Engineering Inspiration - Brake System Glossary

Engineering consultants specialising in braking systems, other engineered systems, specifications, component integration, safety critical systems etc.

Glossary

This section with give you a brief description of all those strange words used in the braking industry.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Abrasive Friction	The mechanical rubbing of the brake pad material directly onto the brake disc, resulting in the mechanical wear of both pad and disc
ABS	Acronym for Anti-Lock Braking system. Anti Lock braking systems sense the speed and rate of deceleration of each of the wheels of a vehicle independently and, through a microprocessor control system, act to prevent lock up of any of the tyres under braking force by cycling the line pressure to the wheel that is approaching lock up. Most current passenger cars are fitted with ABS.
ADR	Australian Design Rules: Standards for Road Vehicles
Air brake	A brake system where all the braking energy is supplied from a separate source, (air under pressure).
Air chamber	A chamber used in an air brake system which contains a rubber diaphragm to converts pressurised air into linear mechanical force to apply a foundation brake.
AK master	A dynamometer test developed by automotive engineers in Europe. This test is for general performance of friction materials.
AK noise	A dynamometer test developed by automotive engineers in Europe. This test is for noise performance of friction materials.
Aluminum Beryllium	A composite material of exceptional stiffness to weight ratio used for Formula One calipers in the late 1990s. Tiny amounts of dust cause lung disease up to 15 years later.
Aluminum Lithium	A composite material of exceptional stiffness to weight ratio currently being used for Formula One calipers.
AMECA	Automotive Manufacturers Equipment Compliance Agency, Inc. This agency certifies the friction level of friction material. The friction level is the last two characters of the edge code; for example, N501H FF.
AMS	Vehicle tests developed by the German automotive magazine Auto Motor und Sport. These tests are intended for high performance sports cars.
Anti-lock Braking System	Anti lock braking systems sense the speed and rate of deceleration of each of the wheels of a vehicle independently and, through a microprocessor control system, act to prevent lock up of any of the tyres under braking force by cycling the line pressure to the wheel that is approaching lock up. Most current passenger cars are fitted with ABS.
Anti-squeal shim	Very thin stiff metallic or composite plates, sometimes coated with a high temperature solid lubricant, inserted between the pad backing plates and caliper pistons on passenger cars to reduce or eliminate brake squeal.
Arjeplog	Frozen lake test site in Northern Sweden, used for ABS and other low mu brake tests.
Asbestos	Impure magnesium silicate with very low thermal conductivity - once used as an insulating material and as one of the components in brake friction materials. Tiny amounts of dust cause lung disease up to 15 years later.
ASR	Acronym for Anti-Spin-Regulation. More commonly called Traction Control. A system to prevent wheel spin under rapid acceleration. Utilising the foundation brakes and engine control.
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Backplate	Steel plate to which friction material is moulded or riveted to produce a disc brake pad.
Bedding	A period of time after installation of new pads or discs before the matting friction parts conform to each other. A process is often defined to minimise this time without causing local overheating.
Bell	See "hat". The tube shape part on a brake disc that connects the braking faces to the mounting face.
Bias bar	A system allowing rapid adjustment of the front to rear braking force on a car. Universal in racing, the bias bar connects the pushrods of dual master cylinders with an adjustable fulcrum allowing crew or driver adjustment of the braking ratio.
Bleeding	A process used to remove trapped air in a hydraulic brake system. Often includes 'Brake Fill'
Blue Spot	A odd number of blue spots appearing on the brake disc caused by over heating. Blue spots are generally hard Austenite with a FCC structure and cause thermal judder.
Bluing	Discolouring of cast iron rotors due to heat. Although bluing is evidence of thermal stress and will lead to reduced rotor life, it is normal under repeated hard braking and is not a cause for concern.
Bonded lining	Brake lining (friction material) attached to the brake shoe with adhesive.
Booster	An device for multiplying the pedal force to give greater brake line pressure. Generally taking the additional energy from a vacuum source but could be a pressurised hydraulic source.
Bracket	see "Carrier". The fixed part of a sliding caliper which carries the pads.
Brake bias	The term used to indicate the ratio between the amount of braking force exerted on the front tyres compared to the rear.
Brake booster	An assist device that amplifies pedal force. In most passenger cars it is a vacuum chamber but hydraulic boosters are also used.
Brake fade	A reduction or loss in braking force due to loss of friction between the disc pad and the rotor. Fade is caused by heat build-up through repeated or prolonged brake application.
Brake lining	Friction material, usually applied to drum brakes.
Brake pressure	See "Line pressure". The hydraulic line pressure achieved at the foundation brake during a brake application. (Often the maximum allowed pressure).
Brake shoe	The crescent shaped part of a drum brake which carries the brake lining. Usually fitted in handed pairs.
Brake torque	The output of a foundation brake. When divided by the tyre radius determines the braking force. Torque is a multiple of the clamp force, friction level and disc effective radius for a disc brake system.
Bundy	Trade name widely used to indicate all rigid steel (or copper) based brake pipe.
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Caliper bridge	The part of a caliper that passes over the disc and carries the tensile load as when the brake is applied.
Caliper	A type of clamp that axially grips a rotating disc to create a torque opposing motion.
Carbon/carbon	A braking system in which both discs and pads are manufactured from carbon composite material. Carbon/carbon brakes offer significant reduction in unsprung mass along with much greater thermal capacity but require high temperatures to give suitable friction levels as such they are not used for road vehicles.
Carrier	see "Bracket" The fixed part of a sliding caliper which carries the pads.
Cast iron	Iron containing between 2% and 4.5% dissolved carbon within its matrix. Used widely on brake components because it has good thermal, noise damping and wear properties. It also happens to be relatively cheap.
Ceramic buttons	Insulating buttons inserted in the face of racing or commercial vehicle caliper pistons to reduce conduction of heat to the brake fluid.
Clamp force	The clamp force of a caliper is the brake line pressure multiplied by the total piston area on one side of the disc. when multiplied by the number of friction interfaces (usually 2), the pad friction level and the disc effective radius gives the brake torque.
Coefficient of friction	see "Mu" A dimensionless indication of the ratio between the applied force and the normal reaction force. Brake friction materials give a coefficient of friction of between 0.35 and 0.5 when clamping onto a iron disc. A tyre will have a coefficient of friction of between 0.7 and 1.0 on a dry tarmac road surface.
Compressibility	All materials are compressible to differing amounts. Highly compressible brake fluids or friction materials require additional input travel and therefore affect pedal feel or response times.
Conduction	One of only three heat transfer mechanisms. Convection and radiation are the other two. Conduction is the transfer of heat by physical contact. For example, some of the heat generated by the braking system is transferred into the surrounding components by conduction.
Convection	One of only three heat transfer mechanisms. Conduction and radiation are the other two. Convection is the transfer of heat by fluid flow. Air forced through a ventilated brake disc dissipates heat by convection.
Cracking	Cracking usually refers to thermal cracking. Applying high loads and high thermal cycling to a brake disc or drum can result in cracking due to the repeated high stress levels. Incorrect sizing or cooling for the given duty is the culprit brought about by unrealistic packaging or weight targets.
Creep groan	A noise that occurs at very low vehicle speed (under 2 mph) when applying or holding the brake on. Creep groan is caused by brake lining "stick-slip". It can often be experienced on vehicles with automatic transmissions or when reaching the gradient limit of a hill hold application.
Cross-drilled rotors	Discs that have been drilled through with a non-intersecting pattern of radial holes. The objects are to provide a number of paths to get rid of the boundary layer of out gassed volatiles and incandescent particles of friction material and to increase "bite" through the provision of many leading edges.
Curved vanes	Curved vanes are shaped as curves to act as more efficient pump impellers and increase airflow through the central portion of the disc. They also act as barriers to the propagation of cracks caused by thermal stress and, as each vane overlaps the next, they dimensionally stabilize the disc. Curved vane discs must be mounted directionally to offer effective cooling.
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Differential bores	The leading edge of a brake pad wears faster then the trailing edge since the frictional force on the pad abutment creates a moment. By providing an optimally designed larger caliper piston at the trailing edge of the pad, wear can be evened along the length of the pad
Differential vanes	Some discs are designed with alternating vanes of different length. This modern design feature has been dictated by flow studies. It was found that the volume of air that a disc can flow increases by alternating the length of the inlet without much of a sacrifice in surface area. The more air a vent flows, the more convective cooling can be realized.
DIH	Drum In Hat, a type of parking brake system used with vehicles that have rear disc brakes. DIH utilizes two drum brake linings inside the hat area of the brake rotor.
Disc brake pad	The component in a disc brake system that is fitted with friction material and clamped against the brake disc (rotor) to cause friction.
Disc brake	A type of brake using a rotor as a surface on which a caliper generates frictional forces.
Disc	see "Rotor" The rotating portion of a disc brake system. Mechanically attached to the axle, and therefore rotating with the wheel and tyre the disc provides the moving friction surface of the system while the pads provide the stationary friction surfaces.
DOT	US Department of Transport. Defined some salient point for brake fluid. DOT3,4 and 5 fluid are widely used.
DOT 5.1	A Glycol Ester based brake fluid meeting the High temperature requirements of the DOT 5 standard but not complying with the low temperature viscosity.
Drum brake	A type of 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 radially.
Drum in hat	A disc design in which the internal surface of the hat serves as a brake drum. Often used as a parking brake.
DST	Detroit Suburban Traffic vehicle test. This test is used to evaluate DTV generation by friction materials.
DTV	Disc Thickness Variation, the variation in thickness between two points on the friction surface of a rotor. It is usually caused by poor alignment of the rotor/caliper or the rubbing of the friction material against the rotor when the brakes are off.
Dust boots	Rubber shields that fit over the exposed portion of the caliper pistons of slide pins to prevent the ingress of dust and road grime.
Dynamic groan	Noise that occurs at high speeds, usually caused by an uneven transfer layer (stick-slip)
Dyno	Dynamometer. A machine used to test for performance, wear, noise, rotor aggressiveness, DTV, and other characteristics of a brake system. A dynamometer is a machine which consists of a motor driving a specified inertia load, to simulate the energy of the vehicle being tested and an apply system that can accurately control the brake pressure and/or torque of the brake being tested.
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EBD	Acronym for Electronic Brake (force) Distribution. A system that controls the line pressure applied to the rear axle brakes under conditions that are similar but not identical to those where a pressure limiting valve or proportioning valve would function. EBD systems typically use software that is calibrated to perform the function of a brake line proportioning valve in combination with ABS system hardware to eliminate the need for a stand-alone valve.
EBS	Acronym for Electronic Braking System: A air or hydraulic braking system controlled entirely by electrical solenoid valves.
ECE	Acronym for Economic Commission for Europe. Produce a number of braking regulations. E.g. ECE Reg 13 or Reg 90
EEC	Acronym for European Economic Community. Similar regulations to ECE but not enforced as widely.
Effective radius	The radius of a brake disc at which the centre of pressure of the brake pad acts. Used to determine the braking torque.
EHB	Acronym for Electro Hydraulic Brake. A hydraulic braking system controlled entirely by electrical solenoid valves.
EMB	Acronym for Electro Mechanical Brake. A braking system without any fluid. Generally thought of as electric motor driven calipers.
ESP	see "VSC" Acronym for Electronic Stability Program. A combination of ABS, ASR and other sensors to improve vehicle stability.
Ester based brake fluid	see "Glycol brake fluid" DOT 3 and 4 brake fluids are based on Alkyl Polyglycol Ether Esters. Also, sometimes referred to as Glycol Ether Borate Ester fluids. DOT 3 and DOT4 fluids are suitable for high performance passenger car use.
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Fade	Loss of braking torque due to a reduction in friction caused by excessive heat build-up.
Fixed caliper	A brake caliper in which two or more pistons are arranged on either side of a rigid body with the disc in the centre. Has good stiffness and better durability than a sliding caliper but is difficult to package on FWD cars and more difficult to bleed.
Flash mold	A moulding process used for friction material. This process utilizes constant volume for each part; the excess material is "flashed" outside the mould cavity.
Floating caliper	see "Sliding caliper" A design in which the brake pistons are all on the inboard side of the disc. Force on the outboard pad is created by allowing the whole assembly to slide on sealed pins. Easy to package on FWD cars and allows a parking brake mechanism to be integrated. Also used for mechanical calipers fitted to air brake vehicles.
Floating disc	A floating or two-piece disc consists of a friction disc mechanically attached to the hat either through dogs or through drive pins. This allows the disc to grow radially without coning and to float axially, greatly reducing stresses and drag.
FMVSS	Federal Motor Vehicle Safety Standards. US standards for dynamometer and vehicle testing.
Foundation brakes	Those parts of the brake system responsible for the generation of wheel braking torque and thermal energy storage and dissipation.
Friction material	Materials used to generate frictional forces in disc or drum brakes.
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Glycol brake fluid	see "Ester brake fluid" DOT 3 and 4 brake fluids are based on Alkyl Polyglycol Ether Esters. Also, sometimes referred to as Glycol Ether Borate Ester fluids. DOT 3 and DOT4 fluids are suitable for high performance passenger car use.
Green fade	When the pad is first placed in service the first few heat cycles will cause the volatile elements of the material to out gas. The process is continuous throughout the service life of the pad, but it is most pronounced in the bedding in process when the out-gassed materials form a slippery layer between the pad and the disc reducing the coefficient of friction to near zero. Once the pads are bedded in out-gassing is so slow as to not be a problem unless the effective temperature range of the pad is exceeded.
Groove	The seal groove in the caliper bore to seal and retract the brake piston.
Grooving	Concentric wear grooves on the surface of a disc caused by debris becoming trapped within the pad material.
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Hat	See "bell". The tube shape part on a brake disc that connects the braking faces to the mounting face.
High-speed judder	Vibration during high speed braking not related to DTV. It is caused by hot spots or foreign material on the disc.
Howl	Noise generated in the range of 500 to 1,000 Hz and caused by stick-slip.
Hygroscopic	The property of readily absorbing water. DOT 3 and 4 fluids absorb and chemically react with water lowering their boiling point. The process slows at about 3% water. High performance fluid only have better dry boiling points as such racing fluids may need changing very regularly. Hydraulic or silicone fluid systems do not absorb water but any water in the system remains free and could boil off at 100 deg C.
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Idiada	Brake proving ground near Barcelona, Spain.
JASO	Japanese Automobile Standards Organization. Produce many standards for braking components.
Judder	Vibration during braking at low speeds. Judder is caused by DTV. See also high-speed judder.
Jump-in	The brake line pressure required before a vacuum servo begins to produce any output.
Knockback springs	Small coil springs fitted inside the caliper pistons of some brakes to prevent the pads from excessive knock back caused by flexing of the suspension upright.
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LACT	Los Angeles City Traffic vehicle test, used to evaluate life (wear) and noise generation of friction materials.
Leading edge	With respect to disc rotation the leading edge is that edge of the pad that first comes into contact with the disc when pedal pressure is applied. Unless multiple pads or differential piston diameters are used the leading edge wears faster than the trailing edge. See "Differential pistons" and "Taper wear".
LHM	Hydraulic oil used in power brake systems, Citroen, Rolls Royce and tractors and others special vehicles.
Line pressure	See "Brake pressure". The hydraulic line pressure achieved at the foundation brake during a brake application. (Often the maximum allowed pressure).
Lining	Another term for friction material.
LSV	A valve fitted to the chassis and connected via an arm to the suspension to modify the rear brake line pressure to take account of load and weight transfer.
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Master cylinder	The hydraulic cylinder that converts the driver's pedal effort into hydraulic fluid pressure for subsequent transmission to the operating end of the braking system.
Metal matrix (MMC)	Term applied to a family of composite materials consisting of metallic cores infused with "whiskers" or "grains" of very stiff non-metallic elements resulting in a light and strong material. The most popular of the metal matrix composites is Aluminium ceramic metal matrix, the ceramic typically but not exclusively being composed of Silicon Carbide, Aluminium Oxides and Boron Carbides. Used for lightweight brake discs where the operation temperatures are well below 400deg C.
MFDD	Mean Fully Developed Deceleration. The steady state deceleration during a stop. Used for legislation testing and comparison with theoretical performance.
MIRA	Motor Industry Research Association. UK based testing and development company.
Modulation	The ability to control and adjust braking force with respect to the input force and travel.
Monobloc caliper	A caliper machined from a single piece of billet, cast or forged material.
Mu	see "Coefficient of friction" The Greek letter always given to a dimensionless indication of the ratio between the applied force and the normal reaction force. Brake friction materials give a coefficient of friction of between 0.35 and 0.5 when clamping onto a iron disc. A tyre will have a coefficient of friction of between 0.7 and 1.0 on a dry tarmac road surface.
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NAO	Non Asbestos Organic friction material composed of less than 10% steel fibre.
OE	This is an industry standard term for that equipment that was installed on the model(s), being referred to in context, at the time of manufacture.
OEM	This is an abbreviation for Original Equipment Manufacturer.
Off brake drag	A condition in which the brake pads do not fully retract when line pressure is released. Off brake drag increases temperature and wear while decreasing acceleration, top speed and fuel mileage. It is caused by either non-optimum seal design, seals that have been hardened by thermal stress or excessive disc run out.
One-piece disc	A disc cast in one piece with its hat or bell. This is the inexpensive way to manufacture a disc and is perfectly adequate for normal use. There are some tricks to the design to reduce distortion.
Open caliper	The design of fixed caliper in which the "window" through which the pads are inserted is structurally open. This design, while less expensive to manufacture, significantly reduces caliper rigidity or stiffness.
Organic	A family of friction materials, often containing asbestos, used for both drum linings and disc pads through the 1980s. Now largely supplanted NAO.
Out-gassing	The boiling off of the volatile elements in friction materials. Out-gassing, while it is continuous over the useful life of the pad, is only noticeable during the bedding in process or when the temperature capability of the pad has been exceeded. Under those conditions the volatiles form a layer between the friction materials and the disc surface, smelling bad and causing "green fade".
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Pad abutments	Mechanical elements that locate the pads in the caliper and provide a hard surface for the pads to slide against.
Pad fade	When the temperature at the interface between the pad and the rotor exceeds the thermal capacity of the pad, the pad loses friction capability due largely to out gassing of the binding agents in the pad compound. The brake pedal remains firm and solid but the car won't stop. The first indication is a distinctive and unpleasant smell, which should serve as a warning to back off.
Pad retraction	To prevent drag and premature pad wear the properly designed seal systems retract the caliper pistons a fraction of a millimetre when the pedal pressure is released. This allows what little disc run out there is to "knock" the pads back from contact with the disc. When everything works right the amount of retraction is so slight that the free play is not noticeable when pedal pressure is applied.
Pad	The stationary element of the disc brake system. The pads, consisting of friction material bonded to steel backing plates are held in place by the caliper and forced against the disc by the caliper pistons when pedal pressure is applied.
Pedal ratio	The brake pedal is designed to multiply the driver's effort. The pedal ratio is the distance from the pedal pivot point to the effective centre of the footpad divided by the distance from the pivot point to the master cylinder push rod. Typical ratios range from 4:1 to 9:1.The larger the ratio, the greater the force multiplication (and the longer the pedal travel).
Pillar disc	Ventilated brake discs where the braking faces are supported on pillars rather than vanes. Offer reduced forced cooling but increased convective cooling. Ideal for lower speed vehicles.
Piston	A movable plug sealed in a bore to convert fluid pressure into force or force into pressure. Used in master cylinders, calipers and wheel cylinders.
Positive mold	A moulding process used for friction material. This process utilizes constant pressure and therefore produces no "flash" of excess material.
Pressure bleeder	A tool allowing rapid bleeding of the system and replenishment of the fluid. Too high a pressure may cause cavitations and the formation of air bubbles rather than their removal. A pressure bleeder should contain a physical separation between the fluid and air either through a flexible diaphragm or otherwise.
Pressure bleeding	A 'brake fill' system where the entire system is filled with fluid under about one bar pressure to force fluid from the bleed nipples without operating the brake pedal.
Proportioning valve	A valve to limit the amount of pressure transmitted to the rear brakes under very heavy braking. Front and rear line pressures are the same until some pre-determined "knee" point is reached above which rear line pressure increases at a lower rate.
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Quad valve	A valve used on an air brake system to isolate the separate circuits should a leak occur.
Radiation	One of only three heat transfer mechanisms. Conduction and Convection are the other two. Radiation is the transmission of energy by the emission of waves. In the case of braking systems, radiant heat is difficult to manage and may cause damage to nearby components, boots, hoses etc.
Reservoir	The container in which brake fluid or air is stored to provide a source of fluid for a brake application.
Residual pressure valve	Some passenger cars, particularly those equipped with drum rear brakes, are fitted with a "residual pressure valve" which functions to ensure that the pads are kept in close proximity to the discs despite run out, knock back, etc. The residual pressure is very small to prevent drag.
Rollback	The amount a caliper piston moves away from the pad backplane when the hydraulic pressure is release. It is effected by the shape of the seal groove but is widely misunderstood since caliper stiffness plays no part in rollback but is usually much higher than the amount of shear.
Rotor scoring	Grooves on the friction surface of the brake rotor, usually caused by the friction material.
Rotor	see "Disc" The rotating portion of a disc brake system. Mechanically attached to the axle, and therefore rotating with the wheel and tyre the disc provides the moving friction surface of the system while the pads provide the stationary friction surfaces.
Run out	The amount of axial dimensional variation of the surface of the disc as it rotates. Measured with a dial indicator, normal specification is up to 0.2mm total run out on larger discs. Excessive run out can result in inefficient braking and perceptible pedal pulsation.
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SAE	Society of Automotive Engineers.
Scorching	Process used by friction material manufacturers to reduce "green fade". It involves burning the face of a new pad to eliminate the resins from the contact face.
Seals	Both air and hydraulic brake components and sealed using electrometric rings in grooves.
Semi-metallic	Friction material composed of 30% to 60% steel fibre to increase the operating temperature range.
Service brakes	Another name for foundation brakes.
Servo	Generally used for a vacuum brake booster. A large diameter can which creates additional braking force in proportion to the pedal force. It's takes it's power from the manifold vacuum or a vacuum pump.
Silicone brake fluid	Brake fluid based on silicone. Used by the military as it can operate in wide ambient temperature conditions and classic and custom cars where paint damage from glycol ester based fluids would be disastrous. While silicone based fluids are not hygroscopic they are subject to "frothing" when subjected to high frequency vibration and when forced through small orifices readily trapping air particles. They are also very compressible as temperature rises and invariable cause longer pedal travels. Silicone fluid offer poor lubrication which may result in component damage and reduced seal life.
Sliding caliper	see "Floating caliper" A design in which the brake pistons are all on the inboard side of the disc. Force on the outboard pad is created by allowing the whole assembly to slide on sealed pins. Easy to package on FWD cars and allows a parking brake mechanism to be integrated. Also used for mechanical calipers fitted to air brake vehicles.
Slotted Disc	Shallow, sharp edged but radiuses bottom grooves milled into cast iron discs to provide leading edges for bite and a path for the fire band of gases and incandescent friction material to be dissipated through. If the slots fill up with pad material, the system is operating at too high a temperature.
Slotted Pad	Radial grooves cut into the surface of the pad to prevent differential thermal expansion between friction material and backplane causing pad warping or bond failure.
Solid disc	A disc cast without vents. Used for light cars or rear brakes where duty is low.
Spring brake	An air brake actuator which includes a very big spring to hold on a parking brake.
Squeal (high frequency)	Noise generated in the range of 5,000 to 18,000 Hz and caused by the positive mechanical feedback excitations from the brake linings.
Squeal (low frequency)	Noise generated in the range of 800 to 5,000 Hz and caused by the positive mechanical feedback excitations from the brake linings.
Squeal	Annoying high-pitched noise associated with some combinations of friction materials at low brake torque values. Reduced by the use of anti squeal plates. Can be improved by a different pad material, but also made worse if the former and current pad materials are incompatible.
Squeeze form casting	A casting process that is a cross between die casting and forging. Liquid aluminium is poured into a die and, just before it begins to solidify, the die is forced closed under very high pressure. Alternatively a second cylinder is filled during the moulding other than the primary injection cylinder that is then compressed at high pressure to increase the moulding pressure. The process reduces porosity and leaves the grain structure more like a forging than a casting - resulting in a stronger and stiffer part.
Stainless steel brake lines	Flexible brake hoses made of extruded PTFE protected by a tightly braided cover of stainless steel wire. They virtually eliminate line swelling under pressure giving shorter stiffer brake pedal feel and allowing smaller master cylinder diameters to be specified. Many do not comply with road regulations as the have no traceability markings and fail flex tests by fraying at the end fittings. Universally used in racing where suspension travel is limited and life is short. Fully complying road parts are available buyer beware.
Stick-slip	A phenomenon caused by a non-linear change in the coefficient of friction. When the mu (coefficient of friction) increases, the two surfaces will stick. As the rotational force increases or the mu decreases, a sudden release or slip will occur between the two surfaces.
Straight vanes	Straight vanes extend in straight lines radially outward from the inner surface to the outer surface of the disc. This design is often used in production automobiles and trucks because the same part can be used on both sides of the vehicle.
Sumitube	Heat shrink tube protection fitted onto brake tube (Bundy) to give extra protection against abrasion or wheel blast.
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Taper wear	Differential wear on a brake pad in a disc brake. Longitudinal taper wear is caused by pad abutment friction and often corrects itself as the frictional material wears away. Radial taper wear is caused by lack of stiffness of the caliper.
Thermal cracking	The precursor to cracking. Small surface cracks caused by thermal stress. By themselves thermal cracks are not a cause for concern but they are a warning sign that the disc is not receiving adequate cooling air and cracks will follow.
Thermal shock	Disc materials, particularly cast iron are degraded not only by the magnitude of temperatures reached, but also by the "delta" temperatures - the speed at which the temperature increases and decreases. Cracks are largely caused by weakening of the bonds between the grains of the metal brought about by rapid change in temperature.
Thickness Variation	Variation in the transfer layer, which initiates brake vibration. While the impact of an uneven transfer layer is almost imperceptible at first, as the pad starts riding the high and low spots, more and more TV will be naturally generated until the vibration is much more evident. With prolonged exposure, the high spots can become hot spots and can actually change the metallurgy of the rotor in those areas, creating ?hard? spots in the rotor face that are virtually impossible to remove.
Threshold braking	Braking at maximum possible retardation in a straight line.
Titanium	A very light, very strong metal will very low thermal conductivity. Almost universally used to make caliper pistons for racing applications in order to reduce heat transfer to the fluid within the caliper.
Traction Control	see "ASR" A system to prevent wheel spin under rapid acceleration. Utilising the foundation brakes and engine control.
Trailing edge	That portion of the pad located away from the direction of rotation of the disc.
Transfer layer	A layer of friction material transferred to the brake disc during braking which creates the frictional characteristics of the brake.
TÜV	TÜV Acronym for Technischer Überwachungs-Verein, German technical monitoring and test company.
Two piece caliper	A caliper manufactured from two essentially mirror imaged parts rigidly bolted together. To perform as well as a monobloc caliper, the assembly must result in a rigid structure by design, bolt selection and materials.
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Vacuum bleeding	A 'brake fill' system where the entire system is subjected to an extreme vacuum connected to the reservoir before fluid is forced in. Used by OEM's to eliminate any bleeding process.
Vanes	The term given to the central webs which serve to separate the inboard and outboard friction surfaces of ventilated discs.
Vapour lock	Any free water in a caliper will boil causing the master cylinder to fully stroke without generating substantial pressure. This may also arise when DOT 3 or 4 fluid boils due to increased water content or a dragging brake.
VCA	Acronym for Vehicle Certification Agency, UK government technical monitoring and test company.
Ventilated disc	A disc cast with internal cooling passages. The norm in racing, high performance and heavy vehicles.
VSC	see "ESP" Acronym for Electronic Stability Program. A combination of ABS, ASR and other sensors to improve vehicle stability.
Wear sensors	To ensure that pads are replaced before they are worn down to the backing plates, several types of wear sensors are employed. Some cars use a electronic wear sensor in the pad. This type of sensor typically is worn through when wear limits are reached, breaking continuity in the sensor circuit. Another less expensive method is where the pad has a thin tab riveted to the pad backing plate that rubs on the disc face and squeals when the wear limit is reached.
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