disc brake

A disc brake converts hydraulic or mechanical pressure into friction by squeezing brake pads against a rotating disc, called a rotor. The rotor is bolted to the wheel hub and turns with the wheel. When you apply the brake, calipers on either side of the rotor push friction material, usually organic or semi-metallic compounds, directly onto the rotor surface. This direct contact stops the wheel more effectively than the older drum brake design.

Rotors come in different configurations. Solid rotors are simplest, used on many road cars where weight and cost matter. Vented rotors have internal cooling fins that allow air to pass through the disc, dissipating heat faster; these appear on heavier vehicles and performance cars. Drilled or slotted rotors further improve cooling and gas evacuation by creating channels in the rotor face. Carbon-ceramic rotors, bonded layers of carbon and ceramic matrix, withstand extreme temperatures and offer longer service life in racing and high-performance applications, though they cost significantly more and require special pad compounds.

Disc brakes excel at heat rejection because both sides of the rotor are exposed to air. This matters during extended braking or on mountain descents where brake fluid can overheat in drum systems, causing fade. Disc systems also resist water ingress better than drums; light rain does not impair braking. However, disc brakes require precision manufacturing and generate more brake dust from pad wear, creating maintenance issues on vehicles where dust accumulation is undesirable.

Brake pads are the sacrificial component. Manufacturers bond friction material to a steel backing plate using resin or sintered metal. Organic pads, made from carbon, rubber, and resin, offer smooth initial bite and low noise but wear faster and perform poorly when hot. Semi-metallic pads contain iron, copper, or graphite mixed into the friction material; they tolerate heat better and last longer but can squeal and produce more dust. Sintered pads, forged from compressed metal powder, resist extreme temperatures and suit off-road and heavy-duty use but demand stiff rotors to avoid cracking.

The caliper, which houses the pads and applies pressure, can be floating, fixed, or sliding. Fixed calipers bolt rigidly to the suspension and contain pistons on both sides of the rotor for even pressure distribution. Floating calipers move slightly during braking, relying on a single piston on one side and mechanical balance to pull the other pad in. Floating designs cost less and tolerate rotor runout better, making them standard on many road vehicles. Wear indicators, small metal tabs bonded to pad backing plates, produce a high-pitched squeal when pads thin to a warning thickness, though not all pads include them.

Service intervals depend on driving style, rotor material, and pad compound. Typical organic pads last 25,000 to 70,000 miles depending on vehicle mass and braking frequency. Rotors usually last two to three pad replacements before thickness limits force replacement. Uneven wear, uneven braking force between wheels, or pulsation under braking signals caliper problems, stuck pistons, or rotor damage. Moisture trapped inside calipers over time promotes corrosion of piston seals and pistons themselves.