As fading (English: to fade = dwindle) or brake fade is an undesirable decrease in the braking action of a mechanical braking system designated by heating of the brake. The coefficient of friction on the brake decreases with increasing temperature. In this case, the braking distance can increase, which is critical to safety, after repeated or prolonged braking . It occurs in everyday life, especially on long and steep downhill runs.
Drum brakes that have an internal servo effect (self-boosting) are particularly sensitive to fading. Fading is less with disc brakes. They have no self-reinforcement and the heat generated when braking is better dissipated.
Partial fading also occurs after installing new brake pads. Since these have not yet adapted (ground in) to the grooves of the brake drum or the brake disc, local overheating can occur in the event of full braking . This is where fading occurs.
The formation of water vapor bubbles in a hydraulic brake system, the brake fluid of which has "aged" due to water absorption, is sometimes incorrectly referred to as fading. The braking effect does not diminish gradually, but the brake lever or brake pedal travel becomes longer and finally the brake fails completely. This technical defect can only occur if braking is interrupted, because only then can water in the brake fluid evaporate due to the pressure relief and the resulting increase in volume push some of the brake fluid back into the brake fluid reservoir.
- Because of their susceptibility to fading, drum brakes are rarely used in motorcycles, larger passenger cars and small vans despite their low maintenance requirements. In small cars, on the other hand, drum brakes are often installed with weaker engines and due to the low vehicle weight.
- The brake is cooled by a supply of air.
- Disc brakes can be equipped with internally ventilated brake discs . Perforated brake discs are common for bicycles.
- Especially on bicycles with disc brakes, the risk of fading for high-temperature-stable brake pads can be reduced, but also with larger brake discs.
- In heavy motor vehicles, a permanent brake , for example the engine brake , an eddy current brake or a retarder , is used for longer braking operations .
- Ceramic brakes are less prone to fading, as the coefficient of friction of the components involved (disc, brake pads) does not drop as sharply with increasing temperature.
- Continuous braking is to be avoided as far as possible. When driving downhill give ride breaks the brakes time to cool in between.
- During longer downhill journeys in motor vehicles, the engine brake is used to reduce the speed by shifting into a lower gear in order to relieve the brakes.
- When driving downhill for longer periods, high speeds are avoided by braking in good time. The required braking power decreases with lower speed.