The braking capacity of a vehicle depends on friction forces developed in the contact area between the vehicle tires and the road. For longitudinal braking, the friction force Fx is expressed as Fx = mu.Fz, where Fz is the normal load and mu is the tire/road friction coefficient. The tire/road friction coefficient depends on the wheel slip (k), expressed as k = S/V, where S is the relative speed between the tire and the road (called also slip speed) and V is the vehicle speed. This mu-k-diagram comprises two parts corresponding to two braking behaviors: In the first part, mu increases to a maximum value called mumax. The braking is 'soft': the more the driver brakes, the more the vehicle decelerates. In the second part, mu decreases to a value called mulocked, which is reached when the wheels are locked and the vehicle slips. Since mumax and mulocked are essential for the knowledge of friction-slip law, methods are developed to estimate them whether from wheel kinematics or from road-surface texture. This paper presents a synthesis of this work, including a collaboration between LCPC and LTDS. Tire/wet road friction depends on two road-surface texture scales: the macrotexture (centimeter- to 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 macro- and 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. It was shown that tire/road friction coefficients can be estimated from road surface texture. Knowledge of road macro- and microtexture can be used, via models, to estimate mumax and mu(locked), which are two important parameters for vehicle handling assessment.
Influence of the road-surface texture on the speed dependency of tire/road friction
Einfluss der Oberflächentextur der Fahrbahn auf die Geschwindigkeitsabhängigkeit der Reibung zwischen Straße und Autoreifen
2005
9 Seiten, 12 Bilder, 4 Quellen
Aufsatz (Konferenz)
Englisch
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