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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.

As part of the British Transport and Road Research Laboratory's research program to determine the relative magnitude of the various factors influencing grip on wet pavements, measurements of skid resistance (braking force coefficient (BFC)) have been made using trailer and front wheel braking techniques. The five sets of smooth treaded tires differed only in the tread polymers which were chosen to cover a range of physical properties. The tires were tested at speeds ranging from 1 km/h (0.5 mph) to 160 km/h (100 mph) on eight surfaces having a wide range of textures. Macrotexture was measured by sandpatch “texture depth” and stereophotogrammetric “profile ratio” techniques. Comparison of the mean BFC/speed curves for each polymer shows a similarity of speed dependence, indicating that on the highway penetration of the water film is the overriding factor rather than a speed dependent viscoelastic effect which has been postulated from laboratory friction measurements. However the order of the BFC shows evidence of viscoelastic behavior, the more resilient polymers giving the lower values. A hypothesis relating to a change in the tire/pavement contact conditions at high speeds is put forward that the high values of BFC on surfaces with large texture depth are partly due to modification of the contact conditions by changes in the viscosity of the lubricant brought about by heating.

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