Self-levelling refers to an automobile suspension system that maintains a constant ride height of the vehicle above the road, regardless of load.
Many vehicle systems including aerodynamic properties, headlights, bumpers, and shock absorption from the suspension, are negatively impacted on a conventional vehicle by changes in load.
There is an inherent conflict in suspension design - if the springs are soft, the car will be comfortable but dramatically affected by load. If the springs are hard, the car will be uncomfortable, but less affected by load.
Numerous manufacturers realize this conflict and have pursued different avenues to achieve both comfort and load capacity simultaneously.
In 1954, Citroën introduced the first self-levelling rear suspension, and then in 1955 pioneered self-levelling of all four wheels, using its hydro-pneumatic system. Since then, millions of Citroën cars have been equipped with self-levelling as an unobtrusive, but integral design feature. The Citroën's dashboard includes a five-position lever which allows the driver to select whether the car would travel with the body in a high or low position. When the engine is turned off, the suspension slowly loses pressure until the car rests on the rubber bump stops. When the engine is restarted it rises back to its pre-selected height.
In 1966, Rolls-Royce licensed Citroën's hydro-pneumatic system to fit to the rear axle of the Silver Shadow.
Mercedes-Benz, Ford, GMC, BMW, Land Rover, Scania AB, and Jaguar have each pursued numerous avenues to address this issue, including air suspension and rear axle mechanical devices.