Most modern vehicles use disc brakes as part of the braking system. Some models only have them on the front while others use them on all four wheels. They provide the stopping action for the vehicle and must be replaced when they are no longer working correctly for the safety of the driver and passengers. Disc brakes provide greater stopping force without overheating than drum brakes. Disc brakes are seen on all four wheels in high-performance or high-end vehicles, while the majority of vehicles use disc brakes on the front and drum brakes on the rear. This works because the front brakes provide the majority of the stopping power for the vehicle, while the rear brakes only assist.
Parts That Make Up the Disc Brakes System
Rotor: round disc that is able to turn as the wheel turns.
Caliper: fits on the disc and includes hydraulic pistons that attach to the master cylinder. It may be a single piston floating caliper or a four piston fixed caliper. The floating caliper moves along in a track or “floats” to center over the rotor. While the piston pushes against the inner pad on this design, the caliper is pushing against the outer pad in the opposite direction. Floating calipers may also be used with two pistons and are often seen on more expensive vehicles for a smoother braking experience. The second option, four piston fixed calipers, are mounted to the support and do not move. With this design, the pistons push against the pads and press them to the rotor. They provide a smooth feel and are highly efficient. They are most often seen on luxury vehicles or high performance models.
Pistons: a disc brake may have only one pair of pistons or it may have multiple pairs to improve the stopping time of a vehicle. A single piston may even operate both pads with a scissor-like design.
Friction pads: are clamped on both sides of the disc and cover much of the disc. Also called brake pads, they feature a metal portion called a shoe and a lining that is attached to the shoe. Linings may be made of different materials and fall into three categories: organic, semi-metallic and ceramic. The material chosen will impact the length of brake life and the amount of noise heard when the brakes are applied. Manufacturers generally keep the exact composition of the lining a secret, but up to 20 different compounds can be used.
Rubber sealing rings: surround the pistons to allow them to move forward as the brake pads wear down to prevent adjustment.
Sensors: Newer models of vehicles often contain sensors that have been embedded into the brake pads. The sensors short circuit when the pads have been worn down too low, which causes a warning light to come on from the instrument panel.
How the Parts Function
The driver puts his foot on the brake pedals inside the car and fluid pressure is sent to the master cylinder and the hydraulic pistons. This forces the pistons to apply pressure to the friction pads against the discs. This motion is only a small distance and the pads barely move away from the disc once the brakes are released. The force used to press the friction pads to the disc may either slow the vehicle or cause it to come to a full stop. It only takes a fraction of a second for the brakes to engage once the brake pedal has been pressed.
Wear and Tear on Disc Brakes
Over time, the friction pads will wear down and become thin. They will create grooves in the disc or rotor that matches the wear pattern on the pads. When the brake pads are replaced, the rotors are sanded to remove the grooves and ridges. This ensures that the new pads fit correctly against the discs. If the pads are not replaced soon enough, the rotors may also need to be replaced.
When you hear a noise that is only audible when the brake is applied, it may mean that the brake pads have worn to the point of needing replacement. Some new vehicles will provide an alert that it is time to replace the pads before they cause too much damage to the rotor. Brakes should be inspected on a regular basis to ensure they are working correctly and no parts need to be replaced.
Symptoms of Problems with Disc Brakes
Squealing when the brake is applied: may be caused by improper installation of brake pads or the wrong mounting hardware, may also be caused by weather or even the way the brake is designed
Sponginess in applying the brakes: may be caused by loose wheel bearings
Change in the way brakes feel when applied: may require brake bleeding to remove air from the system
This article originally appeared on YourMechanic.com as How Disc Brakes Work and was authored by Joyce Morse.