Truck Topics

Know Your Brake System

If your driving history goes back to the days when you were advised to "pump your brakes" in an emergency stop on slick pavement, then the first time you used a vehicle equipped with an anti-lock braking system (ABS), it probably came as a shock to discover the brakes seemingly had a mind of their own.  It required a bit of adjustment on your part to simply apply steady pressure to the pedal and let the system do its thing.

As you probably know, ABS is now required on all on-road vehicles with air brakes and all hydraulic-braked trucks over 10,000 pounds.

Air-braked tractors had to have ABS as of March 1 of ’97, followed by trailers in ’98 and hydraulic-braked straight trucks in ’99.   In the decade since first mandated, ABS has proven its value time and again, with glowing testimonials from drivers who were able to control their vehicles in a panic stop.

Forward-looking trucking fleet managers began buying ABS in the late ’80s because the executives believed they would improve safety. As time went on, they say their reduced accident rates proved them right.
     
However, most fleets declined to spend extra money on then-optional ABS, and few vehicles were built with it until the federal government’s mandate began going into effect in 1996.

The government’s first mandate of ABS was thrown out by a court suit; not until later years did regulators, reassured by successes with latter-day systems, reinstate the requirement for antilock braking performance. 

What is ABS?
ABS is a computerized system that keeps your wheels from locking up during hard brake applications.  It is an addition to the normal brakes. It does not decrease or increase normal braking capability. ABS only activates when wheels are about to lock up.

ABS does not necessarily shorten your stopping distance, but, it does help you keep the vehicle under control during hard braking.

History of ABS
A better way to handle skids while braking had been used on aircraft for decades before it was introduced in ground vehicles.

 Aircraft were subject to the same low-traction conditions as were cars and trucks, and a skidding aircraft—already only marginally steerable—was truly a danger to its occupants and those around it. To combat this problem, many aircraft were equipped with antilock brake systems, known as ABS, which prevented the braking wheels from locking up and skidding on slippery runways.

At first, this was accomplished through elaborate and expensive hydraulic controls which cycled the brakes on and off rapidly, permitting the airplane to be steered under slippery conditions while still allowing a large measure of stopping ability. Later, electronic controls permitted anti-lock action that was more responsive to actual ground conditions.

As the electronic and hydraulic portions of aircraft ABS became smaller and less expensive, truck and automobile manufacturers began to take interest. At first, antilock brake systems were developed only for heavy-duty trucks.

Large tractor-trailer combination vehicles weighing up to 80,000 pounds were especially hazardous to traffic around them when they skidded since they not only moved out of the driver's control, but also articulated, or jack-knifed, and frequently rolled over.

How it works
Today’s electronic antilock braking systems use microprocessor technology to sense when wheels are about to lock up under braking. They sense that the wheel has just begun turning slower than others on the vehicle and momentarily reduce braking force at the affected wheel. Brake pressure is adjusted to a level providing the maximum braking without danger of lockup.

ABS works far faster than the driver can respond to potential wheel lock up. And it only adjusts the brake pressure to the wheels that are in danger of locking up. At all other times the brake system will operate normally.

When the wheel resumes rolling, full braking force is again applied. ABS repeats this process until there’s no longer any need for modulated braking. ABS thus “pumps” the brakes as you’d do in an emergency stop on slippery surfaces. But ABS acts faster than you could, pumping the brakes several times per second.

Depending on the type of system, it adjusts braking force at each wheel or set of wheels, whereas your foot on the brake pedal operates all the brakes at once.

You may or may not be able to stop faster with ABS, but you should be able to steer around an obstacle while braking, and avoid skids caused by overbraking.  

Electronic ABS will also deal with a spinning wheel if the system is set up with “traction control.” ABS then uses the sensors to note when a powered wheel begins spinning and lightly applies the brake at that wheel to stop the spinning.

Braking with ABS
Having ABS on only the tractor, only the trailer, or even on only one axle, still gives you more control over the vehicle during braking.

When only the tractor has ABS, you should be able to maintain steering control and there is less chance of jackknifing. BUT keep your eye on the trailer and let up on the brakes (if you can safely do so) if it begins to swing out.

When only the trailer has ABS, the trailer is less likely to swing out, but if you lose steering control or start a tractor jackknife, let up on the brakes (if you can safely do so) until you gain control.
 
When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:

Use only the braking force necessary to stop safely and stay in control. And brake the same way, regardless of whether you have ABS on the tractor, the trailer, or both.

As you slow down, monitor the tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.

There is only one exception to this procedure. If you always drive a straight truck or combination with working ABS on all axles, in an emergency stop, you can fully apply the brakes.

What ABS won't do:  

*It won't allow you to drive faster, follow more closely, or drive less carefully;

*It won't prevent power or turning skids - ABS should prevent brake-induced skids or jackknifes, but not those caused by spinning the drive wheels (traction-control might help there) or going too fast in a turn;

*It won't necessarily shorten stopping distance - ABS will help maintain vehicle control, but not always shorten stopping distance;

*It won't increase or decrease ultimate stopping power ABS is an “add-on” to your normal brakes, not a replacement of them;

*It won't change the way you normally brake. Under normal brake conditions, your truck will stop as it always stopped. ABS only comes into play when a wheel would normally have locked up because of overbraking;

*It won't compensate for bad brakes or poor brake maintenance.

Resources
www.answers.com
www.howstuffworks.com