Sunday, October 21, 2012

Car care: basics of braking -- Part 2

Proportioning Valves
Proportioning valves may be integral to the master cylinder housing to reduce the weight and complexity of the hydraulic piping, or they can be mounted separately. There are various types of proportioning valves: single, dual pressure sensitive. and deceleration.

When a vehicle brakes, its weight is transferred to the front. The nose will dip as it gets heavier, and the rear will rise as it gets lighter. The rear wheels need less hydraulic pressure, hence the purpose of proportioning valves.

When a large variation in axle loading at the rear takes place such as in station wagons and trucks, load or height sensitive proportioning valves regulate hydraulic pressure to the rear axle in the amount needed. Some vehicles with front/rear split braking systems have a proportioning valve by-pass which allows full pressure to the rear if the front brakes fail.

Brake Booster
The brake booster reduces the effort involved in braking which is, after all, one of driving's most repetitive functions. Mounted on the firewall between the brake pedal and the master cylinder, brake booster varies in size, and can be either single or double diaphragm. There is a vacuum in the manifold of all four-stroke petrol motors. Brake boosters used this to increase the force applied to the master cylinder by three to five times without losing brake sensitivity or response. On diesel engines, an auxiliary vacuum pump is utilised to supply vacuum.

Brake boosters have two chambers: one at the front which is always at a constant pressure, and one at the rear where thepressure varies. These are separated by a pressure plate and valve body, and are sealed from one another by a rubber diaphragm.

A control valve regulates the amount of atmospheric pressure let into the rear chamber. Operated by the pedal pushrod, the valve is directly connected to the out-put push-rod and includes a vacuum valve.

A vacuum non-return or check valve is fitted either to the booster or in the hose from the engine manifold. If the engine stops, this will retain enough vacuum in the booster for up to three brake applications.

If the vacuum supply is totally lost, the control valve assembly and output rod act as a single pushrod. The brakes can still be operated but the driver will have to push a lot harder to stop the vehicle.

Disc Brakes
Because they are very stable and safe, disc brakes are fitted to just about all of today’s passenger vehicles. In its simplest form, a disc brake is a cast iron rotor that turns the wheel, and two fixed pads with friction material bonded to them. When the driver puts the brakes on, pressure from the master cylinder forces the pads against the rotor. The resulting friction develops the braking force needed to slow or stop the vehicle. The force with which the pads clamp the rotor governs the amount ofbrake force generated.

Most of the rotor is exposed to the air so friction heat is easily radiated away. This minimises brake fade and helps keep the braking stable at all speeds. Since the rotor spin with the wheels, they will literally spin themselves dry if they get wet.

The disc caliper holds one or more pistons which force the pads against the disc rotor. Calipers may be either fixed head opposed piston or floating head piston. While fixed head calipers may have four opposed pistons, some floating head calipers have two. Multiple pistons are used when more force is needed like heavy machinery or high performance vehicles. The rotors themselves can be either solid or ventilated design, depending on the application. Since the ventilated type has surface area, it radiates heat faster so it suits heavy duty use.

Disc brakes don't need periodic brake adjustments (to keep the distance between the pads and the rotor constant) because they adjust automatically in use. This is so for all disc brake calipers, even those including integral parking brakes.

Where four wheel disc brakes are installed, the parking brake in many instances has been incorporated into the rear caliper. These days, a growing number of vehicles have a separate brake assembly fitted inside the disc for that purpose.

Drum Brakes
Brake shoes are anchored to a braking plate and lined with friction material. They are mounted inside a brake drum which turns with the wheel. Pressure from the master cylinder forces the shoes against the spinning drum to prevent it from rotating.
The pressure with which the shoes are forced against the drum controls the amount of friction heat and hence the braking force. The most common type of rear drum brake assembly in today's vehicle is the leading and trailing shoe design.

This has gained favour over the earlier duo-servo design though both feature automatic adjusting systems. Both types provide a parking brake operated by a driver's lever connected via cables to a lever-and-strut mechanism that expands the brake shoes against the drum.

The force applied by the driver is usually multiplied by an intermediate lever, and an equaliser ensures the same force is applied to each brake. If the vacuum supply is totally lost, the control valve rod assembly and output rod act as a single pushrod. The brakes can still be operated, but the driver will have to push a lot harder to stop the vehicle.


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