Self-levelling suspension

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 on a conventional vehicle are negatively affected by changes in load, including aerodynamic properties, headlights, bumpers, and shock absorption from the suspension.

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 2003 ORIStruts recognized the need for self-leveling suspension without the use of external controls. ORIStruts then developed the first, completely internal, self leveling strut. The DP3 strut was built with an internal gravity sensitive pendulum that maintains a downward position regardless of the vehicle's tilt. The strut senses the force of gravity around hard turns and automatically adjusts spring rate. ORIStrut's technology is used for recreational purposes but is making it's way into military applications worldwide.

First completely internal self leveling struts

Recreational Uses of ORIStruts

In 1954, Citroën introduced the first self-levelling rear suspension, and then in 1955 pioneered self-levelling of all four wheels, using its hydropneumatic 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 hydropneumatic system to fit to the rear axle of the Silver Shadow.

Land Rover developed a different system for the Range Rover in the early 1970s. The Range Rover was intended to be as comfortable on the road as a normal saloon car yet as capable of heavy off-road use as a traditional Land Rover. This highlighted the contradiction in suspension design as the Range Rover used all-round long-travel coil springs. If these were kept soft to maintain comfort they would compress too much under a heavy paylod, restricting axle travel when off-road and comprimising handling. Land Rover developed a self-energising hydraulic strut as part of the rear suspension. Of similar construction to a hydraulic shock absorber the strut used the motion of the suspension travelling over bumps to pump itself back up to a pre-set height. It was sufficently powerful to regain up to 85% of normal ride height with a full load over the rear axle, and had the advantage of requiring no external power source or a dedicated hydraulic system in the vehicle. The same system was applied to the Land Rover in the 1980s when these vehicles adopted the Range Rover's coil spring suspension. In the 1990s Land Rover, in pursuit of the same blend of on- and off-road ability, developed an air suspension system that was both self-levelling and height adjustable. Originally this was in conjunction with live axles but it is now used with fully independent suspension using wishbones. The original coil-springs-with-hydraulic-strut system is still used on certain models of the current Land Rover Defender.

Mercedes-Benz, Ford, GMC, BMW, Scania AB, and Jaguar have each pursued numerous avenues to address this issue, including air suspension and rear axle mechanical devices.