Drum brakes work on the same principle as disc brakes:
Shoes press against a spinning surface. In this system,
that surface is called a drum.
Figure 1. Location of drum
Many cars have drum brakes on the rear wheels and disc
brakes on the front. Drum brakes have more parts than disc
brakes and are harder to service, but they are less expensive
to manufacture, and they easily incorporate an emergency brake
In this edition of HowStuffWorks,
we will learn exactly how a drum brake system works, examine
the emergency brake setup and find out what kind of servicing
drum brakes need.
Figure 2. Drum brake with drum in
Figure 3. Drum brake without drum in
Let's start with the basics.
The Drum Brake The drum brake may look
complicated, and it can be pretty intimidating when you open
one up. Let's break it down and explain what each piece does.
Figure 4. Parts of a drum
Like the disc
brake, the drum brake has two brake shoes and a piston.
But the drum brake also has an adjuster mechanism, an
emergency brake mechanism and lots of springs.
First, the basics: Figure 5 shows only the parts
that provide stopping power.
Figure 5. Drum brake in operation
When you hit the brake pedal, the piston pushes the brake
shoes against the drum. That's pretty straightforward, but why
do we need all of those springs?
This is where it
gets a little more complicated. Many drum brakes are
self-actuating. Figure 5 shows that as the brake shoes
contact the drum, there is a kind of wedging action, which has
the effect of pressing the shoes into the drum with more
The extra braking force provided by the wedging action
allows drum brakes to use a smaller piston than disc brakes.
But, because of the wedging action, the shoes must be pulled
away from the drum when the brakes are released. This is the
reason for some of the springs. Other springs help hold the
brake shoes in place and return the adjuster arm after it
Brake Adjuster For the drum brakes to
function correctly, the brake shoes must remain close to the
drum without touching it. If they get too far away from the
drum (as the shoes wear down, for instance), the piston will
require more fluid to travel that distance, and your brake
pedal will sink closer to the floor when you apply the brakes.
This is why most drum brakes have an automatic
Figure 6. Adjuster
Now let's add in the parts of the adjuster mechanism. The
adjuster uses the self-actuation principle we discussed above.
Figure 7. Drum brake adjuster in
In Figure 7, you can see that as the pad wears down,
more space will form between the shoe and the drum. Each time
the car stops while in reverse, the shoe is pulled tight
against the drum. When the gap gets big enough, the adjusting
lever rocks enough to advance the adjuster gear by one
tooth. The adjuster has threads on it, like a bolt, so that it
unscrews a little bit when it turns, lengthening to fill in
the gap. When the brake shoes wear a little more, the adjuster
can advance again, so it always keeps the shoes close to the
Some cars have an adjuster that is actuated when the
emergency brake is applied. This type of adjuster can come out
of adjustment if the emergency brake is not used for long
periods of time. So if you have this type of adjuster, you
should apply your emergency brake at least once a week.
The Emergency Brake The emergency brake on a
car has to be actuated by a different power source than the
system. The drum brake design allows for a simple
cable actuation mechanism. Click here
to see an emergency brake in action.
Figure 8. Emergency brake in operation
When the emergency brake is actuated, a cable pulls on the
lever, which forces the two shoes apart.
Servicing The most common service required
for drum brakes is changing the brake shoes. Some drum
brakes provide an inspection hole on the back side, where you
can see how much material is left on the shoe. Brake shoes
should be replaced when the friction material has worn down to
within 1/32 inch (0.8 mm) of the rivets. If the friction
material is bonded to the backing plate (no rivets), then the
shoes should be replaced when they have only 1/16 inch (1.6
mm) of material left.
Photo courtesy of a local AutoZone
store Figure 9. Brake
Just as in disc brakes, deep scores sometimes get worn into
brake drums. If a worn-out brake shoe is used for too long,
the rivets that hold the friction material to the backing can
wear grooves into the drum. A badly scored drum can sometimes
be repaired by refinishing. Where disc brakes have a minimum
allowable thickness, drum brakes have a maximum allowable
diameter. Since the contact surface is the inside of the
drum, as you remove material from the drum brake the diameter
Figure 10. Brake
For more information on all different kinds of brakes,
check out the links on the next page!