"Why It's Good"

Antilock Brakes

By Carey Russ

Are antilock brake systems (ABS) a good thing or not? A recent study concluded that ABS-equipped cars, contrary to expectations, did not have a reduced accident rate compared to cars without antilock brakes. Two possible reasons were cited: driver overconfidence and a lack of understanding of the limitations of ABS.

Neither reason is particularly surprising. Many people believe that a car equipped with antilock brakes will stop in a shorter distance than one without ABS. Not necessarily true! They also may never brake hard enough to make the ABS work. I will look at both potential problems.

First, stopping distance. A car with locked wheels will not stop in as short a distance as a car in which the wheels are at the threshold of locking but not actually locked. And, when the wheels are locked, the driver is strictly along for the ride - steering inputs will do nothing. A skilled driver can, by use of a technique called threshold braking, stop a car as quickly as the same car can stop with ABS. However, the important words in that sentence are "skilled" and "can". Threshold braking involves application of the brakes to a point just before the wheels lock up, and is what an ABS system does with electronics and hydraulics. For humans, it takes skill, familiarity with the vehicle, and practice. Professional racing drivers use it, as ABS is illegal in most if not all racing series - it decreases the skill level required. And, the pros often lock up the brakes, particularly in the heat of battle or in emergency evasive maneuvers. The overwhelming majority of street drivers are far less skilled than pro racers, and have less chance to practice driving their cars at the limit. So, ABS is a definite advantage for them - meaning you and me and 99.99999 percent of the rest of the people on the road.

Why? One word: control. as mentioned above, a car with locked wheels is a car out of control. It will not respond to steering inputs, and it takes a longer distance to stop than a car with the brakes applied just under the lockup threshold. And, it ruins the tires, wearing flat spots in them where the skid occurred. (More than the tires may be ruined if the skid results in a collision.) Antilock brakes allow the driver to steer out of a hazardous situation while braking - BUT the driver must be able to do so. The car will not steer itself. Also, and very important for anyone who learned to drive before ABS, the brakes must be applied hard enough that the antilock equipment is activated.

Those of us who came of age before ABS (and that includes the vast majority of drivers, as widespread ABS technology is only a few years old) were taught NEVER to mash the brake pedal completely to the floor and so lock the brakes. ABS goes against that almost instinctive teaching - if you don't absolutely floor the brake pedal, the ABS circuitry will never be activated. It took me quite a while before I felt comfortable braking hard enough to activate an ABS system - all of my instincts said "if you do this, it's going to hurt!" Because this kind of stop should only be an emergency maneuver and (hopefully) a rare occurrence, relatively few drivers know how hard to brake safely, and how their car will react. That is the second potential problem with ABS systems. There is a simple remedy to this: practice.

ABS panic stops can be practiced. BUT, IF YOUR CAR DOESN'T HAVE ANTILOCK BRAKES, DON'T EVEN THINK OF TRYING THIS!!!! And, if you have a pickup truck with antilock brakes on the rear wheels only, don't try it. Rear-wheel-only antilock systems are designed to prevent rear-wheel lockup in unloaded or lightly-loaded pickups due to the extreme forward weight distribution of such a vehicle. Such systems are better than no ABS at all, but don't allow the steering control under extreme braking that a four-wheel ABS system does. Since the antilock equipment is on the rear wheels only, the front wheels can be locked with such systems.

How to practice ABS panic stops? Find an empty stretch of road, or a large, empty parking lot. Look around and be absolutely sure that there is no one behind you or near you, even at a distance. Then check again to be really, really sure. If you have any passengers, warn them that you will be trying a panic stop. Drive up to 25 mph or so and mash the brake pedal to the floor, as hard as possible. Notice what happens. You will probably notice the brake pedal throbbing under your foot - this is normal, and gives feedback that the antilock system is working. Don't let up when you feel this, and don't pump the brakes as you might in a non-ABS car. Look around again to check for traffic. Try a panic stop again and steer while the car slows. You will notice that the car responds to steering input.

There is one situation in which ABS systems are far superior to even the best professional driver. That is when different tires are on different surfaces, for instance two on pavement and two on the dirt shoulder in an evasive maneuver. Without ABS, this is a very serious situation with little or no control possible. With ABS, it's still serious, but control is possible. But control is up to the driver.

The main advantage of an antilock braking system is increased driver control in an emergency situation. Still, the best way to take advantage of that is to try to stay out of the emergency situation. Pay attention to your surroundings, don't follow cars too closely, and always have at least one escape route if anything happens on the road near you. Know your car's limits and your own limits.

What about stopping distance? That is related to the type and size of the brakes, the brake pad or lining material and friction surfaces, the tire size, compound, and weight loading, the weight and weight distribution of the vehicle, and the surface on which the vehicle is attempting to stop. That will be the subject of another column.