Friction on Wet Surfaces of Tire-Tread-Type Vulcanizates

Abstract

Abstract A laboratory investigation has been made into the variation with temperature of the hardness and resilience of a wide variety of rubber compounds of the tire tread type. The effect of hardness and resilience on the fractional properties of the compounds under wet conditions has also been studied. In the first series of tests the resilience and hardness of 25 compounds were measured over a temperature range 0° to 80° C. All were vulcanized tire tread type compounds, and the basic materials used comprised 14 natural rubbers, 7 styrene/butadiene (SBR) rubbers, 2 butyl, 1 polybutadiene, and 1 ethylene/propylene. The tests showed a marked increase in resilience with increasing temperature for all compounds except the polybutadiene; the hardness of all compounds changed very little with temperature, only a slight decrease being observed over the whole temperature rise. Nine compounds of representative resilience and hardness were selected for a second series of tests in which friction was measured over a temperature range 1° to 40° C on seven surfaces representing roads of different textures. For eight of the compounds, friction values decreased with increase in temperature; for the other compound the friction increased to a maximum value at 30° C. These changes in friction cannot be explained by changes in hardness of the compounds, but they are in accordance with resilience changes, taking into account the different test conditions obtaining in the friction and resilience tests. The friction tests also showed that with the portable skid-resistance tester used to measure friction the sharpness of the projections in the road surface is more important than their size in determining the friction values under wet conditions, even when rubber compounds of low resilience are used. The implications of the findings and their application to the study of friction between tire and road are discussed. In particular, they have a bearing on the correct interpretation of resilience measurements of tire tread materials in relation to friction values under wet conditions.