General

Description
The basis of a braking action is the friction between tires and road surface. To brake a vehicle, the tires have to transmit a frictional force to the road surface. In doing so, there will be slip between the tires and road surface, i.e. the tire peripheral velocity will be slower than the road speed of the vehicle.

The figure shows the relation between the frictional force and slip for a typical case on a dry road surface. The transmitted braking force reaches its maximum value in shaded area A. This is also the regulating range for the ABS system. When a wheel is locked, i.e. at 100 the brake pressure in the wheel brake cylinder should be just sufficient to ensure that slip remains within the cross-hatched are because this is where the highest level of friction can be obtained.

At the same time there will then be a sufficient lateral cornering force to guarantee steering and directional stability.

The amount of slip with which there is maximum frictional force, and the amount of frictional force itself depend chiefly on the tires (material, surface condition, wet conditions, ice), the road speed an the slip angle of the wheels.

ABS detects the frictional conditions between the tires and road surface currently prevailing. It automatically adjusts to each change in road surface/tire relation and makes use of the highest coefficient of friction without allowing the vehicle to slip or skid due to wheel lock.

Non-uniform friction conditions between separate wheels will be held under control as will instantaneous changes in road-surface condition, e.g. through patches of ice.

Braking And Lateral Forces In Relation To Slip