WhoCanFixMyCar explains exactly how you car brakes work
Every time you drive, you rely on your brakes for safety and control. But have you ever wondered how they actually work?
Most drivers are aware of this process only in its most simple form: you push down a pedal and your car begins to slow. However, this simple motion triggers a series of processes which rely on hydraulics and friction to bring your car to a halt.
In this guide, we explain these processes in simple terms, identifying the vital components which allow them to happen.
How do brakes work in a nutshell?
By pressing the brake pedal, you create hydraulic pressure which travels to the caliper or drum at each wheel. This pressure is then used to drive a friction material (brake pads) into a spinning surface that’s connected to each wheel, and the subsequent friction slows the vehicle down.
It’s that simple! Well, sort of…
Brake discs sit between the wheel and hub, providing a surface for the pads to act against. They can come in two forms: vented (two discs joined together) and solid. Usually, vented discs are used at the front end of the car because the braking forces are higher.
Each brake disc is fitted with two pads at either side. These pads have to be extremely durable because they are designed to provide a high amount of friction against the disc.
Drums are more common on older cars, though they still appear at the rear of some older, smaller and lower powered cars. Brake pads are housed within the drum, so when you press the brake the pads are forced towards the outer edge of the drum, slowing the car.
Calipers come in many different sizes and shapes so they will typically vary from car to car. They deploy hydraulically actuated pistons which push the pads into the disc when you push the pedal. The amount of pistons a caliper has will affect the distribution of the braking force - the more there are the bigger the brake pad can be, and the bigger the brake pad is the better the stopping power.
Brake servos work in tandem with the master cylinder to increase the force deployed by the brake pedal. This can happen via a hydraulic pump or the vacuum from the engine.
This component converts the power exerted by your foot pushing the pedal into hydraulic pressure. The movement of the pedal pushes pistons within the cylinder. In turn, this acts on the brake fluid, moving it around the system.
A series of thin pipes connect all the different brake components together to transport brake fluid around the system. Most of these pipes will be made of copper, though in the places where they meet calipers they need to be flexible, so you’ll often find rubber flexi-hoses.
When a wheel ‘locks up’ during braking, wheel sensors in an ABS system will detect this and cause the brakes to pulse on and off very quickly. The ABS pump controls the distribution of brake fluid.
In hydraulic brake circuits, fluid-filled master and slave cylinders are connected by pipes. Here’s how it works:
When you push the brake pedal it depresses a piston that’s located in the master cylinder, forcing fluid along the pipe.
There are slave cylinders at the wheel - the fluid travels to each of these, filling them and forcing the pistons out to apply the brakes.
The combined surface area of all the slave pistons is significantly greater than that of the piston in the master cylinder.
This means that a large amount of force can be exerted by the brakes.
If you’re struggling to picture how all of this works, it’s similar to how a long handled lever can easily lift a heavy object a short distance.
It’s reassuring to know that most modern cars are fitted with twin hydraulic circuits consisting of two master cylinders in case one fails.
Power assisted brakes
In cars that are fitted with power assisted brakes, the effort needed to apply the brakes is significantly less. The main source of power comes from the difference between the outside air and the partial vacuum in the inlet manifold.
In addition to this, the brake pedal also works on a set of air valves. Connected to the master-cylinder piston is a large rubber diaphragm.
When you press the brake pedal, the valve linking the diaphragm to the manifold is closed, and another valve is opened that lets air in from outside.
Since the outside air is at a higher pressure, it forces the diaphragm forward, pushing on the master-cylinder piston. This ultimately assists the braking effort.
All cars have a handbrake which usually acts on the two rear wheels. In the event of hydraulic brake failure, the handbrake system will only offer limited assistance; its main function is as a parking brake.
The lever that you use inside the car pulls a pair of cables that are linked to the brakes by a set of smaller levers, pulleys and guides.
There is a ratchet on the handbrake lever which keeps the brake on when it has been applied.
If you think your brakes need repairing or replacing, you can use WhoCanFixMyCar to find reliable local mechanics.