By Jim Kerr
Even though anti-lock brake systems (ABS) are a common feature on most modern vehicles, there are still a lot of misconceptions about how these systems work and what they can do for drivers. You may have experienced them in operation several times over the past couple of months as slippery roads have tested the braking capability of our vehicles, but what really happens when you step on the brakes?
First of all, Antilock Brakes are designed to prevent the tire from locking up and sliding on the road surface. It doesn’t matter whether the road is wet, ice covered, gravel or good clean pavement. In every braking situation, the brakes are capable of stopping the wheel and tire from rotating. When this happens, the vehicle is no longer under control and ABS allows us to retain control.
Maximum braking occurs when the tire is moving at a speed about 14 to 20 percent slower than the road surface below it. If the difference in speed is less than 14%, the vehicle isn’t braking to full potential. If the difference in speed is greater than 20%, the tire is sliding too much on the road surface and there is very little control.
There are many manufacturers of ABS systems with different hardware and computer configurations, but the theory of operation is similar in all of them. When the driver steps on the brake pedal, the ABS computer monitors wheel deceleration rates with wheel speed sensors. It compares the deceleration rate of the wheel to an internal table to determine if the wheel is about to lock up. When it senses a wheel about to lock, ABS first blocks brake fluid pressure to the brake caliper on that wheel. If the wheel is still decelerating too fast, the ABS will then release some of the brake fluid from the caliper and return it through a pump motor into the inlet port again, where it is ready for the next brake apply.
With the brake fluid pressure released from the caliper, the wheel now begins to rotate again. The ABS then reapplies brake fluid to the brake caliper to make it brake again. This sounds like a slow process but systems are able to do this 15 times a second for every wheel on the car. The rapid clicking sounds you may hear when coming to a stop at a slippery intersection are the sounds of the solenoids cycling to release and apply brake fluid pressure to the wheels.
Many drivers think that a locked wheel will stop faster than a rotating one. This may be true in a couple situations, such as on loose dirt or gravel, or deep packed snow. The locked wheel and tire act like a snow plow and create a pile or hill in front of the tire. This incline, even though small, slows the car quickly. However, with the wheel locked, there is no longer any steering control, so the vehicle may spin out of control or slide off the road.
With ABS keeping the wheels rotating, the driver is able to steer the vehicle and maintain control, while stopping in the shortest possible distance. Another advantage of ABS is that one panic stop without ABS can flat spot the tires. The stationary sliding tire has a flat spot worn on the tread. I have seen brand new tires destroyed in one panic stop because the wheels were locked up.
ABS allows the driver to steer the vehicle while braking. Shorter stopping distances can be achieved if the vehicle is stopped in a straight line because every time the vehicle is steering, additional slip angles are placed on the tires. However, a driver may be able to avoid a collision by steering around it, or may be able to steer where there is better traction so the vehicle can stop quicker. Most drivers tend to automatically hit the brakes and keep the steering wheel straight, but learning to steer while braking can prevent a collision.
Brake assist is another feature that has quietly been added to many vehicles. Brake assist will apply the brakes fully in a panic braking situation so ABS can be fully utilized. When a potential collision situation occurs, most drivers quickly step on the brake pedal but don’t push on it fully until several milliseconds later. The time frame between initially applying the brakes and then fully applying them can allow the vehicle to travel several vehicle lengths further. Brake assist monitors the rate of pedal movement to determine if panic braking is occurring, and it will then fully apply the brakes.
One final thing to remember about braking systems – they still must follow the laws of physics: get the vehicle sliding sideways on a slippery corner and ABS brakes won’t help a lot. That’s what electronic stability control and common sense are made to prevent. Drive as if you didn’t have ABS and you will be a safer driver. Then let it work when you really need it.