An Anti-lock Braking System (ABS) in a car monitors traction and ensures effective braking under pressure rather than skidding.
When the driver exerts a force on the brake pedal, the anti-lock braking systems prevent the wheels from locking up (ceasing rotation) and avoiding uncontrolled skidding. The abbreviation can also stand for Assisted Brake System.
It has been further refined with electronic computerised systems, which help with braking in low traction conditions by applying the brake to one or more wheels to prevent them from locking up in extreme conditions.
ABS is an electronic braking system, which uses computer-controlled hydraulic valves to split and control the level of brake fluid pressure applied to each wheel. When a tire is rolling freely, the ABS hardware applies the brakes at full force. However, when a tire encounters less than full traction (when braking), much of the braking force is routed through the logic module, which modulates that portion of applied brake power needed to allow forward motion by preventing wheel lockup.
The valves are controlled by electrical servomotors based on information received by various wheel sensors throughout the vehicle. The wheel speed information is relayed to the electronic control unit, which modulates the calipers via the ABS pumps. Optimal tractive effort can be achieved depending on road conditions and driver inputs. Braking effort is modulated in response to these factors. The result is that ABS allows the driver to maintain control while braking, cutting down on the need for regular brake pad replacement and minimising stopping distances on all road surfaces and in all weather conditions.ABS is not available on all vehicles. In Australia, it is a legal requirement for all new passenger cars sold since 2003; ABS-equipped vehicles are required to have an ABS light. It's a brake warning lamp that is lit when the system has been deactivated or an audible warning device sounded for 4 seconds every time the vehicle ignition is turned off. ABS systems are not made equal. Further, some vehicle manufacturers only fit the anti-lock braking system only on the rear axle. Rear-wheel ABS is common with pickup trucks and heavy-duty vehicles, which assist drivers in off-road and slippery road conditions. In general, however, statistics show that an anti-lock braking system in cars reduces the chances of a fatal crash.
While most cars have both, they both serve different purposes. The anti-lock brake system is a safety system that allows you, through the power of threshold braking, to have steering control still and be able to steer your car during an emergency stop. The anti-lock brakes help prevent your wheels from locking up as you brake by pumping the fluid out of the master cylinder, so pressure doesn't build up too quickly, many times faster than most drivers. This prevents the tires from locking up, which could lead to skidding. ABS works automatically with brake pressure release so that if a car's speed drops below a certain threshold, it disengages the ABS function temporarily until vehicle speed returns above this threshold. Many people confuse these systems because they use similar parts and components but work in slightly different ways though one depends on the other for optimum braking.
Many people confuse these systems as ABS and Traction control are similar in operation. They both prevent skidding by modulating the brake pressure during an emergency stop and allowing you to steer, but they work differently. Anti-lock braking systems cut engine power when a car is sliding along the road or spinning out of control on slippery surfaces or snow. The reason for this is that if all four wheels lock up under heavy braking, you'll lose steering ability (similar to what happens when your vehicle hydroplanes at high speeds). By releasing some brake pressure based on individual wheel speeds, especially on the front brakes where most of your control input comes from, you'll regain steering capability even with a reduced tire contact patch. Traction control uses the same concepts but works on slippery surfaces like snow instead of wet roads. On some vehicles, this feature is known as electronic stability or "ESP."
There are three types of Anti-lock braking systems.
4 Channel ABS monitors and controls 4 wheel speed sensors. This is the most common type installed on new vehicles to date.
5 Channel ABS also monitors and controls the 5th wheel speed sensor, usually found on four-wheel drive vehicles.
6 Channel ABS. As well as monitoring both front and rear wheels, a control unit will also monitor the accelerator pedal position (to see if you're applying gas or not) as well as yaw rate sensors (which measure your car's "slip angle" in turns), all of which helps control braking pressure at each individual wheel to make sure you don't lose control while braking.
ABS works by having the vehicle speed constantly measured. The computer then decides based on the vehicle speed if it will apply braking pressure or not. If there is no wheel lockup, then braking pressure can be applied to each of the four brakes through hydraulic means (meaning your brake fluid sends a command to the four calipers). If one or more wheels begins locking up (because you entered a turn too fast, for example), the ABS control unit will send electrical impulses that modulate (release and reapply) pressure at different calipers to allow some wheels to rotate faster than others. This reduces skidding and allows you to steer while still reducing your overall stopping distance, causing the vehicle to not exceed the critical threshold of locking up all four wheels. The continued friction and the rotational speed on each wheel ensure optimal braking performance. However, ABS braking does not stop the car faster. The ABS system uses rotational speed sensors on each wheel to detect when the wheel starts to lock, which means your car loses some steering ability. To help control the locking of the tires, brake pressure is momentarily released, allowing you to regain partial steering capability. This process allows you to steer while still braking and decreases braking distances. Anti-lock brakes are one of the greatest safety features created for drivers on today's cars. By using a combination of hydraulic pressure releases and modulating how much these releases happen per individual wheel, loss of tire traction can be greatly reduced. It provides greater driver control in critical situations when skidding or losing control could have dire consequences.
Most Traction control systems use the ABS system to adjust how much brake pressure is released on each wheel when traction loss occurs. The amount of brake pressure release will vary depending on the type of vehicle you are driving. Traction control works in 3 ways.
When it detects that a wheel has lost traction, the computer reduces engine power until stability is returned by reapplying braking pressure to that wheel.
A secondary detection process uses the ABS sensors at all four wheels to detect if one or more tires have started to skid under heavy braking. This happens when your car hydroplanes, for example, and loses contact with the road surface.
An electric motor inside the differential cuts power to the spinning wheel. When the slipping stops, power is applied back to that wheel. Most new cars have some sort of traction control built-in. The more expensive vehicles will come equipped with electronic stability control (ESC) or "ESP" which not only monitors wheels for loss of traction but also watch how each tire performs when steering through a turn.
Every wheel has its own sensor; there's no such thing as using one sensor for all four wheels. Each wheel sends information about how quickly it rotates (the speed at which it turns), whether it spins backwards (indicating lockup), and if pressure is applied to the brakes.
Traditionally, ABS systems have used three sensors: two spinning discs mounted next to each other on either side of a car/truck's brake rotor and a third sensor attached to the body or suspension of the vehicle. The two discs spin past each other, and when one gets close enough, it closes an electronic circuit that tells the ABS system that a wheel is about to stop rotating.
Drifting is a driving technique used to control a driving line without traction to the rear wheels.
There are drivers who do use ABS systems so they can get more power from their brakes. When you're going sideways at 100 km/h, sometimes stability control will cut power to the engine until you're back in line.
The main reason drift race cars use ABS is that it helps with brake distribution during drift events when one wheel is constantly in the air. One example is when a car does a "donut" and has one tyre in contact with the ground at all times.
In most cases, professional drifters shut off their traction control when they drift. For more information on Electronic Stability Control, take a look at the Vic Roads website.