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General

General


The car is equipped with two independent brake systems:


• Hydraulic brake system with Bosch ABS 5.3

• Handbrake system

ABS brakes (Anti-lock Braking System) have 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 brake pressure applied.

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

All Saab 900 models are equipped with the ABS braking system.

The footbrake system is divided into two circuits. One circuit comprises the front right wheel and rear left wheel, while the other comprises the front left wheel and 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.

• Shortest possible braking distance.

• Reduced tyre wear


The forces acting on the tyre consist entirely of braking and/or lateral forces.


1. Direction of travel

2. Braking force

3. Lateral force


To understand how the ABS system provides optimal braking with no loss of directional stability, we need to examine the play of forces acting on a wheel when it is braked.



1. Braking force

2. Stable braking zone

3. Instable braking zone

4. Slip


The curve in the diagram shows the relationship between braking force (expressed as a coefficient) and tyre slip (expressed as a percentage).

Braking force is equivalent to the coefficient of adhesion, that is to say the friction between the tyre 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, 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.



1. Lateral force

2. Stable braking zone

3. Instable braking zone

4. Slip


Maximum braking force (coefficient of adhesion) is reached at a point known as the limit of optimum slip.

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 tyre (e.g. 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 have control over the vehicle.



1. Braking/lateral force

2. Stable braking zone

3. Instable braking zone

4. ABS operating range

5. Limit of optimum slip

6. Slip


Both curves are shown on the above graph which is superimposed with the range within which the ABS system is operative. 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.

Since the ABS prevents the limit of optimum slip from being exceeded, the tyres never enter the unstable zone. At the same time, some lateral force is preserved to ensure that steering control is retained (curve II).