General System Description

Description and Operation

Anti-lock Braking System has been developed to provide optimum braking with no loss of directional stability, under widely varying conditions. The stopping distance of a car is influenced by a variety of factors, including weather conditions, road surface, traffic conditions and the amount brake pressure applied.

The ABS braking system provides up-to-date braking systems with control functions that enable the maximum braking effect to be applied in critical situations, regardless of the road conditions.

All models are equipped with the ABS braking system.
The hydraulic brake system is divided into two circuits. One circuit comprises of the front right wheel and rear left wheel, while the other comprises of the front left wheel and the rear right wheel. The main advantages of the ABS system are:
- No loss of directional stability on braking.
- Steering control retained even during heavy braking.
- Shortage possible braking distance.
- Reduced tire wear.




The curve in the diagram shows the relationship between braking force (expressed as a coefficient) and tire slip (expressed as a percentage).
The braking force is equivalent to the coefficient of adhesion, that is to say; the friction between the tire and the road surface. Each application of braking force gives rise to a certain degree of slip, ranging from 0% when the wheel rolls freely to 100% when the wheel is locked.

When the brake is first applied at 0, the braking force increases sharply, but the degree of slip increases only gradually, up to a certain limit. Beyond that point, the braking force decreases with increasing slip.
The maximum braking force (coefficient of adhesion) is reached at a point known as the limit of optimum slip.

Maximum Braking Force




The section of the curve between 0% slip and the limit of optimum slip is known as the stable braking zone. The section of the curve between the limit of optimum slip and 100% slip is know as the unstable braking zone, as stable braking can not be achieved in 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 lateral force is applied on the tire (i.e. on cornering).

Curve II on the diagram shows lateral force as a function of slip. As can be seen, lateral force falls away sharply with increasing slip. At 100% slip, that is to say; when the wheels lock up, no lateral force remains for steering and the driver will no longer be able to control his vehicle.




Both curves are shown on this next chart, that also shows the range within which the ABS system is operative. During braking, the system allows the braking force to increase to a point just before the limit of optimum slip and then prevents it from increasing further. The system modulates the hydraulic pressure to keep the braking force as close as possible to the optimum value (the limit of optimum slip) regardless of the pressure applied to the brake pedal.

Because the ABS system prevents a degree of slip from exceeding the optimum slip, the car never enters the unstable zone. At the same time, some lateral force is preserved to ensure that steering control is retained.