ABS (Anti-lock Braking System) commonly known as ABS Brakes
The theory behind anti-lock brakes is simple. A skidding wheel (where the tire is sliding or locked up, against the road)
has less traction than a non-skidding wheel. A quick proof is that while burning out (smoking) your tires from a start
causes a slower start than a controlled start. Just watch drag racing. 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 parts that make ABS work:
Speed sensors allow the system to know when a wheel is about to lock up, or in the case of some systems, when a wheel has
locked up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this critical
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 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.
Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back, or else
the brakes cease to function. That is what the pump does; the pump gets the pressure back up after the valves close.
The controller is the magical black box that controls all of this nonsense. This computer chip takes the data from the
sensors and uses that information to decide when the valves need to open or close and when the pump needs to run. If this
goes bad, ABS is disabled, and the car reverts to standard brakes.
ABS at Work
Here is a simple example of ABS at work:
The controller monitors the speed sensors. 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. It might take a car five seconds to stop from 60 mph
under ideal conditions, but a wheel that locks up could stop spinning in less than a second. This would lead to both a loss
of steering control and a longer stopping distance (proof below in a seperate section).
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. This gives the system maximum braking power, as the wheels are
using all of the friction physically avalible.
When the ABS system working you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of
the valves. The system can operate the valves upwards of 15 times a second, much faster than a human foot can pulse the brake
Types of Anti-Lock Brakes
Anti-lock braking systems use different ways depending on the type of brakes in use. Normally the different types of
systems are described by the number of channels (number of valves) and number of sensors.
Here are some of the most common systems:
- Four-channel, four-sensor ABS - 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. This is
the most effective of the ABS setups.
- Three-channel, three-sensor ABS - This 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. The front wheels are
allowed to reach their maximum braking power, and the rear wheels are kept from locking up together, causing a rear end
slide. Because it is possible for one of the rear wheels to lock up but not the other, the braking effectiveness may be
- One-channel, one-sensor ABS - It has one valve, which controls both rear wheels, and one speed sensor, located in the
rear axle. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.
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.
The Flip Side
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. It is possible drives of ABS equipped
cars believe that they can drive faster and still remain safe, though this belief is misfounded and dangerous.
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.
Proof that ABS allows cars to stop sooner that non ABS cars:
This proof is quite simple. When the tires are rolling, they have more friction against the road than tires that have
locked up, thus are sliding. Here are some simple numbers:
The higher the number, the more friction (better stopping power) between tires and the ground.
Physics: Principles with applications 5th edition Douglas C. Giancoli
ASE TECH: BRAKING SYSTEMS