statement of task for this study calls for an examination of these many relationships.

The statement of task also calls for the study to address factors that can affect vehicle safety and scrap tire generation. There is a public interest in tire safety and scrap tire generation, as there is in fuel economy. Some 40,000 motorists die in highway crashes each year, most in passenger cars and light trucks. Thousands more are critically injured. Improving the safety performance of the nation’s highways is a public safety goal. During the past two decades, concerns about the environmental effects of tires, particularly the disposal of scrap tires, have also emerged. While aggressive recycling programs have reduced the entry of tires into the waste stream, the large number of tires discarded each year poses a continuing mitigation challenge.

The rolling resistance, traction, and wear characteristics of tires are not independent of one another, if for no other reason than their association with the tire’s tread. As explained earlier, the tread has a major influence on rolling resistance because it contains much of the viscoelastic rubber in the tire that causes hysteretic energy loss. The same tread deformation contributes to the tire’s traction capabilities. A loss in traction capability because of tread wear is the main reason for tire replacement. When the tread wears and traction capabilities are diminished beyond a point deemed acceptable for safe operation, especially in wet and snow conditions, the tire is normally scrapped—and thus becomes a candidate for the waste stream.

Years of tire testing and experimentation have helped tire manufacturers understand the chemical and physical relationships that affect tire traction, wear resistance, and rolling resistance. This has led to a growing appreciation—but still limited understanding—of how such factors relate to the practical outcomes of vehicle fuel consumption, crash incidence, and tire service life. Data sets examined in Chapter 3 show how rolling resistance can differ significantly from tire to tire and how these differences can translate into differentials in vehicle fuel consumption. The same data sets can be examined to gain a better understanding of the relationships among rolling resistance and other tire performance characteristics, including traction and wear resistance. The results of several statistical analyses of the available data sets are therefore presented in this chapter to explain these relationships.

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