Tyre Adhesion

Tyre Adhesion

The amount of the force applied on a shoe against a drum controls the resistance to rotation of a road wheel. Simultaneously the road surface has to drive the wheel around. This driving force attains its limit when the resistance offered by the brake equals the maximum frictional force generated between the tyre and road which is known as the adhesive force. This force can be determined from the expression :

Adhesive force = Load on wheel x Coefficient of friction

When the limit is reached, the wheel starts to skid, and any extra force on the brake shoe does not increase in the rate of slowing down the vehicle, no matter how good is the braking system. This means that the adhesion between the tyre and road is the governing factor for the minimum stopping distance.



Road adhesion depends on :

• Type of road surface.

• Conditions of surface e.g. wet, dry, icy, greasy, etc.

• Designs of tire tread, composition of tread material and depth of tread.

The stopping distance of a wheel is greatly affected by the interaction of the rotating tyre tread and the road surface. The relationship between the decelerating force and the vertical load on a wheel is known as the adhesion factor, \ia. This factor is very similar to the coefficient of friction, \i, that occurs when one surface slides over the other. In the ideal situation of braking, the wheel should always rotate right up to the point of stopping to obtain the greatest retarding resistance. Typical adhesion factors for various road surfaces are presented in Table 28.1.

Table 28.1. Adhesion factors for various road surfaces.

No.	Road Surface	Adhesion Factor
i—»	Concrete, coarse asphalt dry	0.8
2	Tarmac, gritted bitumen dry	0.6
3	Concrete, coarse asphalt wet	0.5
4	Tarmac wet	0.4
5	Gritted bitumen tarmac wet	0.3
6	Gritted bitumen tarmac greasy	0.25
7	Gritted bitumen, snow compressed dry	0.2
8	Gritted bitumen, snow compressed wet	0.15
9	Ice wet	0.1

It is a common thinking that the shortest stopping distance is achieved when the wheel is locked to produce a skid. This idea is incorrect because experiments have confirmed that the force required to 'unstick' a tyre is greater than the force required to skid it over the surface. A wheel held on the verge of skidding not only provides the shortest distance, but also allows the driver to maintain directional control of the vehicle.