Brake Fade - Fade in Self-assisting Brakes

Fade in Self-assisting Brakes

Various brake designs such as band brakes and many drum brakes are self-assisting: when the brake is applied, some of the braking force feeds back in to the brake mechanism to further self-apply the brake. This is called "positive feedback" or "self-servo". Self-assist reduces the input force needed to apply the brake, but exaggerates fade, since a reduction in pad friction also reduces pad force. In contrast, for a brake without self-assist, such as a conventional disc brake, a loss of pad friction does not change the pad force, so the loss of braking is proportional to just the loss of pad friction.

The self-assist mechanism affects the amount of fade. For example, the Ausco Lambert and Murphy brakes have self-assist roughly proportional to pad friction, so total braking is reduced as roughly the square of the loss in friction. Many other self-assist designs, such as band brakes and many common drum brakes, have exponential self-assist, described by, where is the natural logarithm base, is the coefficient of friction between shoes and drum, and is the angle of engagement between shoes and drum. A small change in friction causes an exponential change in self assist. In many common brakes, a slight increase in friction can lead to wheel lockup with even light application. For example, on damp mornings, drum brakes can lock on first application, skidding to a stop even after the brake pedal has been released. Conversely, a slight decrease in friction can lead to severe brake fade.