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Dual-circuit Brake System

Waiting for the electric braking system?

As long as the completely electrical brake system is not on the market, motor cars will be equipped with hydraulic brakes. The pedal force, almost always reinforced by an assistant, is in this case, transfered, through the brake fluid to the wheel brake- cylinders. To better protect the hydraulic transfer against leakage, two brake circuits working independently of each other, are necessary.

Pedal-strength, directly or amplified, produces pressure

The pressure is generated completely, or for the most part, by the pedal force. This works mechanically on the first piston in the main brake cylinder, and hydraulically on the second one. The thus developing pressure is evenly distributed by metal pipelines and flexible hoses to the individual wheel brake-cylinders. In most vehicles, the engine delivers the power assistance through its inlet manifold pressure or through a pump powered pneumatically or hydraulically by the engine. Starting with the anti-lock-brake-system, there are a large number of electronic automatic control systems for the hydraulic brake.

Front/rear or diagonal

There are only two types of brake circuit layouts:
- Black-and-white distribution,
- Diagonal distribution.
The black-and-white distribution provides for one brake circuit for the front axle and one for the rear axle. The main disadvantage, is the relatively weak remaining braking power through the rear axle, in the event of the front axle brake-circuit being damaged. An advantage is, that the brake pressure in the rear circuit is maintained, even when overheating occurs in the front circuit.In diagonal distribution, the braking power is precisely and evenly supplied. However, with this system, a breaking out of the vehicle is possible, to the side on which the front brake is still applied. This is compensated, e.g., by the negative steering offset, which many manufacturers introduced together with the diagonal distribution.

There are no longer any other systems.

For a very short time, there were, e.g., in Volvo and Rolls Royce, also other systems which catered for the special safety needs of their customers. However, this made a dual-circuit system necessary for the front axle. Not only was a tandem main brake cylinder necessary, but also a double piston assembly for the front disc brakes. These vehicles can be recognised by the twin brake hoses which lead to each of two front wheels. Another example is the first Mercedes G model. With the comprehensive introduction of ABS, these specific features have disappeared, (also in Rolls Royce) because they would have meant, in this case, distinctly higher costs. 02/08

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