BMW Technology - ABS

An anti-lock braking system (ABS) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is to allow the driver to maintain steering control under heavy braking and, in most situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control). Disadvantages of the system include increased braking distances under certain conditions and the creation of a "false sense of security" among drivers who do not understand the operation and limitations of ABS.

When ABS equipped brakes are depressed hard - like in a panic stop - the ABS system pumps the brakes several times per second. ABS helps you maintain control of the vehicle by stopping in a straight line. ABS is very effective at braking in adverse weather conditions like ice, snow or rain.

With ABS, sensors measure the speed at which the wheels are turning. If this decreases rapidly, the electronic steering system reports blocking danger. The pressure of the hydraulics is reduced immediately and then raised to just under the blocking threshold. The process can be repeated several times per second, and you can notice it by the pulsation on the brake pedal.

To improve ABS, BMW places a sensor on every single wheel. The advantage of 4 sensors is the higher regulation precision. By sensing 4 individual rpm's of the wheels, the braking power on the rear axle can be adjusted with a great deal more sensitivity.

BMW's four wheel-speed sensors are in constant contact with all four wheels, providing greater accuracy than systems that use just three sensors. Each sensor carries accurate feedback from a specific wheel independently. A three-sensor system provides an average wheel-speed figure for rear wheels. This average reading can be inaccurate when one side of the car is on dry pavement and the other is on ice.

Some ABS systems pulsate as little as four times a second, while BMW's ABS pulsates 12 - 15 times per second. The pulsation represents a cycle in which pressure is first released from a particular wheel that has begun to lock and then allowed to resume. The faster the cycle, the closer you are to achieving maximum braking efficiency.

BMW's ABS also provides "yaw control," which keeps the car's back end from swinging to the side, by providing the proper balance between maximum braking and stability when one side of the car is on ice and the other is on dry pavement.

BMW's ABS works with other active safety elements to provide accurate steering, balanced handling and Automatic Stability Control + Traction, which ensures wheels always maintain optimum contact with the road. In fact, the wheel-speed sensors that detect and prevent brakes from locking are also used in BMW's ASC+T to detect and prevent wheels from spinning.

While ABS can usually shorten braking distances on wet and icy roads, there are some limitations in other conditions. It may actually take longer to brake on loose stones or fresh snow because they cannot build up in front of the wheels as they would when the wheels are locked.

When stopping a vehicle with ABS in slippery conditions, apply steady pressure to the brake pedal. The ABS automatically pumps the brakes to keep the wheels from locking up. Do not take your foot off the brake pedal if you hear or feel it chatter. That means that the ABS system is working properly and you should continue to steer the car normally. If you don't have ABS, you can achieve the same effect by gently pumping the brakes, pumping more rapidly as the car slows down.

A 1974 BMW 3.0 CSL races for the first time with Anti-lock Braking System (ABS).

Driver assistance systems are designed to help drivers react correctly - without dominating them. Assistance systems first capture a great deal of driving-related data such as the driver's seating position, the vehicle's speed and lateral acceleration as well as the its environment, including traction, outside temperature and more. Next, these data are evaluated and assessed. If the system detects a dangerous situation, it takes countermeasures.

The three most important aspects of driver assistance.

The BMW Group subdivides driver assistance into three areas.

The first is vehicle stabilisation. This is accomplished by control and override systems such as the Anti Blocking System (ABS), Dynamic Stability Control (DSC), or Dynamic Drive, the active roll stabilisation system.

The second is vehicle control. Here, we have such control systems as the Tempomat, our Active Cruise Control (ACC) or the Hill Descent Control (HDC) which aids four wheel drive vehicles as they go downhill. HDC automatically reduces a vehicle's speed to a little over walking tempo when descending very steep hills, holding this speed constant without the driver needing to brake.

The third area of driver assistance is planning and navigating using information systems such as BMW navigation systems or BMW-Assist. BMW-Assist is a telematics system which provides drivers with extensive services ranging from traffic and recreational information to useful services such as breakdown service all the way to lifesaving functions such as the automatic emergency call function.

Article Date: April 13, 2004