By learning the parts and functions of an automatic braking system, car owners are better prepared to deal with brake problems. Gaining a better understanding of brake functions also helps the owner better explain problems to mechanics. A typical auto brake system has disc brakes in the front and either disc brakes or drum brakes in the rear. Each wheel has brake lines linked to the master cylinder. The master cylinder is the brain of the system, and it tells the brakes to stop by pushing hydraulic fluid through the system to the independent brakes at each wheel.
Brake calipers house the brake piston and the brake pads. The master cylinder delivers fluid to the brake calipers, and the fluid pushes on the piston. The piston presses against the pads, and the brake pads press against the rotor. The friction between the pads and the rotor stops the car. Calipers need periodic maintenance, because the boot on the piston can tear, the piston can wear out, and the brake line to the caliper can develop cracks or leaks.
The stopping power of disc brake systems relies on friction. The brake pads assist in providing the friction in a brake system by rubbing against a steel rotor. The brake pads are the first components in the system to show wear. Pads are classified by two types of friction. Abrasive friction involves an organic pad, which breaks the molecular bond in the steel rotor. Organic pads wear out faster and do not resist high temperature glazing. On the other hand, adherent friction depends on a transfer film being deposited on the surface of the rotor. This is done by semi-metallic pads. The friction comes from breaking the molecular bond of the two like materials on the rotor surface. These types of pads wear more slowly and resist high temperatures better. High-performance pads have a semi-metallic base with tiny amounts of organic material and are combined with other materials such as carbon fiber to form more efficient pads.
The brake rotors are attached firmly to the wheel hubs. Brake pads rub against the rotors, and the friction between the rotor and the pad stops the car. Rotors do not need replacing as often as the pads. Rotors vary, but their thickness is measured during inspection, and mechanics determine whether replacement is necessary. Rotors can also be turned, a process that involves grinding the rotor to remove deep grooves and scratches. Deep grooves cause the pads to wear unevenly and more quickly. A brake rotor is composed of various metals, including carbide-forming metals such as vanadium and titanium, iron and even copper. For some rotors, a measurement of just 0.10 inches can require replacement of that rotor.