Wheel joint
A compound joint for vehicle wheels in 2D: free rotation plus a sprung suspension along one axis.
The wheel joint bundles two constraints into one: a hinge that lets the wheel spin freely, and a prismatic axis with a spring and damping that provides the suspension. The result is a wheel that rolls while also travelling up and down over uneven ground, just like a car's strut. A motor on the rotation drives the wheel forward.
It was introduced in Box2D as the successor to the older "line joint" and is the standard tool for 2D driving games and motocross-style physics puzzles. The spring's stiffness and damping decide whether the vehicle feels stiff or bouncy.