How Cars Work · Crash Prevention
Watching the wheels
Three of the most important safety systems in a modern car all rely on this same principle. Each one watches for a different way the tires might cross the slipping line.
Anti-lock brakes (ABS) catches the case where you brake hard and a wheel is about to lock up. When a wheel locks, it stops rolling and starts sliding — and as we just said, sliding gives way less grip than rolling. The car actually takes longer to stop with locked wheels than with rolling-but-barely wheels. Before computers, drivers were taught to “pump the brakes” — press and release rapidly — to keep the wheels just barely rolling. ABS does this automatically, dozens of times per second, on each wheel independently. If a wheel slows down faster than the car is, ABS releases the brake pressure on that wheel for a tiny moment, lets the wheel start rolling again, and then re-applies. The driver feels this as a rapid pulsing in the brake pedal. That pulsing is ABS doing its job.
Traction control watches the opposite case. When you press the gas hard, the engine pumps power into the drive wheels. If the road is icy or wet or covered in loose gravel, the wheel can spin in place — sliding around its axle without gripping the road. That is bad for two reasons. The car doesn’t go forward, and a spinning tire has nearly no grip if you also need to steer. Traction control uses the same wheel sensors as ABS. If a drive wheel suddenly spins faster than the others, the computer cuts engine power for a moment, or applies a brief brake to that wheel, until it starts gripping again.
Stability control watches the trickiest case. Sometimes the whole car starts to slide sideways — the back end fishtails out in a turn, or the front end pushes wide. To catch this, stability control adds another sensor: a small device that measures how fast the car is rotating around its own vertical axis (engineers call this yaw). If the car is rotating more or less than the steering input says it should, the computer applies the brake to one wheel only — usually for less than a second — to twist the car back into alignment. Pulling on just the right front wheel, for example, drags the right side of the car backward, which yaws the car right and can correct a left-sliding skid.
All three systems use the same wheel-speed sensors — small magnetic devices that count the teeth on a ring spinning with each wheel, hundreds of times per second. All three feed into the same computer. All three are different branches of one big watchful system, each catching one of the three ways a tire can lose grip. Each one is a sense-decide-act loop, exactly like the ECU from the last chapter — just pointed at the wheels instead of at the engine.