Measurement of Rolling and Sliding Friction Coefficients of Tread Compounds

Abstract

Abstract We have described two methods to measure friction forces arising from sliding and rolling on a wet surface. In rolling friction we showed that energy of deformation of a braked rubber specimen could be estimated and separated from friction energy. Rolling friction shows a characteristic slip dependence for various compounds. The rise at low slip values can be quantitatively explained. Especially with polybutadiene vulcanizates, at higher slips the friction coefficient decreases; this is attributable to thickness of the water layer between specimen and friction surface and depends on ease of wetting with water. Wetting can be altered by addition of a wetting agent like Pril, and measured by the contact angle of a water drop on the rubber surface. In wet friction, the changes in the water layer outweigh the temperature and velocity dependent shifts expected for adhesion and deformation friction components. A sort of mixed friction occurs on wet surfaces. The true contact surface, that is, surface not covered with water, has a strong effect on the magnitude of friction forces. In general only the highest asperities of the friction surface extend above the water and are in direct contact with the rubber surface. It is therefore understandable that surface asperity has a strong effect on friction coefficients. Finally we have compared friction coefficients determined with our apparatus with those obtained using the Portable Skid Resistance Tester and with results of road tests. The Portable Skid Resistance Tester provides a single point method, which has good reproducibility and whose results correlate well with those on the road at a speed of about 48 km/hr. Our apparatus has the advantage that test conditions such as velocity, temperature, and load can be widely varied. In this way, various actual driving conditions can be approximated and one may seek to determine the mechanism of friction from dependence of friction coefficients on these parameters.