DOT HS 811 201 October 2009

NHTSA Tire Aging Test Development Project

Report 1: Laboratory Roadwheel Testing of Light Vehicle Tires as Purchased New and After Retrieval From Service in Phoenix, Arizona

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161

Phase 1 - Phoenix, Arizona, Tire Study

This publication is distributed by the U.S. Department of Transportation, National Highway TrafficSafetyAdministration, in the interestof informationexchange.Theopinions,findingsand conclusions expressed in this publication are those of the author(s) and not necessarily those of the Department of Transportation or the National Highway Traffic Safety Administration. The United States Government assumes no liability for its content or use thereof. If trade or manufacturers’ names or products are mentioned, it is because they are considered essential to the object of the publication and should not be construed as an endorsement. The United States Government does not endorse products or manufacturers.

i

TECHNICAL REPORT DOCUMENTATION PAGE 1. Report No.

DOT HS 811 201 2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

NHTSA Tire Aging Test Development Project Phase 1 - Phoenix, Arizona, Tire Study; Report 1: Laboratory Roadwheel Testing of Light Vehicle Tires as Purchased New and After Retrieval From Service in Phoenix, Arizona

5. Report Date

October 2009 6.

Performing Organization Code

7. Author(s)

James D. MacIsaac Jr.,1 Sebastien Feve (Formerly),2 Larry R. Evans,2 John R. Harris2 & W. Riley Garrott, Ph.D.1 1National Highway Traffic Safety Administration, 2Transportation Research Center, Inc.

8. Performing Organization Report No.

9. Performing Organization Name and Address

National Highway Traffic Safety Administration Vehicle Research and Test Center P.O. Box B-37 10820 State Route 347 East Liberty, OH 43319-0337

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTNH22-02-D-08062, DTNH22-03-D-08660

12. Sponsoring Agency Name and Address

National Highway Traffic Safety Administration 1200 New Jersey Avenue SE. Washington, DC 20590

13. Type of Report and Period Covered

Final 14. Sponsoring Agency Code

NHTSA/NVS-312 15. Supplementary Notes

Project support and testing services provided by the Transportation Research Center, Inc., Smithers Scientific Services, Inc. and Akron Rubber Development Laboratory, Inc. 16. Abstract

As a result of the TREAD Act of 2000, NHTSA initiated an effort to develop a laboratory-based accelerated service life test for light vehicle tires (often referred to as a “tire aging test”). It is believed that if such a test method was successful, then light vehicle tires could eventually be required to meet standards that would make them more resistant to operational degradation and possibly reduce their failure rate during normal highway service. The development of a potential tire aging test relied upon first examining how tires change during service by measuring their roadwheel performance levels and material properties after varying lengths of service and accumulated mileages. Since the rate of degradation of tire rubber components increases with temperature, NHTSA expected that the “worst case” tires in service in the United States would be found in the southern States. This report describes the rationale used to select Phoenix, Arizona, as the tire collection location, the methodology of the collection, and the results of the roadwheel testing portion of this first phase of the project. The methodology and results of the tire material properties analysis will be described in a following report. In Phase 1 of the project, 101 on-road tires and 8 full-size spare tires of six different models were retrieved from Phoenix after varying amounts of service and compared to 45 new samples of themselves in one of two indoor roadwheel dynamometer tests. Since the cost and duration of the roadwheel tests were critical factors in determining their feasibility for a safety standard, two short-duration/high-intensity tests were utilized. One test, the Stepped-Up Load (SUL) test, used the FMVSS No. 139 Endurance test as a basis and continued to step-up the load during the test in regular increments until tire failure. The other test, the Stepped-Up Speed (SUS) test, used the FMVSS No. 139 High Speed test as a basis and continued to step-up the speed of the test to the speed category (speed rating) of the tire, at which the speed was maintained until tire failure. The results indicate that as on-road and full-size spare tires experience longer durations of service, their resistance to over-deflection (overloading or underinflation), or use at or near their speed category for long periods of time can diminish. 17. Key Words

Tire, tire aging, tire safety, Phoenix, FMVSS No. 139, TREAD Act, spare tire, accelerated service life, tire durability, roadwheel

18. Distribution Statement

This report is free of charge fromNHTSA Web site at www.nhtsa.gov

the

19. Security Classif. (of this report)

Unclassified 20. Security Classif. (of this page)

Unclassified 21. No. of Pages

225 22.

Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

Approximate Conversions to Metric Measures

Symbol When You Know Multiply by To Find Symbol

LENGTH

in inches 2.54 centimeters cmft feet 30 centimeters cmmi miles 1.6 kilometers km

AREA

in2 ft2mi2

square inches square feet

square miles

6.5 0.09

2.6

square centimeters square meters

square kilometers

2 cm 2 m

2 km

MASS (weight)

ounces 28 grams g oz lb pounds 0.45 kilograms kg

PRESSURE

psi psi

pounds per inch2 pounds per inch2

0.07 6.89

bar kilopascals

bar kPa

VELOCITY

mph miles per hour 1.61 kilometers per hour km/h

ACCELERATION

ft/s2 feet per second2 0.30 meters per second2 m/s2

TEMPERATURE (exact)

F Fahrenheit 5/9 (Celsius) - 32C C Celsius

Approximate Conversions to English Measures Symbol When You Know Multiply by To Find Symbol

LENGTH

mm millimeters 0.04 inches incm centimeters 0.4 inches inm meters 3.3 feet ftkm kilometers 0.6 miles mi

AREA

2 cm

km2 square centimeters

square kilometers 0.16 0.4

square inches square miles

in2 mi2

MASS (weight)

g grams 0.035 ounces ozkg kilograms 2.2 pounds lb

PRESSURE

bar kPa

bar 14.50 kilopascals 0.145

pounds per inch2 pounds per inch2

psi psi

VELOCITY

km/h kilometers per hour 0.62 miles per hour mph

ACCELERATION

2 m/s meters per second2 3.28 feet per second2 ft/s2

TEMPERATURE (exact)

C Celsius 9/5 (Celsius) + 32F Fahrenheit F

ii

TABLE OF CONTENTS

LIST OF FIGURES .................................................................................................................................... v

LIST OF TABLES ....................................................................................................................................vii

LIST OF EQUATIONS...........................................................................................................................viii

EXECUTIVE SUMMARY ....................................................................................................................... ix

1.0 INTRODUCTION ................................................................................................................................ 1

2.0 BACKGROUND................................................................................................................................... 3

3.0 METHODOLOGY............................................................................................................................... 7 3.1 Selection of Tire Retrieval Location............................................................................... 7

3.2 NHTSA Phoenix, Arizona, Tire Exchange Program.................................................... 14

3.3 Inspection and Screening of Phoenix-Retrieved Tires.................................................. 15

3.4 Phase 1 Tire Tests ......................................................................................................... 17

3.5 Determining the Service Duration of Test Tires........................................................... 18

3.5.1 Replacement Market Tires........................................................................................ 18

3.5.2 Original Equipment Tires ......................................................................................... 20

3.5.3 Tires Purchased New ................................................................................................ 20

3.6 Determining the Mileage of Test Tires......................................................................... 21

4.0 RESULTS............................................................................................................................................ 23 4.1 Tire Collection Data...................................................................................................... 23

4.2 Nondestructive Inspection ............................................................................................ 26

4.2.1 Tread Depth .............................................................................................................. 26

4.2.2 Shore Hardness of Tread........................................................................................... 28

4.2.3 Shearographic Interferometry ................................................................................... 30

4.3 Roadwheel Testing........................................................................................................ 33 4.3.1 Stepped-Up Load Testing (Based on FMVSS No. 139 Endurance Test)................. 35

4.3.1.1 SUL Results - Type B Tires.............................................................................. 35

4.3.1.2 SUL Results - Type C Tires.............................................................................. 44

4.3.1.3 SUL Results - Type D Tires.............................................................................. 48

4.3.1.4 SUL Results - Type E Tires .............................................................................. 53

4.3.1.5 SUL Results - Type H Tires.............................................................................. 59

4.3.1.6 SUL Results - Type L Tires .............................................................................. 64

4.3.1.7 Shearography Results Versus Stepped-Up Load Roadwheel Performance...... 70

4.3.2 Stepped-Up Speed Testing (Based on FMVSS No. 139 High Speed Test).............. 71

4.3.2.1 SUS Results - Type B Tires .............................................................................. 72

4.3.2.2 SUS Results - Type C Tires .............................................................................. 76

4.3.2.3 SUS Results - Type D Tires.............................................................................. 82

4.3.2.4 SUS Results - Type E Tires .............................................................................. 87

4.3.2.5 SUS Results - Type H Tires.............................................................................. 91

4.3.2.6 SUS Results - Type L Tires .............................................................................. 96

4.3.2.7 Shearography Results Versus Stepped-Up Speed Roadwheel Performance .. 102

5.0 CONCLUSIONS............................................................................................................................... 103

iii

Appendix A. FARS 2001 Query: Light Vehicle Fatal Crashes Where “01 Tires” was Coded as the

“Vehicle Related Factor (1)”.................................................................................................................. 105

Appendix B. Tire Model Cross-Sections ........................................................................................... 107

Appendix C. Tire Exchange Program Form Sample (Filled by Tire Retailer) ............................. 113

Appendix D. Tire and Wheel Information ........................................................................................ 115

Appendix E. Summary of Laboratory Analysis Performed for Both New and Field Tires ......... 116

Appendix F. Roadwheel Tests Excluded From the Dataset (48 Tests)........................................... 117

Appendix G. Test Procedures Stepped-Up Load Test ..................................................................... 136

Appendix H. Test Procedures Stepped-Up Speed Test .................................................................... 137

Appendix I. Codes for Roadwheel End-of-Test Findings............................................................... 139

Appendix J. Type B - SUL End-of-Test Photographs..................................................................... 156

Appendix K. Type C - SUL End-of-Test Photographs..................................................................... 161

Appendix L. Type D - SUL End-of-Test Photographs..................................................................... 166

Appendix M. Type E - SUL End-of-Test Photographs..................................................................... 170

Appendix N. Type H - SUL End-of-Test Photographs .................................................................... 177

Appendix O. Type L - SUL End-of-Test Photographs..................................................................... 183

Appendix P. Type B - SUS End-of-Test Photographs ..................................................................... 188

Appendix Q. Type C - SUS End-of-Test Photographs ..................................................................... 192

Appendix R. Type D - SUS End-of-Test Photographs ..................................................................... 195

Appendix S. Type E - SUS End-of-Test Photographs ..................................................................... 198

Appendix T. Type H - SUS End-of-Test Photographs..................................................................... 203

Appendix U. Type L - SUS End-of-Test Photographs ..................................................................... 206

REFERENCES........................................................................................................................................ 209

iv

LIST OF FIGURES

Figure 1. Cumulative Failure Frequencies From Firestone Investigation [NHTSA EA00-023].... 8

Figure 2. Fatalities and Injuries Resulting from Firestone Tire Failures by State.......................... 9

Figure 3. FARS 2001 Light Vehicle Fatal Crashes Where Tires Were Cited as the "Vehicle-

Related Factors (1)"; National Mean Temperatures 1971-2000; and National Vehicle Miles Traveled in 2001 ................................................................................................................... 13

Figure 4. Inflation Pressure at Collection Versus Tire Age (109 Phoenix Tires)......................... 25

Figure 5. Average Tread Depth Versus Age (75 Phoenix Tires and 31 New Tires) .................... 27

Figure 6. Average Tread Depth Versus Mileage (75 Phoenix Tires and 31 New Tires).............. 28

Figure 7. Average Tread Shore Hardness Versus Age, New and Phoenix On-Road Tires (84

Phoenix Tires and 5 New Tires) ........................................................................................... 29

Figure 8. Average Tread Shore Hardness Versus Mileage, New and Phoenix On-Road Tires (84

Phoenix Tires and 5 New Tires) ........................................................................................... 30

Figure 9. Example of Bead-to-Bead Shearography Results (Tire N0224) ................................... 31

Figure 10. Bead-to-Bead Shearography Separation @ 50 mbar Vacuum Versus Age, New and

Phoenix On-Road Tires (45 New Tires, 106 Phoenix Tires)................................................ 32

Figure 11. Bead-to-Bead Shearography Separation @ 50 mbar Vacuum Versus Mileage, New

and Phoenix On-Road Tires (45 New Tires, 106 Phoenix Tires) ......................................... 33

Figure 12. Type B - SUL Time to Failure Versus Age................................................................. 36

Figure 13. Type B - SUL Time to Failure Versus Mileage .......................................................... 37

Figure 14. Cumulative Severity of Stepped-Up Load Roadwheel Test........................................ 39

Figure 15. Type B - SUL Severity Versus Age and Mileage ....................................................... 40

Figure 16. Type B - SUL Severity Versus Service Factor............................................................ 41

Figure 17. Type C - SUL Time to Failure Versus Age................................................................. 44

Figure 18. Type C - SUL Time to Failure Versus Mileage .......................................................... 45

Figure 19. Type C - SUL Severity Versus Age and Mileage ....................................................... 46

Figure 20. Type C - SUL Severity Versus Service Factor............................................................ 47

Figure 21. Type D - SUL Time to Failure Versus Age ................................................................ 49

Figure 22. Type D - SUL Time to Failure Versus Mileage .......................................................... 50

Figure 23. Type D - SUL Severity Versus Age and Mileage ....................................................... 51

Figure 24. Type D - SUL Severity Versus Service Factor ........................................................... 52

Figure 25. Type E - SUL Time to Failure Versus Age................................................................. 54

Figure 26. Type E - SUL Time to Failure Versus Mileage .......................................................... 55

Figure 27. Type E - SUL Severity Versus Age and Mileage........................................................ 56

Figure 28. Type E - SUL Severity Versus Service Factor............................................................ 57

Figure 29. Type H - SUL Time to Failure Versus Age ................................................................ 60

Figure 30. Type H - SUL Time to Failure Versus Mileage [Estimated for On-Road Tires]........ 61

Figure 31. Type H - SUL Severity Versus Age and Mileage ....................................................... 62

Figure 32. Type H - SUL Severity Versus Service Factor ........................................................... 63

Figure 33. Type L - SUL Time to Failure Versus Age................................................................. 65

Figure 34. Type L - SUL Time to Failure Versus Mileage .......................................................... 66

Figure 35. Type L - SUL Severity Versus Age and Mileage........................................................ 67

Figure 36. Type L - SUL Severity Versus Service Factor............................................................ 68

Figure 37. Shearography Results Versus Stepped-Up Load Roadwheel Performance (24 New

Tires, 61 Phoenix tires) ......................................................................................................... 71

Figure 38. Type B - SUS Time to Failure Versus Age................................................................. 72

v

Figure 39. Type B - SUS Time to Failure Versus Mileage .......................................................... 73

Figure 40. Type B - SUS Severity Versus Age and Mileage........................................................ 74

Figure 41. Type B - SUS Severity Versus Service Factor............................................................ 75

Figure 42. Type C - SUS Time to Failure Versus Age................................................................. 77

Figure 43. Type C - SUS Time to Failure Versus Mileage .......................................................... 78

Figure 44. Type C - SUS Severity Versus Age and Mileage........................................................ 79

Figure 45. Type C - SUS Severity Versus Service Factor............................................................ 80

Figure 46. Type D - SUS Time to Failure Versus Age................................................................. 82

Figure 47. Type D - SUS Time to Failure Versus Mileage .......................................................... 83

Figure 48. Type D - SUS Severity Versus Age and Mileage ....................................................... 84

Figure 49. Type D - SUS Severity Versus Service Factor............................................................ 85

Figure 50. Type E - SUS Time to Failure Versus Age ................................................................. 87

Figure 51. Type E - SUS Time to Failure Versus Mileage........................................................... 88

Figure 52. Type E - SUS Severity Versus Age and Mileage........................................................ 89

Figure 53. Type E - SUS Severity Versus Service Factor ............................................................ 90

Figure 54. Type H - SUS Time to Failure Versus Age................................................................. 92

Figure 55. Type H - SUS Time to Failure Versus Mileage .......................................................... 93

Figure 56. Type H - SUS Severity Versus Age and Mileage ....................................................... 94

Figure 57. Type H - SUS Severity Versus Service Factor............................................................ 95

Figure 58. Type L - SUS Time to Failure Versus Age ................................................................. 97

Figure 59. Type L - SUS Time to Failure Versus Mileage........................................................... 98

Figure 60. Type L - SUS Severity Versus Age and Mileage........................................................ 99

Figure 61. Type L - SUS Severity Versus Service Factor .......................................................... 100

Figure 62. Shearography Results Versus Stepped-Up Speed Roadwheel Performance (21 New

Tires, 43 Phoenix Tires)...................................................................................................... 102

Figure 63. Tire with ID Number and Barcode............................................................................ 115

vi

LIST OF TABLES Table 1. NHTSA Safety-Related Tire Defect and Compliance Campaigns Since 1966 ................ 3

Table 2. Top Five Rates of Combined Firestone Tire Recall Deaths and Injuries per Million

Light Vehicle Registrations and Million State Residents ..................................................... 10

Table 3. Top Five Normal Daily U.S. State Mean and Maximum Temperatures* ...................... 10

Table 4. Top Five States for Total Light Vehicle Tire-Coded Fatal Crashes in 2001 FARS Query

............................................................................................................................................... 11

Table 5. Top Five States for Total Light Vehicle Tire-Coded Fatal Crash Rate in 2001 FARS

Query by PPM Registered Vehicles ..................................................................................... 12

Table 6. Tire Data Collected by Dealers in Phoenix, Arizona, at Time of Tire Retrieval............ 15

Table 7. Phase 1 Tire Models Selected for Testing ...................................................................... 16

Table 8. Example of VRTC Inspection of a Phoenix-Retrieved Tire........................................... 17

Table 9. Phase 1 Roadwheel Test List .......................................................................................... 18

Table 10. Overall Phase 1 Tire Test Distribution (Six Tire Models)............................................ 18

Table 11. Non-Service Time Between Tire Manufacture and Roadwheel Testing...................... 21

Table 12. Phase 1 Roadwheel Test Age Distribution (Six Tire Models)...................................... 21

Table 13. On-Road Tire Samples (Qty.) and Average Inflation Pressure by Vehicle Make and

Model .................................................................................................................................... 24

Table 14. Average Full-Size Spare Tire Inflation Pressure by Vehicle Make and Model ........... 25

Table 15. Phase 1 Roadwheel Tire Distribution ........................................................................... 34

Table 16. SUL Test - Type B Tire End-of-Test Finding(s) .......................................................... 43

Table 17. SUL Test - Type C Tire End-of-Test Finding(s) .......................................................... 48

Table 18. SUL Test - Type D Tire End-of-Test Finding(s).......................................................... 53

Table 19. SUL Test - Type E Tire End-of-Test Finding(s) .......................................................... 58

Table 20. SUL Test - Type H Tire End-of-Test Finding(s).......................................................... 64

Table 21. SUL Test - Type L Tire End-of-Test Finding(s) .......................................................... 69

Table 22. Variables With Significance Greater Than 0.60 to Failure – Stepped-Up Load Test .. 70

Table 23. SUS Test - Type B Tire End-of-Test Finding(s) .......................................................... 76

Table 24. SUS Test - Type C Tire End-of-Test Finding(s) .......................................................... 81

Table 25. SUS Test - Type D Tire End-of-Test Finding(s) .......................................................... 86

Table 26. SUS Test - Type E Tire End-of-Test Finding(s)........................................................... 91

Table 27. SUS Test - Type H Tire End-of-Test Finding(s) .......................................................... 96

Table 28. SUS Test - Type L Tire End-of-Test Finding(s)......................................................... 101

Table 29. Stepped-Up Speed Test Correlation With Service in Phoenix, Types E and H ......... 101

Table 30. Corresponding Test Wheel for Each Test Tire ........................................................... 115

Table 31. Stepped-Up Load (SUL) Roadwheel Test Conditions ............................................... 137

Table 32. Stepped-Up Speed Test Conditions ............................................................................ 137

vii

LIST OF EQUATIONS

Equation 1: Service Duration of Replacement Market Tires........................................................ 19

Equation 2: Service Duration of Original Equipment (OE) Tires ................................................ 20

Equation 3: Service Duration of Brand New Tires....................................................................... 21

Equation 4: Estimated Mileage for Replacement Tires ................................................................ 22

Equation 5. Arrhenius Equation.................................................................................................... 35

Equation 6. Severity Calculation .................................................................................................. 38

Equation 7. Service Factor Calculation ........................................................................................ 41

viii

EXECUTIVE SUMMARY

During the September 2000 Congressional hearings regarding the Firestone tire failures on Ford light trucks and SUVs, members of Congress expressed concerns that the then-current Federal Motor Vehicle Safety Standards (FMVSS) did not evaluate how well tires perform after being subjected to environmental variables, such as heat, over time, which can accelerate degradation. The National Highway Traffic Safety Administration (NHTSA) was asked to consider the feasibility of requiring a “tire aging test” (i.e. accelerated service life test for tires) that could evaluate the risk of failure at a period later in the life of a tire than the current regulation, which only evaluates new tires. As a result of the committee’s actions, the Transportation Recall, Enhancement, Accountability, and Documentation (“TREAD”) Act [H.R. 5164, Pub. L. No. 106-414] was enacted on November 1, 2000. The TREAD Act contained provisions mandating NHTSA to “revise and update” the passenger car and light-truck tire safety standards (however, the legislation did not mandate specific requirements).

In response to the TREAD Act, the agency examined the effectiveness of the current tire safety standards and embarked on an ambitious tire test development program that culminated in the new Federal Motor Vehicle Safety Standard (FMVSS) No. 139 , “New pneumatic radial tires for light vehicles.” Though three methods of evaluating long-term tire durability were evaluated during the development of FMVSS No. 139, data from agency as well as industry evaluations of the three methods demonstrated the need for additional test development. Of primary need was a better understanding of service-related tire degradation that could serve as the “real-world” baseline for the development of a laboratory-based accelerated service life test (herein referred to as “tire aging test”). These needs were the basis of the NHTSA Tire Aging Test Development Project, which started in late 2002. Phase 1 of the project would study how tires change during actual service as measured by changes in their roadwheel performance levels and quantitative material properties when compared to new versions of each tire. Since the rate of degradation of tire rubber components increases with temperature, NHTSA expected that the “worst case” tires in service in the United States would be found in the southern States. It was thought that designing a tire aging test to simulate service in a severe environment that has high relative tire failure rates would offer the best margin of safety nationwide. Per this approach, Phoenix, Arizona, was selected as the location for the collection of on-vehicle tires for analysis. This decision was based on polymer chemistry theory, data from the Firestone tire recall, trends in NHTSA crash data systems, and practical considerations. The tire models were selected from popular sizes, brands, OE fitments, etc. in the year 2002 and narrowed down by contacting the manufacturers to help confirm the conformity of production of tire models during the 1998 to 2003 timeframe.

This report details the laboratory roadwheel dynamometer results of 101 on-road tires and 8 full-size spare tires retrieved from Phoenix of varied ages and mileages that were compared to 45 new, unused versions of themselves. After retrieval from Phoenix, tires were rigorously inspected to screen out tires with damage or repairs. The resulting test population was run to ultimate failure on one of two indoor roadwheel dynamometer tests. One test, the Stepped-Up Load (SUL) test, used the FMVSS No. 139 Endurance test as a basis and continued to step-up the load during the test in regular increments until tire failure. The other test, the Stepped-Up Speed (SUS) test, used the FMVSS No. 139 High Speed test as a basis and continued to step-up the speed of the test to the speed category (speed rating) of the tire, which was then held until tire

ix

failure. The results from the two tests were correlated against the absolute age and mileage (estimated for replacement tires), as well as against normalized functions of age and mileage.

While all tires retrieved from service in Phoenix were performing effectively at the time of collection, as the age and/or mileage of the tires increased, the overall performance of the tires in the SUL roadwheel test in terms of “time to failure” (also load at failure, cycles to failure, etc.) decreased for the six tire models tested. The results of the SUL roadwheel test suggest that tires become less resistant to over-deflection (i.e., operation of the tire while underinflated and/or overloaded) with increasing amounts of service. The results from the SUS test indicated little effect of age and mileage on time to failure (also speed at failure, cycles to failure, etc.) for four of the six tire models tested. However, the other two models exhibited significant reductions in performance in the SUS test while at or near their speed rating with increasing amounts of service. The eight full-size spare tires exhibited some reduction in performance in both tests when compared to new tires, though less than tires of the same age with actual mileage.

Since 95% of NHTSA light-vehicle tire defect and compliance campaigns since 1966 have involved the tread/belt region of the tire, and the upgrade of the Federal tire safety standards was prompted by occurrences of tread and belt #2 detachments in the field, the ability of the SUS test to generate this specific type of failure at a relatively high rate (29.7%) was attractive. However, the SUS test did not show a strong correlation to tire service, in fact failing almost as many brand new tires by tread and belt #2 detachment as on-road tires. It is theorized that the embrittled rubber components in aged tires are only moderately sensitive to the high stresses imparted by the nominal loading and high speeds of the SUS test. Also, the SUS test only generated these types of failures at sustained speeds over 160 km/h (99 mph), which is not indicative of typical highway use in the United States. The SUL test demonstrated a clear relationship between performance in the test and the amount of service a tire had experienced. It is theorized that the embrittled rubber components in aged tires are particularly sensitive to the high strains imparted by the over-deflection of the tires at the highway-like speeds of the SUL test. The SUL test is also more realistic in terms of real-world operating conditions (i.e., operation at 120 km/h [75 mph] with varying levels of underinflation and/or overloading). Therefore, the SUL test appears to be the better candidate of the two tests evaluated in assessing the effects of service on tires, especially if the test is terminated before the load is stepped-up to a level that produces only thermally induced failures.

At its most basic, a tire is a flexible composite structure of rubber, fabric, and metal that is pressurized with a degradative gas and subjected to millions of cycles of flexing under multi-axial loads. An adequate safety margin must be built into each new tire such that its tread wears out and it is removed from service before the structure yields from degradation and fatigue. Since all tires in this study were performing effectively in the field at the time they were collected, the results of the SUL and SUS roadwheel testing indicate that tires can lose a significant percentage of their new-tire capacity during service (i.e., with increasing age and mileage) and still retain structural integrity in real-world operation. However, the results also indicate that as on-road and full-size spare tires experience longer durations of service, their resistance to over-deflection (overloading or underinflation), or use at or near their speed category for long periods of time can diminish.

x

1.0 INTRODUCTION

On September 12, 2000, the U.S. Senate Committee on Commerce, Science, and Transportation conducted a hearing on the recall of 14.4 million Firestone Radial ATX, Radial ATX II, and Wilderness AT tires on specific models of Ford, Mercury, and Mazda light trucks and SUVs. During these hearings, members of Congress expressed concern that the then-current Federal Motor Vehicle Safety Standards (FMVSS) did not evaluate how well tires perform when significantly underinflated or after being subjected to environmental variables, such as heat, which accelerate aging.[1] As a result of the committee’s actions, the Transportation Recall, Enhancement, Accountability, and Documentation (“TREAD”) Act [H.R. 5164, Pub. L. No. 106-414] was enacted on November 1, 2000. Section 10 of the TREAD Act contained provisions mandating the NHTSA to “revise and update” the passenger car and light-truck tire safety standards (however, the legislation did not mandate specific test requirements). During the consideration and enactment of the TREAD Act, members of Congress placed particular emphasis on improving the ability of tires to withstand the effects of factors such as tire heat build up, low inflation, and aging (i.e., service-related degradation). With regards to aging, the agency was asked to consider the feasibility of requiring a “tire aging test” (i.e., accelerated service life test for tires) that could evaluate the risk of failure at a period later in the life of a tire than the current regulation, which only evaluates new tires.

In response to the TREAD Act the agency examined the effectiveness of the current passenger vehicle tire safety standards, which had not been substantially revised since their issuance in 1967, and determined the following:

“While the durability and performance of tires have improved, the conditions under which tires are operated have become more rigorous. Higher speeds, greater loads, extended lifetimes of tires, longer duration of travel and shifting demographics of vehicles sales have all contributed to much greater stresses and strains being placed upon today's radial tires than those endured by earlier generation radial tires. The characteristics of a radial tire construction in conjunction with present usage and purchasing patterns render the existing required minimum performance levels in the high-speed test, endurance test, strength test, and bead-unseating test ineffective in differentiating among today's radial tires with respect to these aspects of performance.”[2]

NHTSA conducted tire safety research in support of what would become the new Federal Motor Vehicle Safety Standard (FMVSS) No. 139,1 “New pneumatic radial tires for light vehicles.” During this effort, agency researchers conducted comprehensive literature reviews and had numerous consultations with industry regarding the long-term effects of service on radial tire dura

1 Previously, passenger tires were regulated by the FMVSS No. 109 (“Passenger car tires”) and light-truck tires under the separate FMVSS No. 119 (“Tires for vehicles other than passenger cars”). The FMVSS No. 119 had less severe test conditions than the FMVSS No. 109 and did not include a high-speed or bead unseat test for tires. The FMVSS No. 139 unifies regulation of the majority of passenger and light-truck tire designs for vehicles with a gross vehicle weight rating of 10,000 pounds or less. This new standard became mandatory on September 1, 2007, for non-snow tire designs and became mandatory on September 1, 2008, for designated snow tire designs. Optional compliance was permitted before those dates.

1

bility. The agency concluded that while most tire manufacturers conduct some form of accelerated service life testing on their tires (“tire aging tests”), their approaches varied widely and a single industry-wide recommended practice did not exist. As part of the FMVSS No. 139 development research, the agency conducted an evaluation of multiple laboratory-based accelerated service life tests for tires that were based on either industry submissions or previous agency test experience. In the March 5, 2002, Notice of Proposed Rulemaking (NPRM) section related to tire aging for FMVSS No. 139 (67 FR 10050), the agency proposed three alternative tests that could be used to evaluate a tire's long-term durability.[1] These approaches can be characterized as: 1) 24 hours of roadwheel conditioning followed by an adhesion (peel strength) test between the belts; 2) an extended duration roadwheel endurance test with oxygen rich inflation gas; and 3) an oven aging conditioning period followed by a roadwheel endurance test. However, based on the results of this initial evaluation, as well as comments and data from industry, the agency decided to defer action on the proposal to add an aging test to the new FMVSS No. 139 until further research was conducted. To conduct this further research, the agency initiated its NHTSA Tire Aging Test Development Project in late 2002.

Phase 1 of the NHTSA Tire Aging Test Development Project consisted of the analysis of six different tire models collected from service on privately owned vehicles in the Phoenix, Arizona, metropolitan area during the spring of 2003. This study was conducted to provide a better understanding of service-related tire degradation and to serve as the “real-world” baseline for the eventual development of a laboratory-based accelerated service life test for tires (often referred to as a “tire aging test”). As part of the Phase 1 effort, the performance of 109 tires retrieved from service in Phoenix, Arizona, of varying age and mileage were compared to 45 new tires of the same type and model in one of two laboratory roadwheel tests. All raw data used in the report are available in the public NHTSA Phoenix Dataset 5.0.[3]

2

2.0 BACKGROUND

Gardner & Queiser (2005) state the following with regard to tire structural failures:

“With a sufficiently high applied stress or strain, every material is subject to failure. In use, tire materials are subjected to the effects of fatigue, wear, heat, corrosion, and other external damage. The magnitude and frequency of the externally applied forces and moments can ultimately cause the rubber to fatigue and tear or cause steel and polyester cords to rupture. The process of tire structural failure can progress slowly, or be essentially immediate; it depends upon the structure itself, the magnitude and rate of imparted energy, and external conditions such as temperature.”[4]

The primary goal of the U.S. Federal tire safety standards is to specify tire performance requirements for structural integrity and resistance to operational conditions that the government has determined minimally necessary for new tires to possess. What is unknown is whether or not these new tire performance requirements translate into adequate tire durability throughout the entire service life of the tire. NHTSA’s Office of Defects Investigation maintains a searchable database of NHTSA safety-related defect and compliance campaigns, which contains records back to 1966 (including tire recalls2). This database can be queried through a Web interface or the records downloaded in raw form.[5] A cursory analysis of these records indicates that over 95% of the campaigns that involve full-size tires and list the component/region involved, cite the tread/belt area of the tire as the faulty component. Therefore, in consideration of these results, long-term tire safety is primarily a function of tread- and belt-region durability.

Table 1. NHTSA Safety-Related Tire Defect and Compliance Campaigns Since 1966 Stated Reason for Recall Number of Tires Recalled Percent of Recalled Tires Tread/Belt 58,161,358 95.7% Sidewall 1,170,380 1.9% Bead 462,855 0.8% Other 458,080 0.8% Unspecified 476,632 0.8%

As tires last longer, the concerns over long-term safety-related durability increase. In 1973 the average tread life of a passenger car tire in the United States was approximately 24,000 miles. By 2004, this number had risen to approximately 44,700 miles.[6] Using an average miles traveled by passenger vehicles of 9,992 miles in 1973 and 12,497 miles in 2004, the average tire service life was calculated to be around 2.4 years in 1973 and 3.6 years in 2004.[7, 8] This suggests a roughly 49% increase in average tire service life over the previous 30 years. Over this time period no revisions to the Federal tire safety standards were made. Today, the tread life of tires has

2 “RMA [Rubber Manufacturers Association] estimates that there have been about 295 tire recalls in this country. Only four of these recalls have involved more than 1 million tires and only 51 recalls have involved over 10,000 tires. Furthermore, RMA estimates that 142 recalls have involved less than 1,000 tires.” Comments of the RMA, On Notice of Proposed Rulemaking: Motor Vehicle Safety; Disposition of Recalled Tires, February 19, 2002, p. 2, Docket Document ID: NHTSA 2001-10856-0004, www.regulations.gov

3

http:www.regulations.gov

increased such that some tires are currently being offered with limited warranties of up to 160,000 km (100,000 miles).[9] These warranties are usually limited to a time period such as six years or older from the date of manufacture, or five years or older from the date of purchase. As average tire tread life has increased, so has concern about the feasibility of predicting long-term tire durability with short-duration tests of new tires. As recognized by the Rubber Manufacturers Association (RMA):

“RMA notes that objective, meaningful laboratory tests for assessing high speed and endurance performance of tires is certainly beneficial to safety. However, such tests are necessarily conducted over a small portion of time in comparison to the years of service provided by tires on a motor vehicle.”[10]

For instance, the agency noticed in the 2000 Firestone tire investigation, as well as other tire investigations, that tire designs that eventually proved defective generally tended to perform adequately in the first couple of years of service and only began to degrade in performance after that time.3, 4 The limitations of the current approach of only testing new tires are best elucidated in a research paper from the Uniroyal Inc. Research Center:

“Government agencies, private institutions and tire manufacturers all have made tremendous efforts to improve the performance of tires. The accelerated wheel tests are the most common approach to assessing tire service life. For example, the government specifies wheel tests like the 'Stepped-up Speed' (SUS) and 'Stepped-up Load' (SUL) [i.e. FMVSS High-Speed and Endurance tests]. These tests, however, would be classified as the short term tests which are aimed primarily at evaluating the structural strength and heat generation characteristics of the tires without any consideration of chemical reaction which happen in a long term of service.[sic] The long term durability is influenced by the material changes due to oxidation, adhesion retention as affected by water vapor, etc.”[11]

In modern terms, the average passenger tire service life predicted for 2004 was 3.6 years, or about 31,500 hours.[6] Assuming that approximately 4.5% of the average tire’s life is spent in actual rolling operation5, this would translate into approximately 1,400 hours and 44,700 miles of

3. “Based on a review of a sample of complaints received by the agency 's Office of Defects Investigation, complaint dates for tires are typically two to three years later than the model year of the vehicle on which they are equipped. This indicates, based on available data, that tire mileage may have been in the 20,000 to 30,000-mileage range when the complaint was submitted.” [Footnote #37], Docket Document ID: NHTSA-2000-8011-0019, U.S. Federal Docket Management System (FDMS), from: http://www.regulations.gov. 4. For example, in NHTSA ODI’s engineering analysis of the recalled Firestone ATX and Wilderness AT tires, they noted that: “...tread separations failures rarely occur in the focus tires (ATX / Wilderness AT) until at least three years of use.. .” Engineering Analysis Report and Initial Decision Regarding EA00-023: Firestone Wilderness AT Tires, Oct. 2001, p. 6. 5. U.S. DOT/Research and Innovative Technology Administration. (2001). Bureau of Transportation Statistics, Highlights of the 2001 National Household Travel Survey: 29.1 miles per day per person / 55.1 minutes per day of travel per person = average speed of 31.7 mph. 12,497 miles per year vehicle use / 31.7 mph = 394 hours per year of vehicle use. www.bts.gov/publications/highlights_of_the_2001_national_household_travel_survey/html/section_02.html

4

www.bts.gov/publications/highlights_of_the_2001_national_household_travel_survey/html/section_02.htmlhttp:http://www.regulations.gov

total rolling operation. Assuming an average vehicle speed of 31.77 mph in the United States,6 an average 16-inch passenger tire would experience approximately 35 million deflection cycles during service.7 To attempt to account for this, the minimum performance tests in Federal safety standards subject new tires to conditions on the curved indoor roadwheel that are more severe than normal operating conditions on a flat road surface. However, unlike actual tire service, the straight-line indoor roadwheel tests in the safety standard do not input cornering and camber forces into the tire and do not evaluate the structural durability of a tire after it has experienced long-term material property degradation under cyclic fatigue.[12]

Though some tire manufacturers have recently issued chronological tire service life recommendations, a tire’s service life has traditionally been indicated to the consumer by “wear-out” of the tread.[13] “Wear-out” is defined for the purposes of this report as all or a substantial portion of the tread of the tire having a depth of less than the legal minimum as specified in State regulations.8 Worn-out tires are a potential safety hazard, affecting the driver’s ability to control the vehicle on wet or slippery roads. On the positive side, Tread Wear Indicators (TWI), also known as “wear-bars” are required by Federal standards to be molded into the tire tread to alert motorists when the tire has worn to a tread depth of 2/32 inch. Vehicle servicers are trained to identify worn-out tires, which are easily detectable by inspection. However, tread wear-out is not an assured means of limiting the service life of tires. A 2005 study by the Rubber Manufacturers Association (RMA) of over 14,000 scrap tires from seven States indicated that 7% of tires were over 8 years of age when removed from the vehicle (i.e., more than double the 3.6-year average service life).[14] Even after their first removal from service, an estimated 30 million used tires are resold to consumers for continued use each year in the United States.[15]

Tires are also removed from service for visible damage or detectable performance deficiencies from incursions with road hazards, or abuse such as operation while underinflated or overloaded, operation with misalignment, etc. Incursions with road hazards can be a safety hazard, possibly leading to a tire failure at speed and a loss of vehicle stability or crash. However, the manner and means by which a tire experiences a road hazard (nails, debris, potholes, etc.) are so varied that tests to simulate these events are difficult to design and performance criteria difficult to set. The FMVSS for light-vehicle tires contain a plunger strength performance test for tires that specifies a minimum static breaking energy. Though the plunger test was designed to evaluate the strength of the reinforcing materials (mainly fabrics) in the tires, a tire must possess a sufficiently strong construction to pass the test such that it will also possess some inherent level of resistance to road hazards, abuse, and operational conditions. In-service tire pressure surveys conducted by the agency and other sources have determined that operation of the tire while underinflated is a widespread problem in the United States.[16, 17] This issue was specifically addressed after the TREAD Act with a requirement for all new passenger vehicles to be equipped with a tire pres

6 Product of speed and percentage time across all fourteen cycles (31.77 mph), Final Technical Support Document Fuel Economy Labeling of Motor Vehicles: Revisions to Improve Calculation of Fuel Economy Estimates, Page 49, U.S. Environmental Protection Agency, EPA420-R-06-017, December 2006. 7 Most popular replacement passenger tire size 2007 (source RMA Preliminary 2007): P225/60R16 = (44,700 mi 63,360 in/mi * 1 rev / 2 Pi radians) / 12.75 in Dynamic Radius = 35.4 million cycles 8 “In the U.S., 42 states consider 2/32nds the minimum tread depth, California and Idaho consider 1/32nd the mini-mum, and Arkansas, Montana, New Mexico, North Dakota, South Carolina and West Virginia have no standards.” The Importance Of Tire Tread Depth And Placement For Wet Traction, William Blythe, Ph.D., William Blythe, Inc.

5

sure monitoring system indicating severe underinflation in the tires (FMVSS No. 138). Also, an attempt to address overloading of the tires was addressed after the TREAD Act through revised vehicle placard requirements.

Statistically, instances where a tire fails during operation by a sudden, catastrophic failure such as complete or partial tread separation, sidewall failures (blow-outs and zipper failures), bead failures, etc. are rare. However, depending upon the vehicle and the driving conditions it may be difficult for the driver to maintain vehicle control when a sudden catastrophic tire failure occurs. These instances can result in a loss of control, which may in turn result in the vehicle leaving the roadway. It is logical therefore to ask whether or not the thermo-oxidative degradation and fatigue cracking of internal rubber components observed in tires retrieved from service contribute to a decrease in a tire’s resistance to operational conditions. In other words, would 1-year-old passenger vehicle tires have less chance of a structural failure while operated underinflated or overloaded, or during long periods of high-speed use, than the 7% of passenger vehicle tires over 8 years of age?[14]

If the service-related internal degradation of tires does contribute to tire failures, whether in conjunction with or independent of a road damage event or abuse such as underinflation, are there any indications of this degradation to the driver? In the past, the exterior condition of the tire such as surface cracking, powdering, fading, etc. may have indicated that the tire had experienced significant degradation and should be removed from service, despite possessing remaining legal tread depth. However, advances in compounding such as anti-oxidants, anti-ozonants, protective waxes and oils, and exterior surface treatments mitigate much of the exterior degradation of tires. Therefore, while the degradation of the tire’s internal components, especially at the belt edges, is a primary concern, motorists and tire servicers have little means of assessing a tire other than inspection of the exterior surfaces.

Accordingly, the first goal of tire-aging research was to develop a better understanding of service-related tire degradation over time. Once this goal had been achieved, the second goal was to develop an accelerated, laboratory-based tire test that simulates real-world tire aging. NHTSA tire-aging research to-date has been divided into four phases: Phase 1 addressed the first of these goals – a better understanding of how tires change over time. Phases 2 through 3 focused on developing an accelerated, laboratory-based tire test that simulates real-world tire aging and evaluates the remaining structural durability of the aged tires. Phase 4 was a validation of the final tire-aging test, completed on a large cross-section of tire models of various service types.

6

3.0 METHODOLOGY NHTSA began its research by conducting the “Phase 1 Tire Aging Field Study” to: Gain a better understanding of service-related tire degradation (“tire aging”);

Determine if service-related degradation was quantifiable and if so, which parameters are

good indicators of a tire’s aged state. In this pursuit, a broad list of tire material properties and whole tire roadwheel tests were to be studied;

Establish a real-world aged tire profile for use in the development of a laboratory-based

accelerated service life test for tires (“tire-aging test”); and Determine if there is a correlation between the static rate of loss of inflation pressure for

new tires and tire degradation (aging) rates in both the field and laboratory tests. The methodology used to select, collect, and test tires in Phase 1 is outlined in the following section.

3.1 Selection of Tire Retrieval Location NHTSA’s decision as to where to collect field tires was based upon four factors: polymer chemistry theory, data from the Firestone tire recall, trends in NHTSA crash data systems, and practical considerations. Tire aging (service-related degradation) is expected to occur due to changes in tire materials, particularly the tire rubber and its bonds to adjacent components. Two main types of changes were expected: 1) Mechanically induced changes in the rubber due to the stresses and strains developed by the tire while supporting the load both statically and under the cyclic deflections of operation; and 2) oxidation of tire rubber components that occurs also during both static and dynamic use. Mechanically induced changes in tire rubber and interfaces were expected not to vary substantially from geographic location to geographic location. However, the oxidation of the tire rubber is a chemical reaction. These reactions have been generally been shown to conform to an Arrhenius-type reaction rate, which as a rule of thumb states that for every 10 degrees C (18 degrees F) increase in temperature, the rate of the chemical reaction doubles.[18, 19, 20, 21, 22] Therefore, it is reasonable to expect that increasing ambient temperature will increase the rate at which the oxidation of tire rubber will occur. Based on the principles of polymer chemistry, NHTSA expected that the “worst case” tires in service in the United States to be found in the southern States. To confirm this prediction, NHTSA researchers examined data from the Firestone Radial ATX, Radial ATX II, and Wilderness AT tire recall. In the agency’s engineering analysis of the recalled tires, the following text accompanied Figure 1:

7

“The cumulative tread separation claims frequency trends for the tires recalled by Fire-stone in August 2000 and the focus tires are shown by plant of production in Figure 16. These trends show a distinct ordering by plant. (Footnote: For each of the tires, the earliest failures and the highest failure rates occurred in the hottest states.) These claims data show that the ATX and Wilderness AT tires manufactured at Decatur began to fail after between one and two years of service, with the ATX tires reaching a claims frequency of about 1,000 ppm. The failure trends for the Wilson tires began to develop after 2-3 years, with the claims frequency for the Wilson ATX tires approaching 200 ppm. The failure trend took longest to develop in the Joliette tires (after 3-4 years), with the claims frequency for the Joliette ATX tires approaching 100 ppm.”[23]

Figure 1. Cumulative Failure Frequencies From Firestone Investigation [NHTSA EA00-023]

Two particular issues were raised by the Firestone Investigation. First, the earliest and highest failure rates occurred in the hottest States. Second, even the recalled tire model with the highest failure rate (“Decatur ATX”) did not begin to fail until 1 to 2 years of service in these States. Figure 2 shows reported fatalities and injuries from failures of these tires by State. As this figure shows, the largest numbers of fatalities were in the States of Texas, California/Florida, Arizona,

8

and New Mexico (in that order). Note that, in accordance with chemical theory, these are all southern States that have higher average temperatures than the national average. The States with the largest numbers of injuries were Texas, Florida, California, Mississippi, and Arizona (again, in that order). Again, all of these States are southern States that have higher average temperatures than the national average.

Figure 2. Fatalities and Injuries Resulting from Firestone Tire Failures by State Graphic Courtesy of the Public Citizen Firestone Tire Resource Center [24]

Having more vehicles on the road could reasonably be expected to result in higher cumulative totals of fatalities and injuries due to the recalled Firestone tires. Therefore the death and injury totals for each State in the figure above were combined and divided by the number of light vehicle registrations in 2001 or by State population (Table 2). Per the figure, the three highest rates of combined deaths and injuries by light vehicle registrations were in Mississippi, Arizona, and Texas (in that order). The three highest rates by population were in Mississippi, Arizona, and New Mexico. As seen in Table 2, both of these lists are dominated by southern States with higher than average temperatures.

9

Top 5 States

Table 2. Top Five Rates of Combined Firestone Tire Recall Deaths and Injuries per Million

Light Vehicle Registrations and Million State Residents

2001 Light

Vehicle Registrations

Population (2000

Census)

Deaths Injuries Combined Rate Per

Million Light Vehicles

Combined Rate Per

Million State Residents

Mississippi 1,899,850 2,910,540 3 122 65.8 42.9 Arizona 3,896,783 6,166,318 15 61 19.5 12.3 Texas 13,812,966 23,507,783 35 195 16.7 9.8 New Mexico 1,326,277 1,954,599 7 14 15.8 10.7 Nevada 1,189,737 2,495,529 5 12 14.3 6.8

An independent analysis of NHTSA’s defect database for the Firestone investigation concluded in 2001 that 49% of these crashes known to Public Citizen occurred during hot summer months of June, July, and August.[24] Therefore, annual and summer temperatures were examined. U.S. National Oceanic and Atmospheric Administration (NOAA) temperature data from 1971-2000 indicated that Florida, Arizona, and Louisiana had the three highest mean and maximum daily temperatures for that period, as well as the highest mean temperatures during the summer months of June, July, and August. The three highest daily maximum temperatures were in Arizona, Texas, and Louisiana. Arizona not only had the highest daily maximum temperature during the three summer months, but if the weather station data from the mountain-top city of Flagstaff, Arizona, is excluded9 (Flagstaff Pulliam Airport has an elevation of 2134.5m [7,003 ft] above sea level), the maximum daily temperatures are over 100F during those three months.[25]

Table 3. Top Five Normal Daily U.S. State Mean and Maximum Temperatures*

Top 5 States

Normal Daily Mean Temp (F)

Normal Daily Max Temp (F)

June-August Normal Daily

Mean Temp (F)

June-August Normal Daily Max Temp (F)

Florida 72.0 81.1 81.8 90.2 Louisiana 67.5 77.3 81.7 90.8 Arizona 63.8 77.4 81.7 96.2

Arizona (Excluding Flagstaff)

68.3 81.5 86.3 100.2

Texas 65.6 76.7 81.5 92.2 Oklahoma 60.5 71.0 80.7 91.2 *Temperature data averages 1971-2000, NOAA.[26]

The NHTSA Fatality Analysis Reporting System (FARS), which contains data on all vehicle crashes in the United States that occur on a public roadway and involve a fatality, lacked sufficient detail to be of much use during the planning of the project. A 2003 analysis of tire-related

9 “Flagstaff, Arizona is located at an elevation of around 7,000 feet. Because of this fact, temperatures in the sum-mer tend to be significantly cooler than in much of the rest of Arizona.” Washington, DC: National Oceanic and Atmospheric Administration. (2008). Retrived from Web site: www.wrh.noaa.gov/fgz/science/hot.php?wfo=fgz.

10

www.wrh.noaa.gov/fgz/science/hot.php?wfo=fgz

crashes in FARS by the agency found that tire problems10 are noted after the crash and do not always imply crash causation.[27] Despite these limitations, it is useful to examine the FARS11 data in an attempt to confirm the trends seen in the Firestone Investigation data that were the basis for the choice of a tire retrieval site. In the year 2001, there were 494 light-vehicle fatal crashes where code “01 Tires” was cited for "Vehicle Related Factors (1)" (raw data in Appendix A). A “Vehicle Related Factor” is defined in FARS as “Pre-existing Vehicle Defects or Conditions Noted.” Again, this variable only indicates that the police accident report noted or cited the presence of a condition (either in an element on the form designed for doing so or in the narrative). The variable does not necessarily indicate a cause or imply a role in the crash, which could have been caused by excessive speed in a curve with a vehicle that had excessively worn tires rather than every crash being a result of a structural tire failure. For the “01 Tires” code, the following example is given in the 2001 FARS Coding and Validation Manual[28]:

01 Tires o Excludes improper tire pressure, which is due to driver irresponsibility.

An analysis of the fatal light-vehicle crashes coded with the “01 Tires” was completed by month and State to confirm trends. The top five States in 2001 for fatal crashes are listed in Table 4 by month. The three highest fatal crash cumulative totals were California, Texas, and Arizona (in that order).

Table 4. Top Five States for Total Light Vehicle Tire-Coded Fatal Crashes in 2001 FARS Query

Top 5 States Jan

uary

Febr

uary

Mar

ch

Apr

il

May

June

July

Aug

ust

Sept

embe

r

Oct

ober

Nov

embe

r

Dec

embe

r

Tot

al

California 8 7 8 8 15 12 13 11 6 7 6 9 110 Texas 0 0 4 2 5 5 12 7 6 5 5 4 55 Arizona 1 1 1 4 1 6 4 5 5 1 1 2 32

North Carolina 1 1 1 1 5 6 3 3 5 2 1 2 31 Georgia 1 2 1 5 8 3 1 1 3 1 1 0 27

If the total fatal crashes in Table 4 are divided by the number of registered light vehicles in each State in 2001, the top three States were Wyoming, Arizona, and Vermont. While Wyoming and Vermont had the fewest registered light vehicles of any States at 237,847 and 516,928 respec

10 “In the FARS system, tire problems are noted after the crash, if they are noted at all, and are only considered as far as the existence of a condition. In other words, in the FARS file, we don’t know whether the tire problem caused the crash, influenced the severity of the crash, or just occurred during the crash. For example, (1) some crashes may be caused by a tire blowout, (2) in another crash, the vehicle might have slid sideways and struck a curb, causing a flat tire which may or may not have influenced whether the vehicle rolled over. Thus, while an indication of a tire problem in the FARS file gives some clue as to the potential magnitude of the tire problem in fatal crashes, it can neither be considered the lowest possible number of cases nor the highest possible number of cases.” 11 The FARS system can be queried at www-fars.nhtsa.dot.gov/QueryTool/QuerySection/SelectYear.aspx

11

tively (which may influence the rate calculations), Arizona was 29th at 3,896,783. So the FARS data for 2001 indicate a relatively high rate of light-vehicle fatal crashes in Arizona where tires were cited as a vehicle-related factor.

Table 5. Top Five States for Total Light Vehicle Tire-Coded Fatal Crash Rate in 2001

FARS Query by PPM Registered Vehicles

Top 5 States Jan

uary

Febr

uary

Mar

ch

Apr

il

May

June

July

Aug

ust

Sept

embe

r

Oct

ober

Nov

embe

r

Dec

embe

r

Tot

al

Wyoming 0.0 0.0 5.6 0.0 3.7 0.0 1.9 1.9 1.9 0.0 1.9 0.0 16.8 Arizona 0.3 0.3 0.3 1.0 0.3 1.5 1.0 1.3 1.3 0.3 0.3 0.5 8.2 Vermont 0.0 0.0 0.0 0.0 0.0 0.0 1.9 1.9 0.0 1.9 1.9 0.0 7.7 North Carolina 0.2 0.2 0.2 0.2 0.8 1.0 0.5 0.5 0.8 0.3 0.2 0.3 5.3 Tennessee 0.0 0.2 0.2 0.6 0.6 0.8 0.6 0.2 0.2 0.6 0.6 0.0 4.6

In Figure 3 the cumulative fatal crashes from 2001 are plotted by month with national mean temperature (1971-2000) and national vehicle miles traveled12 in 2001.[26, 29] There is a clear trend of many more frequent fatal crashes coded with tires as a vehicle factor in the summer months when national temperatures are higher, but so is the total amount of mileage travelled by motor vehicles. Therefore, while states with high average annual temperatures had the highest rates of deaths and fatalities during the Firestone recall, the increase in tire-coded crashes in the summer months could be a result of higher temperatures or increased vehicle miles traveled, or both. Further statistical analysis of FARS data from other years would be needed to fully evaluate the effects of seasonal high temperatures.

12 National vehicle miles traveled encompasses all vehicles in the U.S. fleet, not just light vehicles. However there is an assumption that commercial vehicle mileage is mostly consistent throughout the year, and light vehicle mileage is the primary quantity fluctuating.

12

0

10

20

30

40

50

60

70

80

Janu

ary

Febr

uary

Mar

ch

Apr

il

May

June

July

Augu

st

Sept

embe

r

Oct

ober

Nov

embe

r

Dec

embe

r

Fata

l Cra

shes

: M

ean

Tem

pera

ture

(F)

190,000

200,000

210,000

220,000

230,000

240,000

250,000

Est.

Veh

icle

Mile

s Tr

avel

led

(Mill

ions

)

FARS Fatal Tire Crashes National Mean Temp (F) National Miles Travelled

Figure 3. FARS 2001 Light Vehicle Fatal Crashes Where Tires Were Cited as the "Vehicle-

Related Factors (1)"; National Mean Temperatures 1971-2000; and National Vehicle Miles

Traveled in 2001

One practical factor in selecting a tire retrieval location was the likelihood of finding a population of older tires. Given the data from the Firestone recall, FARS, and temperature data, it was clear that the retrieval location would be in a State along the Southern border of the United States. Additionally it was felt that the arid climate of the Southwestern United States, which diminishes need for minimum tire tread depth to facilitate wet and snow traction, as well as a relatively less aggressive roadway aggregate in that region, would result in longer tire service lives (i.e., older tires). Since Arizona was in the arid Southwest and in top three States in terms of combined death and injury rate during the recall, in the top three States in terms of annual and summer max and mean temperatures, and in the top three States in 2001 FARS tire-coded fatal crashes, Arizona was considered the best candidate for a tire aging study. The Phoenix, Arizona, metropolitan area was selected as the collection location for the following reasons:

1. Maricopa County (Phoenix metro area), Arizona, was a large population center with over 3 million residents13 and possessed a large infrastructure of tire retail centers.

13. U.S. Census Bureau. (2000). Maricopa County population: 3,072,149 (2000 U.S. Census), www.census.gov/.

13

http:www.census.gov

2. Phoenix, Arizona, has an annual normal daily mean temperature of 23.4° C (74.2° F).[30] Phoenix also has a mean number of 169 days with a maximum temperature of 32.2° C (90° F) or higher.[31]

3. Phoenix, Arizona, has an arid climate with an annual precipitation of only 21.06 cm (8.29 in), thus increasing the likelihood of finding older tires in service due to diminished need for tread depth.[32]

3.2 NHTSA Phoenix, Arizona, Tire Exchange Program Phase 1 of the NHTSA Tire Aging Test Development Project consisted of the collection of 12 different tire models from use on private vehicles in the Phoenix, Arizona, metropolitan area. In late 2002, NHTSA researchers used tire industry statistics on the most popular brands, sizes, tread designs, etc. to construct a preliminary list of models for the study. Tire manufacturers were contacted for help in narrowing down models to those that were in production from 1998 to 2003, had no ‘significant’ design changes14 in that period, and were available for public purchase in Arizona. During the February 25th to March 17th, 2003, timeframe the agency sent two successive teams of Federal and contractor staff from the NHTSA Vehicle Research and Test Center (VRTC) in Ohio to Maricopa County (Phoenix), Arizona. The teams’ assignments were to identify tire retail locations for collection centers, establish storage and transportation logistics, train retail staff on retrieval procedures, and launch the tire collection program. One large tire retailer with multiple locations and six vehicle dealerships agreed to participate in the tire collection program. A centrally located warehouse and a small fleet of moving vans were leased to facilitate short-term tire storage and transportation. After the first 3 weeks of the program were completed, all staff departed from Phoenix and the program was administrated remotely by the agency until the end of April 2003. At the conclusion of the project, all Phoenix-collected tires were shipped back to the VRTC for processing and distribution to the various test labs.

The vehicle sample population was primarily comprised of random vehicles entering tire retail locations as well as past customers of the businesses that were contacted by the retailer’s employees for interest in participation. (A small number of hard-to-locate tires were retrieved from vehicles on auto dealer lots.) For vehicles entering a collection location, dealer service personnel checked the tires against a collection list updated by NHTSA each day. If the tires matched the exact specifications on the collection list, met age/mileage targets, and the vehicle had current Arizona license plates, the dealer contacted agency staff via phone for transaction authorization. If transaction authorization was secured, the dealer would explain the program and offer the member of the public a new set of tires with a road-hazard warranty at no charge in exchange for their current tires (including full-size spares), which would become Government property. Mini-spare tires were not collected. All interactions with the customers and vehicles, as well as all financial transactions were handled directly by the local dealers, who were paid via government

14 Since multiple variants of each tire model can exist simultaneously, this confirmation of design consistency with the manufacturers for each tire model was only marginally effective. As can been seen in Appendix B, three of the six tire models studied had what would be considered design changes or line-to-line build variations during the 1998-2003 time period. A more effective way to select test tire samples of a given model may be to limit tires to the same SKU (Stock Keeping Number), which often changes if the tire design has been modified.

14

purchase card transactions. Study participants (vehicle owners) were not required to fill in any forms or questionnaires and did not provide any personal data. Table 6 documents the vehicle and tire information collected by the dealer. A total of 493 on-road tires of 12 different models (Models “A-L” in Appendix C) were collected from local residents’ vehicles to assure an adequate sample of tire models, ages, and mileages. The used tires were retrieved regularly from the stores and stored in a local warehouse to await shipment.

Table 6. Tire Data Collected by Dealers in Phoenix, Arizona, at Time of Tire Retrieval Category

Tire

Vehicle

Collection Location

Test Sidewall information (DOT code / brand / model / size / load index / speed rating) Inflation pressure Position on vehicle Original date of sale if known Had Arizona license plate (Yes / No) Vehicle identification number (VIN)* Make / model Production year Mileage Store identification number Date of retrieval

* VIN redacted from public release. Used to confirm vehicle information such as make, model, etc.

3.3 Inspection and Screening of Phoenix-Retrieved Tires Following the Phoenix tire collection, the tires were shipped to the NHTSA VRTC in East Liberty, Ohio, for processing. VRTC staff separated full-size spare and non-Arizona tires from the on-road tire population through searches of tire collection sheets, vehicle identification numbers, photos, and vehicle registrations. The vehicle identification number (VIN)15 was decoded using the PCVINA© for Windows (R. L. Polk and Co) software. The online vehicle registration search service CARFAX© was used to confirm vehicle model information and registration history. Despite all vehicles having Arizona license plates, about 10% of sample tires were eliminated from testing because the vehicle was not registered in Arizona for the entire service life of tire. This was done to prevent tires that may have been in service in other lower-temperature regions of the country from confounding the results of the analysis of Phoenix area tires.

Though 12 tire models were collected in Phoenix, it was very difficult to find tires in every age/mileage group targeted for collection. In the end, an acceptable distribution of age and mileage could only be obtained for five “original equipment” (OE) tire models (i.e., the tires on the vehicle when the vehicle was purchased new) and one “replacement market” tire model. This result can be attributed to the technicians’ ability to watch for a specific vehicle model when searching for OE tires, but not for a specific vehicle model when searching for replacement market tires. Not knowing which vehicle models to look for made many replacement tire models difficult or impossible to locate in a usable sample size. The six Phoenix-collected tire models with the best distribution of age and mileage were selected for testing and are listed in Table 7.

15 Redacted from public datasets to protect vehicle owner identification.

15

Table 7. Phase 1 Tire Models Selected for Testing NHTSA Tire

Type B

C D E H L

OE Fitment?

Yes

Yes Yes Yes No Yes

Tire Brand

BFGoodrich

Goodyear Michelin Firestone Pathfinder* General

Tire Model

Touring T/A SR4 Eagle GA LTX M/S Wilderness AT ATR A/S Grabber ST

Tire Size

P195/65R15

P205/65R15 P235/75R15XL P263/75R16 LT245/75R16 255/65R16

Load Range

89

92 108 114 120/116E 109

Speed Rating

S

V S S Q H

*Manufactured for the Discount Tire Company by the Kelly-Springfield Tire and Rubber Company, a subsidiary of the Goodyear Tire and Rubber Company

Following collection in Phoenix the tires were inspected, de-stoned, and photographed from multiple views at the VRTC before being sent for testing at the testing laboratories. A barcode labeling system (Appendix D) allowed for tracking of all tires and wheels throughout the test program. Any suspect areas of the tire discovered during pre-screening were permanently marked on the tire, annotated, and photographed. An example of typical inspection results for a tire with damage can be seen in Table 8.

16

Tread Side 2 Sidewall Serial Side Sidewall Opposite Serial Side

Repair Close-up DOT Code New Version

Table 8. Example of VRTC Inspection of a Phoenix-Retrieved Tire Serial Side Opposite Serial Side Tread Side 1

This pre-screening process separated out many unsuitable test samples (tires with excessive wear, any damage or repairs, wrong load range, etc.) and helped researchers determine test assignments. In order to not unfairly bias the roadwheel test results, only tires free from visi-ble damage and repairs (no patches, plugs, exposed belt edges, etc.) were assigned to road-wheel testing. Tires in good shape but having a repair or puncture (such as tire N0249 in Table 8) were assigned for materials testing instead of roadwheel testing. A full bead-to-bead shearographic inspection of the tire prior to any testing allowed the test labs to find any repairs or foreign objects in the tires that were not identified during visual inspection.

3.4 Phase 1 Tire Tests The Phase 1 test tires were subjected to either testing on an indoor 1.7-m (67-inch) laboratory roadwheel or a cut-tire analysis of component materials properties at independent, accredited tire testing laboratories under contract with the agency.[33] This report details the results of the 154 tires subjected to the tests in Table 9. Subsequent reports will detail the results of the material properties analysis (tests detailed in Appendix E).

17

Table 9. Phase 1 Roadwheel Test List Test Category Test

Nondestructive Inspection Visual Inspection for Cracks, Punctures, and Repairs Shearographic Interferometry Tread Depth Tread Durometer

Laboratory Roadwheel Testing Stepped-Up Speed to Structural Failure Stepped-Up Load to Structural Failure

Table 10 documents the distribution of the Phase 1 tires between the various tests.

Table 10. Overall Phase 1 Tire Test Distribution (Six Tire Models) Tire Position Roadwheel Tests Materials Tests Total

New 45 37 82 On-road 101 71 172 Spare 8 1 9 Total 154 109 263

3.5 Determining the Service Duration of Test Tires The week and year of manufacture for all tires in the study was known from the DOT code (Tire Identification Number - TIN) on the tire sidewall. Also known was the date the tire was collected from service in Phoenix, allowing a calculation of a tire’s chronological age at the time of collection. This method of calculating “service duration” would therefore include both the duration of pre-installation storage for a tire (i.e., time a new tire spent in storage/transport prior to installation on the vehicle), as well as the tire’s actual duration of service in Phoenix. Another time period considered was the duration tires spent in storage/transport between collection in Phoenix or as purchased new, and actual testing on the roadwheel. All of the chronological information available on the tires was considered in the development of the final age variable “DOT_Age” used in the analysis. The various rationales are detailed below.

3.5.1 Replacement Market Tires The tire vendors were able to provide the original date of sale for 19 of the 51 roadwheel tested replacement market tires collected from Phoenix, making it possible to determine both the duration of pre-installation storage/transport and actual duration of service in Phoenix. On average, these 19 tires spent 0.22 0.12 years, or about 1 to 4 months in storage/transport as new before being installed on a vehicle in Phoenix (Table 11). This calculation was not possible for the other 32 replacement market tires that did not have the original date of sale information. Since the average duration of pre-installation storage appears to be brief, the effects of non-pressurized storage/transport were thought to be negligible, and the duration of pre-installation storage could not be determined for all replacement tires, the “DOT_Age” variable for replacement market tires was calculated from chronological age at the time of collection as follows:

18

“DOT_Age” = Date of Collection - Middle Day of the Week/Year of Manufacture Equation 1: Service Duration of Replacement Market Tires

Since the DOT_Age variable for replacement tires includes a brief duration of pre-installation storage/transport as well as the tire’s actual duration of service in Phoenix, the result is to somewhat understate the effects of the actual duration of service in Phoenix. However, this measure of age is thought to be the most representative of consumers’ experiences with replacement tires in service. For instance, a typical tire in service with a chronological age of 3 years would likely have experienced a few months of pre-installation storage/transport, and a few months less than 3 years of actual service.

Following collection in Phoenix, the average time between retrieval of replacement market tires and the invoice date of the roadwheel test (invoiced at the end of each month) was about 1.05 0.22 years. This is due to the fact that Phoenix-retrieved tires went through extensive pre-test inspections, and were tested in many phases as the two roadwheel test methods were developed and refined. Since tires were subjected to roadwheel testing over the entire period of each month, half the duration of the 1-month invoicing cycle (0.041 years) was subtracted from the average post-collection storage time to adjust for the invoicing cycle, yielding an estimate of about 1.01 0.22 years of average post-collection storage time. These non-service storage/transport time figures are again documented in Table 11.

Though the amount of post-collection storage time is known for all of the roadwheel tested tires, it was not added to the service duration variable (DOT_Age) due to the fact that once tires were retrieved from service in Phoenix and dismounted from the vehicle rim (i.e., depressurized), the thermo-oxidative degradation of tire rubber compounds would have slowed by at least an order of magnitude.16 This is an important note, since multiple independent analyses of the raw data in this report have inadvertently included storage time in Ohio with the actual service period of the tires in Phoenix, dramatically lessening the calculated effects of service in Phoenix and assigning 1 year of service in Phoenix to brand new tires with no service. If the magnitude of the material property degradation of internal components was similar between unmounted tires stored in warehouses and mounted tires in actual service in Phoenix, then tens of millions of new tires in current inventories throughout the United States would experience marked material property degradations before being sold to consumers, and that has not been observed.

16 That is not to say that tire material properties do not change