812 Grip control

Factors influencing the ability of a tyre to grip the road when being braked are:

a) the vehicle speed, b) the amount of tyre wear, c) the nature of the road surface, d) the degree of surface wetness.

Vehicle speed (Fig. 8.4) Generally as the speed of the vehicle rises, the time permitted for tread to ground retardation is reduced so that the grip or coefficient of adhesive friction declines (Fig. 8.4).

Tyre wear (Fig. 8.5) As the tyre depth is reduced, the ability for the tread to drain off water being swept in front of the tread is reduced. Therefore with increased vehicle speed inadequate drainage will reduce the tyre grip when braking (Fig. 8.5).

Road surface wetness (Fig. 8.6) The reduction in tyre grip when braking from increased vehicle speed drops off at a much greater rate as the rainfall changes from light rain, producing a surface water depth of 1 mm, to a heavy rainstorm flooding the road to a water depth of about 2.5 mm (Fig. 8.6).

Road surface texture (Fig. 8.7) A new tyre braked from various speeds will generate a higher peak coefficient of adhesive friction with a smaller fall off at the higher speeds on wet rough surfaces compared to braking on wet smooth surfaces (Fig. 8.7).

Fig. 8.5 Effect of speed on relative tyre grip with various tread depth when braking on a wet road

Fig. 8.6 Effect of speed on relative tyre grip with various road surface water depths

Fig. 8.7 Effect of speed on the coefficient of adhesive friction with both wet rough and smooth surfaces

Fig. 8.5 Effect of speed on relative tyre grip with various tread depth when braking on a wet road

Fig. 8.7 Effect of speed on the coefficient of adhesive friction with both wet rough and smooth surfaces

The reduction in the coefficient of adhesive friction when braking with worn tyres on both rough and particularly smooth wet surfaces will be considerably greater.

8.1.3 Road surface texture (Fig. 8.8) A road surface finish may be classified by its texture which may be broadly divided in macrotexture,

Microtexture

Smooth Sharp Smooth Sharp or or or or polished harsh polished harsh

Smooth Sharp Smooth Sharp or or or or polished harsh polished harsh

Microtexture

Fig. 8.8 Terminology and road surface texture which represents the surface section peak to valley ripple or roughness, and microtexture which is a measure of the smoothness of the ripple contour (Fig. 8.8). Further subdivisions may be made; macrotexture may range from closed or fine going onto open or coarse whereas microtexture may range from smooth or polished extending to sharp or harsh.

For good tyre grip under dry and wet conditions the road must fulfil two requirements. Firstly, it must have an open macrotexture to permit water drainage. Secondly, it should have a microtexture which is harsh; the asperities of the texture ripples should consist of many sharp points that can pene trate any remaining film of water and so interact with the tread elements. If these conditions are fulfilled, a well designed tyre tread will provide grip not only under dry conditions but also in wet weather. A worn road surface may be caused by the hard chippings becoming embedded below the soft asphalt matrix or the microtexture of these chip-pings may become polished. In the case of concrete roads, the roughness of the brushed or mechanically ridged surface may become blunted and over smooth. To obtain high frictional grip over a wide speed range and during dry and wet conditions, it is essential that the microtexture is harsh so that pure rubber to road interaction takes place.

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