You'll find disc brakes on the front axle of all modern cars, while many more expensive or higher-performance cars will have disc brakes fitted on the rear axle as well.
How does a disc brake work?
There are three main components in a simple disc brake assembly. Firstly, the brake disc itself - which sits between the wheel and the wheel hub. The other two key components are the brake caliper, which contains hydraulically actuated pistons, and the brake pads.
The brake caliper is mounted to the car's axle housing, suspension upright or trailing arm, and the brake pads sit on either side of the disc within the caliper. When you press on the brake pedal, hydraulic fluid in the brake system is pressurised and pushes the pistons out of the caliper. This presses the brake pads, which are metal plates covered with friction material on the disc side, against the surface of the disc. This generates friction and slows the car down.
Cooling is provided by the ambient air flowing over the disc brake's components, which helps prevent them from overheating. When the brake overheats, usually after several hard stops or after a long hill descent while braking, its stopping power reduces - a condition known as 'brake fade'. Many disc brake set-ups feature ventilated discs and air ducts in order to improve cooling and reduce the chance of brake fade.
Are all disc brake set-ups the same?
There are two key elements that vary from disc brake to disc brake. Many cars, for example, use a 'floating' caliper. These feature a caliper support, bolted to the suspension or axle, which locates the caliper over the disc. The caliper has one piston and 'floats' on pins and bushings, which allows both brake pads to evenly the clamp the disc when the brake is applied. Sliding calipers are similar in design but the caliper body rides in machined slots in the support instead of on pins.
Otherwise, the brake system will rely on fixed calipers that are rigidly bolted to their attachment point. These fixed calipers typically have two pistons, one for each side, but can feature more. Increasing the number of pistons grants several benefits, such as permitting bigger brake pads to be used.
You may also encounter high-performance 'floating' brake discs. These have a separate brake rotor, clamped by the pads, which bolts to a 'hat' or 'bell' that then attaches to the hub. This arrangement allows the disc to expand easily when it gets hot, reducing the chance of warping or cracking.
The discs themselves are made from cast iron in most applications but some high-performance set-ups may use carbon-ceramic discs. These are lighter, more durable and resistant to corrosion. They are, however, far more expensive.
What are inboard disc brakes?
This refers to a braking set-up where the disc brakes are mounted inside the car's chassis or body, instead of behind the wheels. These offer several benefits, including being easier to package and improved ride and handling - because the brakes are no longer bolted to and part of the suspension system, reducing unsprung mass.
Inboard disc brakes can be far harder to service, however, and are typically only found in classic racing or road car applications.
Are disc brakes better than drum brakes?
Drum brakes rely on moving brake shoes into contact with the interior of a rotating drum to slow a wheel, unlike a disc brake that clamps pads to a rotating disc.
While drum brakes can provide plenty of stopping power, they are prone to overheating quickly during heavy use. A disc brake, which is not enclosed like a drum brake, is far better at dissipating the heat generated during braking.
Disc brakes are also far easier to service and require little maintenance - whereas many drum brakes can require intermittent adjustment to deliver proper function.
A brief history of disc brakes
The concept of disc brakes has been around for a long time, with British manufacturer Lanchester trialling a system in 1902. Problems with materials led that particular concept to go no further, however.
As cars got faster, manufacturers began seeking improved and more reliable stopping power. Consequently, in the late 1940s and early 1950s, disc brake systems began being developed and pressed into action - most prominently in Jaguar's C-Type race car, from 1952 onwards.
The first mass-produced car with disc brakes arrived not long after, in the form of the 1955 Citroen DS.