Learner Zone
Motor Vehicle HubBrakes
In automotive terms, a brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle or to prevent its motion, most often accomplished by friction.
Most modern cars have brakes on all four wheels that are operated by a hydraulic system. The type of brake can be either disc or drum.
The front brakes play a greater part in stopping the car than the rear ones, as braking throws the car’s weight forward to the front wheels. Many cars therefore have disc brakes at the front (which is generally more efficient) and drum brakes at the rear.
All-disc braking systems are typically used on high-performance cars, while all-drum systems are used on older or smaller models.
A typical dual-circuit braking system acts on both front wheels and one rear wheel. Pressing the brake pedal forces fluid out of the master cylinder along the brake pipes to the slave cylinders at the wheels; the master cylinder has a reservoir that keeps it full.
Watch these short videos to learn more about the basic principles of brake discs and brake drums.
discs
A brake disc has a disc that turns with the wheel. The disc is straddled by a caliper, in which there are small hydraulic pistons worked by pressure from the master cylinder. The pistons press on friction pads that clamp against the disc from each side to slow or stop it. The pads are shaped to cover a broad sector of the disc.
There may be more than a single pair of pistons, especially in dual-circuit brakes. The pistons move only a tiny distance to apply the brakes. They have no return springs.
The image depicts the basic type of brake disc with a single pair of pistons. There may be more than one pair, or a single piston operating both pads through different types of calipers (swinging or sliding).
drums
A drum brake has a hollow drum that turns with the wheel. The open back is covered by a backplate on which there are two curved shoes carrying friction linings. The shoes are forced outwards by hydraulic pressure moving pistons in the brake’s wheel cylinders which presses the linings against the inside of the drum to slow or stop it.
Each shoe has a pivot at one end and a piston at the other. A leading shoe has the piston at the leading edge relative to the direction in which the drum turns. The rotation of the drum pulls the leading shoe against it when it makes contact, improving the brake effect.
Some drums have twin leading shoes, each with its own hydraulic cylinder. Others have one leading and one trailing shoe with the pivot at the front. This design allows the two shoes to be forced apart by a single cylinder with a piston in each end. This is simpler but less powerful than the two-leading-shoe system.
In either type, return springs pull the shoes back when the brakes are released. Shoe travel is kept as short as possible by an adjuster. Older systems have manual adjusters that need to be turned from time to time as the friction linings wear. Modern brakes have automatic adjustment.
Drum brakes may wear if they are applied repeatedly within a short time – they heat up and lose efficiency. Discs, with their open construction, are less prone to wear.
hydraulics
A hydraulic brake circuit has fluid-filled master and slave cylinders that are connected by pipes.
Pushing the brake pedal depresses a piston in the master cylinder which forces fluid along the pipe. The fluid travels to slave cylinders at each wheel and fills them, forcing pistons out to apply the brakes. Fluid pressure distributes itself evenly around the system. The combined surface ‘pushing’ area of all the slave pistons is much greater than that of the piston in the master cylinder. As a result the master piston has to travel several inches to move the slave pistons the fraction of an inch it takes to apply the brakes. This arrangement allows great force to be exerted by the brakes in the same way that a long-handled lever can easily lift a heavy object a short distance.
Most modern cars are fitted with twin hydraulic circuits that have two master cylinders in case one fails. Under heavy braking so much weight may come off the rear wheels that they lock and cause a dangerous skid. For this reason the rear brakes are deliberately made less powerful than the front.
Most cars have a load-sensitive pressure-limiting valve that closes when heavy braking increases hydraulic pressure to a level that might cause the rear brakes to lock, and prevents any further movement of fluid to them.
There are also complex anti-lock systems that will sense how a vehicle is decelerating and whether any wheels are locking. Such systems apply and release the brakes in rapid succession to stop them locking.
anti-lock & power assist
Watch this ten-minute video to learn more about the basic principles of anti-locking brake systems (ABS).
Power assistance reduces the effort needed to apply the brakes. The source of power is usually the pressure difference between the partial vacuum in the inlet manifold and the outside air.
The servo unit that provides the assistance has a pipe connection to the inlet manifold. A direct-acting servo is fitted between the brake pedal and the master cylinder. The brake pedal pushes a rod that in turn pushes the master cylinder piston.
The brake pedal also works on a set of air valves, and there is a large rubber diaphragm connected to the master cylinder piston. When the brakes are off, both sides of the diaphragm are exposed to the vacuum from the manifold. Pressing the brake pedal closes the valve linking the rear side of the diaphragm to the manifold, and opens a valve that lets in air from outside. The pressure of the outside air forces the diaphragm forward to push on the master cylinder piston which assists the braking effort.
When the pedal is released, the space behind the diaphragm is reopened to the manifold, so the pressure drops and the diaphragm falls back.
Some cars have an indirect-acting servo fitted in the hydraulic lines between the master cylinder and brakes. The unit can be mounted anywhere in the engine compartment instead of directly in front of the pedal. It relies on manifold vacuum to provide the boost. Pressing the brake pedal causes hydraulic pressure build up from the master cylinder, a valve opens and triggers the vacuum servo.
handbrake
Aside from the hydraulic braking system, all vehicles have a mechanical handbrake acting on two wheels.
The handbrake gives limited braking if the hydraulic system fails completely, but its main purpose is as a parking brake. The handbrake lever pulls a cable or pair of cables linked to the brakes by a set of smaller levers, pulleys and guides whose details vary greatly from car to car. A ratchet on the handbrake lever keeps the brake on once it is applied. A push button disengages the ratchet and frees the lever.
On drum brakes, the handbrake system presses the brake linings against the drums.
