You may never have really considered how your brakes work, other than knowing that pressing the middle pedal makes the car come to a halt. But it's always worth understanding how the they actually function, so we look at the component parts and how they work together.
How brake discs are made
These metal discs bolt between the wheel and hub and provides the friction surface for the pads to act against. Brakes can either be solid (one piece) or vented (effectively two discs joined by a series of veins) for aiding cooling.
Vented discs are commonly used on the front of cars where the braking forces are higher and are subject to greater temperatures.
Brake Pads – Fitted in pairs to each disc, the pads are made of a very hard wearing compound that provides excellent heat resistant properties and the ability to provide a high level of friction against the brake disc.
Brake Drums – Not as common on modern cars, but still fitted to the rear of some smaller, and lower powered cars. The brake pads or 'shoes' are housed within the drum, and pressing the brakes forces the shoes outwards onto the inner edge of the drum which slows the car.
Brake Caliper – Calipers come in many shapes and sizes and employ one or more hydraulically actuated pistons which force the pads into contact with the disc when the brake pedal is pushed. The more pistons a caliper has the more evenly distributed the braking force is across the pad, and the larger the pad surface can be. The larger the pad the greater the friction acting on the disc therefore the better the stopping power.
Brake Servo (or Booster) – These act with the master cylinder to increase the force applied by the brake pedal via either vacuum from the engine (or from a vacuum pump on diesels) or via a hydraulic pump. Without the servo the brakes feel very hard, and require much more effort to be able to slow the car. The servo only works with the engine running.
The master cylinder converts the action of you pressing on the brake pedal into hydraulic pressure. As you press the pedal it moves pistons within the cylinder which in turn acts on the brake fluid forcing it around the system.
The master cylinder will have the brake fluid reservoir attached to the top of it, and will be connected to the brakes via a network of brake lines.
These are a series of thin pipes which connect the various components together to transfer the brake fluid around the system. The majority of the pipes are made of copper but where they meet the brake calipers they need to be flexible (to allow the wheels to turn) so you'll commonly find rubber 'flexi-hoses'.
Anti-lock brakes work by detecting when a wheel 'locks up' under braking. Wheel sensors detect when one or more wheels are no longer rotating and pulse the brakes on-and-off incredibly quickly – You'll feel it through the pedal as a hard vibration. The ABS pump controls the distribution of brake fluid, and controls the action of the ABS.
So what happens when you press the brake pedal?
As you press the brake pedal the force generated by your leg is amplified via mechanical leverage within the pedal assembly, then amplified further by the action of the brake servo.
The mechanical force of pressing the pedal is converted into a hydraulic force by the master cylinder. This forces hydraulic (brake) fluid around the braking system via a network of brake lines.
The fluid forces small pistons within the brake calipers to push the brake pads onto the brake discs, and it's this clamping force which slows the car down.