Goodyear Medalist Lecture. Rubber Friction and its Relation to Tire Traction

Abstract Rubber friction differs from that of hard solid materials in that it is not linearly related to the normal load and it depends strongly on sliding speed and temperature. There exists an interrelation between these two variables on their effect on the friction coefficient, first observed for the viscosity of liquids and generally described by the universal WLF transformation equation. The friction coefficient at a constant load is then described by a so-called master curve. Such master curves have been obtained on different types of surface and for gum rubbers as well as filled rubbers on wet and dry surfaces and it is shown that they may also be obtained on ice. The shape of the curve and position on the log(aTv) axis depends on the polymer and the track surface structure indicating that two distinct processes determine the friction: adhesion friction akin to a molecular relaxation process and a deformation process in which energy is lost due to the cyclic deformation of the rubber by the surface asperity. To obtain such a master curve it is necessary to keep the experimental speeds so low that the temperature rise in the contact area can be neglected. In practical tire tests, sliding speeds are high and hence the temperature rises with speed. Since the WLF equation is a negative function of temperature, the range of log(aTv) is limited. If thermocouples are used as sliders on rubber, it is shown that the experimental curves as function of speed can be transformed into a part of a master curve. For compound development friction tests, a limited range of track temperatures and speeds are sufficient to ensure a high correlation with road test data. A single point laboratory measurement may correlate with road tests if carefully chosen. More usually, it leads to misleading conclusions. Side force measurements at a reasonably large slip angle also reflect the friction coefficient and are a useful laboratory tool to evaluate the traction properties of tread compounds.