We all know that pushing down on the brake pedal slows a
car to a stop. But how does this happen? How does your car
transmit the force from your leg to its wheels? How does it
multiply the force so that it is enough to stop something as
big as a car?

Layout of typical brake system

In this edition of HowStuffWorks,
the first in a six-part series on brakes, we will follow the
chain of events from the pedal to the wheel, explaining all of
the parts of the brake system along the way. This part of the
series will cover the basic concepts behind car brakes and
examine the workings of a simple brake system. Additional
articles cover the rest of the components of a car brake
system, detailing how each one operates.

Brake Basics When you depress your brake
pedal, your car transmits the force from your foot to its
brakes through a fluid. Since the actual brakes require a much
greater force than you could apply with your leg, your car
must also multiply the force of your foot. It does this in two
ways:

Mechanical advantage (leverage)

Hydraulic force multiplication

The brakes transmit the force to the tires using
friction, and the tires transmit that force to the road
using friction also. Before we begin our discussion on the
components of the brake system, let's cover these three
principles:

Leverage

Hydraulics

Friction

Leverage The pedal is designed in such a way
that it can multiply the force from your leg several times
before any force is even transmitted to the brake fluid.

Force multiplication

In the figure above, a force F is being applied to the left
end of the lever. The left end of the lever is twice as long
(2X) as the right end (X). Therefore, on the right end of the
lever a force of 2F is available, but it acts through half of
the distance (Y) that the left end moves (2Y). Changing the
relative lengths of the left and right ends of the lever
changes the multipliers.

Hydraulic Systems The basic idea behind any
hydraulic system is very simple: Force applied at one point is
transmitted to another point using an incompressible
fluid, almost always an oil of some sort. Most brake
systems also multiply the force in the process. Here you can
see the simplest possible hydraulic system:

Simple hydraulic
system

In the figure above, two pistons (shown in red) are fit
into two glass cylinders filled with oil (shown in light blue)
and connected to one another with an oil-filled pipe. If you
apply a downward force to one piston (the left one, in this
drawing), then the force is transmitted to the second piston
through the oil in the pipe. Since oil is incompressible, the
efficiency is very good -- almost all of the applied force
appears at the second piston. The great thing about hydraulic
systems is that the pipe connecting the two cylinders can be
any length and shape, allowing it to snake through all sorts
of things separating the two pistons. The pipe can also fork,
so that one master
cylinder can drive more than one slave cylinder if
desired, as shown in here:

Master cylinder with two
slaves

The other neat thing about a hydraulic system is that it
makes force multiplication (or division) fairly easy. If you
have read How a Block and
Tackle Works or How Gear Ratios
Work, then you know that trading force for distance is
very common in mechanical systems. In a hydraulic system, all
you have to do is change the size of one piston and cylinder
relative to the other, as shown here:

Hydraulic
multiplication

To determine the multiplication factor in the figure above,
start by looking at the size of the pistons. Assume that the
piston on the left is 2 inches (5.08 cm) in diameter (1-inch /
2.54 cm radius), while the piston on the right is 6 inches
(15.24 cm) in diameter (3-inch / 7.62 cm radius). The area of
the two pistons is Pi * r^{2}.
The area of the left piston is therefore 3.14, while the area
of the piston on the right is 28.26. The piston on the right
is nine times larger than the piston on the left. This means
that any force applied to the left-hand piston will come out
nine times greater on the right-hand piston. So, if you apply
a 100-pound downward force to the left piston, a 900-pound
upward force will appear on the right. The only catch is that
you will have to depress the left piston 9 inches (22.86 cm)
to raise the right piston 1 inch (2.54 cm).

Friction Friction is a measure of how hard
it is to slide one object over another. Take a look at the
figure below. Both of the blocks are made from the same
material, but one is heavier. I think we all know which one
will be harder for the bulldozer to push.

Friction force versus
weight

To understand why this is, let's take a close look at one
of the blocks and the table:

Friction at the microscopic
level

Even though the blocks look smooth to the naked eye, they
are actually quite rough at the microscopic level. When you
set the block down on the table, the little peaks and valleys
get squished together, and some of them may actually weld
together. The weight of the heavier block causes it to squish
together more, so it is even harder to slide.

Different materials have different microscopic structures;
for instance, it is harder to slide rubber against rubber than
it is to slide steel against
steel. The type of material determines the coefficient of
friction, the ratio of the force required to slide the
block to the block's weight. If the coefficient were 1.0 in
our example, then it would take 100 pounds of force to slide
the 100-pound (45 kg) block, or 400 pounds (180 kg) of force
to slide the 400-pound block. If the coefficient were 0.1,
then it would take 10 pounds of force to slide to the
100-pound block or 40 pounds of force to slide the 400-pound
block.

So the amount of force it takes to move a given block is
proportional to that block's weight. The more weight, the more
force required. This concept applies for devices like brakes
and clutches,
where a pad is pressed against a spinning disc. The more force
that presses on the pad, the greater the stopping force.

Coefficients

An
interesting thing about friction is that it usually
takes more force to break an object loose than to keep
it sliding. There is a coefficient of static
friction, where the two surfaces in contact are not
sliding relative to each other. If the two surfaces are
sliding relative to each other, the amount of force is
determined by the coefficient of dynamic
friction, which is usually less than the coefficient
of static friction.

For a car
tire, the coefficient of dynamic friction is much
less than the coefficient of static friction. The car
tire provides the greatest traction when the contact
patch is not sliding relative to the road. When it is
sliding (like during a skid or a burnout), traction is
greatly reduced.

A Simple Brake System Before we get into all
the parts of an actual car brake system, let's look a
simplified system:

A simple brake
system

You can see that the distance from the pedal to the pivot
is four times the distance from the cylinder to the pivot, so
the force at the pedal will be increased by a factor of four
before it is transmitted to the cylinder.

You can also see that the diameter of the brake cylinder is
three times the diameter of the pedal cylinder. This further
multiplies the force by nine. All together, this system
increases the force of your foot by a factor of 36. If you put
10 pounds of force on the pedal, 360 pounds (162 kg) will be
generated at the wheel squeezing the brake pads.

There are a couple of problems with this simple system.
What if we have a leak? If it is a slow leak,
eventually there will not be enough fluid left to fill the
brake cylinder, and the brakes will not function. If it is a
major leak, then the first time you apply the brakes all of
the fluid will squirt out the leak and you will have complete
brake failure.

The master cylinder on modern cars is designed to deal with
these potential failures. Be sure to check out the article on
How
Master Cylinders and Combination Valves Work, and the rest
of the articles in the brake series (see the links on the next
page), to learn more.