Good road texture essential
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• Tyre tread depth generally has only a small effect on braking force coefficient in wet weather if the road surface texture provides adequate drainage paths. according to a new report by the Road Research Laboratory, although in some instances a tread depth of below lram has a serious effect on the coefficient. On smoother surfaces tyre wear has a more pronounced effect on braking force. But at higher speeds on smooth surfaces a full-depth tread gives very little improvement in locked-wheel coefficients over a smooth tyre.
Tests were carried out by the RRL on six different wetted surfaces covering a range of textures comparable with those of normal highways. These comprised smooth concrete, mastic asphalt, fine textured asphalt, mixed aggregate macadam, Bridport gravel macadam and asphalt to motorway specification. For the test a Jaguar F-type car was used, the braking system of which had been modified to enable front wheel brakes to be applied alone at a controlled rate until the wheels locked. The tyres were of cross-ply construction and braking force measure ments were made initially with tyres having a full tread depth of 8.4mm and were repeated until the tread pattern had completely disappeared. Peak and locked-wheel coefficients were measured as a function of speed and tread depth over the speed range. 50 km /h (30 mph) to 130 km /h (80 mph).
On fine textured asphalt the peak braking force co-efficient was high and not affected by variations in tread depth to any extent. At 130 km /h the effect of tread depth at peak braking force was, however, very pronounced, the braking effectiveness of a fully worn tyre being reduced by about 80 per cent.
In the tests on the smooth polished surface of mastic asphalt the peak braking force coefficient was high and fairly constant at 50 km /h for tread depths in excess of 2mm but fell off very rapidly with treads of lesser depth. The decrease became less pronounced with increasing speeds up to 130 km /h. On smooth concrete the braking characteristics were similar to those obtained on mastic asphalt.
On the rough and harsh surface of asphalt to motorway specification fairly high coefficients were obtained even at higher speeds, although at 130 km /h there was a marked decrease in coefficient at tread depths below I.5mm. On the rough but polished surface of Bridport gravel coefficients were little affected by either speed or tread depth but the locked wheel coefficient was relatively low. The rough surface on mixed aggregate provided similar characteristics to those produced by Bridport gravel except that the coefficients tended to be higher.