Tire endurance as measured by performance on the National Highway Traffic Safety Administration (NHTSA) Stepped Up Load (SUL) test is shown to be a function of both tire construction and the extent of oxidation in the skim and wedge rubber regions of the tire, as measured by peel strength or elongation to break retention. Tire constructions can be distinguished by speed rating. Tires with higher speed ratings (> S) tend to have relatively high times-to-failure (TTF) in the SUL test and are relatively insensitive to rubber oxidation. SUL TTFs for tires with speed rating of S and lower tend to be much more sensitive to rubber oxidation. For these tires, the SUL TTF decreases linearly with aging time in the field. The rate of loss of SUL performance is proportional to the rate of loss of rubber properties. The large variability observed in the SUL results from field aged tires can be explained by the natural variability in oxidation aging rates observed for these tires. For oven aged tires, the correlation between SUL and rubber oxidation is more complex. Initially, the SUL failure time does not change much with rubber oxidation. At a critical oxidation level, however, the SUL failure time begins to drop rapidly with rubber oxidation approaching the behavior of the field tires at high levels of oxidation. The reason for the difference in behavior between the oven aged and field aged tires is the lack of mechanical damage in the belt edge in the oven aged tires relative to the field aged tires.

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