A rocker panel is a three or four inch piece of metal that runs along the bottom of the car body underneath the doors. Rocker panels are usually coated with a rock proof protectant which rubberizes the exterior surface before the car is painted. If you have mud flaps behind your wheels, this protects your rocker panels, as well as your fenders and your doors. Rocker panels are often made of chrome plating, and enhance the car.