HIGH AND LOW-SPEED SKIDDING RESISTANCE: THE INFLUENCE OF TEXTURE ON SMOOTH AND RIBBED TYRE FRICTION

The Transport Research Laboratory (TRL), sponsored by the Highways Agency (HA), has recently conducted research into the relationship between friction and speed and the influence of surface type and texture depth. Peak and locked-wheel wet friction were measured using ASTM standard smooth and ribbed tyres and a 1mm nominal water depth at five speeds between 20 and 130 km/h at nearly 150 sites. Low-speed skid resistance was measured using a SCRIM (sideway-force coefficient routine investigation machine). Texture depth was also recorded using a laser-based sensor. The sites were selected to include the major surfacing types used on main roads in the UK and to cover a range of texture depths and levels of low-speed skid resistance. The relationship between locked-wheel friction and speed was similar for the two tyres, but the effect of the road surface texture depth was less pronounced with the ribbed tyre. The high-speed friction recorded with the ribbed tyre could be predicted using the same model as that developed for the smooth tyre, if the ribbed tyre is considered as providing "extra" texture depth, in addition to that provided by the road surface. This model applied to all the impermeable surfaces tested, but consistently under-estimated the performance of porous surfacings. High peak friction levels are maintained at all speeds for some surfaces, but the peak friction falls rapidly with increasing speed for surfaces with low texture depth using the smooth tyre. The ratio between locked-wheel and peak friction values was similar for many surfaces and falls from approximately 70% at 20 km/h to 50% at 110 km/h. This implies that for low water depths the capacity of the ribbed tyre to displace water can partly compensate for the low-texture of the road surface. Although further research is needed using real car tyres and physically realistic water depths. The ratio between the locked-wheel and peak friction measurements indicates that the potential benefit to cars fitted with ABS in maintaining higher friction levels increases at high speeds. However, on surfaces with inadequate texture depth, low friction levels can result. (A)