Why and how ABS brakes work


By James R. Van Liere

Special to The SUN

Because winter officially arrives tomorrow and snowy and icy roads are inevitable, a discussion on the theory behind automatic braking systems might be in order.

This is not a highly technical explanation of the electrical and mechanical systems involved in an automatic braking system, but a simple discussion of the principles of why automatic braking systems work.

An automatic braking system is based on two concepts from the world of physics, namely, the coefficient of static friction (CSF) and the coefficient of dynamic friction (CDF), sometimes called kinetic or sliding friction.

To better explain these two concepts in the real world, I imagine most of us at one time or another have tried to slide a heavy box across a floor. As you probably noticed, it takes more force to start the sliding process, but once the box starts to slide, it becomes much easier.

The reason for this is that the CSF must be overcome to start the box to move, but once it starts to move, it becomes easier because the CDF comes into play.

Of course, the above example is assuming a “dry” surface between the box and the floor; however, if the surface between the two objects is lubricated, i.e., the floor is covered in oil, then the coefficients will change to reflect this condition. Unfortunately, this CSF is a hindrance when sliding boxes, but works well when trying to stop a moving vehicle.

Another example of these two coefficients coming into play is trying to get a toboggan to slide with three or four people on it. The toboggan does not move until someone gives it a small shove to overcome the static friction factor, then the dynamic friction factor takes over and away you go.

Hypothetically, and by the numbers, if the CSF is 0.5 and the CDF is 0.3, then the force to start sliding a 100-pound cardboard box on a dry concrete floor is 0.5 times 100 pounds equals 50 pounds, and the force to keep it sliding is 0.3 times 100 pounds equals 30 pounds. Of course, these figures would change if the surface in the example above were lubricated.

Quite obviously, there are many combinations of different materials sliding relative to one another, and the various surfaces may have different degrees of roughness, all of which leads to many values for these coefficients.

As an example, a car tire on dry asphalt has a CSF in the range of 0.5 to 0.8, with an average value of approximately 0.72. For the same tire on ice, the values for the CSF range from 0.15 (the lowest I have seen) to 0.40.

These values will depend on the tire tread, the condition of the tire and the actual condition of the ice. (As a rule, after driving in the mountains in all kinds of inclement weather for 60 years, I have found that traffic always moves as fast as the vehicle with the baldest tires.)

Now, how does all this theory apply to an automatic braking system (also known as ABS)? Years ago, before the advent of ABS, people driving on snow and ice learned how to “pump” their brakes, which essentially meant alternating applying and releasing the brakes so that the vehicle would slow down gradually without sliding. In essence, they were using the CSF. Of course, this took practice, and some were better at it than others were.

Automatic braking systems improved upon this “pumping” method by using a computer to apply or “pump” the brakes about 15 times a second. By doing this, the computer is taking advantage of the CSF and not letting the tires slide, thus, not allowing the CDF to come into play.

The downside to ABS, if one can consider it a downside, is that they may be noisy. Years ago, when automatic braking systems were installed on vehicles, a few people were killed using this new braking system. They did not realize noise from automatic braking systems was a normal thing and took their foot off the brake pedal. Of course, they now had no brakes and crashed.

The point is: when using ABS brakes, you should continue to stand on the brake pedal and hold the steering wheel straight until the vehicle comes to a stop. If the road has steep crown, the vehicle may tend to slide slightly sideways, but this is nothing to worry about (unless you are sliding off a cliff). For the most part, automatic braking systems will keep the vehicle in a straight line, unless you turn the steering wheel. In this case, the vehicle will tend to turn.

If your vehicle has an automatic braking system and you have never really used them in an emergency or on a gravel, asphalt or concrete road, or on a snow-packed or icy road, I suggest you try them out on these various surfaces at low speed (say about 20 mph) in a non-traffic area. By doing this, you will develop a feel and know what your braking system will sound like and will not panic if an emergency situation arises.

I hope that this simple article has been of value, that you now know a little bit more about ABS, and will not end up in a ditch on some lonely road.