Function

Function

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 tyre 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

Lateral force/Slip

Diagram 2 shows lateral force as a function of slip.

Vertical axis = Lateral force

Horizontal axis = Slip

The maximum braking force is reached at a point known as the limit of optimum slip. The section of the curve between zero slip and the limit of optimum slip is called the stable braking zone, and the section of curve between the limit of optimum slip and 100% slip is called the unstable braking zone, as stable braking cannot be achieved within this zone. This is because the wheel quickly becomes locked after the limit of optimum slip has been reached, unless the braking force is immediately reduced.

Slip also occurs when the tyre is called upon to transmit a lateral force, e.g. on cornering. Curve 2 shows how the lateral force falls away sharply with increasing slip. At 100% slip, i.e. when the wheels have locked up, no lateral force remains for steering and the driver will no longer be able to control the vehicle.

The ABS control module measures the speed of each rear wheel and compares the speeds to determine whether the car is being driven straight ahead or cornering.

1.	Braking/lateral force

2.

Stable

3.

Unstable

4.	ABS operating range

5.	Limit of optimum slip

6.

Slip

Both curves are shown on the above graph, and the range within which the ABS system is operative has been superimposed. During braking, the system allows braking force to increase to a point just before the limit of optimum slip and then prevents it from increasing further. The system then modulates the hydraulic pressure to keep the braking force as close as possible to the optimum value (the limit of optimum slip) regardless of how hard the brake pedal is depressed.

Thus, because the ABS system prevents the degree of slip from exceeding the limit of optimum slip, the car never enters the unstable braking zone. At the same time, some lateral force is preserved to ensure that steering control can be maintained (curve II).

Depending on the friction of the road surface and the speed of the car, the ABS control module regulates the permissible degree of slip between 7 and 18%.