Editing Brake fade (section)

==Fade in self-assisting brakes==
[[Image:AirBrake.gif|thumb|right|Truck air brakes can fade due to expansion if improperly adjusted.]]

Various brake designs such as [[band brake]]s and many [[drum brake]]s are self-assisting: when the brake is applied, some of the braking force feeds back into the brake mechanism to further self-apply the brake. This is called "positive feedback" or "self-[[Servomechanism|servo]]". Self-assist reduces the input force needed to apply the brake, but exaggerates fade, since a reduction in pad friction material height or thickness also reduces pad force. In contrast, for a brake without self-assist, such as a conventional disc brake, a loss of pad friction material does not change the pad force, so there is no necessary loss in the brake torque reaction for a given amount of input force.

The self-assist mechanism affects the water pump and the amount of fade. For example, the [[Ausco Lambert disc brake|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 brake]]s and many common [[drum brake]]s, have exponential self-assist, described by <math>e^{\mu \theta}</math>, where <math>e</math> is the natural logarithm base, <math>\mu</math> is the [[coefficient of friction]] between shoes and drum, and <math>\theta</math> 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.