Also known as ABS (and some people say ABS braking, which is as bad as saying ATM machine), Anti-lock Braking Systems allow a vehicle to reduce it's speed with far better control than with conventional brakes.

The theory is that, when rapidly braking, the more traction you have, the more control you have because the tyres are actually still turning and not sliding on the road surface.

ABS brakes actually consist of four main components. The speed sensors, the pump, a controller and valves. All four work together to slow the wheel as fast as possible without allowing it to lock.

The controller is the system that keeps an eye on the speed sensors and controls the valves. It's a small computer sitting in the car itself.

The sensors purely monitor the speed on the wheel, passing acceleration or deceleration data to the controller.

The valves sit in three stages; open, which allows brake pressure to increase, closed, which stops more pressure and maintains an equal pressure on the brake and release, which allows pressure to decrease to the brake.

The pump is what increases the pressure on the brake lines.

While applying ABS brakes, you may feel the pedal pulse in and out. This is the controller increasing and decreasing pressure on the brake itself to keep the decelerating wheel at maximum slowing power without locking.

As a safety tip, when driving a car with ABS, don't pump the pedal. If you need to stop in an emergency hit the brake as firmly as possible and let it pulse like crazy and do all the work. This let's you concentrate more on actually steering to avoid that idiot in the 4 wheel drive that just cut you off.

Finding out that your car does not have anti-lock brakes the hard way is not the sort of thing you want to do. Instead of waiting to discover that your vehicle lacks anti-lock brakes at a time when you desperately need them, I suggest asking someone knowledgeable about the vehicle in question (i.e. whoever you're borrowing it from, or whoever's selling it to you).

The main reasons for knowing this information before driving are:

  • Assuming that your vehicle does have anti-lock brakes when it does not can lead to some rather nasty surprises — think "car wrapped around tree", "multi-car pile up", and "skidding into oncoming traffic".
  • Assuming that your vehicle does not have anti-lock brakes when in fact it does makes for inefficient braking — the car is trying to lurch-stop itself while you're sitting there pumping the brake pedal. If the car has an ABS, just let the car do its thang.

Of course, it's better to assume that your car lacks an ABS than it is to assume it has one. Pump-braking when the car has anti-lock brakes only decreases your stopping time a little, whereas locking the wheels and skidding around on the road can lead to loss of property or life. Don't do that.

I do concede that you may find yourself in a situation that has not given you the opportunity to ask about the vehicle's brake status (i.e. driving your drunk-ass friend home, or carjacking). If you find yourself in such a situation, it is best to try to determine whether or not the car has an ABS before finding out The Hard Way.

A good way to do so is to brake suddenly and see whether the car's wheels lock up and you skid (no ABS), or whether it jerkily lurches to a stop (yes ABS). Before performing the sudden-stop test, make sure you're in a suitable location — you want a dry, even driving surface, free from pedestrians and other vehicles. It is not good to do this test on wet leaves, ice, or small children. Remember, the point is not to see how long it takes you to come to a stop, but rather to make the car use its ABS if it has one, and for as short a time as possible.

Finally, the age of the vehicle is not a good way to determine whether or not it has an ABS! Don't depend on how old you think the car is, nor how old your friend says it is. Do the test if you're unsure, or if your drunk-ass friend is a liar and a menace.

[ ] (r) 2004.1.18@10:31 sideways says as any professional driver will tell you, ABS simply is not as good as well controlled non-abs. But given that 99% of people aren't willing to spend the time and effort involved in learning to brake effectively in a nonABS car...

ABS stands for Anti-Lock Braking System. It solves the problem of wheels stopping rotating when the brakes are applied hard, such as in an emergency situation, or when roads are wet. A computer monitors the speed the wheels are rotating at, and when it senses one is locking up, it pulses the brakes on and off, keeping the wheel slowing but not stopping. The effectiveness of the system is often derided by idiots, who never brake hard enough to activate the system, or when they do, back off because the pedal starts vibrating to indicate the system is working. Probably due to ignorance connected to not reading the owner's manual.

Contrary to what many people think, the Anti-lock Braking System does NOT improve the efficiency of the braking (actually, the opposite effect occurs on some surfaces, like gravel), but it allows the driver to retain a certain amount of steering ability even when braking fully.

Theoretically, a similar effect could be created on cars lacking ABS by furiously tapping the brakes, but doing it as well as the computer even when you're not in an emergency situation is impossible.

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

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 data.


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.


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 pedal.

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:

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


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