Citroen XM's Hydractive

XM's Hydractive system. Note that there are totally 3 spheres in the rear axle. All the suspensions are interconnected with high-pressure hydraulic which is supplied by the engine-driven pump.

Unlike adaptive damping, Citroen's famous Hydractive suspension is fast-reacting, can vary individual axles' spring rate and damping rate. Let's see how it works:

The Hydractive, which appeared in the XM as optional equipment since 1989, was based on the company's traditional "Hydro pneumatic suspension". The latter has a large sphere at the top of each shock absorber. Within the sphere there are 2 compartments filling with compressed air and high-pressure fluid respectively, separated by an elastic barrier. The gas acts as conventional suspensions' spring while the fluid acts as damper. Shock from the wheel transmit via the fluid into the elastic barrier, than compress into the gas compartment. The gas absorbs the energy and release back to the fluid, which smooth the reaction by its damping effect. Remember, in the "Hydro pneumatic suspension" there is no spring and damper. The sphere does the jobs of both.

Now comes to the sophisticated Hydractive suspension. It still employs the spheres at each corner, but added with an extra sphere (central sphere) at each of the axles, linking by fluid between the spheres at left and right wheel. The front and rear central spheres also link each other. In other words, the front and rear, left and right suspensions are all interconnected by high-pressure fluid.

With the addition of central spheres, the total volume of gas and fluid increases. Therefore the suspension can provide softer spring and damping rate that requires by comfortable ride. Moreover, like the conventional Hydro pneumatic suspension, the fluid can flow from one wheel to another, one axle to another, thus further smoothen the ride.

When the car need stable handling and roll resistance, valves in the central spheres close, thus isolating the wheel's spheres. As the volume of gas and fluid has been reduced, stiffer spring and damping rate are obtained.

The setting is not just bounded to "soft" and "hard" only, since there are many valves associated in the central spheres. The more valves close, the stiffer the suspension becomes. In fact, Citroen added more valves to the Hydractive 2 system in 1993 in order to create more levels of setting and smoother transition between soft and hard.

What made the Hydractive so effective is its fast-reacting brainpower. A computer analyses the data acquired by speed, g-force, throttle, brakes and gearbox sensors, then decides the most suitable setting and activates the valves via solenoids and sophisticate electronics.

Because the energisation of the solenoid valves takes as long as half a second while de-energisation need merely 2 milliseconds, Citroen use the de-energisation to actuate the closure of valves, thus making the change from soft to hard setting far quicker than vice versa. This is logical, as we would always require stiff suspension as soon as we start driving hard. We don't always need a comfortable ride as soon as we ease off the throttle.