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
In this edition of HowStuffWorks,
the last in a six-part series on brakes, we'll learn all about
anti-lock braking systems -- why you need them, what's in
them, how they work, some of the common types and some
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 (see Brakes: How
Friction Works for more). 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.
There are four
main components to an ABS system:
- Speed sensors
Anti-lock brake pump and
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.
There is a valve in
the brake line of each brake
controlled by the ABS. On some systems, the valve has three
- In position one, the valve is open; pressure from
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.
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.
is a computer in the car. It watches the speed sensors and
controls the valves.
ABS at Work
There are many different
variations and control algorithms for ABS systems. We will
discuss how one of the simpler systems works.
The controller monitors the speed sensors at all times. It
is looking for decelerations in the wheel that are out
of the ordinary. Right before a wheel locks up, it will
experience a rapid deceleration. If left unchecked, the wheel
would stop much more quickly than any car could. It might take
a car five seconds to stop from 60 mph (96.6 kph) under ideal
conditions, but a wheel that locks up could stop spinning in
less than a second.
The ABS controller knows that such a rapid deceleration is
impossible, so it reduces the pressure to that brake
until it sees an acceleration, then it increases the pressure
until it sees the deceleration again. It can do this very
quickly, before the tire can
actually significantly change speed. The result is that the
tire slows down at the same rate as the car, with the brakes
keeping the tires very near the point at which they will start
to lock up. This gives the system maximum braking power.
When the ABS system is in operation you will feel a
pulsing in the brake pedal; this comes from the rapid
opening and closing of the valves. Some ABS systems can cycle
up to 15 times per second.
Types of Anti-Lock Brakes
systems use different schemes depending on the type of brakes
in use. We will refer to them by the number of channels --
that is, how many valves that are individually controlled --
and the number of speed sensors.
- Four-channel, four-sensor ABS - This is the best
scheme. There is a speed sensor on all four wheels and a
separate valve for all four wheels. With this setup, the
controller monitors each wheel individually to make sure it
is achieving maximum braking force.
- Three-channel, three-sensor ABS - This scheme,
commonly found on pickup trucks with four-wheel ABS, has a
speed sensor and a valve for each of the front wheels, with
one valve and one sensor for both rear wheels. The speed
sensor for the rear wheels is located in the rear axle.
This system provides individual control of the front
wheels, so they can both achieve maximum braking force. The
rear wheels, however, are monitored together; they both have
to start to lock up before the ABS will activate on the
rear. With this system, it is possible that one of the rear
wheels will lock during a stop, reducing brake
- One-channel, one-sensor ABS - This system is
commonly found on pickup trucks with rear-wheel ABS. It has
one valve, which controls both rear wheels, and one speed
sensor, located in the rear axle.
This system operates the same as the rear end of a
three-channel system. The rear wheels are monitored together
and they both have to start to lock up before the ABS kicks
in. In this system it is also possible that one of the rear
wheels will lock, reducing brake effectiveness.
This system is easy to identify. Usually there will be
one brake line going through a T-fitting to both rear
wheels. You can locate the speed sensor by looking for an
electrical connection near the differential on the rear-axle
- Should I pump the brake pedal when stopping in
You absolutely should not pump
the brake pedal in a car with ABS. Pumping the brakes is a
technique that is sometimes used in slippery conditions to
allow the wheels to unlock so that the vehicle stays
somewhat straight during a stop. In a car with ABS the
wheels should never lock in the first place, so pumping the
brakes will just make you take longer to stop.
In an emergency stop in a car with ABS, you should apply
the brake pedal firmly and hold it while the ABS does all
the work. You will feel a pulsing in the pedal that may be
quite violent, but this is normal so don't let off the
- Do anti-lock brakes really work?
brakes really do help you stop better. They prevent wheels
from locking up and provide the shortest stopping distance
on slippery surfaces. But do they really prevent accidents?
This is the true measure of the effectiveness of ABS
Institute for Highway Safety (IIHS) has conducted
several studies trying to determine if cars equipped with
ABS are involved in more or fewer fatal accidents. It turns
out that in a 1996
study, vehicles equipped with ABS were overall no less
likely to be involved in fatal accidents than vehicles
without. The study actually stated that although cars with
ABS were less likely to be involved in accidents fatal to
the occupants of other cars, they are more likely to be
involved in accidents fatal to the occupants of the ABS car,
especially single-vehicle accidents.
There is much speculation about the reason for this. Some
people think that drivers of ABS-equipped cars use the ABS
incorrectly, either by pumping the brakes or by releasing
the brakes when they feel the system pulsing. Some people
think that since ABS allows you to steer during a panic
stop, more people run off the road and crash.
Some more recent information may indicate that the
accident rate for ABS cars is improving, but there is still
no evidence to show that ABS improves overall safety.
For more information on anti-lock brakes and related
topics, check out the links on the next page!
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