Rocker panels are the outer sill panels under the doors.

In engine design, a rocker is a component which transfers motion from a cam lobe to a valve or pushrod. It is pivoted at its centre and 'rocks' back and forth as the cam rotates – hence its name.

Rack and pinion is a type of steering system consisting of a gear wheel (connected to the steering column) and a flat toothed component, the rack. As the pinion turns the rack is pushed sideways, and this motion is used to turn the steered wheels.


A rollcage is a network of strong tubes which protects the occupants in the event of an accident.

Separate roll cages are fitted to competition cars. Often they also help to stiffen to bodyshell.

In road cars, roll over protection is built into the bodyshell.

Mitsubishi Evo X rolls in a corner

Roll is the outward lean of a vehicle in cornering.

Roll occurs when the centre of gravity of the vehicle is above the roll axis, an imaginary line about which the car tilts. This is almost always true.

Hydraulic systems like Citroen's Hydractive can be set up to eliminate roll or even tilt a vehicle inwards (as used on the Mercedes F300 Life-Jet and tilting trains such as the Pendolino).

Compare pitch and yaw.