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Abrasive Friction
The mechanical rubbing of the brake pad material directly onto the rotor
disc,
resulting in the mechanical wear of both pad and rotor.
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 tires under braking force by cycling the line pressure to the
wheel that is approaching lock up. Most current passenger cars are fitted
with ABS.
Adherent Friction
The transfer of a thin layer of brake pad material, which sticks (adheres)
to the rotor face. The layer of pad material, once evenly established on
the rotor, is what rubs on the brake pad. The bonds that are broken, for
the conversion of Kinetic to Thermal energy, are formed instantaneously
before being broken
again.
Aluminum Beryllium
A composite material of exceptional stiffness to weight ratio used for
Formula One calipers (and Ilmor/Mercedes engine blocks) in the late 1990s.
Now outlawed on health grounds.
Aluminum Lithium
A composite material of exceptional stiffness to weight ratio currently
being used for Formula One calipers.
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 tires 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 plates
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.
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. Now outlawed on health grounds.
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Backing plate
The steel portion of a disc brake pad which contacts the caliper piston(s)
and to which the friction material is bonded. The backing plate provides
the necessary stiffness and mechanical strength to the pad system. Its
dimensions, flatness and surface finish must be closely controlled. The
longer the pad, the thicker the backing plate must be. Backing plates of
less than about 3 mm should be viewed with suspicion unless the pad is
very short.
Bedding in
Also called “breaking in” or “break-in”. There are two types of "bedding
in" with regard to brakes:
1. Bedding of the friction material : All friction materials contain volatile
elements used as binders. In the initial thermal cycling of the material
these volatiles boil off, forming a gaseous layer between the friction material
and the brake. A bedded pad will exhibit a layer of discolored material from
1.5 to 3 mm thick.
2. Bedding of the disc : Before using a new disc, all machining and preservative
oils must be completely removed following the disc manufacturer's recommendations.
Usually it involves washing with soap and water or using one of the proprietary "brake
clean" compounds. The disc should then be mounted and checked for run out.
It should be bedded in with a number of moderate stops with lots of cool
off time between stops, gradually increasing the severity of the stops until
the entire surface of the disc is evenly discolored. This will prevent thermal
shock, distortion and the formation of "hot spots" (regional deposition of
pad material which results in a permanent transformation of the cast iron
underneath the deposit) and ensure maximum disc life.
Bell : See "hat".
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.
Bite
The speed at which the friction material reaches its maximum coefficient of
friction when braking is initiated. The amount of bite is a compromise. Too
much bite makes initial modulation difficult. Too little causes a delay in
braking. In racing, different drivers prefer pads with different degrees of
bite.
Blades
See "vanes".
Bleeding
The process of removing existing brake fluid from the hydraulic system while
simultaneously replacing it with fresh fluid. This is typically done to remove
overheated fluid and/or air bubbles from the hydraulic circuit(s) shortly following
hard use, but should be performed on a regular basis as well due to the natural
tendency of brake fluid to absorb water over time.
Bluing
Discoloring 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.
Brake bias
The term used to indicate the ratio between the amount of brake torque exerted
on the front brakes compared with the rear. Brake bias is normally expressed
as a percentage of brake torque at one end of the car to the total brake torque,
as in "60% front".
Brake booster
A vacuum assist device that amplifies pedal force. In some cases, this assist
is accompanied by small increases in pedal travel and reductions in pedal firmness.
However, due to its compact design and efficiency, it is virtually universal
on passenger cars.
Braking efficiency
The ratio of actual deceleration achieved on a given surface compared with
the theoretical maximum.
Brake line pressure
The instantaneous hydraulic pressure within the brake lines. Brake line pressure
in pounds per square inch is the force applied to the brake pedal in pounds
multiplied by the mechanical pedal ratio (plus any booster assist where applicable)
divided by the area of the master cylinder piston in square inches. For the
same amount of pedal force, the smaller the master cylinder and/or the greater
the mechanical pedal ratio and/or booster contribution, the greater the brake
line pressure and the longer the pedal travel.
Braking torque
Braking torque in pounds feet on a single wheel is the effective rotor radius
in inches times clamping force in pounds times the coefficient of friction
of the pad against the rotor (a unit-less value) all divided by 12. Braking
torque is the force that actually decelerates the wheel and tire. To increase
the braking torque it is necessary to increase the line pressure, the piston
area (clamping force), the coefficient of friction, or the effective rotor
radius. Increasing the pad area will not increase the braking torque.
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Caliper
The "hydraulic clamp" portion of a disc brake system. Manufactured from either
ferrous or non-ferrous material and bolted firmly to the suspension upright
(or "knuckle") the caliper holds the pads in place and, through the action
of hydraulic pistons actuated by the master cylinder, forces them against the
rotating surface of the disc when pressure is applied to the brake pedal.
1. 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 center. Due to its inherent
stiffness the fixed caliper is the only design suitable for racing categories
where it is allowed and is the preferred design for high performance cars.
However, it's commensurate increased size, cost, and weight prevents its
widespread use on passenger vehicles.
2. Floating caliper : A design in which a single or dual piston is located
inboard of the disc and the outer body of the caliper slides on suitable
surfaces in reaction to piston pressure. The piston forces the inboard pad
against the disc while the sliding outer body clamps the outboard pad against
the disc. The inherent lack of rigidity in the design, compared to fixed
caliper design, combined with the friction inherent in the sliding outer
body makes this design less suitable for racing and high performance use.
The design is well suited for use with front wheel drive as the absence of
any outboard pistons allows greater negative (inward) wheel offset. In all
applications, this caliper type is simpler to manufacture and affords more
packaging flexibility for zero or even negative scrub radius front suspension
designs. It is sometimes used in the rear on an application that has a fixed
design in the front.
3. 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.
4. Closed caliper : The design of fixed caliper in which the "window" through
which the pads are inserted is structurally reinforced by a bridge.
5. Caliper bridge : The structural reinforcement across the open face of
a fixed caliper. In order to be effective the bridge must be rigidly bolted
in place with high tensile fasteners.
Caliper Pistons
The hydraulic cups that transmit line pressure to the
pads to clamp the pads against the rotating disc. Manufactured from Aluminum,
steel or Titanium and sealed in the caliper bores, the mechanical design
of the piston is critical. The inescapable drag of the pad against the disc
tends to "cock" the piston in its bore so the piston to bore clearance, thermal
coefficients of expansion between piston and caliper as well as seal design
and location are crucial. Care should be taken in using pistons or seals
from a supplier other than an OEM. In all cases that a Stoptech part is listed
as a direct replacement for the OE part, it will work as well as the OE part.
Carbon/carbon brake
A braking system in which both discs and pads are manufactured from carbon
composite material. Utilized in every form of racing where they are not outlawed,
carbon/carbon brakes offer significant reduction in rotating mass and inertia
along with much greater thermal capacity and dimensional stability in use.
The disadvantages include cost, a certain amount of lag time while heat builds
up (especially in the wet) and some difficulty in modulation. Contrary to popular
belief, the coefficient of friction is no better than that of state of the
art carbon metallic pads and cast iron discs. A major advantage on super speedways
is the reduction of gyroscopic precession on corner entry.
Carbon metallic
This is a trademark of the Performance Friction Corporation. As such, it refers
to pad friction compounds containing large percentages of pure carbon along
with various metallic elements. Pioneered by Performance Friction Corporation,
these compounds offer very constant coefficients of friction vs. temperature
characteristics along with increased thermal capacity. The disadvantage is
that, since they both operate at higher temperatures and their temperature
rises to operating temperature faster than other compounds, they increase thermal
shock to the disc and increase thermal conduction to the caliper pistons and
brake fluid. As a result, it is not recommended to use drilled discs with carbon
metallic pads.
Cast iron
Metallic iron containing more than 2% dissolved carbon within its matrix (as
opposed to steel which contains less than 2%) and less than 4.5%. Because of
its cost, relative ease of manufacture and thermal stability cast iron (sometimes
referred to as "gray cast iron" because of its characteristic color, but is
actually a more specialized material for brake applications) is the material
of choice for almost all automotive brake discs. To work correctly, the parts
must be produced at the foundry with tightly monitored chemistry and cooling
cycles to control the shape, distribution and form of the precipitation of
the excess carbon. This is done to minimize distortion in machining, provide
good wear characteristics, dampen vibration and resist cracking in subsequent
use.
Ceramic buttons
Insulating buttons inserted in the face of racing caliper pistons to reduce
conduction of heat to the brake fluid. Not currently in use as Titanium buttons
have proved more effective.
Ceramic coatings
Some racing calipers feature a ceramic coating sprayed onto the interior surfaces
as a radiation barrier to reduce heat transfer from the disc and pads to the
caliper and fluid.
Clamping force
The clamping force of a caliper in pounds is the brake line pressure multiplied
by the total piston area of the caliper in a fixed caliper and two times the
total piston area in a floating design. To increase the clamping force it is
necessary to either increase the line pressure or the piston area. Increasing
the pad area or the coefficient of friction will not increase clamping force.
Coefficient of friction
A dimensionless indication of the friction qualities of one material vs. another.
A coefficient of 1.0 would be equal to 1g. The higher the coefficient, the
greater the friction. Typical passenger car pad coefficients are in the neighborhood
of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range. With most pads the
coefficient is temperature sensitive so claims that do not specify a temperature
range should be viewed with some suspicion. The optimum is to select a pad
with a virtually constant but decreasing coefficient over the expected operating
range of temperatures. As a result, the driver does not have to wait for the
pad to heat up before it bites, and the pad fade will not be a factor so that
modulation will be easy (see "plot shape").
Compressibility
All materials are compressible. Under enough pressure the rock of Gibraltar
will compress to some extent. It is important that the friction material of
the pad not compress significantly under the expected clamping force. If it
does, pad wear will be uneven and braking efficiency will be compromised. Compressibility
is seldom mentioned in advertising. It should be. Compressibility of a given
material and the wear rates are the two primary factors that are considered
in determining the size of a pad for a given application.
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 automobile braking system is transferred
to the caliper pistons and thence to the brake fluid by conduction. Some of
it is also transferred to the hub, upright (knuckle) bearings, and wheels in
the same way. Two-piece or floating discs reduce conduction to the hub, and
other parts because of the intervening hat. Conduction is also the strategy
used in all designs to move the heat from the disc pad interface to the vanes
and in some designs to provide the initial heat sink for the energy generated
at the start of a braking cycle. The sacrifices made if this is the primary
strategy used for heat transfer are the weight and inertial penalty in a rotating
part.
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 can be considered
to be a fluid in a thermal model of a brake system when it is moving and is
contact with the heated surfaces of the disc or drum. In the case of a solid
disc the air moving over the surface of the disc is very random and turbulent,
but still functions to provide some cooling. In the case of a ventilated disc,
by the pressure of a forced air duct or by induced flow that is a result of
the centrifugal acceleration of the air already in the vent of a rotating disc,
air flows through the vents. The air absorbs thermal energy along the vent
path. In this way, the heat generated by the braking system of an automobile
is transferred to the moving air stream and away from the brake disc.
Cracking
Cracking is primarily due to heat cycling that weakens the cast iron discs.
The exact mechanism of this failure is disputed. Cast iron discs are formed
with the excess carbon being precipitated in the form of carbon plates or flakes
dispersed throughout the ferrite (iron) matrix. What is believed to happen
is that when discs are operated above about 900º F, the carbon becomes
more flexible or "fluid" in its shape partly due to the thermal expansion of
the enclosing ferrite matrix. Then, as the disc cools relatively rapidly back
below about 900º F the carbon is trapped in a changed more random shape
then when it was first cast. This creates internal stress on the part and continuously
transforms the disc by relieving the stress through the cracking. The cracks
begin by appearing between carbon flakes. Nodular or ductile iron would resist
this cracking due to the excess carbon being precipitated in a spheroidal form,
but it, like other alternative materials do not have the mechanical properties
needed to function ideally in a brake disc application. In discs that are cast
to resist cracking through chemistry and controlled cooling at the foundry,
cracking will still occur, but more slowly and take the form of heat checks
on the surface. In some cases cracks will begin at the periphery of the disc
and propagate inwards. In this situation, propagation can be delayed by drilling
small holes at the end of the cracks (stop drilling). We do not recommend this
however, because if the cracks continue to propagate unnoticed, catastrophic
mechanical failure will result. Replace disc at the first sign of cracks at
the outer edge of any size. A historic note, the original purpose of the curved
or angled vane disc was to prevent cracks from propagating by imposing a solid
vane in the path of the crack. The cooling function was secondary.
Cryogenic treatment
A thermal process in which metallic components are
slowly cooled to near Kelvin temperature and then equally slowly returned
to room temperature. Proponents claim that the grain structure is refined
by the process. There is considerable doubt about the effectiveness of the
process. Evidence is largely anecdotal and to date no scientific and quantitated
engineering studies have been published.
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Differential bores
he leading edge of a brake
pad wears faster then the trailing edge. This is due to the migration of
particles of incandescent friction material carried from the leading to trailing
edge of the pad. In effect the trailing portion of the pad is riding on a
layer of incandescent material. 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
Disc
The rotating portion of a disc brake system. Mechanically attached
to the axle, and therefore rotating with the wheel and tire the disc provides
the moving friction surface of the system while the pads provide the stationary
friction surfaces. Except for racing, discs are normally manufactured from
one of several grades of cast iron. Some European front drive passenger cars,
where the rear brakes do very little work, are using aluminum metal matrix
rear discs to save weight. Most professional racing cars use carbon/carbon
discs.
1. 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.
2. Floating disc : The norm in racing, the floating or two-piece disc consists
of a friction disc mechanically attached to the hat either through dogs or
through drive pins. Properly designed this system allows the disc to dilate
(grow radially) without distortion and to float axially, greatly reducing
drag.
3. Solid disc : A disc cast as a solid piece suitable for light cars not
subjected to extreme braking.
4. Ventilated disc : A disc cast with internal cooling passages. The norm
in racing, high performance and heavy vehicles.
Drilled or 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. The advent of carbon metallic friction
materials with their increased temperatures and thermal shock characteristics
ended the day of the drilled disc in professional racing. They are still
seen (mainly as cosmetic items) on motorbikes and some road going sports
cars. Typically in original equipment road car applications these holes are
cast then finished machined to provide the best possible conditions by which
to resist cracking in use. But they will crack eventually under the circumstances
described in another section (see Cracking). Properly designed, drilled discs
tend to operate cooler than non-drilled ventilated discs of the same design
due the higher flow rates through the vents from the supplemental inlets
and increased surface area in the hole. That's right, inlets. The flow is
into the hole and out through the vent to the OD of the disc. If discs are
to be drilled, the external edges of the holes must be chamfered (or, better
yet, radiused) and should also be peened.
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.
Dust boots
Rubber shields that fit over the exposed portion of the caliper
pistons to prevent the ingress of dust and road grime. As no known rubber
compound will withstand the temperatures generated by racing brakes, dust
boots are not used in racing and should be removed before truly hard driving
for extended periods.
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EBD
Acronym for Electronic Brake Distribution.
See “Electronic Brake Distribution”.
Effective temperature range
The range of operating temperatures within
which a pad material remains effective or consistent. As with coefficient
of friction, this should be used for comparative purposes only as measurement
procedures very between manufacturers and pad temperatures are strongly affected
by disc mass and rate of cooling.
Electronic Brake Distribution
Electronic Brake Distribution systems differentially
control the line pressure applied to the front and rear axle brakes under
conditions that are similar 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. In
most cases, disabling a vehicle's ABS will also disable the EBD function.
Check with the vehicle manufacturers documentation on the status of this
important function if you plan to disable an ABS system. See "Proportioning
valve".
Ether based brake fluid
"Normal" 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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Friction consistency
The variation in coefficient
of friction over a range of repeated stops. Minimum variance allows efficient
brake modulation and happy race car drivers.
Friction Mechanisms
For a pad and disc to function as a brake there has
to be the conversion of kinetic energy to heat. There are two primary models
of the mechanism of this conversion; both involve the breaking of bonds to
release energy. In the case of the abrasion model the bonds broken are the
ones already existing in a materials. The bonds are broken due to the chafing
or abrasion of a harder material or particle in direct contact with it. The
second model is the adhesion-breakage model where temperature and pressure
at the interface between the pad and disc surface cause the fusion of one
material to the other or the diffusion of one material into the other. In
this case, the instantaneous bonds formed in the process are broken releasing
energy. Pad materials function using both models at the same time or at different
times. The abrasive mechanism predominates at lower temperatures but is also
necessary to control build-up of low melting point pad materials at elevated
temperatures where the adhesion-breakage mechanism is thought to predominate.
The adhesion-breakage model requires a transfer layer of pad material to
be established on the disc surface to function unless the brake system is
designed so that the disc is fusing into the pad. The latter is the case
with many high dusting European automotive designs where the disc wears observably
as the pad wears. The iron in these discs is typically a "softer" more dampened
form of cast iron.
Fade
Loss of braking efficiency from excessive thermal stress. There are
three separate and distinct types of brake fade:
1. 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.
2. Fluid boiling : When the fluid boils in the calipers, gas bubbles are
formed. Since gasses are compressible, the brake pedal becomes soft and "mushy" and
pedal travel increases. You can probably still stop the car by pumping the
pedal but efficient modulation is gone. This is a gradual process with lots
of warning.
3. 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 outgassed 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.
Fireband
The name given to the boundary layer of outgassed volatiles and
incandescent particles of friction material that rotate with the disc.
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Glycol brake fluid
See "ether based brake fluid"
Grooves
See "slotted"
Grooving
A wear pattern of concentric grooves on the surface of a disc.
This can be caused by inclusions within the pad material, inappropriate pad
material for the operating conditions, poor initial machining of the disc,
and/or improper bedding in procedure. Not a major cause for concern on passenger
cars.
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Heat checking
The precursor to cracking. Heat checks
are actually surface cracks caused by thermal stress. By themselves heat
checks are not a cause for concern but they are a warning sign that the disc
is not receiving adequate cooling air and cracks are sure to follow. See
also the section "Cracking".
Hydraulic ratio
The ratio of fluid displacement by the master cylinder
to fluid displaced in the caliper pistons. Hydraulic ratio is an important
factor in the pedal effort equation, the higher the ratio, the less pedal
effort is required (and the longer the pedal travel to achieve a given clamping
force). The stiffer the caliper and the stiffer the pad, the higher the hydraulic
ratio that can be employed.
Hygroscopic
The property of readily absorbing water. All non-silicon based
brake fluids are hygroscopic in nature. The adsorption of a minute amount
of water will dramatically lower the boiling point of brake fluid. For this
reason brake fluid should be completely replaced annually or more frequently
in conditions of severe use. In professional racing, the fluid is replaced
at least daily.
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Knockback springs
Small coil springs fitted inside
the caliper pistons of some brakes to prevent the pads from excessive knock
back from flexing of the suspension system or run out in the discs. If the
disc run out is within specification and the upright/axle assembly is sufficiently
rigid, there should be no need for knockback springs. However, when operating
conditions are severe with regard to either generated side force or bumps,
they may be required on the best of designs.
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Leading edge (of pad)
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".
Line pressure
See "Brake line pressure".
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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 (calipers).
Material transfer
Please see "Friction Mechanisms", where beneficial material
transfer is discussed in context with the adhesion-breakage model. Otherwise,
when the operating temperature of the pad (particularly organic pads) is
exceeded, friction material may be deposited onto the surface of the disc
in a non-uniform manner while degrading the braking capacity and causing
noticeable roughness. The only cure is to either upgrade the pad material
or increase the cooling (or both). "Pick up" should never be removed with
ordinary sandpaper, which uses aluminum oxide as the abrasive. The same is
true of sand blasting - don't do it. The correct way to remove pick up is
by grinding (not turning) the disc. When that is not practical, the major
portion can be removed by scraping and the remained sanded off with garnet
paper.
Mechanical pedal ratio
The brake pedal is designed to multiply the driver's
effort. The mechanical pedal ratio is the distance from the pedal pivot point
to the effective center 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)
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 Aluminum Ceramic metal matrix, the ceramic typically
but not exclusively being composed of Silicon Carbide, Aluminum Oxides and
Boron Carbides, which are well suited for use in racing calipers. Also, lightweight
disc have been made for Original Equipment and aftermarket applications using
Silicon Carbide and Aluminum Oxides, but with only limited success due to
two factors, the first being a low maximum operating temperature of the materials
mentioned of around 900o to 1000o F. Second the much greater expansion rate
of the typically aluminum based MMC material results in thermal distortion
or cracking. One OE application actually has small slots and stop drill holes
positioned radially around the disc periphery. In all cases of MMC discs,
the primary friction mechanism is the adhesion-breakage model. Please refer
to the section, "Friction Mechanisms".
Modulation
The term given by the process by which the skilled driver controls
the braking torque to maintain maximum retardation without locking wheels.
Because the human being modulates most efficiently by force rather than displacement,
effective brake modulation requires minimum pedal travel and maximum pedal
firmness.
Monobloc caliper
A caliper machined from a single piece of billet, cast
or forged material.
Multi pad systems
Caliper systems utilizing multiple pistons (either four
six or eight) with separate pads and abutment systems for each pad. The design,
almost universal in professional racing, provides multiple leading edges
for better "bite". At the same time the very short pads ensure even longitudinal
wear without the necessity for differential piston diameters.
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OE
This is an abbreviation for Original Equipment.
Please see the section "Original Equipment". Sometimes it is used as an abbreviation
to refer to the Original Equipment Manufacturer (but more correctly referred
to as the OEM).
OEM
This is an abbreviation for Original Equipment Manufacturer.
Off brake drag
A condition in which the caliper pistons 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.
Original Equipment
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.
Organic (pad material)
A family of friction materials, often containing
asbestos, used for both drum linings and disc pads through the 1980s. Now
largely supplanted by semi-metallic materials with better temperature characteristics,
but new non-asbestos organic (NAO) compounds can be found as well.
Out-gassing
The boiling off of the volatile elements in friction materials.
Outgassing, 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
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.
Pad abutments (or pad abutment plates)
Mechanical elements that locate
the pads in the caliper and provide a hard surface for the pads to slide
against. Non-ferrous (Aluminum or MMC) calipers, which do not provide a hard
and smooth surface to locate the ends of the pads and provide an efficient
sliding surface, should be viewed with great suspicion.
Pad retraction
To prevent drag and premature pad wear the properly designed
seal systems retract the caliper pistons a few thousandths of an inch 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.
Pick up
See the section "Material Transfer".
Plot shape
The shape of the friction plot during a long brake application.
It is easier and more efficient for the driver to add pedal pressure than
to remove it. Therefore the easiest pad to modulate exhibits a high initial
bite followed by a gradual decrease in coefficient throughout the stop. If
the level of friction rises throughout the stop, brake modulation will be
very difficult.
Pressure bleeder
A tool allowing rapid bleeding of the system and replenishment
of the fluid. Pressure bleeders should never be used on racing or high performance
cars as the rapid forcing of the fluid through small passages may cause cavitation
and the formation of air bubbles rather than their removal. At no time should
a pressure bleeder be used which does not contain a physical separation (either
through a flexible diaphragm or otherwise) between the brake fluid and the
pressurizing agent (air).
Proportioning valve
Its function is 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. After this
point, rear line pressure, while it still increases linearly with pedal effort,
increases at a lower rate (slope) than front. The purpose is to avoid rear
wheel lockup and the attendant unstable condition. It is not a good idea
to remove the proportioning valve from a road going automobile for highway
use. If you feel that you must do so, the best way is to remove the OEM rear
brake line proportioning valve completely and replace it with one of the
adjustable units manufactured by Tilton Engineering or Automotive Products.
Do not place a second proportioning valve in line with the OEM unit. Note
that ABS software in some cases can be calibrated to perform the proportioning
function, eliminating the need for a stand-alone valve. This feature if present
in an ABS system is sometimes referred to as Electronic Brake Distribution
(EBD) because it functions, as the name implies, to differentially control
the applied line pressure on the front and rear axle brakes under conditions
that are similar to the line proportioning valve. In most cases, disabling
the ABS systems will also disable the EBD function.
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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, thermal
energy is emitted by both the discs and the pads at elevated temperature.
In the case of the disc, as temperature increases, radiation is the predominant
heat transfer mechanism. The focus in disc designs is on the vent or the
vanes because it is something that can be changed significantly at a reasonable
cost. While radiation is a function of the material choices made, keeping
in mind the other functional parameters necessary for the application. A
large portion of this radiant energy can be reflected into the air stream
by "radiant barriers", such as ceramic coatings on the internal surfaces
of calipers.
Release characteristics
The opposite of "bite", release characteristics
become important when braking into turns either on the track or on the road.
If the braking torque does not decrease linearly with decreasing pedal pressure "trail
braking" becomes difficult at best.
Reservoir
The container in which brake fluid is stored to provide a source
of fluid for the master cylinder(s). The reservoir must have sufficient volume
to allow fluid displacement equivalent to wearing the pads down past the
backing plates. It must also be sealed to prevent the absorption of moisture
by the highly hygroscopic brake fluid. Typically the reservoir cap is fitted
with an elastomeric bellows open to atmosphere but sealed from the fluid.
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 (2-4psi.)
so off brake drag is not a problem for street use.
Rotor : see "Disc".
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 0.000 to 0.005" total run out. Excessive run out can result in inefficient
braking and perceptible pedal pulsation.
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Seals
Caliper pistons are sealed in their bores
by elastomeric rings seated in grooves. The seals fulfill a secondary function
of slightly retracting the pistons when line pressure reduced to zero at
the end of braking. This prevents "off brake pad drag", reducing both temperature
and wear. Both the compound and the mechanical design of these seals are
critical. The cross section of properly designed caliper seals is square,
not round. "O" rings cannot be substituted.
Seal grooves
The caliper seal grooves can be either in the caliper bore
or on the piston (or both). The mechanical design of the grooves is critical
to ensure optimum piston retraction. The cross section of a proper caliper
piston seal groove is trapezoidal, not square.
Semi-metallic : Friction materials compounded with significant amounts of
metallic elements to increase the operating temperature range.
Silicon brake fluid
Brake fluid based on silicon. While silicon based
fluids are less hygroscopic than ether based fluids, they are subject to "frothing" when
subjected to high frequency vibration and when forced through small orifices.
This makes them unsuitable for racing or high performance use.
Sliding caliper
See "Caliper, Floating Caliper".
Slotted
1. Disc : Shallow, sharp edged but radiused 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.
2. Pad : Radial grooves molded or cut into the surface of the pad to provide
a path for fire band dissipation and to double the number of leading edges
and improve bite. Some long pads also have a longitudinal groove.
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 aluminum 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 molding other than the primary injection
cylinder that is then compressed at high pressure to increase the molding
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 TeflonÔ protected
by a tightly braided cover of stainless steel wire. Because these hoses virtually
eliminate line swelling under pressure, and because they offer superior mechanical
protection for the brake line itself, they are universally used in racing
applications to reduce pedal travel, increase pedal firmness and allow more
efficient brake modulation. A few manufacturers offer stainless steel braid
protected hoses of extruded Teflon which meet all of the DOT requirements
for passenger car use. Several more offer hoses which claim to meet the specification
but do not. Let the buyer beware.
Stiffness
Stiffness is the resistance of a material or a structure to
deformation. It is not the same as strength. The stiffness of a material
is indicated by its "modulus of elasticity" - the measure of the elasticity
of the atomic bonds within the material. It is essential that calipers (and
caliper mountings) be stiff. Which is why metal matrix composite materials
are used for racing calipers.
Strength
Strength is the resistance of a material or structure to rupture.
It is defined as the stress required to rupture the atomic bonds of a material.
It is not the same as stiffness.
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Taper wear
Uneven wear of brake pads caused by
geometry, by the difference in temperature between leading and trailing edges
and/or by lack of stiffness in the caliper. When pads are taper worn, the
first increments of caliper piston travel are used up in forcing the pad
against the disc, increasing pedal travel. Additionally the piston tends
to cock in its bore resulting in bore scoring and wear.
1. Radial taper
Radial taper is apparent when the pad is viewed from either
end. The linear speed between pad and disc is greater at the periphery of
the disc and so the outer surface of the pad wears faster. In addition any
tendency for the caliper to "open up" under pressure like a clamshell results
in more pressure being placed on the outer portion of the pad, further increasing
relative wear. For this reason many pads are trapezoidal in plan view with
less surface area toward the inside.
2. Longitudinal taper
Longitudinal taper is apparent when viewed from
either the inner or outer surface of the pad. The trailing section of the
pad is partially floated in the boundary layer of outgassed volatiles and
incandescent particles of friction material torn from the leading section.
The leading edge of the pad will therefore always run hotter and wear faster
than the trailing edge. This phenomenon is more pronounced in long pads and
is one of the major reasons why racing calipers are designed with a multitude
of small pads. (See "Multi piston calipers").
Thickness Variation (TV)
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.
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.
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.
Trail braking
The process in which the skilled driver "trails off" the
brakes as he enters a corner, thus combining braking and turning in the initial
phase of the corner and maximizing the total traction available from the
tires. The technique, universal in racing, although not always admitted,
also effectively lengthens the straight preceding the corner.
Trailing edge (of pad )
That portion of the pad located away from the direction
of rotation of the disc.
Transfer Layer
An even layer of brake pad material on the rubbing surface
of the rotor disc. Note the emphasis on the word even, as uneven pad deposits
on the rotor face are the number one, and almost exclusive cause of brake
judder or vibration.
Two part (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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Vanes
The term given to the central webs which
serve to separate the inboard and outboard friction surfaces of ventilated
discs.
1. Straight vanes : Straight vanes are the easiest to manufacture. They
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.
2. Curved vanes: Curved vanes are shaped as curves to act as more efficient
pump impellers and increase mass 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 are more expensive to produce that straight vanes
and must be mounted directionally. They are universally used in racing where
carbon/carbon brakes are prohibited.
3. Islands : Some designs utilize "islands" to separate the friction surfaces
rather than vanes. Properly designed the island system is dimensionally stable
but inefficient from the viewpoint of airflow.
4. 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.
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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. As such, it needs to be replaced if the light has
come on. There is another less expensive method used where the pad has a
thin but stiff tab riveted to the pad backing plate that rubs on the disc
face and squeals when the wear limit is reached. In some modern race cars
used in long distance events, calipers are fitted with more complex electronic
sensors and circuitry to warn the drivers and, by telemetry, the crew of
the pad condition
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