MAN Truck & Bus
How well braking forces are transferred depends on the traction and this in turn on the brake slip (sliding motion) between tyres and road. As slip increases, the usable traction also initially increases (stable range). As greater braking force is applied, the slip continues to increase and traction is reduced. This ultimately results in the wheels locking (unstable range). An ABS anti-lock braking system prevents the wheels from locking by reducing the braking force. Any vehicle equipped with ABS remains directionally stable and steerable even during emergency braking on slippery road surfaces, since the wheels do not lock (exception: Off-road ABS). On truck-trailers and articulated trucks, the ABS system also prevents jack-knifing of the trailer during emergency braking. ABS has been required by law since 1999 on articulated trucks and truck-trailers above 3.5 tons overall weight and on buses with more than eight passenger seats.
Anti-lock braking systems consist of speed sensors, an electronic control unit and pressure control valves. The braking pressure in each brake cylinder is reduced or held constant as necessary. The speed sensor determines the wheel's rotation speed, which the electronic control unit converts into vehicle speed (reference speed). Microprocessors in the control unit use the speed and reference speed to calculate the slip on each individual wheel. Wheel delay and wheel slip signals are then used to determine the tendency of the individual wheels to lock. The pressure control valves are activated and regulate the brake pressure in the individual brake cylinders. Two-, three- and four-axle ABS systems are available. depending on the commercial vehicle and the number of controlled axles. The best braking behaviour is achieved when the tractor and trailer/ semi-trailer have a separate ABS system.