The Antilock Braking System, or ABS, is a system which seeks to prevent you from locking up wheels on your vehicle through braking. It does this through the use of a hydraulic pump which is installed in line between your brake master cylinder and the brake slave cylinders. This pump reduces braking force when a sensor detects that a wheel has stopped moving, causing the wheel to unlock. The primary benefit of the ABS system is that you can slam on the brakes without skidding, which permits you to steer the vehicle no matter how hard you are attempting to brake.
ABS works by watching the speed of the vehicle's wheels, and when one or more wheels stops while others are still turning, the brakes are reduced on those wheels to avoid a skid condition. This is especially helpful in conditions which reduce traction, like snow or rain (although stopping on ice is still pretty much impossible) and greatly reduces driver safety in a panic situation, in which the driver is likely to lock up the brakes if left to their own devices.
ABS is not always used at all wheels of the vehicle, or if it is, all wheels do not necessarily operate independently. For instance, many small Ford pickups have ABS on the rear wheels only to try to prevent fishtailing due to having very little weight over the back wheels, thus very little traction and very little braking power. Many vehicles, in fact most vehicles with ABS made prior to the 21st century, have "three channel" ABS which provides independent antilock to each front wheel, but handles the rears together rather than separately. Ideally, however, the vehicle has "four channel" ABS, which can unlock an individual sticking wheel on the front or the rear of the vehicle.
Locking up the brakes reduces brake effectiveness and increases stopping distance, which leads to a greater tendency to crash into things, so ABS is potentially very helpful. The biggest benefit of ABS is that it permits steering of the vehicle while braking at maximum force, because it allows the brake system to automatically apply varying amounts of braking force to each connected wheel. This can also be used for yaw control. A truly modern ABS will stop a car faster than even a talented human driver on any surface except potentially dry tarmac.
Unfortunately, ABS has not been found to reduce accidents. While no formal study has yet been done as to why, it is suspected to be because people expect ABS to get them out of situations from which there is no salvation.
Early ABS systems universally produce a longer stopping time than a talented human driver on any surface. They can sometimes cope very poorly with ice and even the best systems have problems with bumps which drive the wheels off of the ground, causing them to decelerate every time they lift off the road surface. They also produce inferior stopping power in most situations on snow or gravel because when a wheel is locked up on those surfaces it causes the snow or gravel to build up in front of the wheel and assist braking. ABS has come a long way since its invention and these days some of the more advanced ABS systems can actually detect that they are plowing through snow or gravel and they will lock up the wheel momentarily in order to create that buildup to assist in stopping, but those older systems have not gone away.
The most important thing to know if you have antilock brakes is that you should never pump the brakes. With non-ABS braking, pumping the brakes is the most effective way to stop. Unfortunately, it doesn't mesh well with the operation of ABS, and it can cause your brakes to fail entirely. Don't do it! Some newer systems are resistant to this effect, but it is still to be avoided as a matter of practice on vehicles equipped with ABS.
ABS Components and functional description
The ABS system runs on a limited number of parts. The major elements of the system are shown here, although you can't see the brake calipers/drums or the wheel sensors in this picture.
The short description of the diagram is that the blue shape is the brake booster, the light grey one is the pump/valve body, the brick-colored box is the ABS controller; the black lines are brake lines and the brick-colored ones are wires. This picture displays a left-hand-drive car. Axles are shown just to give you an idea of what is going on; obviously cars don't just have two straight axles.
Many of us already know how a brake booster works, and the function of a master cylinder; I won't cover those here.1 The magic begins when you get to the pump/valve unit. This doesn't have to be a single unit, but it almost always is; it contains the valves for the separate ABS "channels", meaning brake lines. One brake line can feed multiple brakes, and this is common practice for the rear brakes on a car. Most vehicles use a "dual" master cylinder which has two pistons in it; the frontmost and larger one runs the front brakes through separate lines, while the rearmost and smaller one runs the rear brakes through a single line which splits off to the two sides somewhere near the rear of the vehicle. Subaru uses a master with two identically-sized cylinders; one cylinder drives the right front and left rear brakes, while the other drives the left front and right rear. This way you can lose either side of the system and still have both a front brake and something like balance in braking.
In an antilock braking system, both sides of the master go to the ABS pump. This pump contains a valve for each channel; most cars today have four-channel ABS, which means each wheel is controlled independently, and thus the system has four valves. The valves may open to as many as three positions. When the valve is open, fluid passes from the master cylinder to the brake (and back) as normal. When closed, it prevents the user from adding more braking force to the specific wheel (or wheels) if they should press on the pedal. In the third position, it actually releases pressure from the chosen wheel. Most systems have only the first and third position, which is why the brake pedal "pulses" when the ABS is taking effect; the valve is switching rapidly between connecting the pedal to the brake, and releasing pressure from that part of the system. Some of the newest ABS systems instead switch between the release and blocked position, using only the pump in the valve unit to put pressure back into the system, and therefore the pedal does not pulsate. This pedal pulsation doesn't detract from the effectiveness of the system on its own, but many drivers are unnerved by it and let up on the pedal, which of course is the sign to the car that you don't want to stop any more.
The basic operation of the system goes something like this: The wheel speed sensors (which could be mounted at the differential or at the wheel hub) are always determining the rate of rotation of the wheels, although they don't think of it in terms of speed, but in terms of the number of pulses per second. This is usually accomplished using a magnetic sensor next to a metallic wheel. The wheel has notches in it and as the notches pass the sensor the magnetic field is interfered with, causing a field to be extended and collapsed rapidly. This induces a current in the sensor wire, which is measured by the ABS controller unit. This unit talks to the pump/valve body and tells it what to do.
When the controller detects that a wheel has stopped moving, it checks to see if the brake pedal has been pressed, usually by watching the signal from the brake light switch. This switch is mounted to the brake pedal assembly and is used (as the name suggests) to illuminate the brake lights when the pedal is pressed. If the brakes are being pressed, then the ABS controller takes action. It tells the valve unit to release pressure from the brake attached to the stopped wheel, which causes it to stop braking. This is the "anti-lock" in antilock brakes. This is all that the simplest systems do. Slightly more complicated systems also watch to see if a wheel is moving substantially slower than the other wheels, but has not yet stopped - this is a sign that a wheel is about to lock up.
Other uses of ABS
ABS is also used to provide yaw control. Similar to traction control, which seeks to ensure that you don't burn rubber, yaw control tries to make sure that the car is going where you want it to go, based on the input from the steering wheel, brake pedal, and gas pedal. Without getting overly complicated, the car watches the brake light switch, throttle position sensor, and a steering rotation sensor, as well as at least one accelerometer which is usually located inside the yaw control computer. This tells the car which way it is actually moving, so that it can tell if (for example) it's starting to go sideways. The ABS pump is engaged and the valves are used to control the brakes without the user even having to do anything. The yaw control system will then apply braking force to individual wheels, while at the same time causing the engine to produce more or less power. This allows not only the slowing down of individual wheels, but also the speeding up of them as well. For example, you could brake the rear right wheel at maximum, apply power to the rear left wheel, and brake the front right wheel at half power in order to make the car turn around to the right.
ABS can also be used to enhance the capabilities of a limited slip differential, or even to mimic one entirely! Volkswagen/Audi uses Torsen ("torque-sensing") differentials in vehicles with 4Motion/Quattro, and theirs have a maximum slip ratio of 5:1. This means that if one wheel is moving five times faster than the other, then it acts like an ordinary open differential and just slips. These vehicles detect when the slip angle is approaching this ratio and use the ABS to slow down the slipping wheel. The latter approach is used in some cars by Porsche; a viscous limited slip is heavy and "mushy" while a clutch-type limited slip wears out and must be rebuilt, and a torsen differential is expensive. Porsche uses a standard open diff and just uses the ABS on the slipping wheel. This does wear out the brakes faster, but it's only done when a wheel is slipping, and it's a lot easier to replace some brake pads than it is to replace clutches in your differential.
- 1. At least, not in depth; for the unintiated, the booster is a box with a diaphragm which helps you push down the brake pedal, powered by vacuum, while the master cylinder feeds brake fluid to the slave cylinders at the wheels, which pushes pads or shoes against discs or drums (respectively) and causes braking.