The Speed and Temperature Dependence of Rubber Friction and Its Bearing on the Skid Resistance of Tires

Abstract

The observation that the ranking of tire tread compounds in skid tests is largely independent of the testing conditions, such as speed, type of surface, whether the surface is dry or wet and whether locked-wheel braking or cornering tests are carried out is explained by means of the temperature and speed dependence of rubber friction.

It is shown that speed and temperature are in every case related by the WLF equation, provided the sliding speeds are sufficiently low for the temperature rise in the contact area either to be negligible, or to be known. Master curves, therefore, can be obtained under all relevant conditions, and these describe completely the friction between the rubber and the surface under investigation. The shape of the master curve depends on the type of rubber, on the black filler content, on the type of track surface, and on the extent of lubrication.

The range of combined temperature — speed variable, log a_Tv, achievable in tire skid experiments, is small because of the opposing nature of speed and temperature effects. The crossing of the friction — log a_Tv curves of different polymers on any one surface is unlikely. If it does occur, as in the case of the comparison of oil extended rubbers with unextended rubbers of the same polymer, it shows itself also in a reversal of the ranking in a skid trial at high speeds.

Because the temperature range is limited on ice, the part of the master curve corresponding to high a_Tv values comes into play. Since in this region the ranking of polymers, particularly NR and SBR, is reversed, a similar reversal in ranking is also observed in tire skids.

The likely improvement which a particular compound can make to the skid resistance of tires can, therefore, be estimated correctly from simple laboratory comparisons.