ABS BRAKES: MYTH OR MAGIC?
BY WILLIAM C. PETERS
The 1996 edition of NFPA 1901, Automotive Fire Apparatus, if adopted in its present form, will require the installation of an antilock braking system (ABS) on all fire apparatus, where available. Some wonder if this $5,000 feature is really worth the cost.
To answer this question for myself [and at the invitation of the Phoenix (AZ) Fire Department], I traveled nearly 3,000 miles to the General Motors Desert Proving Grounds in Mesa, Arizona, to find out. Sedans, utility vehicles, and fire apparatus (both with and without ABS) were put to the test. The results were dramatic, to say the least!
To duplicate the worst possible braking conditions, General Motors has constructed a skid pad of polished volcanic stone. After the underground sprinklers are turned on and the water truck thoroughly soaks the track, the pad has the same friction surface characteristics as ice!
THE DEMONSTRATIONS
The first demonstration vehicles on the skid pad were Phoenix Fire Department sedans, both with and without ABS brakes, traveling side-by-side at 35 mph. On signal, they both initiated a panic stop. The ABS-equipped sedan continued in a straight line and came to a smooth stop. The wheels continued to turn, which would have allowed the driver to steer around any obstacle. The wheels on the non-ABS vehicle immediately locked, and the inertia carried the car forward as it skidded out of control to the side of the pad. With locked wheels, the driver was unable to correct the skid.
After another pass with the water truck, the same test was conducted with the utility vehicles (light trucks). Once again, the ABS vehicle came to a controlled stop whereas the non-ABS vehicle unit skidded out of control.
The next test–fire apparatus–was the one I really wanted to see. The first pumper was equipped with ABS brakes and an electrical driveline retarder. As the apparatus entered the skid pad at 35 mph, the brakes were applied rapidly. Just as with the smaller vehicles, the pumper slowed to a controlled stop. This proved not only that the ABS system controlled the vehicle`s foundation brakes effectively but that the interface with the retarder worked, preventing it from locking the driveline and rear wheels. This safety feature is very important if an auxiliary braking device is being used on the apparatus.
As predicted, the non-ABS pumper skidded like a 20-ton metal brick, with all wheels locked until it finally came to rest at a 45-degree angle. If this had occurred anywhere except in the middle of the wide-open spaces, it certainly would have struck parked cars, utility poles, or whatever was in its way.
The most dramatic demonstration and probably the most challenging to apparatus drivers was the split-coefficient exercise. Here, the apparatus approaches the pad with only two wheels on the slippery surface; the other side is on normal pavement. The situation simulates conditions that could exist if one-half of the driving lane were covered with snow, ice, loose gravel, or wet leaves.
At 35 mph, the driver of the ABS-equipped pumper applied his brakes and required very little effort to bring the apparatus to a controlled stop while traveling in a straight line. The non-ABS pumper, while duplicating this maneuver, went wildly out of control, doing a 180-degree spinout and dumping its entire hose load on the skid pad when it came to rest in front of the spectators.
Through the courtesy of the Phoenix Fire Department and General Motors, the participants were all able to drive and personally experience the stopping charcteristics of the ABS-equipped units as well as the split-coefficient stopping exercise. (We were not able to perform the spinout, but it was too hot to repack the hosebed after each person, anyway!)
HOW ABS WORKS
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 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.
Several variations of ABS are available; some control each wheel individually and others, by axle. On a typical system, as the air brakes are applied, the computer samples speed information from each wheel sensor and determines if more or less air pressure should be applied to the brakes at that 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.
If a component fails in the ABS, the affected circuit returns to normal braking while the remainder of the system continues to operate as designed. Should the total system fail, the entire vehicle returns to standard braking without losing any of its normal stopping ability.
DRIVER TRAINING
The apparatus driver must be trained in how to use ABS. Unlike the “fanning the brakes” procedure (rapidly apply and release) taught for many years for attempting to stop on a slippery surface, the driver of an ABS-equipped unit must apply firm pressure to the brake pedal and allow the system to work. Pumping the brakes tends to confuse the computer with regard to what you want to do and will increase stopping distance.
While specifying equipment such as ABS brakes and retarders can help provide the firefighter with a safer apparatus, the driver`s skill and attitude are most important for safely operating an emergency vehicle–personal characteristics such as speed control, anticipating hazardous conditions, and exercising caution when approaching intersections are just a few.
The participants at the Phoenix program were shown and practiced other methods for avoiding collisions. The serpentine course (zigzag through a line of cones) helped to acquaint the driver with the vehicle`s turning capabilities and to increase awareness of where the vehicle was in relation to stationary objects. A controlled braking exercise helped the students to develop the ability to achieve maximum braking while still being able to control the direction of the vehicle. Another-collision avoidance technique presented was the evasive maneuver, which teaches that less distance is required to steer around a fixed object than to panic-stop.
While I witnessed a few rubber cones “killed” in this process, we all departed with some newfound knowledge that will certainly enhance driver safety.
WHAT IS THE ANSWER?
To answer the question raised in the title of this article, “ABS Brakes: Myth or Magic?” I would have to answer no. There is no myth involved. The system works and, in my opinion, is well worth the money to have it included in your apparatus specifications.
Magic? No, there is no magic involved here. It is a very real, finely tuned system that helps the driver contend with panic-stop situations under poor road conditions. The bottom line is that ABS works, but it does not relieve the driver of responsibility to exercise due care, drive defensively, and maintain a positive attitude while operating an emergency vehicle.
My sincere thanks and appreciation are extended to the members of the Phoenix Fire Department who instructed us and Captain Joe Gorraiz, the driving training officer, who invited me to participate in this exciting program. n
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), the booklet Final Farewell to a Fallen Firefighter: A Basic Fire Department Funeral Protocol, and two apparatus chapters in The Fire Chief`s Handbook, Fifth Edition (Fire Engineering Books, 1995).
