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.
The ABS System
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.
There are four main components to an ABS system:
- Speed sensors
- Pump
- Valves
- Controller
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.
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 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 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.
Anti-Lock Brake Types
Anti-lock braking 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 effectiveness.
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 housing.
BS Questions
Should I pump the brake pedal when stopping in slippery conditions?
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 brake.
Do anti-lock brakes really work?
Anti-lock 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 systems.
The Insurance 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.
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