A Disc Brake Glossary
Like most areas of automotive engineering, the world of disc brakes is crowded with jargon, unique terms and specialised 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 maximise braking power while preventing the wheels from locking and skidding.
AFTER MARKET COMPONENT
Automotive replacement component provided by a company other than the original equipment supplier.
AUTOMATICALLY ADJUSTING BRAKES
A system designed to compensate for wear by adjusting brake shoes or calipers automatically.
BACKING PLATE
Steel plate on which wheel cylinders(s), brake shoes and other drum brake parts are mounted.
BLEEDING
Method of purging air from a brake system's hydraulic lines and cylinders. Air is compressible and contaminates brake fluid. It is released via a "bleeder valve" on each wheel cylinder.
BONDED LINING
Brake lining (friction material) attached to the brake shoe with adhesive.
BRAKE BALANCE
The ratio of braking force distributed between the front and rear wheels.
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 linings (pads).
BRAKE DRUM
The basis of a drum brake system. It is a circular metal component which rotates with the road wheel. A set of brake shoes which are fixed in position and act on the drum by expanding.
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 braking action via the brake pedal.
BRAKE FLUID
Liquid formulated specifically to be used in hydraulic brake systems.
BRAKE HOSE
Flexible rubber (or synthetic) hose used to join hydraulic brake components.
BRAKE LINE
Rigid tubing which links various hydraulic components in a brake system.
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.
BRAKE SHOE
Correctly, the shoe is the metal component used to push the friction lining against the drum in a drum brake system, but the term is now used to refer to the whole shoe and pad assembly.
CALIPER
A type of clamp which grips a disc rotor to create friction and thereby generate stopping power.
CROSS-DRILLED DISC
Disc with friction surfaces which have been drilled with rows of holes to improve cooling, reduce weight and provide an escape route for dirt and gasses which can be wedged between the pads and disc. High-performance rotors can be both cross-drilled and slotted.
CURVED VANE DISC
Ventilated rotor in which the cooling channels (or vanes) have been curved to increase their ability to pump out hot air and cool the disc. Curved vane rotors are more efficient than conventional ventilated rotors and, as a side benefit, tend to be stronger.
DISC BRAKE
The most popular and effective type of automotive brake. It uses a rotor (a round grey metal plate) which is squeezed by a caliper 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
Alternative 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.
DUAL CIRCUIT BRAKES
Safety design incorporated on modern cars which ensures there are two largely independent hydraulic brake circuits. Some dual circuit systems are more sophisticated than others.
DUO-SERVO DRUM BRAKE
A self-energising drum brake that has servo action when travelling forwards or backwards.
DYNAMIC WHEEL LOADING
The amount of weight being forced onto each road wheel as a result of the car pitching, rolling or squatting during acceleration, braking or cornering.
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 brake system to create friction and therefore stopping power.
GLAZING
The process whereby a brake lining or disc rotor becomes smooth and glossy due to excess heat.
HEAT DISSIPATION
The 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 areas on a rotor caused by extreme heat.
HERRINGBONE PATTERN
Pattern found on the surface of a disc which has been poorly machined.
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.
HYGROSCOPIC
A characteristic whereby something tends to absorb water. Brake fluid is hygroscopic.
MANUAL BRAKES
Braking system which does not use power-assistance to magnify the pedal effort. Manual brakes are becoming increasingly rare on 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. See also Scrap Thickness.
NON-ASBESTOS LINING
Friction material which uses no asbestos, thereby being easier on public health (breathing asbestos dust can cause the disease asbestosis). Sometimes non-asbestos linings can be more abrasive, accelerating rotor wear.
ORIGINAL EQUIPMENT
Industry term for a component supplied with a new car or as an official replacement part. Known as OEM or "Original Equipment Manufacturer" parts, they are not necessarily produced by the car-maker in question.
OUT-OF-ROUND
Effect where a disc is no longer true to its original shape, as a result of either 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).
PULLING
Tendency of a vehicle to pull to one side under braking.
PULSING
Uneven or stutter-like force transmitted through the brake pedal during braking, usually caused by problems with disc rotors or linings.
RESERVOIR
Chamber connected to the master cylinder (usually by hoses) and used for storing hydraulic fluid.
ROTOR
Alternative 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.
Scrap 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. Scrap Thickness is usually determined by the vehicle manufacturer.
SELF-ENERGISING
Property of a drum brake, whereby the braking force is increased by the braking action of the shoes against the drum.
SLOTTED DISC
Type of disc brake rotor which has a series of slots or grooves across its friction surfaces. These are designed to improve the bite of the pads and break down the build up of gas and dirt which can occur between pad and rotor. High-performance rotors can be both slotted and cross-drilled.
SOLID ROTOR
Disc rotor with solid metal between the two friction surfaces.
SPONGY PEDAL
Pedal which feels spring-like, perhaps due to the presence of air in the hydraulic system.
STATIC WHEEL LOADING
The amount of weight forced onto each road wheel in a stationary car, as a result of a car's (usually uneven) weight distribution.
SWEPT AREA
Total friction area contacted by the pads during one revolution of the rotor.
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
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.
