How Car Brakes Work

Every braking system on the road, from a city car to a laden artic, comes down to one principle: friction.

It must be fairly obvious what the braking system is for. It allows the vehicle to be slowed in varying degrees, and also provides some method of holding a stationary vehicle without having to continually apply the service, or foot, brake. The braking system on any motor vehicle, whether it be the smallest glass fibre two seater city car or the fully laden heavy haulage tractor unit and trailer, relies on the basic principle of friction.

It must be mentioned at this point that the majority of heavy commercial vehicles and buses have supplementary braking systems which do not use friction. Instead they have various devices which have a direct effect on the engine or exhaust, or alternatively a system utilising the principles of electromagnetism mounted into the drivetrain.

Two systems on a typical set-up

Returning to the average family car, two entirely separate systems are fitted: the service or foot brake system, and the park or hand brake system.

With the service brake, the brake pedal is connected to the brake master cylinder, and when the pedal is pressed, hydraulic pressure is applied through the rigid metal pipes and the flexible brake hoses to the brake cylinders or brake calipers mounted at each wheel. The calipers and cylinders are absolutely no different in their principle or operation to the huge hydraulic rams seen on earthmoving machines or in the tipping gear of trucks; it is only their appearance that is so different.

Simply, hydraulic pressure exerted within a sealed space with a moveable section at one end — a piston — can be used to move or operate another mechanism, in the case of a brake system, a friction device. All that is needed is for the friction device, a pad or shoe, to be pressed against a rotating body, a disc or drum, in turn attached to the wheel assembly. As the exerted pressure is increased, so is the clamping effort on the disc or drum, thus slowing its rotation.

The materials involved

The metal construction of the pad or shoe needs to be faced with a material which not only provides an effective degree of friction and a high resistance to heat — an unavoidable by-product of friction — but which also has excellent wear characteristics without producing premature wear in the disc or drum.

The maximum amount of hydraulic pressure generated depends not only on how hard the driver presses the brake pedal, but also on the relationship between the diameters of the master cylinder and the cylinders or calipers at each wheel.

Why the dual circuit matters

One of the most important features of the modern service brake system is the dual circuit. With the old single circuit designs, all the hydraulic lines were shared, so that if a leak occurred all brake effort was lost. The dual system offers split circuits so that in the event of failure two brakes will still work, albeit with much greater pedal travel. The split will normally be arranged so that the two remaining brakes will be one at the front and one at the back, usually diagonally opposed.

Regulations and cable tension

With the advent of the dual circuit system, less emphasis is placed on the second brake system fitted to the motor vehicle, the park or hand brake. The car now effectively has three brake systems: two almost separate hydraulic service circuits, and one completely separate mechanical park brake.

Regulations dictate that the park brake must be purely mechanical in operation, the most common being a centrally mounted lever in the passenger compartment coupled to the rear brake assemblies via a cable arrangement. Pulling on the handbrake tensions the cables, pulling levers inside the brake assembly and applying the shoe or pad against the disc or drum. As the park brake must be able to be applied and locked in the on position, a ratchet mechanism is incorporated in the handbrake lever, so the brake can be set and only released when required by the driver.