### Anti-lock Brakes

One of the greatest contributions to automotive safety was the advent of anti-lock braking systems. Anti-lock braking systems (ABS) allows maintaining directional control of the vehicle during braking. While benefits from ABS can be derived on dry pavement driving, the most substantial benefits are witnessed under adverse traction conditions.

Braking systems operate on the principle that motion energy is removed from the vehicle in the form of heat and dissipated. The brake calipers squeeze the brake pads against the rotors and slow the rotors. This does not stop the vehicle; the friction of the road surface against the tires is what actually slows the vehicle. The brakes merely provide the retarding force for the tires. If the tires can not maintain a level of traction with the road surface, the best braking system can not slow the vehicle.

If during braking, 1 or more tires hit a section of low traction, the braking force applied by the calipers will overwhelm the available traction at the tire contact patch. As a result the tire will slide instead of roll. If we look at the contact patch of the tire as the car rolls down the road, we would see that the tire has a relative speed of zero compared to the ground. Under braking the relative may increase so there is a slight percentage of slip between the tire and the road surface. A small percentage of slip is acceptable and friction force will rise, slowing the car. If the percentage rises too high and the tire is no longer rolling, the friction force drops tremendously and the tire can not provide lateral or longitudinal traction.

Driving in the wet or snow, loose gravel or sand, or any other kind of low traction surface can cause the tires to lock and loose directional stability. ABS monitors the rotation of the tires and compares the speed of each. If the speed of 1 or more tires drop drastically below that of the others during braking, the ABS controller will cut hydraulic pressure to that wheel until it is rotating at the same speed as the others. This will provide the best chance of maintaining directional control of the vehicle.

ABS can not perform miracles. If the laws of physics are exceeded, the car can leave the roadway. ABS can only help to maintain control. Go too fast into a turn and mash the brakes, ABS or not, the tires can only do so much and control may be lost. Driving too fast in the rain or snow is a recipe for trouble. ABS is a tool to make driving safer, not a cure-all for bad driving habits.

ABS can be useful in dry ground driving in the same way it is in low traction situations.

Despite impressive test track performance, the on-the-road safety benefits of passenger car anti-locks are disappointing. They haven't cut the frequency or cost of crashes resulting in insurance claims for vehicle damage. Recent studies by government, industry, and the NHTSA found that cars equipped with anti-lock brakes are in more fatal single-vehicle crashes than cars without anti-locks. It's not clear why this is the case, but it is suspected that many drivers don't know how to use anti-locks effectively. Trained to brake gently on slippery roads or pump brakes to avoid a skid, drivers have to "unlearn" old behavior and use hard, continuous brake pressure to activate anti-locks.

If you anticipate driving a lot on slick roads, such brakes may be a worthwhile choice. But remember that anti-locks aren't "super" brakes allowing you to stop on a dime under all circumstances. And don't take risks you'd avoid if you did not have the anti-locks.