Stopping distance variation with road surface temperature and tire wear

Stopping distance measurements are affected by a large quantity of noise factors that contribute to make them not comparable when measured in particular conditions. For automotive companies and tire manufacturers is not feasible, in terms of costs and available time, to control such noise factors, of which ones of the most important are: - Road surface temperature during tests; - Tire wear. As regards road surface temperature, currently the state of the art of tire standards that take it into account are SAE J2909 [1] and ISO 21994:2007 [2]. Regarding testing temperature, the ISO standard states that: - The ambient temperature should stay between +5 and +35 degrees C, and its variation should be less than 10 degrees C during a testing session. - The road surface temperature should stay between +10 and +40 degrees C, and its variation during a sequence of measurements should be less than 10 degrees C. - Measurement performed within acceptable range can be compared only if temperature difference between one another is less than 10 degrees C. As previously stated automotive and tier one companies cannot wait such a range of environmental conditions to perform and compare their tests (and, in addition, the need to compare tests from months, or even years apart can rise). On the other hand, SAE J2909 gives a polynomial formulation that, starting from the adhesion coefficient of the track used, gives a polynomial corrective formula to adjust measured stopping distances to the desired road surface temperature. However, this implies the need to measure the road surface friction coefficient, with a standard reference tire and a standard method (ASTM E 1136 - 1137 [3]) every time before tests. Moreover, both ISO and SAE standards do not take into account that road surface temperature can have different influence on different types of tire for what regards compound and tread pattern (e.g. on summer, all season and winter tire). It is very important, for a testing proving ground, to have the opportunity to compare test performed in different environmental conditions or to estimate tire performance at different road surface temperature. In addition, also tire wear should be taken into account while performing stopping distance tests. In particular, experimental evidence shows that measured stopping distances are shorter after a certain number of braking maneuvers performed on the same tire. For this reason it is important to understand when stopping distance measurements give more repeatable results on a single tire during a brake test session, and which burnishing procedure to perform in order to save time and improve measurement repeatability. In this paper conclusion of six months experimentation and measurement on five different summer tires and one all season are reported. From data analysis, a corrective formula for stopping distance measurement with road surface temperature is proposed. This formula is compared to the other ones present in literature and will be validated for future use. Furthermore some important considerations on the effect of tire wear, intended as the number of braking maneuvers performed on the same tire, are done.