ABS: Understanding Anti-Lock Brakes
By MARK KOLSKY, Technical Editor
July 1997 One of the best ways to survive an accident is not to have one at all. But since no one has come up with a way to avoid accidents all together, more consumers are requesting air bags, anti-lock braking systems and other safety features to help save their lives in an accident. By the turn of the century, virtually every new vehicle will use some form of anti-lock brake system (ABS). Verifying proper operation of this safety equipment is an important check in both the estimating and collision repair process. This feature covers the basics in ABS diagnosis.
The Origins of ABS
Anti-lock braking was originally designed for trains in the early 1900s, and developed for jet aircraft after World War II. In the late 1960s, car manufacturers began to adapt ABS for use in luxury automobiles. However, early prototype systems were severely limited by mechanical and analog technologies of the time.
Advances in electronics technology allowed car manufacturers to develop highly reliable anti-lock braking systems that can be economically installed in a wide variety of vehicles.
How They Work
Applying excessive pressure on the brake pedal, or panic slamming the brake pedal, can cause wheels to lockup and possibly send the vehicle careening into a terrifying skid. Excessive brake pedal pressure often occurs in an emergency or adverse situations, such as on wet or icy roads. With an anti-lock braking system, computerized sensors located at each wheel will monitor deceleration when the brake is applied. The sensor will detect any wheel lockup and trigger the hydraulic system to automatically pump the brakes up to 15 times per second. Threshold braking is the term used to describe how ABS works.
A driver will press down on the brake pedal until he or she senses that wheel lockup is imminent. At that point, the driver will ease up on the pedal pressure just enough to avoid the feel of lockup, and repeat the process. ABS eliminates the need for the driver to manually pump the brake pedal.
Threshold braking should not be confused with pumping the brakes. In threshold braking, the driver is applying pressure to the brakes all of the time. While pumping the brakes is an alternative that most people think of first, each pedal pressure and release can eventually cause wheel lockup. Unfortunately, threshold braking requires practice, and is a technique with which most drivers are unfamiliar.
The dynamic technology in ABS allows the driver to be master of threshold braking. All the driver has to do is apply pressure on the brake pedal, and the ABS system will mechanically threshold brake evenly and smoothly. The well-developed and complex ABS feature allows the driver to maintain control of the vehicle and shorten the stopping distance.
Automakers would like us to believe that anti-lock brakes are the mechanical wonder of the century, and possibly they are. For example, General Motors has manufactured more than 12 million systems since 1991.
However, with all of this positive technical support from the carmakers there are also negative statistics including one from a recent study performed by the Insurance Institute for Highway Safety (IIHS).
The IIHS study states, "Cars with anti-lock brakes are more likely to be involved in fatal crashes than cars without them." However, the systems themselves do not seem to be the problem.
There are many factors to be considered when reading facts and figures about anti-lock braking systems. Injuries and deaths reveal alarming statistics; however, according to several industry studies, ABS systems have also saved more lives in emergency situations than standard braking systems. Notwithstanding, there are some concerns about the complexity of ABS systems:
* First, there are more cars equipped with anti-lock brakes on the road today. This is a new feature for drivers who have never driven with anti-lock brakes before. Possibly, drivers panic while mashing down on the brake pedal to avoid an accident in emergency situations, and thus propel their vehicle into a potentially dangerous skid, especially under adverse road conditions.
* Drivers are still pumping the brakes during sudden stops, or not steering away from obstacles even when the ABS system gives them the ability to do so.
* Also, more importantly, people are driving at higher speeds and placing themselves at greater risk of having an accident. More often, drivers are used to applying their brake pedal with a certain foot pressure in order for the vehicle to come to a stop. Drivers should be aware of how anti-lock braking systems work, so as not to apply as much pressure or use the new system as a safety crutch.
* Finally, some drivers actually believe that they can travel at almost any speed and have the assurance that their anti-lock brakes will securely stop the vehicle on a dime, and without an accident.
Technical Information From NHTSA
Anti-lock brakes eliminate the screeching sound that most drivers are accustomed to hearing when they apply excessive pressure on their brakes, according to a recently published technical report from the National Highway Traffic Safety Administration (NHTSA). Since anti-lock brakes prevent wheel lockup, there is no screeching sound. The absence of a screech means the ABS system is working. Also, according to NHTSA, the typical anti-lock system includes the following major components:
* Wheel speed sensors. These measure wheel speed and then transmit this information to an electronic control unit.
* The electronic control unit. This receives information from the sensors, determines when a wheel is about to lockup and activates the brake pressure modulator. Also, it detects any malfunction of the ABS.
* The brake pressure modulator. This reduces, holds and restores pressure to one or more brakes, independent of the driver's brake pedal effort.
* Some anti-lock systems control only the two rear wheels; others control all four wheels of the vehicle. In general, the four-wheel systems provide better stability and control during braking, but are more expensive.
* In the event of a malfunction in the anti-lock system, a warning lamp on the instrument panel alerts the driver that the ABS system is in need of repair. However, the vehicle's normal brakes will still function.
NHTSA is working with motor vehicle dealers to improve the understanding among consumers of anti-lock braking systems.
Activating ABS Systems
According to NHTSA, manufacturers currently offer two types of ABS systems: four-wheel anti-lock systems available on cars and trucks, and rear-wheel anti-lock (RWAL) systems offered on some trucks. Under normal driving conditions, ABS brakes do not operate any differently than conventional brakes. Motorists should operate the brake pedal as they normally would, gently pushing it down while applying steady pressure.
To correctly activate four-wheel ABS, step or push down hard on the brake pedal and hold it. Do not pump the brakes. Just keep your foot pressed firmly on the brake pedal, and let the anti-lock brake system work for you. The motorist may feel the brake pedal vibrate, or notice some noise, but this is normal.
RWAL systems are activated the same way as four-wheel ABS, by holding the brake pedal down. RWAL systems differ from four-wheel ABS in that the front wheels have conventional disc brake systems. Upon brake application, the motorist may feel the front wheels begin to lockup. If this happens, the motorist should momentarily release and reapply the brakes as necessary in order to avoid wheel lockup.
The driver may want to practice an ABS-activated stop just to get a feel for the brake pedal and how the system works. However, always be sure this is done in a safe area, such as a vacant parking lot clear of any obstacles.
The motorist should remember that in vehicles equipped with ABS, the system is always available, but if the driver does not maintain substantial pressure on the brake pedal, the system will not work properly, if at all.
An ABS Advantage
According to NHTSA, the important capability of ABS systems is control. Under most driving conditions, ABS provides the driver with improved vehicle steerability in order to get around an object while braking, and provides improved vehicle stability while performing these functions so that the vehicle is less likely to spin out of control.
Also, according to NHTSA, in all braking situations, stopping distance is critical. Under good driving conditions, the motorist should follow at least 23 seconds behind the vehicle in front of them. This time should be doubled if weather and/or road conditions are hazardous. These requirements are no different for vehicles equipped with ABS systems.
Under normal operating conditions, a vehicle equipped with an ABS system stops by using the conventional braking system. In severe braking conditions, however, the ABS system is automatically activated when any one of the vehicle's wheels approaches a lock condition. The system determines when ABS activation is required through wheel speed sensors and an electronic brake module (EBCM).
The sensors continuously relay information concerning the rotational speed and acceleration of each wheel to the EBCM, which compares these inputs with calibrated values of allowable wheel slip and other pertinent vehicle dynamics information. If and when conditions warrant, the EBCM then signals the ABS system's hydraulic modulator to begin ABS action.
The system modulates pressure to the affected wheel(s) by first isolating the wheel brake from further pressure build-up. This is performed by closing the solenoid bypass valve and energizing an electric motor that closes the displacement cylinder check valve assembly as pressure is released.
Stopping Distance
According to General Motors Delco Chassis Division, braking performance is one of the most important contributors to driving safety. There are several hazardous situations in which an ABS system is advantageous. These conditions include: a vehicle in front of the motorist suddenly stops in heavy traffic; a child or animal runs into the motorist's path or a road obstruction suddenly comes into view, especially at night. All of these situations require a high level of braking performance, especially under adverse road conditions.
Also, according to Delco Chassis Division, ABS systems will greatly enhance the performance of conventional hydraulic brake systems in several critical operations.
First, ABS systems can provide shorter controlled stopping distances in hazardous conditions, such as on wet and slippery roads, than most drivers can achieve without an ABS system, by providing optimum braking pressure at each wheel.
Second, it enhances steerability, allowing the driver to steer the vehicle in the desired direction during braking.
Finally, and perhaps most importantly, the ABS system enhances vehicle stability by preventing individual wheel lock.
The amount of improvement in stopping distance depends on many factors, including the road surface, severity of braking, initial vehicle speed, etc. On some surfaces, such as roads covered with deep snow or gravel, braking distances can be longer with an ABS system, because the wheels keep rolling along the surface of the snow or gravel, rather than locking and plowing the snow or gravel as the vehicle slides to a stop. The driver would, however, still have the control benefits of the ABS system.
NHTSA notes that even a vehicle with a failed ABS system meets the federal safety standard for stopping distance.
Important Facts
A motorist should never operate an ABS-equipped vehicle more aggressively than a vehicle not equipped with an ABS system. A motorist should always leave enough room in order to stop safely.
There are no effective anti-lock brake systems available, in either OEM or aftermarket, for installation on older vehicles. ABS is designed to work together with the base hydraulic brake system, and is tailored to each specific vehicle in order to achieve optimum performance. For these reasons, the American Coalition for Traffic Safety does not recommend advertised aftermarket ABS products.
Additionally, ABS performance may be affected by a change in tire size. Always consult and follow the recommendations contained in the vehicle owner's manual regarding a replacement tire size.
Anti-lock braking systems may add to automotive safety; however, driveability is also a part of safety. If motorists continue to drive at a high rate of speed or carelessly in adverse weather conditions, it is unlikely that they will be able to safely stop the vehicle in emergency situations. A possible solution could be for the manufacturers to make sure consumers understand how to use their products safely, rather than adjusting the product.
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