Fixed and sliding/floating calipers.
In most applications, the disc brake caliper assembly is bolted to the vehicle axle housing, or steering knuckle, and clamps the brake pads onto the rotors to slow the vehicle. There are two main types of calipers: fixed calipers and sliding or floating calipers FIGURE 32-7. Sliding or floating calipers are the most common type used in passenger vehicles because they are easier to build and are more compact. All calipers are fitted with a bleeder screw on the top of the piston bore to allow for the removal of air within the disc brake system as well as to help in performing routine brake fluid changes.
Fixed calipers are rigidly bolted in place and cannot move or slide. This makes their application of braking forces more precise than floating calipers. They commonly have one to four pistons on each side of the rotor FIGURE 32-8. When the brakes are applied, hydraulic pressure forces the pistons on both sides of the caliper inward, causing the brake pads to come in contact with the rotor FIGURE 32-9. Once the pad-to-rotor clearance is taken up, the hydraulic pressure rises equally on each side of the caliper, applying both brake pads equally.
The sliding or floating caliper has brake pads located on each side of the rotor, but all of the pistons are only on one side, usually the inside of the rotor. Thus, the hydraulic force is generated on one side of the rotor, but the unique design applies equal braking force to both sides of the rotor. This distribution of force is possible because the caliper is mounted on pins, or slides, that allow it to move (float or slide) from side to side as necessary. This movement allows pistons on one side of the rotor to generate force on both sides of the rotor at the same time.
Fixed calipers with multiple pistons.
Fixed caliper being applied.
Sliding/floating caliper application.
When the brakes are applied, hydraulic pressure forces the piston toward the rotor. This takes up any clearance between the brake pad and rotor and starts to push the pad into the rotor. Since the caliper is free to move on the pins or slides, it gets pushed away from the rotor, pulling the outer brake pad into contact with the outside of the rotor. Once all clearance is taken up on the outer brake pad, the clamping force will increase equally on both brake pads, applying the brakes FIGURE 32-10.