Principal Considerations on Tire Wear

The prerequisite for tire wear is relative motion between the tire and road in their common area of contact. Such relative motion occurs, for example, when a tire is flattened on entering the area of contact and can also take place when the tire exerts a force on the ground, i.e., during propulsion, braking and, particularly, cornering. The mechanism of abrasion when the wheel is not rolling freely becomes complicated by the fact that, because of the elasticity of the tire, its circumferential velocity with respect to the vehicle can differ from the travelling velocity, and that, furthermore, the tire can have a velocity component normal to its plane without skidding. In other words, slip without noticeable abrasion is possible. The only case discussed here will be when the wheel describes a curved path. The elasticity problem involved has been treated, among others, by Fromm, von Schlippe, and Temple; the essential facts are best visualized by means of the model experiment reproduced in Figure 1. In this experiment, a solid rubber wheel rolled around a Perspex disk, and the area of contact was illuminated by internal reflection. The deformation of the equator (marked in black) shows that two regions can be distinguished in the area of contact. In the front part, the equator is distorted into a curve parallel to the path, i.e., into a circle, and one can easily see that no relative motion between wheel and track occurs here, but that the wheel adheres to the track. The tangential stress necessary to deflect the wheel is maintained by friction until the stress becomes so high that the limiting friction is reached. At this point the second or rear part of the area of contact develops, in which the wheel slides back sideways. Abrasion should, therefore, be expected only in the rear part of the area of contact. It will decrease with increasing elasticity of the tire, but nothing will be gained thereby, since the tire has, after all, to take up the forces arising during driving. In the present case, this involves the side force holding the vehicle on the curve, and what matters is the dependence of abrasion on this side force.