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
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.
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.
How Anti-Lock Brakes Work
Stopping a car
in a hurry on a slippery road can be very challenging.
Anti-lock braking systems (ABS)
take a lot of the challenge out of this sometimes
nerve-wracking event. In fact, on slippery surfaces, even
professional drivers can't stop as quickly without ABS as an
average driver can with ABS.
Location of anti-lock brake components.
Getting The ABS Concept
The theory
behind anti-lock brakes is simple. A skidding wheel
(Where the tire contact patch is
sliding relative to the road) 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.
Inside ABS
There are four main components to an ABS
system.
Speed sensors.
Pump.
Valves.
Controller.
Anti-lock brake pump and valves.
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 one, the valve is open;
pressure from the master cylinder is passed right through to
the brake.
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.
We give our thanks to
HOW STUFF WORKS for allowing us to use their info and
graphics!
For more information on how engines
or anything else in this world works, visit
www.howstuffworks.com.
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