Today, most new vehicles have some form of Anti-lock Brake System (ABS). This innovation has significantly improved the ability of the average driver to safely control and stop the vehicle in slippery conditions, panic stops and other dangerous situations.
The reason that an ABS can increase the driver’s control of a vehicle in these situations is that it prevents the wheels from “locking up”, or ceasing to rotate, during braking maneuvers. This improves the driver’s control and the safety of the situation in two ways. First, with the wheels locked up, the driver loses steering ability. With an ABS, the driver has a much better chance of maintaining steering control of the vehicle. A second reason is that rolling wheels being braked just short of ceasing to rotate will, under most conditions, result in a shorter stopping distance.
The origin of the ABS concept can be traced to early aviation. When landing, a pilot needs to stop the aircraft quickly, since runway lengths are typically limited. As mentioned above, the fastest way to stop a vehicle with wheel braking is to brake the wheels just short of the point where they cease to rotate. This technique is somewhat difficult to execute, as the primary feedback to the driver (or pilot) as to exactly where that point is occurs when the wheels lock up.
Of course, once the wheels lock up, braking power decreases, steering control is lost, and the tires are subjected to extreme stress, strain and temperature, which can quickly result in blown out tires. Once the wheels lock up, the driver will typically release brake pressure, allowing the wheels to start rotating again, then increase pressure again, trying to find that maximum braking force. This increase and decrease in pressure is commonly known as “pumping the brakes.”
The ABS performs that “pumping” action automatically by sensing wheel rotational speeds and adjusting the brake fluid pressure to achieve optimum braking force for the vehicle, within the capability of the system. In a vehicle equipped with an ABS, the driver should not “pump the brakes”, but should instead apply firm, steady pressure to the brakes and let the ABS do its job.
ABS is not a “silver bullet” for all braking situations. For instance, in soft sand or deep snow an ABS can actually increase stopping distance over the locked wheel condition. However, even in those situations, an ABS will significantly contribute to maintaining the vehicle’s ability to be steered. Studies over the years consistently show that vehicles with an ABS fare better in a variety of emergency situations than those without.