What is brake fade? I read the term a lot in new-vehicle reviews, particularly those of high-performance cars. – Fred
The friction between a brake disc or drum and a brake pad is used to slow a vehicle.
By applying the brakes you are essentially converting the motion of the vehicle into heat. In hybrid cars, this energy is converted into electricity to recharge the batteries.
In a conventional car, it is dissipated into the air as it passes over the brake system. If that heat is not removed or more is created than can be removed, the brake pad material, commonly composed of various materials glued together into a solid piece, will reach a point where the glue holding it together starts to boil or smoke, and the resulting gases act as a lubricant between the two braking surfaces and braking effectiveness is lost.
When you apply the brake pedal you cause pressure to be exerted upon the brake system. In a disc brake system, the pads are squeezed against a rotating disc (rotor) attached directly to the wheel. If you have ever driven a bicycle with hand brakes, you have watched this very action as the brake pads pushed against the rims wheel when you squeezed the brake levers. In the case of a drum brake, the brake pads are pushed out against the inside of a "drum," which is attached to the wheel.
In either case, fade occurs when there is a loss of friction between the brake material and the surface of the rotor or drum.
Brake fade commonly occurs during sustained or frequently repeated heavy applications of the brakes. You might have noticed this in reviews of high-performance vehicles because they are capable of, and often tested at higher speeds, meaning more braking is necessary to bring them to rest.
But fade can, and does occur, in everyday vehicles, usually when towing is involved. This occurs because of the extra weight of the object being towed, often more total weight than the brake system was designed for. It can also occur in vehicles with worn or poorly adjusted brakes descending long hills.