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 the change in attitude coming from changes in load - specifically a heavy load in the backseat or trunk. This change in attitude affects aerodynamic properties, headlight aim, bumpers, shock absorption from the suspension and the vehicle's performance in a collision.

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 France in 1954, Citroën introduced the first self-levelling rear suspension on a production car, and then in 1955 pioneered self-levelling of all four wheels, using its hydropneumatic system. These cars maintain an exact height over the road when the engine is on - height control valves attached to the roll bars via linkages would open to add or drain fluid from the suspension, and when the desired height was reached the valve would automatically close due to its design. Later models would use electronic height sensors and motors so adjustment could be achieved with the engine off.

This system allowed the suspension to achieve an unusually soft ride quality.

Since then, millions of fairly inexpensive Citroën cars have been equipped with self-levelling as an unobtrusive, but integral design feature. The Citroën's dashboard (later console or fascia mounted controls) includes a position lever which allows the driver to select whether the car would sit with the body in high, intermediate, normal or low positions, the extremes used for maintenance such as changing wheels or hydraulic system work. Up until 1995 when they added "antisink" to the range when the engine was turned off, the suspension slowly lost pressure until the car rested on the bump stops. When the engine was restarted it rose back to its pre-selected height. The addition of anti-sink added 2 non-return valves and an extra accumulator so that when hydraulic pressure was lost the valves would close and keep the remaining fluid in the system, and leaving the car resting at a normal height when parked.

...
Wikipedia