Description:
Below is an explanation of this system's operation
The Brake System
The braking system is the most important system in your
car. If your brakes fail, the result can be disastrous. Brakes are
actually energy conversion devices, which convert the kinetic energy
(momentum) of your vehicle into thermal energy (heat). When you step on
the brakes, you command a stopping force ten times as powerful as the
force that puts the car in motion. The braking system can exert thousands
of pounds of pressure on each of the four brakes. In modern systems, the
master cylinder is power-assisted by the engine. All newer cars have dual
systems, with two wheels' brakes operated by each subsystem. That way, if
one subsystem fails, the other can provide reasonably adequate braking
power. Safety systems like this make modern brakes more complex, but also
much safer than earlier braking systems.
The brake system is composed of the following basic
components: The "master cylinder" which is located under the hood, and is
directly connected to the brake pedal, converts your foot's mechanical
pressure into hydraulic pressure. Steel "brake lines" and flexible "brake
hoses" connect the master cylinder to the "slave cylinders" located at
each wheel. Brake fluid, specially designed to work in extreme conditions,
fills the system. "Shoes" and "pads" are pushed by the slave cylinders to
contact the "drums" and "rotors" thus causing drag, which (hopefully)
slows the car.
In recent years, brakes have changed greatly in design.
Disc brakes, used for years for front wheel applications, are fast
replacing drum brakes on the rear wheels of modern cars. This is generally
due to their simpler design, lighter weight and better braking
performance. The greatest advantage of disc brakes is that they provide
significantly better resistance to "brake fade" compared to drum type
braking systems. Brake fade is a temporary condition caused by high
temperatures generated by repeated hard braking. It occurs when the pads
or shoes "glaze" due to the great pressure and heat of hard use. Once they
cool, the condition subsides. Disc brakes allow greater air ventilation
(cooling) compared to drum brakes. Drum brakes are not internally
ventilated because if they were, water could accumulate in them. Disc
brakes can rapidly fling off any water that they are exposed to, and so
they can be well ventilated.
"Boosters" are present in "power brake" systems, and use
the engine's energy to add pressure to the master cylinder. "Anti-lock"
(ABS) systems, originally developed for aircraft braking systems, use
computer controlled valves to limit the pressure delivered to each slave
cylinder. If a wheel locks up, steering input cannot affect the car's
direction. With ABS, no matter how hard the pedal is pressed, each wheel
is prevented from locking up. This prevents skidding (and allows the
driver to steer while panic-braking).
As impressive as these advances are, the basic process of
converting a vehicle's momentum into (wasted) heat energy has not changed
since the days of the horse and buggy. To stop a horse drawn carriage, the
driver would pull on a lever which would rub on the wheel. But today, with
the advent of regenerating brakes on electric vehicles, new ways of
recapturing this lost energy are being developed. In these types of
electric cars, when you step on the brakes, the motor switches into
"generator mode", and stores the car's momentum as chemical energy in the
battery, to be used again when the light turns green!
Disc Brakes
Disc brakes use a clamping action to produce friction
between the "rotor" and the "pads" mounted in the "caliper" attached to
the suspension members. Inside the calipers, pistons press against the
pads due to pressure generated in the master cylinder. The pads then rub
against the rotor, slowing the vehicle. Disc brakes work using much the
same basic principle as the brakes on a bicycle; as the caliper pinches
the wheel with pads on both sides, it slows the bicycle. Disc brakes offer
higher performance braking, simpler design, lighter weight, and better
resistance to water interference than drum brakes.
Disc brakes, like many automotive innovations, were
originally developed for auto racing, but are now standard equipment on
virtually every car made. On most cars, the front brakes are of the disc
type, and the rear brakes are of the "drum" type. Drum brakes use two
semi-circular shoes to press outward against the inner surfaces of a steel
drum. Older cars often had drum brakes on all four wheels, and many new
cars now have 4-wheel disc brakes.
Because disc brakes can fling off water more easily than
drum brakes, they work much better in wet conditions. This is not to say
that water does not affect them, it definitely does. If you splash through
a puddle and then try to apply the brakes, your brakes may not work at all
for a few seconds! Disc brakes also allow better airflow cooling, which
also increases their effectiveness. Some high performance disc brakes have
drilled or slotted holes through the face of the rotor, which helps to
prevent the pads from "glazing" (becoming hardened due to heat). Disc
brakes were introduced as standard equipment on most cars in the early
seventies.
Brake Drums
The brake drum is a heavy flat-topped cylinder, which is
sandwiched between the wheel rim and the wheel hub. The inside surface of
the drum is acted upon by the linings of the brake shoes. When the brakes
are applied, the brake shoes are forced into contact with the inside
surface of the brake drums to slow the rotation of the wheels.
The drums are usually covered with fins on their outer
surfaces to increase cooling. They are not cooled internally, because
water could enter through the air vent cooling holes and braking would
then be greatly impaired.
Drum brakes are found on the rear wheels of most older
cars, but they are increasingly being fazed out in favor of rear disc
brakes. Drum brakes were standard equipment on all four wheels of most
cars until the early 70's.
Brake Calipers
The caliper works like a C-clamp to pinch the pads onto
the rotor. It straddles the rotor and contains the hydraulic "slave
cylinder" or "wheel cylinder" piston(s). One caliper is mounted to the
suspension members on each wheel. The caliper is usually mounted onto the
spindle, allowing it to deliver the torsional force of the wheel to the
chassis via the control arms. Brake hoses connect the caliper to the brake
lines leading to the master cylinder. A "bleeder valve" is located on each
caliper to allow air bubbles to be purged from the system.
"Floating caliper" disc brakes, the most common variety,
allow the caliper to move from side to side slightly when the brakes are
applied. This is because only one pad moves (in relation to the caliper).
Some calipers contain two or four seperate pistons. These calipers are
fixed in place; i.e., there is no lateral movement like the floating
caliper, the pistons take up the slack on each side of the rotor. These
are called "dual cylinder" or "dual piston" calipers, and are standard
equipment on many performance cars.
Wheel (Slave) Cylinder
Wheel cylinders, also called the "slave" cylinders, are
cylinders in which movable piston(s) convert hydraulic brake fluid
pressure into mechanical force. Hydraulic pressure against the piston(s)
within the wheel cylinder forces the brake shoes or pads against the
machined surfaces of the drum or rotor. There is one cylinder (or more in
some systems) for each wheel. Drum brake wheel cylinders are usually made
up of a cylindrical casting, an internal compression spring, two pistons,
two rubber cups or seals, and two rubber boots to prevent entry of dirt
and water. This type of wheel cylinder is fitted with push rods that
extend from the outer side of each piston through a rubber boot, where
they bear against the brake shoes. In disc brakes, the wheel cylinder is
built into the caliper. All wheel cylinders have bleeder screws (or
bleeder valves) to allow the system to be purged of air bubbles.
As the brake pedal is depressed, it moves pistons within
the master cylinder, pressurizing the brake fluid in the brake lines and
slave cylinders at each wheel. The fluid pressure causes the wheel
cylinders' pistons to move, which forces the shoes or pads against the
brake drums or rotors. Drum brakes use return springs to pull the pistons
back away from the drum when the pressure is released. On disc brakes, the
calipers' piston seals are designed to retract the piston slightly, thus
allowing the pads to clear the rotor and thereby reduce rolling
friction.
Parking (Emergency) Brakes
The parking brake (sometimes called the emergency brake)
is a cable-activated system used to hold the brakes continuously in the
applied position. The parking brake activates the brakes on the rear
wheels. Instead of hydraulic pressure, a cable (mechanical) linkage is
used to engage the brake shoes or discs. When the parking-brake pedal is
pressed (or, in many cars, a hand lever is pulled), a steel cable draws
the brake shoes or pads firmly against the drums or rotors. The release
lever or button slackens the cables and disengages the brake shoes. The
parking brake is self adjusting on most systems. An automatic adjuster
compensates for lining (brake shoe) wear. On many cars, the parking brake
is used to re-adjust the brake shoes as they wear in, or when the shoes
are replaced. In these systems, the adjustment is made by repeatedly
applying the parking brake while backing up.
The parking brake can be useful while driving up hills:
If you're driving a manual transmission car, and you pull up to a stop on
an incline, you might notice that you don't have enough feet to operate
the clutch, brake, and gas at the same time. In other words, you will
likely roll backwards slightly while getting started again. If a someone
pulls up right behind you, this can be a problem. Your parking brake is
useful in this situation: Apply the parking brake after you stop. When you
want to go, release the clutch while pressing the gas, and release the
parking brake. This keeps you from having to quickly switch your left foot
from the brake to the clutch, or your right foot from the brake to the gas
pedal. A little practice, and you'll be able to do it smoothly. Also,
remember if you pull up behind someone who is stopped on a hill, give them
extra room to roll back a little. Especially if it's a truck.
Some cars have no parking brake release! They
automatically release the parking brake when the car is placed in drive or
reverse.
Remember, it's a good idea to test the parking brake
periodically and keep it in good condition. It may save your life if the
main braking system
fails!
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