Like most areas of automotive engineering, the world of disc brakes is crowded with jargon, unique terms and specialized concepts. The full list of words in use runs into the hundreds, but here’s a brief glossary of essentials.
ABS / Antilock Braking System -- Also known as anti skid brakes, modern ABS systems electronically monitor the speed of the wheels and regulate the hydraulic pressure accordingly. The aim is to maximize braking power while preventing the wheels from locking or skidding.
Bleeding -- Method of purging air from a brake systems hydraulic lines & cylinders. Air is compressible and contaminates brake fluid. It is released via a “bleeder valve” on each wheel cylinder.
Brake Balance -- The ratio of braking force distributed between the front and rear brakes.
Brake Disc (or Rotor) -- The basis of a disc brake system: a round metal disc which rotates with the road wheel and, in order to generate braking power, is clamped by a caliper holding two friction lining (pads).
Brake Fade -- Reduction (or complete loss) of braking performance, usually caused by too much heat in the system.
Brake Feel Sensation transmitted to the driver during a brake action via the brake pedal.
Brake Fluid -- Liquid formulated specifically to be used in hydraulic brake fluid systems.
Brake Lining -- Common name for Friction Material
Brake Pad -- The component in a disc brake system which is fitted with brake lining and clamped against the brake disc to cause friction.
Caliper -- A type of clamp which grips a disc rotor to create friction and thereby generate stopping power.
Disc Thickness Variation -- A variation in thickness between two points on the friction surface of a disc rotor (usually caused by poor manufacture, poor machining or rubbing of the rotor against the caliper when the brakes are “off”.
Discard Thickness -- Alternate term for Scrap Thickness
Drum Brake -- A type of older – but still popular and effective – automotive brake in which a circular drum rotates around a set of brake shoes which are fixed to the hub and act on the drum by expanding.
Friction Material -- Material which is pushed against a disc by a shoe or caliper to generate friction.
Friction Surfaces -- Any of the surfaces designed to rub together in a new brake system to create friction and therefore stopping power.
Glazing -- process whereby a brake lining or disc rotor becomes smooth and glossy due to excessive heat.
Heat Dissipation -- A process whereby braking components rid themselves of heat caused by friction. The heat in a disc system is mostly dissipated into the surrounding air. Dissipation can be accelerated by various forms of ventilation.
Heat Spots -- Shiny dark spots on a rotor caused by extreme heat.
Hydraulic System -- The delivery system of a modern braking set up. It uses fluid to transmit the force applied at the pedal to the wheel cylinders, where it can be converted back into mechanical energy to activate the brake shoes or disc calipers.
Hydroscopic -- A characteristic whereby something tends to absorb water. Brake fluid is hydroscopic.
Manual Brakes -- Braking system which does not use power – assistance to magnify the pedal effort. Manual brakes are becoming increasingly rare on the road cars.
Master Cylinder -- The engine – room of a brake system, where the force applied at the pedal is converted into hydraulic pressure so that it can be sent to each wheel cylinder.
Minimum Thickness -- The thickness at which a disc rotor must be discarded. Through wear and machining a disc rotor becomes thinner over time; as a result it becomes less able to dissipate heat and more prone to warping and other problems. The minimum thickness is usually determined by the vehicle manufacturer.
Out-of-Round -- Effect where a disc is no longer true to its original shape, as a result of warping, inconsistent wear or other damage. This can cause pulsing, grabbing, additional noise and lowered performance.
Parallelism -- A term which refers to the relationship between the two friction surfaces on a disc brake rotor. It is critical that the surfaces are parallel, particularly with ABS, as the slightest shudder can confuse the anti lock system.
Power-Assisted Brakes -- Brakes which use power – assistance (usually from engine vacuum) to magnify the pedal force, thereby reducing driver effort and increasing braking power.
Proportioning Valve -- Hydraulic control designed to stop the rear wheels from locking up (rear wheels become “light” under heavy braking and therefore more likely to skid.
Pulsing -- Uneven or shudder – like force transmitted through the brake pedal during braking, usually caused by problems with disc rotors or lining.
Reservoir -- Chamber connected to the master cylinder (usually by hoses) and used for storing hydraulic fluid.
Rotor -- Alternate name for brake disc.
Run-Out -- Rotors which are warped or out – of – true have excess “run out”, meaning the surface varies or wobbles as it rotates around a fixed point.
Spongy Pedal -- Pedal which feels spring – like, perhaps due to the presence of air in the hydraulic system.
Ventilated Rotor -- Disc rotor which has a series of fins (or cooling passages) between the two friction surfaces to aid in heat dissipation.
Wear Sensor -- Device to warn a driver (usually via a dashboard light) that the brake linings need replacement.
Wheel Cylinder -- A small cylinder located at each wheel to convert hydraulic pressure back into mechanical force in order to apply the brakes.
Wheel Lockup The action of a wheel which is skidding, that is to say, one which has ceased rotating even though the car is still in motion.
Wheel Sensor -- A device which electronically monitors the speed at which a wheel is rotating. Usually it forms part of an anti lock braking system, though nowadays wheel sensors increasingly supply information for traction control systems as well.
Glossary information provided by Dick Ridgeway Arvin Meritor
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