INFLUENCE OF THE ROAD-SURFACE TEXTURE ON THE SPEED DEPENDENCY OF TIRE / ROAD FRICTION
Tire/wet road friction depends on two road-surface texture scales: the macrotexture (centimeterto millimeter dimensions) is responsible for the speed dependency of friction, and the microtexture (submillimeter dimensions) is responsible for the friction level at any speed. This paper deals firstly with the description of road-microtexture, the macrotexture scale being well documented. Considering the fact that rubber energy-loss controls tire/wet road friction and that angularity of road-surface asperities influences greatly tire deformation, the second author has proposed a description method derived from the motif-combination technique. This method defines two parameters related to the sharpness of asperity summits and the relative position of these summits. A model has been then proposed by the first author to describe the friction-speed curve from roadsurface macroand microtexture descriptors, tire-related characteristics (rubber properties, wear) and road wetness. The description is based on the Stribeck curve. The speed-dependency friction model is applied to the analysis of car-braking tests. Comparisons between calculated and measured friction-coefficients at different braking speeds are presented and discussed.
A contribution to non-contact skid resistance measurement
Over the past decades a wide range of routine measurement devices has been developed, all of them measuring the friction force between a rubber wheel and the (wetted) road surface. At the same time, many efforts have been undertaken to predict skid resistance solely from texture measurements. This paper presents a concept of contactless skid resistance measurement which is based on optical texture measurement and consists of two components: (1) measurement of the pavement texture by means of an optical measuring system and (2) calculation of skid resistance based on the measured texture by means of a rubber friction model. The basic assumptions and equations underlying the theoretical approach are presented. Two skid resistance-measuring devices were chosen to prove the theoretical approach: one laboratory device called Wehner/Schulze machine which corresponds to a locked-wheel braking test and another, the ViaFriction® device of ViaTech AS which measures the skid resistance under controlled longitudinal slip and corresponds to ABS braking conditions. The results are very promising although in the case of the ViaFriction® device only a few surfaces could be tested. A close relation between measured and predicted friction coefficients could be found. Thus, a strong indication can be provided that skid resistance could be measured without contact in the future.
The Friction of Hard Sliders on Lubricated Rubber: The Importance of Deformation Losses
A study has been made of the friction of hard spheres and cones on a well lubricated rubber surface under conditions where relatively large deformations are produced. It is found that with spheres the sliding friction is almost the same as the rolling friction. Earlier work has shown that in the latter case the friction arises primarily from hysteresis losses in the rubber and it is concluded that, under the experimental conditions described, the main source of friction in lubricated sliding arises from the same cause. With conical sliders it is not possible to make a direct comparison with rolling experiments, but a simple calculation suggests that here again the friction is largely due to deformation losses in the rubber. For cones of small semi-angle there is a large increase in friction but this is associated with penetration of the lubricant film and tearing of the rubber. The friction results are in close agreement with measurements described by Miss Sabey for spheres and cones sliding on wet rubber at relatively high speeds (c. 6 ft sec-1). This suggests that in her experiments that water film provides very effective lubrication and that the friction arises largely from hysteresis losses in the rubber itself. As a general conclusion this study of the friction of lubricated rubber suggests that where interfacial adhesion is small and where relatively large local deformations occur the friction may well be dominated by deformation losses in the rubber. Under these conditions we may therefore expect the friction of hard spherical or conical sliders on rubber to be greater for rubbers of low Young's modulus and high hysteresis losses.