How Disc Brakes Slow a Car: Pads, Rotors and Calipers

A disc brake slows a car by clamping friction pads against a metal disc, called a rotor, that spins with the wheel. The friction turns the car's energy of motion into heat, which the rotor then gives off to the air. Pressing the pedal sends hydraulic fluid pressure to each wheel to do the clamping. Below is each part in order, plus how discs compare with older drum brakes.

From pedal to wheel

When the driver presses the pedal, a piston in the master cylinder builds pressure in the brake fluid. Fluid is almost impossible to compress, so that pressure travels along the brake lines to a caliper at each wheel. EduMech, an engineering education site, describes the caliper pistons as converting hydraulic pressure into mechanical force. Wikipedia adds that hydraulic actuation is the most common system on motor vehicles and that modern cars split the circuits between axles as a safety measure.

Pads, rotor and caliper

• Rotor: a flat, circular disc bolted to the wheel hub, usually made of grey cast iron
• Pads: friction-lined blocks, one on each side of the rotor
• Caliper: the housing that holds the pads and the piston or pistons that push them

Wikipedia states that the caliper contains at least one cylinder and two pads, one on each side of the disc. Most calipers have one piston, although some high-performance cars use as many as ten.

Fixed and floating calipers

A fixed caliper has pistons on both sides of the rotor, which clamp it from either side. Wikipedia describes these as more complex and expensive, and less tolerant of imperfections in the disc. A floating (or sliding) caliper has a piston on one side only. The piston pushes the inner pad onto the disc, and the reaction pulls the caliper body along pins so the outer pad presses the other face. Floating designs with a single piston can stick if dirt, corrosion or dried grease gets onto the sliding parts.

How the pads let go

Disc brakes do not need big return springs. EduMech explains that when the pedal is released and pressure drops, the elastic deformation of the piston seals pulls each piston back slightly, which frees the pad from the rotor. If air gets into the fluid, it compresses and weakens the pedal, so technicians use a bleed screw on the caliper to remove it.

Vented, slotted and drilled rotors

Heat is the main design problem. Wikipedia says ventilated rotors have internal vanes between two friction faces, allowing air to flow through and carry heat away; these are typically used on the front, where most braking work is done. Slotted rotors help clear dust and gas, and drilled rotors help shed water, though the same source notes that cross-drilled rotors can develop cracks through repeated heating and cooling. Racing and high-end cars may use composite or carbon-ceramic rotors.

Discs against drums

A drum brake presses shoes outward against the inside of a spinning drum. Discs cool better, which reduces brake fade, the loss of stopping power when parts get too hot, and they recover faster when wet. According to Wikipedia, discs also have no self-servo action, so braking force rises in proportion to pedal pressure; this gives better feel and helps avoid wheel lock-up. Discs cost more to make, and many lightweight cars still use drums at the rear to save cost and weight and to simplify the parking brake.

A short history

Frederick Lanchester patented a caliper-type disc brake in 1902, but its copper lining wore quickly on the rough roads of the time, Wikipedia says. The Jaguar C-Type won the 1953 24 Hours of Le Mans as the only car in the race with disc brakes. The Citroën DS of 1955 is described as the first sustained mass-production car with modern disc brakes, and the 1963 Studebaker Avanti had them at the front as standard.

This article is general information about how the parts work, based on the sources listed below.

Sources

• Disc brake, Wikipedia
• Disc Brakes Explained, EduMech