Glossary of Breaking Terms

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

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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”.
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.
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.
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.
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.
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.

Fundamentals of Brake Calculation

Brake Calculations

There are many books on brake systems but if you need to find a formula for something in particular, you never can. This page pulls them together with just a little explanation. They should work for any two axle vehicle but it’s YOUR RESPONSIBILITY to verify them. Use them at your risk…..

 

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VEHICLE DYNAMICS

Static Axle Load Distribution

Note: this changes with the loading of the vehicle so laden and unladen figures are often different.

Relative Centre of Gravity Height

Dynamic Axle Loads (Two Axle Vehicles Only)

The changes in axle loads during braking bears no relationship to which axles are braked. They only depend on the static laden conditions and the deceleration.

Note: The front axle load cannot be greater than the total vehicle mass. The rear axle load is the difference between the vehicle mass and the front axle load and cannot be negative. It can lift off the ground though. (Motorcyclists beware)!

STOPPING THE VEHICLE

Braking Force

The total braking force required can simply be calculated using Newton’s Second Law.

Wheel Lock

The braking force can only be generated if the wheel does not lock because the friction of a sliding wheel is much lower than a rotating one. The maximum braking force possible on any particular axle before wheel lock is given by:

 

Brake Torque

Having decided which wheels will need braking to generate sufficient braking force the torque requirements of each wheel need to be determined. For some legislation the distribution between front and rear brakes is laid down. This may be achieved by varying the brake size or more likely using a valve to reduce the actuation pressure.

 

FOUNDATION BRAKE

Disc Effective Radius

The effective radius (torque radius) of a brake disc is the centre of the brake pads by area.
For dry discs it is assumed to be:

 

For full circle brakes it is:

Note: the difference is because full circle brakes contact on the full face but caliper pads are not usually a quadrant but have square sides (Given the variability of friction the difference is not important in practice).

 

Clamp Load

The clamping load is assumed to act on all friction surfaces equally. For dry disc brakes it doesn’t matter whether the brake is of the sliding type or opposed piston. Newton’s Third Law state every force has an equal and opposite reaction and a reaction force from a sliding caliper is the same as an opposed piston one.

 

Brake Factor

Ball ramp brakes have a self servoing effect rather like a drum brake. The brake factor multiplies the output torque.

 

Brake Sensitivity

High factor brakes become very sensitive to manufacturing tolerances and lining friction variations. A measure of sensitivity is the amount the brake factor varies for a change in lining friction. It can be calculated:

 

GENERATING BRAKING

System Pressure

Pressure is a function of the required clamp load and the piston area. Remember on an opposed piston disc brake it’s only the area on one side of the disc.

Servo Booster

Servo characteristics are defined graphically. The output will have at least two slopes but will also have a dead band at the bottom.

Pedal Force

The pedal ratio is calculated to the centre of the foot pad. The pedal return springs may make a significant contribution to the overall pedal force. Especially at full travel.

REAL LIFE DECELERATION & STOPPING DISTANCE

The deceleration used in calculations is a steady state one called MFDD (mean fully developed deceleration). It assumes the vehicle is either braking or not. In practice it takes a time for the system pressure to rise and the friction to build up. This is not the driver reaction time but the system reaction time. Where a calculation requires a stopping distance or an average stop deceleration then this delay must be taken into account. For calculations a linear build up over 0.6 second is used ie 0.3 second delay.

For testing the following graph show the requirements for 71/320/EEC and ECE R13.

BRAKE HEATING

Stop Energy

The energy dissipated in a stop is the sum of energy from three sources, kinetic, rotational and potential.

Kinetic Energy

Assuming the stop is from the test speed down to zero then the kinetic energy is given by:-

Rotational Energy

The rotational energy is the energy needed to slow rotating parts. It varies for different vehicles and which gear is selected however taking 3% of the kinetic energy is a reasonable assumption.

Potential Energy

The potential energy is the energy gained or lost by stopping on a hill.

Braking Power

Only when the brake is applied (but rotating) is energy being dissipated in the brake system. Some of the stop energy is dissipated in the tyre as wheel slip. Managing the ideal wheel slip is the ultimate goal of ABS development but here assume 8%. The energy to each brake depend on the number of brakes and the proportion of braking on each axle.

In order to calculate the power we need to know the brake on time:

The power is then given by:

This is the average power, the peak power at the onset of braking is double this.

Dry Disc Temperature Rise

These calculation are based on that given in the following reference:

Brake Design and Safety 2nd edition by Ruldolf Limpert

Single Stop Temperature Rise

In order to approximate the temperature rise of the disc an assumption as to where the energy is going has to be made. Initially most of the heating takes place in the disc, however this can then be rapidly cooled by surrounding components and the air stream. The calculation assumes 80% goes to the disc.

Heat flux into one side of the disc:

Single stop temperature rise is:

Fade Stop Temperature Rise

The temperature rise after repeated stopping can also be approximated, although so many variables exist it is suggested this is only used for basic optimisation work.

After a number of stops:

PARKING ON AN INCLINE

Axle Loads

When parking on an incline the lower axle has a higher load than it does on the level.

The rear axle load is the difference between the vehicle mass and the front axle load.

Traction Force

If the braked wheel is very light on an incline then it is possible the tyre will slip before the brake. Hill hold is usually required with the vehicle facing both up and down the hill. The traction force required to park the vehicle is:

Where only one of the two axles is braked the limiting slope is:

LOSSES FROM CABLE OPERATED BRAKES

Cable losses are not inconsiderable and vary depending on the number and angle of bends. A typical cable supplier uses the following calculation to calculate cable efficiency:

HYDRAULIC BRAKES

Brake Fluid Volume Requirements

When an hydraulic brake is applied fluid is required to move through the pipes. If the fluid source is a master cylinder it has a finite capacity. The following components need fluid:-

Foundation Brake Requirements

Brake fluid is required to take up running clearance.

It is also needed to compensate for lack of stiffness of the brake housing. For a disc brake the following approximation can be used:

Pads Compressibility

Pad compressibility varies between hot and cold conditions. Worst case figures are 2% cold and 5% hot at a pressure of 16MPa. The fluid required is given by:

Rubber Hose Expansion

The rubber hose expansion coefficient is usually taken as

Steel Pipe Expansion

Pipe expansion is very small and unlikely to be of interest however it should be noted that it is proportional to the cube of the diameter, so using bigger pipe than necessary on a system with a fixed fluid volume will cause longer travel for two reasons, the stiffness of the pipe and more importantly the additional fluid compression losses.

Master Cylinders Losses

Fluid losses in master cylinders increase with bore size and pressure. A reasonable assumption can be found by using the following:

Fluid Compression

Fluid compression varies with temperature and the type of fluid used.

The fluid needed to take account of compression is calculated:

It is usual to allow about 3% for trapped air in the circuits that can’t be removed by bleeding. This air is squashed totally flat during braking.

DYNAMOMETER INERTIA

When testing Brakes on a dynamometer it is important to calculate the inertia requirements.
Many brakes do not run at the same speed as the wheels so it is important to understand how the brake will be mounted on the rig.
Ignoring the inertia of the wheels the required dynamometer inertia is given by

What is the basic of cutting brake and it's working operation ?

  • Basics of Cutting Brake

    Cutting brakes are a system of levers, switches, or pedals that allow you to lock up individual brakes in order to stop one wheel and then use the other wheels to drive the vehicle, thus pivoting around that locked wheel. The result is a tremendously tight turning radius, and they can be implemented in a variety of ways.
    Kit
    One thing to be extremely careful with is using cutting brakes at high speeds and on the street, as it can have deadly consequences.

  • Working Operation

On many tractors the brake is divided into two pedals, one for the left rear wheel and one for the right rear wheel. If you need to stop, you step on both pedals at once. But if you want to turn really sharp, you step on one pedal and turn the steering wheel, and the tractor will spin around the wheel that is locked up.

Kit

When you want to turn sharply in your 4×4, simply engage a cutting brake for one of the rear wheels and this will let the front axle pull you around the turn. This works very well in a vehicle that either has a selectable rear locker and/or a transfer case that allows you to engage the front axle only. If you have a full-time locker in the rear like a Detroit, you need to engage the front axle only, otherwise the rear tires will drive through the cutting brakes. If you have a rear selectable locker like an ARB, Ox, or E-locker that gives you an open differential when unlocked, you can unlock the rear locker but still engage four-wheel drive when you use one rear cutting brake. This will send the power to the three unlocked wheels and you will pivot around the locked one.

 

Kit

In some situations you can achieve an even tighter turning radius if you can unlock the rear locker, put your transfer case in front-wheel-drive only, and lock the cutting brake on the rear tire opposite of the direction you want to turn. This will allow the inside rear tire to actually turn backwards as you pivot around the outside rear tire while your front tires pull you around. However, this maneuver often requires that you be pointed up a hill, and you must let the front tires spin and actually lose traction so you can slide back around the locked tire.

Kit

Another option is to use cutting brakes as a cheap traction tool. Say you have a cutting brake at each rear wheel, but you do not have a locking differential. You could be driving up a hill and one rear and one front wheel start to spin until you stop moving forward. By applying the cutting brake to the spinning wheel, the open differential will send power to the other wheel, and if it has traction it will begin pulling you up the hill. This is sort of a poor man’s traction control.

 

Kit

Of course placement of the lever needs to be easy for the driver to reach. Many buggies have them mounted between the seats, between the driver’s legs, or between the driver seat and the sidebars.

Kit

Kit

Another cool trick we’ve seen is using a line lock (also known as a roll control or roll stop) such as this one. It can be plumbed right on the axle and is activated either electronically or by switching a valve. Simply step on the brake pedal to apply the brakes, flip a switch that activates a solenoid, or close the line-lock valve-depending on the design you’re using-and it holds the pressure in the wheel cylinder to lock the wheel.

Kit

We have also seen trail-rig builders taking tricks from the tractors by putting dual brake pedals in their rigs. This allows them to lock either the front axle or the rear axle. Locking both brakes on the axle still helps in doing tight turns with the front axle driving only (commonly referred to as a “front dig”), but not as well as locking each individual corner.