Braking force/slip

Diagram 1 shows the braking force as a function of slip.

Vertical axis = Braking force

Horizontal axis = Slip

The braking force is equivalent to the coefficient of adhesion or the friction between the tire and the road surface. Each application of braking force gives rise to a certain degree of slip. The slip of a freely rotating wheel is expressed as nil (0%) and of a locked wheel as 100%.

When the brake is first applied at zero slip, braking force increases sharply but the degree of slip increases only gradually up to a certain limit. Beyond that point, braking force decreases with increasing slip.

Information on the speed of each wheel is obtained from the wheel sensors. The ABS control module measures the reference speed, normally on the fastest rear wheel. The reference speed is then used to determine whether any wheel is exceeding the limit of optimum slip.

1. Lateral force
2. Stable
3. Unstable
4. Slip