, 1968, P. 0. Box 189,

Albany, N. Y. 12201, 80 pp.

[21] Transportation Research Review, 2nd Qtr. 1968, Cornell Aeronautical Laboratory,

Inc. , Buffalo, N. Y. 14221.
APPENDIX A. DAVIDSON LABORATORY SITE DATA SHEET

KEY SHEET Date 1st Sample No. Last Sample NoTEST LOCATION

Name

Address:

City: _

County: State

Route: or Miles to nearest town or city:

Direction:

Approval from: Time/date of request for approval

Class of Road: surface no. lanes

Position:

Gal /month:

Area: Residential

Commercial

Industrial

AIR PUMP CHECK AT STATION

Setting on pump 20 25 30 35 1*0 Reading In tire

WEATHER INFORMATION

Date Time Weather Information

Sample no- at time

of reading
D m

1 -1

1

1 DDDS

j ; su J . n i Dr °| °, = n n *

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5 ?

D

n

J || DD D |

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g|

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rn < s ^

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z.

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3

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-JXO z "

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1- H

Ul 3 uj 3 : i

3 H J I Z fr z ,_ K h- a o o UJ ° u. < 5 "* H a £ -i £ -i a. ft ft j

' g dd D DD D DD D DD

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ft u a. u S! u S! u £

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in 3

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*** o- X

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*

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ft < 5

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£ S a 5 £ £ *

a- -< < < N -i M tt *<I in a. o. <[ a. < y -> a o. *<

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i z 5. i

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u a a
APPENDIX D. TRAFFIC INSTITUTE TIRE USE AND CONDITION DATA COLLECTION FORM

TIRE USE AND CONDITION

It accident, give number only. Otherwise complete tdent i f y ing data belo

MM.A l li.:. (lollu.iy, o.isis, ,.1.17.1) MILK POST HOURqiooA

DATEt)

MONTH OcJ.OCCUPANTS / glOry H Wet Snow D Ice

ACCIDENT NUMBER 73C****

STATE MAKEOLbSYEAR MODEL u

DRIVER AGE SEX ITT)

"Where did you get on the Illinois Tollway?" ^* J J ) £L&

AIR TEMPERATURE SO

"From there did you have tire trouble before you got here?" U ^es >3 No "Does this car have ^jYes

power steering?" Q No D Yes

(If yes) 'Did a police officer ask details about it?"

f=^ N

"How far has the car been driven since it was stopped for an hour or more?" *** O

/ / miles

"On this trip on the Illinois Tollway, before you got here, Q yes was the car parked in a parking space at a service area?" ^ No

"Where do you intend to leave the Illinois Tollway?' -y» -. »/r<l^ jPA -t- jJ

R1CHT LEFT Front Rear Rear Front

RtCAP? Yes }£ No *es jg No DYes ^No Q Yes g}No

BRANT) Goooystz // // 41

1 1 Ki :.ame Police Special. // II II

SI /.I tw- tsll II II

PLY AND PLY RAT INC UMfS D II 1

»

SIDF.WALL CRACKS No It II 1 1

BULGES, BLISTERS No l' ll 1 1

PRESSURE XI ±9 Zl J-1

TUBE? Q Yes ^No OYea fl^No G Ves JJfNo Yes ^No

CROOVE

DEPTH

Inside // 9 9 tc

Middle ID 9 to 10

Outside ft to II li_

Describe

UNEVEN WEAR NO rio A/0 No

WEICHT ON TIRE

(If weighed) 1 X00 lb. t/ZO ib. 11.24 /3 20 ib

Estimated WEICHT OF OCCUPANTS

FRONT SEAT XoS* ib lb. lb. REAR SEATS - lb. lb. lb.

Estimated WEICHT OF CARGO

TRUNK CAR TOP

lb. TRAILER--Type TRAILER WEIGHT ON CAR """" * lb

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