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Types of brakes: apparatuses used to slow or stop a
moving vehicle.
Drum brake: mechanism that slows and stops a car by
fiction, by pressing brake shoes against a drum.
Drum: cylindrical part attached to the wheel, against
which the brake shoes are pressed to stop the car.
Brake lining: frictional part on the outside edges of
the brake shoes.
Return spring: part of the brake mechanism that
returns the brake shoes to their initial position.
Piston: cylindrical part that transmits the pressure
to and receives pressure from the brake shoes.
Wheel cylinder: type of roller that applies a uniform
pressure to the wheel then the brake is activated.
Brake shoe: part on which the brake lining is
mounted.
Brake pads: part activated by the piston.
Wheel hub: central part crossed by the axel.
Stud: metal pin.
Disk: round, flat, piece of metal, pressed against
the wheel to slow or stop the car.
Brake line: system liquid-transporting tubes.
Splash shield: protector that prevents dirt from
fouling the braking system.
Disc brake: mechanism that slows and stops a car by
friction, by pressing a disk against the wheel axel. |
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Power brakes (also known as power assisted
brakes) are designed to use the power of the engine and/or
battery to enhance your braking power. Whilst you can
generate a fair amount of force using your foot, using
systems from elsewhere in the car to help you apply even
more force means that you get more powerful brakes as a
result.
The four most common types of power brakes are:
1) Vacuum
suspended system works when you press the brake pedal, the
push rod to the master cylinder opens a vacuum control valve. This
allows vacuum pressure (normally from the intake manifold) to "suck"
on a diaphragm inside the vacuum assist unit. This extra vacuum
suction helps you to produce more force at the pedal end of the
brake system.
2) Air suspended
3) Hydraulic booster
systems usually utilize pressure from the
power steering system to augment pressure on the master
brake cylinder.
4) Electro-hydraulic booster
systems use an electric motor to pressurize the hydraulic system
downwind of the brake pedal which has the effect of amplifying the
internal pressure in the whole system. With vacuum-assist brakes, no
engine means no assistance.
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Brake master cylinders are complicated
affairs involving finely manufactured parts, minute
tolerances, springs, o-rings and rubber seals. When you step on the brake, its
connected to the main plunger. As this is pushed into the
master cylinder it acts on the components inside. The rear
plunger is the
first one to start moving. As it moves forward, brake fluid
from the reservoir is sucked in through the fluid intake and
return port. At the same time, fluid is sucked in through
the equalization port. As the second circuit rear seal
passes the intake and return port, it creates a fixed volume of fluid
between the rear and front plungers. The more you step on
the brake pedal, the more this fluid is now forced out into
the second brake circuit to apply those brakes. At the same
time, the pressure building up in this area overcomes the
strength of the first circuit return spring and the front
plunger begins to move too. As with the rear plunger,
it too sucks fluid from the reservoir until the first
circuit rear seal passes the fluid intake and return port, trapping fluid between it and the front
of the master cylinder. This fluid is then forced out into
the first brake circuit, applying those brakes.
When you take your foot off the brakes, the return springs
push the plungers back into their neutral position. Fluid
returns to the brake fluid reservoir and the system goes
back to an un-pressurized state.
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ABS at Work
Stop without skidding, and maintain control
of the vehicle. That's the premise of ABS. It was first
introduced in the 1980's and has been undergoing constant
refinement ever since. The system is typically comprised of 4
ABS rings, 4 sensors, an ABS computer and a number of
pressure-management circuits in the brake lines. The ABS rings
are attached either to the wheels, or more often, to the brake
discs.
The sensors are magnetic field sensors which are held very
close to the ABS rings and can detect the slight change in
magnetic field as the teeth on the ring pass them. The
pulsing field tells the ABS computer that the wheels are
spinning, and how fast they're spinning.
When you brake, the wheel rotation starts to slow down. The
ABS computer "listens" to the input from the sensors and can
detect if one wheel is slowing down much quicker than the
others - the precursor to the wheel locking up. When the computer
detects this condition, a pressure regulator in the brake
circuit interrupts the pressure in the brake lines by
momentarily reducing it so that the brakes release just
enough to give the wheels a chance to keep spinning rather
than locking up. The computer then instructs the regulator
to re-apply full pressure and again measures the wheel
rotation. This on/off/measure cycle happens around 15 to 30
times a second. If the ABS kicks in, you'll feel it through
the brake pedal as a vibration because the pulsing in the
brake circuit affects all the components.
There are many different variations and control algorithms for
ABS systems. We will discuss how one of the simpler systems
works.
The controller monitors the speed sensors at all times. It is
looking for decelerations
in the wheel that are out of the ordinary. Right before a wheel
locks up, it will experience a rapid deceleration. If left
unchecked, the wheel would stop much more quickly than any car
could. It might take a car five seconds to stop from 60 mph under ideal
conditions, but a wheel that locks up could stop spinning in
less than a second.
The ABS controller knows that such a rapid deceleration is
impossible, so it reduces the pressure to that
brake until it sees an acceleration, then it increases the
pressure until it sees the deceleration again. It can do this
very quickly, before the
tire can
actually significantly change speed. The result is that the tire
slows down at the same rate as the car, with the brakes keeping
the tires very near the point at which they will start to lock
up. This gives the system maximum braking power.
When the ABS system is in operation you will feel a
pulsing in the brake pedal; this comes from the rapid
opening and closing of the valves. Some ABS systems can cycle up
to 15 times per second.
There are four main components to an ABS system:
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Speed sensors
-
Pump
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Valves
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Controller
Speed Sensors
The anti-lock braking system needs some way of knowing when a
wheel is about to lock up. The speed sensors, which are located
at each wheel, or in some cases in the
differential, provide this information.
Valves
There is a valve in the brake
line of each brake controlled by
the ABS. On some systems, the valve has three positions:
In position two, the valve
blocks the
line, isolating that brake from the master cylinder. This
prevents the pressure from rising further should the driver
push the brake pedal harder.
In position three, the valve
releases
some of the pressure from the brake.
Pump
Since the valve is able to release pressure from the brakes,
there has to be some way to put that pressure back. That is what
the pump does; when a valve reduces the pressure in a line, the
pump is there to get the pressure back up.
Controller
The controller is a computer in the car. It watches the speed
sensors and controls the valves.
Anti-Lock Brake Types
Anti-lock braking systems use different schemes depending on the
type of brakes in use. We will refer to them by the number of
channels and the number of speed sensors.
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Four-channel, four-sensor ABS - This is
the best scheme. There is a speed sensor on all four wheels
and a separate valve for all four wheels. With this setup,
the controller monitors each wheel individually to make sure
it is achieving maximum braking force.
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Three-channel, three-sensor ABS
- This scheme, commonly found on pickup trucks with
four-wheel ABS, has a speed sensor and a valve for each of
the front wheels, with one valve and one sensor for both
rear wheels. The speed sensor for the rear wheels is located
in the rear axle.
This system provides individual control of the front
wheels, so they can both achieve maximum braking force. The
rear wheels, however, are monitored together; they both have
to start to lock up before the ABS will activate on the
rear. With this system, it is possible that one of the rear
wheels will lock during a stop, reducing brake
effectiveness.
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One-channel, one-sensor ABS -
This
system is commonly found on pickup trucks with rear-wheel
ABS. It has one valve, which controls both rear wheels, and
one speed sensor, located in the rear axle.
This system operates the same as the rear end of a
three-channel system. The rear wheels are monitored together
and they both have to start to lock up before the ABS kicks
in. In this system it is also possible that one of the rear
wheels will lock, reducing brake effectiveness.
This system is easy to identify. Usually there will be
one brake line going through a T-fitting to both rear
wheels. You can locate the speed sensor by looking for an
electrical connection near the differential on the rear-axle
housing.
Skid Control
On a wet or greasy road
surface where the traction is severely reduced, an ABS system
can pulse the brakes and prevent lockup much better than a human
can. The whole point of brakes is to slow you down. To do
that they rely on friction in two places - between the brake
pads and the rotors, and between the tires and road surface. If
one of those factors is taken out of the equation, the brakes
become useless. The most typical situation is that a driver will
panic-react to something and step on the brakes with as much
power as they can muster. The brake system amplifies this power,
grabs hold of the brake rotors and the wheels stop turning
almost instantly. This causes the tires to now skid across the
road surface, and as they do so, they become subject to dynamic
attrition. In other words, if a tire is rotating and gripping
the road, the "stick" factor is much higher than if the wheel is
locked and skating across the same surface. So that's what ABS
does - in an emergency, it ensures that the wheels don't lock up
but instead keep spinning so that the tires maintain grip with
the road. This is where the real benefit of ABS comes into play.
If you're going to attempt to avoid an accident, the best thing
to do is to try to steer around it. If your tires are skidding
on the road surface, you can point your wheels pretty much
wherever you want because the actual direction you end up going
will have nothing to do with the wheels and everything to do
with the direction you
were traveling, combined with the camber of the road.
Once the tires lose grip, all bets are off. With ABS, if those
wheels keep turning and the tires keep gripping, then when you
ham-fistedly grab the steering and yank it to one side, the car
will still turn and you might be able to avoid the accident. So
that's the true essence of ABS - to maintain control over the
direction of the car.
Snow, Ice and Gravel
ABS by its very nature is designed to stop the wheels from
skidding by allowing them to keep turning. On deep packed
snow and ice, that's exactly what they're going to do -
skid, so ABS effectively removes a considerable amount of
your braking in an emergency in these conditions. It's why
some cars have ABS disable systems for snow and ice, and
it's why ice racers yank the fuse to the ABS system before
they even get in a car to race.
The
ABS Education Alliance, a group aiming to help educate
drivers on how ABS will best benefit them, has this to say
on the subject:
Even in fresh snow conditions, you gain the advantages of
better steerability and stability with four-wheel ABS than
with a conventional system that could result in locked
wheels. In exchange for an increased stopping distance,
the vehicle will remain stable and maintain full steering
since the wheels won't be locked. The gain in stability
makes the increase in stopping distances an
acceptable compromise for most drivers.
So the short answer to this debate is that ABS is worse in
snow and ice for overall stopping distance, but better for
controlability.
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The three main types
of brake fluid now available are DOT3, DOT4 and DOT5. DOT3 and
DOT4 are glycol-based fluids, and DOT5 is silicon-based. The
main difference is that DOT3 and DOT4 absorb water, while DOT5
doesn't.
One of the important characteristics of brake fluid is its
boiling point.
Hydraulic
systems rely on an incompressible fluid to transmit force.
Liquids are generally incompressible while gases are
compressible. If the brake fluid boils (becomes a gas), it will
lose most of its ability to transmit force. This may partially
or completely disable the brakes. To make matters worse, the
only time you are likely to boil your brake fluid is during a
period of prolonged braking, such a drive down a mountain --
certainly not the best time for brake failure!
As a DOT3 or DOT4 brake fluid absorbs water, its boiling
point decreases. It can absorb water from the air, which is why
you should avoid opening your car's brake fluid reservoir. For
the same reason, you should always keep containers of brake
fluid tightly sealed.
DOT5 fluid does not absorb water. This means the boiling
point will remain relatively stable, but it also means that any
water that does get into your brake system will tend to form
pure water pockets, which could cause brake
corrosion.
Two other important things about brake fluid: DOT3 and DOT4
eat paint, so don't spill it on your car. Also, none of the
different types of brake fluid should be mixed. They can react
badly with each other and corrode your brake system.
D.O.T Ratings
All brake fluids are DOT rated. Your owners
handbook for your car or motorbike probably tells you to use
DOT3 or DOT4 from a sealed container. The DOT ratings are a
set of minimum standards the fluid must adhere to in
order to get the rating, and thus work in your braking
system. The following table shows the various properties of
DOT ratings. Remember that the values here are the minimum
values. Most manufacturers make sure their product far
exceeds minimum ratings.
| Boiling Point |
DOT 3 |
DOT 4 |
DOT 5 (silicone-based) |
DOT 5.1 (non-silicone based) |
| Dry |
4010F |
4460F |
5000F |
5000F |
| Wet |
2840F |
3110F |
3650F |
3650F |
The "dry" and "wet" boiling points in the
table above are for brake fluid which is fresh from the
bottle (dry) and which has a 10% water content (wet). A DOT
study in 2000 discovered that on average, the brake fluid in
a vehicle absorbs about 2% water every 12 months.
The two types of brake fluids shown in the table are
DOT3/DOT4/DOT5.1 which are glycol (Polyalkylene Glycol
Ether) based, and DOT5 which is silicone based. DOT3 and
DOT4 fluids are interchangeable - the only real difference
is their boiling point. Theoretically you could interchange
DOT4 and DOT5.1 fluids too but I wouldn't recommend it.
DOT3/4/5.1 and DOT5 fluids cannot be mixed or interchanged
under any circumstances. They mix like oil and water and the silicon based fluids can destroy the
seals in brake systems which rely on the moisturizer
additives that are present in DOT3/4/5.1 fluids.
Other things you ought to know about silicone based fluids:
- they are resistant to absorbing water, which is why their
wet boiling points are so high. Problem is that any water
content eventually pools in the low spots of the brake
system and causes rust.
- they don't strip paint.
- they are not compatible with most ABS system because they
doesn't lubricate the ABS pump like a glycol based fluid.
- putting this fluid in systems which have had DOT3/4 fluid
in will cause the seals in the caliper and master cylinders
to malfunction.
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The
method by which the force from your hand or foot reaches the
brake itself is all to do with the brake actuator system.
Cable-Operated
A cable
is connected to a lever at each end. You press on one lever
with your foot or squeeze it with your hand, and it pulls
the lever at the other end. On the back of the brake-end
lever there's an elliptical cam which rotates inside a
circular cup in the brake shoe. As the long axis of the
ellipse rotates, it forces the brake shoes to move apart. In
the case of a bicycle brake, the brake-end of the cable just
pulls the two calipers together.
Solid Bar Connection
One step up from
the cable-operated, and found on the rear brake of
older motorbikes, the solid bar connection. This allows the
use of mechanical advantage to amplify your
force on the pedal or lever before it gets to the brakes
themselves. Typically these systems are used on drum brakes
with the elliptical actuator described above. The
disadvantage of this system is that it needs hinge and pivot
points that match the position of the suspension components.
Single-Circuit Hydraulic
Another step up and we get to the type of
brake system used on most cars and motorbikes today. Gone
are the cables and bars, replaced instead with a system of
plungers, reservoirs and hydraulic fluid. Single-circuit
hydraulic systems have three basic components - the master
cylinder, the slave cylinder and the reservoir. They're
joined together with hydraulic hose and filled with a
non-compressible hydraulic fluid.
When you press your foot on the brake, or squeeze the brake
lever, you compress a small piston assembly in the master
cylinder. Because the brake fluid does not compress, that
pressure is instantaneously transferred through the
hydraulic brake line to the slave cylinder where it acts on
another piston assembly, pushing it out. That slave assembly
is either connected to a lever to activate the brakes, or
more commonly, is the brake caliper itself, with the slave
cylinder being the piston that acts directly on the brake
pads. Because of the arrangement of the slave cylinder, heat
from the brakes can be transferred back into the brake
fluid.
Dual-Circuit Hydraulic
Dual-circuit hydraulic systems are available
on high-end luxury vehicles and newer motorbikes, in
particular BMW bikes. These have two separate circuits. One
is the command circuit - that's the one you act on with your
hand or foot. The second is a separate circuit controlled by
an onboard computer, and that's the one which is actually
connected to the brakes. As you apply the brakes, you're
sending a pressure signal via the command circuit to the
brake computer. It measures the amount of force you're
applying, and using a servo / pump system, applies the same
force to the secondary circuit to activate the brakes. If
you do something stupid like trying to slam on the brakes at
100mph, the computer will realize that this would result in
a skid or spin, and will not send the full pressure down the
secondary circuit, instead deciding to use it's speed and
ABS sensors to determine the optimal brake pressure to
maintain control of the vehicle. The advantage of a
dual-circuit system is that the command circuit never gets
heat transferred into it because it is totally separated
from the brakes themselves. The disadvantage of course is
that you now have two hydraulic circuits to maintain.
Brake-By-Wire
The most advanced system of brakes to date
are brake-by-wire. These are a direct copy of some styles of
racing brakes and are very similar to the dual-circuit
hydraulic system described above, but instead of the command
circuit being hydraulic, its replaced with electronics. The
brake pedal or lever is connected to a hypersensitive
rheostat (measures electrical resistance). The more you push
it, the greater the electrical signal sent to the brake
computer. From there on, it performs just like the secondary
circuit described above. The advantage to this system is
that the brake pedal or lever can be placed just about
anywhere you like as it no longer is encumbered by the
plumbing that goes with a hydraulic circuit. To combat
driver complaints of "lack of feel" in the brakes, most
brake-by-wire systems have a reverse feedback loop built in.
This measures the pressure being applied to the brakes on
the secondary circuit, and actuates an electrical resistor
in the pedal or lever assembly to provide resistance. This
is needed because there is no physical connection to
any part of the brake system at all.
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Brake pads are steel backing plates with friction material bound
to the surface facing the brake disk. These pads convert the
kinetic energy of the car to thermal energy through friction.
When a brake pad touches a drum or a rotor, it becomes heated.
This causes it to transfer small amounts of friction material to
the disc or pad. The brake rotor and disk, which now have
friction material on them, will stick to each other and provide
stopping power.
The Four Main Types of Brake
Pads
Semi-Metallic
Semi-metallic brake pads contain a mix of 30 to 65 percent of
metal and typically include chopped steel wool or wire, iron
powder, copper or graphite mixed with inorganic fillers. They
also contain friction modifiers that bond all the components
together. These pads have a reputation of being durable and of
having excellent heat transfer; however, they wear rotors down
quickly, are noisy and may not perform up to par in cooler
temperatures.
Non-Asbestos Organic
Sometimes listed as organic or NAO, these types of brake pads
are made from fibers such as glass, rubber, carbon and Kevlar.
In addition, non-asbestos organic brake pads have filler
materials and high-temperature resins. These pads are softer and
quieter than other types of pads, but they wear faster and
create more brake dust.
Low-Metallic NAO
These types of brake pads are made from an organic formula mixed
with small (10 to 30 percent) amounts of copper or steel to help
with heat transfer and provide better breaking. With the added
metal, there is more break dust and they might be slightly
noisier.
Ceramic
These are composed of ceramic fibers, nonferrous filler
materials, bonding agents and possibly small amounts of metal.
They are lighter in color and more expensive than other brake
pads and are cleaner and quieter. In addition, they offer
excellent braking without wearing down the rotors. Most
ceramic-based linings perform well in a wide variety of areas;
but for some, other materials work just as well if not better.
Ceramic is not a generic term for a type of friction material
it is a description that covers a wide spectrum of friction
materials. The only thing they have in common is that they
contain some kind of ceramic as an ingredient.
Finding the Best Brake Pad
Some manufacturers believe in a best-fit method of installing
brake pads: no one brake pad will perform the best on each and
every vehicle on the market. Finding the brake pad that works
best for your car may take trial and error to discover which
material and types of brake pads you prefer.
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In a disc brake, the brake pads
squeeze the rotor instead of the wheel, and the force is transmitted
hydraulically
instead of through a cable. Friction between the pads and the
disc slows the disc down.
A moving car has a
certain amount of kinetic energy, and the brakes have to
remove this energy from the car in order to stop it. How
do the brakes do this? Each time you stop your car, your
brakes convert the kinetic energy to heat generated by
the friction between the pads and the disc. Most car
disc brakes are vented.
Vented disc brakes have a set of
vanes, between the two sides of the disc, that pumps air through
the disc to provide cooling.
Disc brakes are again a
two-part system. Instead of the drum, you have a disc or
rotor, and instead of the brake shoes, you now have
brake caliper assemblies. The caliper assemblies contain
one or more hydraulic pistons which push against the
back of the brake pads, clamping them together around
the spinning rotor. The harder they clamp together, the
more friction is generated, which means more heat, which
means more kinetic energy transfer, which slows you
down.
Standard disc brakes have one
or two cylinders in them - also know as one or two-pot
calipers. Where more force is required, three, or more
cylinders can be used. The disadvantage of disc brakes is
that they are extremely intolerant of faulty workmanship or
bad machining.
The floating rotor
Standard brake rotors are cast in a single piece which bolts
directly to the wheel or drive plate. If the mounting
surface of your wheel or drive plate isn't perfectly flat,
you'll get vibration at speed. Floating rotors are typically
cast in two pieces - the rotor and the carrier. The carrier
is bolted to the wheel and the rotor is attached to the
carrier using float buttons. The other method of floating a
brake rotor is to have the rotor bolted directly to the
wheel itself without a carrier, but the bolts have float
buttons built into them.
These buttons allow the brake
rotor some freedom to move laterally, but restrict the
angular and rotational movement as if they were bolted
directly to the wheel. This slight lateral motion which can
be less than 0.03mm, is just enough to prevent vibration in
the brake system. Because the calipers are mounted solidly,
and warping or misalignment in the wheel or brake rotor
mounting face can be compensated for because the rotor will
"float" laterally on the float buttons. This side-to-side
vibration is separated from the carrier by the float buttons
themselves, so none of the resulting motion is transferred
into the suspension or steering.
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A DRUM brake is a brake in which the friction is caused by a set
of shoes or pads that press against the inner surface of a
rotating drum. The drum is connected to a rotating wheel.
In the first drum brakes in cars the shoes were mechanically
operated with levers and cables.
The shoes in drum brakes are subject to wear and the brakes
needed to be adjusted regularly. In the 1960s and 1970s brake
drums on the front wheels of cars were gradually replaced with
disc brakes.
Another type of drum brake is where a friction belt is wrapped
around the outside of the drum and tightened. This type was used
for the parking brake on the central drive shaft. This type of
band brake is also used in automatic transmissions and aerobic exercise cycling equipment.
Single Leading Edge
Two semicircular
brake shoes sit inside a spinning drum which is attached to
the wheel. When you apply the brakes, the shoes are expanded
outwards to press against the inside of the drum. This
creates friction, which creates heat, which transfers
kinetic energy, which slows you down. The actuator in this case is the blue
elliptical object. As that is twisted, it forces against the
brake shoes and in turn forces them to expand outwards. The
return spring is what pulls the shoes back away from the
surface of the brake drum when the brakes are released. See
the later section for more information on actuator types.
The "single leading edge" refers to the
number of parts of the brake shoe which actually contact the
spinning drum. Because the brake shoe pivots at one end,
simple geometry means that the entire brake pad cannot
contact the brake drum. The leading edge is the term given
to the part of the brake pad which does contact the
drum, and in the case of a single leading edge system, it's
the part of the pad closest to the actuator. The
shoes are pressed outwards and the part of the brake pad
which first contacts the drum is the leading edge. The
action of the drum spinning actually helps to draw the brake
pad outwards because of friction, which causes the brakes to
"bite". The trailing edge of the brake shoe makes virtually
no contact with the drum at all. This simple geometry
explains why it's really difficult to stop a vehicle rolling
backwards if it's equipped only with single leading edge
drum brakes. As the drum spins backwards, the leading edge
of the shoe becomes the trailing edge and thus doesn't bite.
Double Leading Edge
The drawbacks of the single leading edge
style of drum brake can be eliminated by adding a second
return spring and turning the pivot point into a second
actuator. Now when the brakes are applied, the shoes are
pressed outwards at two points. So each brake pad now has
one leading and one trailing edge. Because there are two
brake shoes, there are two brake pads, which means there are
two leading edges. Hence the name double leading edge. |
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In cars with disc brakes on all four
wheels, an emergency brake has to be actuated by a separate
mechanism than the primary brakes in case of a total primary
brake failure. Most cars use a cable to actuate the
emergency brake.
Some cars with four-wheel disc brakes have a separate
drum brake
integrated into the hub of the rear wheels. This drum brake is
only for the emergency brake system, and it is actuated only by
the cable; it has no hydraulics.
Other cars have a lever that turns a screw, or
actuates a cam, which presses the piston of the disc brake.
The theory behind anti-lock brakes is simple. A
skidding
wheel has less traction
than a non-skidding wheel. If you have been stuck on ice, you
know that if your wheels are spinning you have no traction. This
is because the contact patch is sliding relative to the ice. By keeping the wheels from
skidding while you slow down, anti-lock brakes benefit you in
two ways: You'll stop faster, and you'll be able to
steer
while you stop.
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The single-piston floating-caliper disc
brake is self-centering and self-adjusting. The
caliper is able to slide from side to side so it will move to
the center each time the brakes are applied. Also, since there
is no spring to pull the pads away from the disc, the pads
always stay in light contact with the rotor (the rubber piston
seal and any wobble in the rotor may actually pull the pads a
small distance away from the rotor). This is important because
the pistons in the
brakes are
much larger in diameter than the ones in the master
cylinder. If the brake pistons retracted into their
cylinders, it might take several applications of the brake pedal
to pump enough fluid into the brake cylinder to engage the brake
pads.
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Squealing brakes are a sign of one of two things : the
friction material is all gone and you're jamming the backing
plate against the brake rotor, or the fit of the brake pad
against the caliper piston isn't as snug as it could be.
Either way, the squealing is the result of an extremely
high-frequency vibration between the pad, the caliper piston
and the brake rotor. Some vehicles have problems with squeaky
brakes right from the factory. In those cases, simply
changing brake pad manufacturer can often cure the problem
as the different pads will have a slightly different
harmonic frequency, which is harder to attain.
Solving brake squeal
A good way to solve brake squeal is to put
some copper-based grease on the BACK of your brake
pads. Copper grease is extremely resistant to pressure
and heat and if you get any on the front of your pads,
you'll need new pads and rotors or discs. The idea is that
it creates a small pocket of sticky lubrication between the
front side of the brake pistons and the back side of the
brake pads. This is usually enough to prevent the
high-frequency squeal.
Copper grease and rubber
While copper grease works
well in the short term to solve brake squeal, long-term, it
has an adverse affect on the rubber dust seals of the
caliper pistons. This can lead to the seal deteriorating or
failing completely. If that happens, it leaves the piston
and it's surface exposed to the very elements from which it
should be protected.
The other solution to brake squeal
While the ultra high frequency vibration is
one cause of brake squeal, the other biggie is related to
suspension alignment. Driving on badly-maintained roads,
mountaineering through pot-holes or kerbing your wheels all
make the suspension move around in ways it was never really
designed to cope with, and this in turn leads to the
suspension bushes becoming stressed. Normally, re-aligning
the wheels on a vehicle is corrected by mechanical
adjustment only. If the mounting rubbers are not de-stressed
first, then it leads to the transfer of the sound generated
during braking into the chassis and body which then
amplifies it to where we can hear it. If you have squealing brakes
that copper grease doesn't solve, look into a proper
suspension realignment and possibly new suspension bushes.
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