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.
Towards contactless skid resistance measurement
Monitoring of skid resistance is an important component of maintaining road networks. 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 on a variety of grounds to predict skid resistance solely from texture measurements. We present a concept of contactless skid resistance measurement which is based on optical texture measurement and consists of two components: measurement of the pavement texture by means of an optical measuring system; and calculation of the skid resistance based on the measured texture by means of a rubber friction model. We describe the basic assumptions underlying the theoretical approach and present the model based on the theory of Persson. Two skid resistance measuring devices are chosen to prove the theoretical approach: one laboratory device called Wehner/Schulze (W/S) machine which corresponds to a blocked-wheel braking test and the ViaFriction device of ViaTech AS which measures the skid resistance under controlled longitudinal slip and corresponds to ABS braking conditions. We describe the measurement devices and experiments conducted. 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. The 95 % prediction interval is +/- 0.04 and the variance 0.02. Thus, a strong indication could be provided that skid resistance could be measured without contact in the future.
MICROTEXTURE ROUGHNESS EFFECT ON PREDICTED ROAD-TYRE FRICTION IN WET CONDITIONS
Abstract Purely theoretical means were used for predicting the locked wheel and sideways force coefficients of friction on a coarse-textured and a finertextured road surface. The predictions were made directly from surface texture measurements, the measured damping factor of the tread rubber and a theoretical analysis of the surface water using Reynolds' equations of shear and sinkage together with a mechanolattice analysis of the sliding rubber. The reasonable accuracy of the theoretical predictions indicates the high precision possible should empirical factors be later introduced into the method.