Disc Brakes

Disc brakes are the dominant friction-braking system on modern passenger cars, slowing a wheel by squeezing a pair of friction pads against a flat metal disc, or rotor, that spins with the wheel. They became the standard because they convert a vehicle's kinetic energy into heat more efficiently and more consistently than the older drum design, and because they shed that heat readily into the passing air. Reliable, predictable stopping power, even after repeated heavy use, is the reason discs are fitted to the front of virtually every car built today and increasingly to all four wheels.

The mechanism is straightforward and robust. The rotor is bolted to the wheel hub so that it rotates with the wheel. Straddling its outer edge is a caliper, which houses one or more hydraulic pistons and a pair of brake pads. When the driver presses the pedal, the master cylinder pressurises brake fluid, that pressure acts on the caliper pistons, and the pistons clamp the pads hard against both faces of the spinning disc. The friction between pad and disc slows rotation, and the resulting heat radiates and convects from the large, exposed surface of the rotor.

This heat management is the system's defining advantage. Because the rotor is exposed to airflow, it cools quickly, which makes disc brakes far more resistant to brake fade, the loss of stopping power that occurs when friction components overheat. Discs also clear water more effectively, recover faster after wet driving, and are simpler to inspect and service, since the pads and disc face are visible once a wheel is removed. Pad wear is easy to check and replacement is comparatively quick.

Several refinements push the design further. Ventilated discs are cast with internal vanes between two friction faces, pumping air through the rotor to dissipate heat during hard or sustained braking. Cross-drilled and slotted discs help vent gases and clear debris from the pad surface, and are common on performance cars. At the high-performance end, carbon-ceramic discs use a carbon-fibre-reinforced silicon-carbide matrix that tolerates extreme temperatures, weighs far less and lasts far longer, though at considerable cost.

Disc brakes are not without drawbacks. They are generally more expensive than drum brakes, the exposed disc and pads can corrode or collect grit, and incorporating a fully effective parking brake into a disc setup is more complex than with a drum. For these reasons some manufacturers still fit drum brakes to the rear of inexpensive cars, where braking loads are lower. The brake caliper is the key actuating component of the system, and pad and disc condition are central to any thorough brake inspection.