81 Tractive and braking properties of tyres 811 Tyre grip

Tyres are made to grip the road surface when the vehicle is being steered, accelerated, braked and/or negotiating a corner and so the ability to control the tyre to ground interaction is of fundamental importance. Road grip or friction is a property which resists the sliding of the tyre over the road surface due to a retardant force generated at the tyre to ground contact area. The grip of different tyres sliding over various road surface finishes may be compared by determining the coefficient of friction for each pair of rubbing surfaces.

The coefficient of friction may be defined as the ratio of the sliding force necessary to steadily move a solid body over a horizontal surface to the normal reaction supporting the weight of the body on the surface (Fig. 8.1).

Frictional force i.e. Coefficient of friction (u) = —---:—

Normal reaction

where u = coefficient of friction F = frictional force (N) W = normal reaction (weight of body) (N)

Strictly speaking, the coefficient of friction does not take into account the surface area tread pattern which maximizes the interlocking mechanism

Rubber block

Rubber block

Fig. 8.1 Sliding block and board f

Fig. 8.1 Sliding block and board

Relative movement

Fig. 8.2 Variation of friction with relative movement

Relative movement

Fig. 8.2 Variation of friction with relative movement between the flexible tread elements and road. Therefore when dealing with tyres it is usual to refer to the coefficient of adhesive friction. The maximum coefficient of adhesive friction created between a sliding tread block and a solid surface occurs under conditions of slow movement or creep (Fig. 8.2). This critical stage is known as the peak coefficient, up, and if the relative movement of the rubber on the surface is increased beyond this point the friction coefficient falls. It continues to fall until bodily sliding occurs, this stage being known as the sliding coefficient us. Sliding friction characteristics are consistent with the behaviour of rolling tyres.

A modern compound rubber tyre will develop a higher coefficient of friction than natural rubber. In both cases their values decrease as the road surface changes from dry to a wet condition. The rate of fall in the coefficient of friction is far greater with a worn tyre tread as opposed to a new tyre as the degree of road surface wetness increases (Fig. 8.3).

It has been found that the frictional grip of a bald tyre tread on a rough dry road surface is as good or even better than that achieved with a new tread (Fig. 8.3). The reason for this unexpected result is due to the greater amount of rubber interaction with the ground surface for a given size of contact patch. It therefore develops a larger reaction force which

Dry Wet

Surface conti ¡lion

Fig. 8.3 Effect of surface condition on the coefficient of adhesive friction with natural and synthetic rubber using new and bald tyre treads

Dry Wet

Surface conti ¡lion

Fig. 8.3 Effect of surface condition on the coefficient of adhesive friction with natural and synthetic rubber using new and bald tyre treads opposes the movement of the tyre. Under ideal road conditions and the amount of deformable rubber actually in contact with the road maximized for a given contact path area, the retarding force which can be generated between the tyre and ground can equal the vertical load the wheel supports. In other words, the coefficient of adhesive friction can reach a value of 1.0. However, any deterioration in surface roughness due to surface ridges being worn, or chip-pings becoming submerged in asphalt, or the slightest amount of wetness completely changes the situation. A smooth bald tyre will not be able to grip the contour of the road, whereas the tyre with a good tread pattern will easily cope and maintain a relatively high value of retardant force.

When transmitting tractive or braking forces, the tyre is operating with slip or creep. It is believed that the maximum friction is developed when a maximum number of individual tread elements are creeping at or near an optimum speed relative to the ground. The distribution by each element of the tread is not equal nor is it uniform throughout the contact patch. The frictional forces developed depend upon the pressure distribution within the contact patch area and the creep effects. Once bodily slippage begins to occur in one region of the contact area, the progression to the fully sliding condition of the contact area as a whole is extremely rapid.

Under locked wheel conditions, the relative sliding speed between a tyre tread and the road is the speed of the vehicle. If, however, the braking is such that the wheels are still rotating, the actual speed between the tyre tread and the road must be less than that of the vehicle. Even on surfaces giving good braking when wet, maximum coefficients occur at around 10-20% slip. This means that the actual speed between the tyre tread and the road is around one eighth of the vehicle speed or less. Under these conditions, it is possible to visualize that the high initial peak value occurs because the actual tyre ground relative speed relates to a locked wheel condition at a very low vehicle speed (Fig. 8.3).

The ability to utilize initial peak retardation under controlled conditions is a real practical asset to vehicle retardation and, because the tyre is still rolling, to vehicle directional control.

Braking effectiveness can therefore be controlled and improved if the wheels are prevented from completely locking in contrast to the wheels actually being locked when the brakes are applied. Thus when braking from different speeds (Fig. 8.4) it can be seen that the unlocked wheels produce a higher peak coefficient of adhesive friction as opposed to the locked condition which generates only a sliding coefficient of adhesive friction. In both situations the coefficient of adhesive friction decreases as the speed from which braking first commences increases.

Tread on wet concrete

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1 1

Speed Ifcm/h}

AO 60 BO 100

Speed Ifcm/h}

Fig. 8.4 Effect of speed on both peak and sliding coefficient of adhesive friction

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