Brake calipers are essential to your car's ability to stop and are arguably one of the most important automobile brake parts. Most cars today have disc brakes, at least for the front wheels, anyway. But a lot of cars and trucks are now using disc brakes in the rear, too. In a disc-braking system the car's wheels are attached to metal discs, or rotors, that spin along with the wheels. The job of the caliper is to slow the car's wheels by creating friction with the rotors.
The brake caliper fits over the rotor like a clamp. Inside each caliper is
a pair of metal plates bonded with friction material -- these are called brake pads. The outboard brake pads are on the outside of the rotors (toward the curb) and the inboard brake pads on the inside (toward the vehicle). When you step on the brake, brake fluid from the master cylinder creates hydraulic pressure on one or more pistons in the brake caliper, forcing the pads against the rotor. The brake pads have high-friction surfaces and serve to slow the rotor down or even bring it to a complete halt. When the rotor slows or stops, so does the wheel, because they're attached to one another.
Older cars and trucks used drum brakes, where the motion of the wheels is slowed by friction between a rotating drum and brake shoes mounted inside the drum. This friction caused heat and gases to build up inside the drum, which often resulted in a loss of braking power known as brake fade. Because the brake pads in disc brake systems are external to the disc rather than contained within a drum, they are more easily ventilated and heat doesn't tend to build up quite as fast. For this reason, drum brakes have been largely replaced in modern vehicles by disc brakes; however, some less expensive cars still use drum brakes for the rear wheels, where less stopping power is required.
There are two main types of calipers: floating (or sliding) calipers and fixed calipers. Floating calipers move in and out relative to the rotor and have one or two pistons only on the inboard side of the rotor. This piston pushes the entire caliper when the brakes are applied, creating friction from the brake pads on both sides of the rotor. Fixed calipers, as the name implies, don't move, but rather have pistons arranged on opposing sides of the rotor. Fixed calipers are generally preferred for their performance, but are more expensive than the floating kind. Some high-performance fixed calipers have two or more pairs of pistons (or "pots") arranged on each side of the rotor -- some have as many as six pairs total.
Special tools are useful when working with brake calipers, especially when replacing the brake pads. We'll talk about that in the next section, and then discuss the different types of brake calipers available for different types of vehicles.