Mechanical Fade is most commonly associated with drum brakes and not disc brakes. In a drum brake, the application of the lining is outward toward the rotating drum's friction surface. As the brake drum heats up, it expands outward. This expansion will increase the drum's diameter, moving it away from the lining application. The expansion of the brake drum causes a need for increased lining travel and increased travel of the application device. If expansion is great enough, it can cause the application device to bottom out and the brakes to fail. A disc brake lining application is at a right angle to the rotating disc and expansion of the disc is outward toward the application rather than away from it. For this reason disc brakes have better fade resistance.
Brake drum expansion is probably the most concerning aspect of brake fade in S-cam braked heavy trucks. In an S-cam brake, the application device is the brake chamber. A typical brake chamber has a maximum pushrod stroke capability of about 2.5 inches. Cold pushrod stroke measurements on a truck with Automatic Brake Adjusters (ABA's) are commonly found to be around 1.75 inches. So, a typical brake chamber will have a reserve stroke of around 0.75 inches. As a brake drum expands away from the brake linings, the cold pushrod stroke will increase. A brake drum temperature of 400 degrees can increase pushrod stroke by as much as 0.5 inches. The force output of a brake chamber is not linear, and a typical brake chamber reaches its knee point where force will begin to drop at about 1.75 inches. If the brake drum becomes hot enough, the brake chamber can bottom out and the brake chamber force output at that point will be 0.
Drum brakes are also very susceptible to brake fade because they are self-energizing. In a self-energizing brake, when the lining is applied to the drum, the drum rotation tries to pull the lining and shoe along with it. This self-energizing action increases the force applied to the lining friction surface. When fade reduces friction at the friction surface in a self-energizing brake, a compounded reduction in applied force at the friction surface results in a compounded reduction in braking. The two common types of self-energizing drum brakes used in automotive applications are the Leading-Trailing drum brake and Duo-Servo drum brake. The Leading-Trailing drum brake has a fixed anchor on one side of the shoes. The Duo-Servo drum brake has "floating" anchorage. In a Leading-Trailing brake, only the leading shoe is self-energized. In a Duo-Servo brake, both shoes are self-energized. Therefore, a Duo-Servo drum brake can have a greater reduction in applied force at the friction surface than a Leading-Trailing drum brake and is more susceptible to fade.
Brake drum expansion is probably the most concerning aspect of brake fade in S-cam braked heavy trucks. In an S-cam brake, the application device is the brake chamber. A typical brake chamber has a maximum pushrod stroke capability of about 2.5 inches. Cold pushrod stroke measurements on a truck with Automatic Brake Adjusters (ABA's) are commonly found to be around 1.75 inches. So, a typical brake chamber will have a reserve stroke of around 0.75 inches. As a brake drum expands away from the brake linings, the cold pushrod stroke will increase. A brake drum temperature of 400 degrees can increase pushrod stroke by as much as 0.5 inches. The force output of a brake chamber is not linear, and a typical brake chamber reaches its knee point where force will begin to drop at about 1.75 inches. If the brake drum becomes hot enough, the brake chamber can bottom out and the brake chamber force output at that point will be 0.
Drum brakes are also very susceptible to brake fade because they are self-energizing. In a self-energizing brake, when the lining is applied to the drum, the drum rotation tries to pull the lining and shoe along with it. This self-energizing action increases the force applied to the lining friction surface. When fade reduces friction at the friction surface in a self-energizing brake, a compounded reduction in applied force at the friction surface results in a compounded reduction in braking. The two common types of self-energizing drum brakes used in automotive applications are the Leading-Trailing drum brake and Duo-Servo drum brake. The Leading-Trailing drum brake has a fixed anchor on one side of the shoes. The Duo-Servo drum brake has "floating" anchorage. In a Leading-Trailing brake, only the leading shoe is self-energized. In a Duo-Servo brake, both shoes are self-energized. Therefore, a Duo-Servo drum brake can have a greater reduction in applied force at the friction surface than a Leading-Trailing drum brake and is more susceptible to fade.