ABS and Stability Control

ABS Facts
Tires have much less grip when they're sliding than when rolling. Race car drivers are trained to balance the tires on the verge of lockup: right on the grip peak. Under emergency braking, ABS does just about the same thing: It releases brake pressure just before the tire starts to slide and reapplies pressure when grip returns. It does this many times per second to keep the tire at its peak grip. All you have to do is hold the brake pedal to the floor.


Without ABS, if you attempt to steer during emergency braking, at least one tire is almost certain to lock up. If it's a front tire, the car won't turn and will slide into whatever you're trying to avoid. If a rear tire locks up, the car may spin out.

Stability Control Facts
Stability control uses the same idea in reverse. Stability control applies the brakes on the end of the car with too much grip. Let's say a driver encounters a patch of ice while accelerating onto a right-turning interstate on-ramp. The rear tires lose grip and the car begins to spin out. Engineers call this "oversteer," and stock car drivers call it "loose." The front tires have too much grip in relation to the rears. The stability control computer applies the left front brake or maybe both front brakes. This reduces the traction of the front tires to bring them in line with the rears.


Most stability control systems also will offset loss of front grip. Change the scenario to a right turn on a two-lane road with water-filled "wagon-wheel ruts." With this car, the front tires lose grip first and the car slides nose-first toward on-coming traffic. To the driver, it feels as if the steering column had snapped or perhaps the left front tire has blown. Engineers term this "understeer," while stock car drivers call it "push." This time the rear tires have too much grip in relation to the fronts. So the computer applies the right rear brake. If the driver is pushing the brake, some systems will release brake pressure on the over-taxed left-front tire. This reduces the traction of the rear tires and helps the car turn.


All stability control systems also include traction control (but far from all traction control systems include stability control). If the car is fishtailing or losing front grip because the driver is too hard on the accelerator, traction control cuts engine power (sometimes by interrupting electricity to the spark plugs) or applies the brakes to the driven wheels, or both.