Ball Joint

What it does

A ball joint is exactly what it sounds like — a ball inside a socket, just like your shoulder. It connects the steering knuckle (where the wheel bolts on) to the control arm. Because it's a ball in a socket, it lets the wheel move up and down over bumps AND pivot left and right for steering simultaneously. The ball sits in a greased plastic cup inside a steel housing. When the grease runs out or the plastic cup wears, the ball rattles around in the socket — you hear clunking, the car wanders, and at full failure the wheel can fold under the car.