Antilock Braking Systems Revisited

Antilock Braking Systems Revisited


It was early December. I had made arrangements to get away with my family for a weekend in New England. When we left home, a light rain was falling, and the pavement was wet. As we traveled farther north, the rain became sleet and freezing rain. Although most drivers knew that the road surface was slick, it didn`t slow down the 55- to 60-mph pace at which everyone was driving.

As I rounded a curve, I experienced the horror of seeing a car a few hundred feet ahead, spinning almost in slow motion, out of control. He ultimately stopped, facing me in my lane!

I stomped on the brake pedal and in milliseconds scanned my mirrors for a way out. The telltale humming sound and vibration told me that the antilock braking system (ABS) was working to stop my van. While maintaining control, I was able to execute a quick evasive maneuver to the left to pass the obstacle stopped in front of me. After safely passing this potential target, I looked in my mirrors again to see other vehicles skidding in all directions and trying to avoid impact.


Recent bad publicity by the insurance industry about the failure of ABS to reduce the loss of life in the civilian sector has had some in fire circles asking, Did the National Fire Protection Association Equipment Committee act too quickly by requiring ABS on all fire apparatus? Why can some drivers experience such positive results with this equipment while others condemn it for not performing up to expectations? The answer in one word: TRAINING!

When John Q. Public purchases an automobile, the salesperson touts all of the latest features, especially items like antilock brakes that “magically” save lives. When John Q. picks up his car, does he faithfully open the new car manual and review how the ABS works or how it should be used? Probably not. As long as he is able to turn on the headlights and air conditioner, use cruise control, and adjust the sound system, he`s happy. The rest of those operational and maintenance items are overlooked.

Last year, my department purchased new, four-wheel-drive, sport-utility vehicles for the line chiefs. With each unit came the driver`s instruction manual and a videotape that fully explained how and why the antilock braking system worked. Yet, I have received repair complaints that the brakes “vibrate and make noises” on these vehicles, which is completely normal when the ABS is working on a hydraulic brake system.

In my article “ABS Brakes: Myth or Magic?” (Fire Engineering, January 1996), I reported that after seeing demonstrations and hands-on training of apparatus with and without ABS brakes on a skid pad, the obvious conclusion was that no myth or magic is involved. The ABS is a finely tuned system that helps the driver contend with panic stops under poor road conditions.


The ABS on air-brake-equipped fire apparatus has four basic components:

1. The electronic control microprocessor. This miniature computer compares and processes a signal it receives from each wheel and, in turn, controls the air pressure to the brake chamber from the modulation valve. Evaluation and control take place in milliseconds, and the pressure is adjusted and changed as conditions warrant.

2. Exciter or pulse ring. Attached to the axle or wheel hub, this device turns at the same speed as the wheel.

3. Wheel speed sensor. A small induction coil mounted in close proximity to the pulse ring, it generates an impulse to the electronic controller, where the microprocessor analyzes the pulses and determines the speed at which each wheel is turning.

4. Modulation valves. They control air pressure to the brake chambers on command from the electronic control. As quickly as five times a second, these valves can apply, release, or hold air pressure to the brake chamber, which controls the braking action at each wheel.

These components are all wired together using a shielded harness to prevent interference and have several common electrical components such as relays, fuses, and circuit breakers to accomplish the job.

On a typical system, as the air brakes are applied, the computer samples speed information from each wheel sensor and determine if more or less air pressure should be applied to the brakes at a particular wheel. If the vehicle is attempting to stop in a situation where one side is on a dry road surface and the other side is locking in a skid, such as on ice or wet leaves (split coefficient), the computer rapidly releases and applies the brakes of the locked wheels to maintain rotation and help bring the vehicle to a safe, even stop. The wheels on the dry surface will continue the normal steady brake application.

For the ABS to work properly, the brakes should be firmly applied and the computer should be allowed to complete its apply-and-release function. If the driver begins pumping the brakes (as we have been taught to do for many years), the ABS will not function as intended. Many drivers who are inexperienced with the operational characteristics of ABS feel that it takes longer to stop because when they “stomp” on the brakes, the tires don`t screech, smoke, and throw gravel! The ABS was designed to allow the tires to turn while the apparatus is slowing down–to maintain the rolling friction with the road. This allows the driver to maintain steering and helps prevent the apparatus from “swapping ends” when the rear brakes lock.

Tests conducted by ABS manufacturers, using professional drivers, proved time and time again that an ABS-equipped vehicle stops in the same or shorter distance than one without the system.

Of course, if a vehicle operator reports that the apparatus does not appear to be stopping properly, it is the duty of the maintenance officer or shop to remove the unit from service and investigate the cause. Avoid the temptation to dismiss the complaint as a misunderstanding of how ABS works. Reduced air pressure, worn brake linings, excessive stroke travel requiring brake adjustment, air leaks, or wheel seal leakage could all cause braking problems. A full evaluation must be performed to ensure vehicle safety.

* * *

Before the operator of any new vehicle gets behind the wheel, he should review all manufacturer literature and manuals. This recommendation, of course, is not restricted to the ABS system but applies to all instructions pertaining to operations and maneuvers.

The driver training program should include acquiring the “feel” of stopping an ABS-equipped vehicle and the methods for avoiding collisions by practicing evasive maneuvers. Drivers must be taught to drive defensively, to reduce the potential for accidents. Most importantly, the drivers of emergency vehicles must request the right-of-way and not just take it!

Like any other tool, ABS, if properly understood and used, will work effectively to help prevent accidents and injuries. Training and understanding are the keys to applying this technology.

WILLIAM C. PETERS is battalion chief, supervisor of apparatus and equipment, and a 20-year veteran of the Jersey City (NJ) Fire Department. He is a member of the International Association of Fire Chiefs Apparatus Maintenance Section, Local 1064 of the International Association of Fire Fighters, and the Fire Engineering editorial advisory board. Peters is the author of Fire Apparatus Purchasing Handbook (Fire Engineering Books, 1994), two chapters on apparatus in The Fire Chief`s Handbook, Fifth Edition (Fire Engineering Books, 1995), and the booklet Final Farewell to a Fallen Firefighter: A Basic Fire Department Funeral Protocol.

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