How Cars Work
Crash Prevention
CHAPTER SEVEN
We left Chapter 6 with a question. What can the car do, now that it can think?
This chapter answers part of that question.
The most useful thing the computer does in a modern car is watch for trouble. It watches the wheels. It watches the road. It watches how the car is moving through space. And when something starts to go wrong, it reacts in milliseconds — much, much faster than any human driver could.
The result is that drivers today get into fewer crashes, and when they do crash, they walk away more often. Modern cars are remarkably hard to roll, hard to spin out, hard to skid past their braking limit. None of that is because the laws of physics changed. It is because there are small computers wedged into the corners of the car, watching every wheel hundreds of times per second, ready to do something faster than you can blink.
This chapter is about those computers and what they do.
How tires grip the road
To understand what these safety systems are doing, you have to understand one fact about tires.
A tire grips the road by friction — the same friction that made your hands warm when you rubbed them in Chapter 5. The rubber tread of the tire pushes against the bumps and roughness of the pavement, and grips. Up to a point, this grip is rock solid. The tire rolls along, and the road pushes the tire forward (when you accelerate), backward (when you brake), or sideways (when you turn).
But there is a limit.
Push too hard — brake too hard, accelerate too hard, turn too sharply — and the tire crosses a line. Instead of gripping the road, it slips. The rubber starts sliding across the pavement instead of rolling along it. And here is the key fact: a sliding tire grips much less than a rolling one.
You can feel this with your sneakers. On a smooth floor, you can walk normally — your shoe grips, you go. But if you try to stop suddenly while running on the same floor, your shoe might skid out from under you. Once a shoe is sliding, it has way less grip. Same physics.
Every safety system in this chapter exists for one reason: to keep the tires on the right side of that line. To keep them rolling, not sliding. Below the limit, not over it.