Wheels and Tyres

Braking SystemsBraking RequirementsThe brakes must be strong enough to stop the vehicle with in a minimum distance in an emergency.It should also be consistent with safety.The driver must have proper control over the vehicle during emergency and the vehicle must not skid.The brakes must have good antifade characteristics. i.e their effectiveness should not decrease with constant prolonged application.Classification of BrakesPurposeLocationConstructionMethod of actuationExtra braking effortPurpose primary brakes secondary brakesLocationLocated either at transmissionLocated at the wheelsConstructionDrum brakesDisc brakesMethod of actuationMechanical brakesHydraulic brakesElectric brakesVacuum brakesAir brakes

In this type, a brake drum is attached concentric to the axle hub whereas on the axle casing is mounted a back plate.The back plate is made of pressed steel sheet and is ribbed to increase rigidity and to provide support for the expander, anchor and brake shoes.Two brake shoes are anchored on the back shoes. Friction linings are mounted on the brake shoes.

Drum Brakes

Drum Brake AssemblyBrake cylinder assembly:Backing plateBrake shoe Brake shoeHandbrake cableTension springsRetracting springsHandbrake leverNext >

Drum fits over wheel hub or drive flange.The drum provides a friction surface for the brake linings. It also dissipates heat, and can support wheel bearings or road wheel.Drums sometimes have fins to aid cooling.Brake DrumNext >Disc Brakes

Comparison of Disc and Drum brakesIn disc brakes friction s/fs are directly exposed to the cooling air, whereas in the drum type friction occurs on the internal s/f.Friction pads in case of disc brakes are flat as compared to curved friction linings in case of drum brakes, in disc brakes uniform wear of friction pads and not subjected to any bending.Disc brakes weigh less than their conventional drum type.Disc brakes have comparatively better anti-fade characteristics.Compared to the drum type, disc brakes are simple in design. There are very small number of parts to wear.It is very easy to replace the friction pads when required, compared to the drum type where the brake linings have to be either riveted or fixed with adhesives to the brake shoes.Frictional area of pads in disc brakes is very less as compared with conventional drum type brakes.

Introduction: Hydraulic Brakes

This system uses a brake fluid (Preferably ethylene glycol) to transfer the pressure applied by the operator from the controlling unit to the actual brake mechanism, which is usually at or near the wheel of the vehicle.

Most vehicles in India prefer to have drum brakes on rear wheels while disk brakes on the front wheels attached to the hydraulic controlling unit

Construction

Brake pedal or lever

A pushrod (also called an actuating rod)

A master cylinder assembly containing a piston assembly (made up of either one or two pistons, a return spring, a series of gaskets/ O-rings and a fluid reservoir)

Reinforced hydraulic lines

Brake caliper assembly consisting of one or two hollow aluminum or chrome-plated steel pistons (called caliper pistons), a set of thermally conductive brake pads and a rotor (also called a brake disc) or drum attached to an axle

The system is filled with a glycol-ether based brake fluid

System Operation

As the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder causing fluid from the brake fluid reservoir to flow into a pressure chamber

This forces fluid through the hydraulic lines toward one or more calipers where it acts upon one or two caliper pistons sealed by one or more seated O-rings which prevent the escape of any fluid from around the piston

Heat is dissipated through brake pads which are made up of special heat tolerant materials such as kevlar or sintered glass

On releasing the brakes, the rubber sealing rings acts as return springs and retract the pistons and the friction pads away from the disc.

Important Considerations

Hydraulic systems are smaller and less expensive than the air brake systems

Hydraulic fluid should be in-compressible. Also the hydraulic system should be air tight such that no vapor is introduced in the system

Hydraulic fluid must resist vaporization at high temperatures

The fluid that is used should be non-corrosive for the surrounding material

Elimination of Brake Fade

Below are the components in a hydraulic disc brake system.Brake reservoir: Contains hydraulic brake fluidMaster cylinder: Device that pumps the fluid from the reservoir to brake lines that run throughout the vehicleBrake lines: Rubber or steel braided hoses that run from the master cylinder to each brake caliperBrake caliper: A steel housing that mounts on a fixed point of the brake rotor that contains a piston and brake padsBrake piston: A round rod that extends and pushes against a brake pad when hydraulic fluid is fed from the master cylinderBrake pad: A metal backing pad with a semi-metallic overlay that grips the steel rotorBrake rotor: A steel disc mounted to each wheel and hub that the pads grasp to stop the wheels from rotating

Brake FluidBoiling point.Viscosity.Effect on rubber.Corrosive action.Storage stability.

Air SuspensionGenerally two types of air springsBellows type springPiston type.

Working of Air SuspensionFour air springs, which may be either bellows type of piston type are mounted on the same position where gradually the coil springs are mounted.Air compressor takes atmospheric air and compresses it to a pressure of about 240MPa, at which pressure the air in accumulator tank is maintained, which has a safety valve.This high pressure air goes through the lift control valve and the levelling valves, to the air springs.The lift control valve is operated manually by means of a handle on the control panel, through a cable running from the valve to the handle.

Working of Anti lock brake systemThe hydraulic unit is the central component of an ABS system.Each of the four wheels has a speed sensor, which measures the rotational speed of the wheel.This information is monitored by ECU which opens and closes the magnetic valves at the right time.If the wheel is about to lock under heavy braking, the system continues to reduce the hydraulic pressure on that wheel alone, until the threat of locking is past.Working of Anti lock brake systemOnce the wheel is turning freely again, the hydraulic pressure is increased.This increase and release of pressure continues until the driver reduces the force on the brake pedal or until the tendency to lock is overcome.ABS is incorporated in some of the cars to prevent skidding and to rove braking.Ex: Mercedes C200 CDI, Ford Monde, Honda Accord.