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.
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In this article, 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 associated problems.
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.
Inside ABS
There are
four main components to an ABS system:
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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:
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 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
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.
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.
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.
ABS Questions
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.
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.
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.
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