Disc Brake Basics Here is the location of
the disc brakes in a car:
The main components of a disc brake are:
The brake pads
The caliper, which contains a piston
The rotor, which is mounted to the hub
Parts of a disc
The disc brake is a lot like the brakes on a bicycle.
Bicycle brakes have a caliper, which squeezes the brake pads
against the wheel. In a disc brake, the brake pads squeeze the
rotor instead of the wheel, and the force is
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
Self-Adjusting Brakes 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.
Self-adjusting disc brake
Older cars had dual or four-piston fixed-caliper designs. A
piston (or two) on each side of the rotor pushed the pad on
that side. This design has been largely eliminated because
single-piston designs are cheaper and more reliable.
Emergency Brakes 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.
Disc brake with parking
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.
Servicing Your Brakes The most common type
of service required for brakes is changing the pads.
Disc brake pads usually have a piece of metal on them called a
Photo courtesy of a local Autozone
store Disc brake
When enough of the friction material is worn away, the wear
indicator will contact the disc and make a squealing sound.
This means it is time for new brake pads.
There is also an inspection opening in the caliper
so you can see how much friction material is left on your
Disc brake inspection
Sometimes, deep scores get worn into brake rotors.
This can happen if a worn-out brake pad is left on the car for
too long. Brake rotors can also warp, that is, lose their
flatness. If this happens, the brakes may shudder or vibrate
when you stop. Both of these problems can sometimes be fixed
by refinishing (also called turning or machining) the
rotors. Some material is removed from both sides of the rotors
to restore the flat, smooth surface.
Refinishing is not required every time your brake shoes are
replaced. You need it only if they are warped or badly scored.
In fact, refinishing the rotors more often than is necessary
will reduce their life. Because the process removes material,
brake rotors get thinner every time they are refinished. All
brake rotors have a specification for the minimum allowable
thickness before they need to be replaced. This spec can be
found in the shop manual for each vehicle.
For more information, check out the links on the next page.