Citroën's hydropneumatic suspension system makes use of the same principles as the Hydragas system we looked at earlier, but with a couple of small changes that make a big difference.

Same But Different

  The Hydragas system, as we have already seen, basically comprises of two connected rubber balls. The suspension compresses one ball, which is full of fluid. Said fluid is pushed into the next ball, which contains pressurised nitrogen gas. The addition of this fluid taking up room in the ball means that the gas is compressed, providing the springing. The valve between the two spheres deals with damping, so you don't need a damper in addition to the unit. Ok? The Hydropneumatic system takes the basics of this idea a stage further.

  In Citroën's system, the first rubber ball is replaced with a piston sliding in a tube - you may remember that we mentioned this kind of system has become quite popular for some Monster Trucks. In the Citroën version, though, the nitrogen gas is still held in a sphere. At this point, you can see how everything is pretty familiar: Suspension movement pushes piston down tube, shoves fluid into ball containing gas, compression of gas provides springing. Fairly simple. Other than the fact that a mineral oil is used for the fluid instead of a water-based mixture, and the fact that all four suspension units are linked together (as opposed to individual side's front and rear units, as is more common on Hydrolastic/Hydragas), it's all stuff we've covered.

  Here's the big, important bit: Citroën have a pump in the pipe system linking the suspension units together. Where Hydragas is sealed with a given amount of fluid, the Hydropneumatic system can add or remove fluid, opening up massive possibilities for suspension adjustment. First of all, ride height becomes infinitely adjustable at will between fully compressed and fully extended. With additional valving, the same effect can be applied to each suspension unit individually.

  This not only allows for things like load leveling (raising the rear to compensate for a heavy cargo in the back) and anti-roll (raising the suspension on one side while cornering to cancel out bodyroll), it was also possible to raise a wheel off the ground individually, allowing for wheel changing without a jack.

  The system first appeared on the Citroën DS in the '50s. Despite some initial reliability problems (mainly due to the fluid being easily contaminated - the change to mineral oil was to counter this), the system worked well. The smooth ride on rough surfaces combined with the ability to raise the suspension made for a suprisingly successful rally car, despite a relative lack of power. The ability to maintain speed over rough terrain, even with a wheel torn off, was the secret. A DS entered in the 1968 London-Sydney marathon only missed winning after having a head-on collision with a drunk driver. Up until this point the car, running on standard suspension and a detuned engine (to cope with crappy fuel), had a day's lead on the next competitor - remember, by this time the design was a decade-and-a-half old.

  Citroën incorporated several innovations into their hydraulic system, including anti-lock brakes, powered clutch and gearshift, as well as the anti-roll system. They even did R&D work on a hydraulically-actuated airbrake and power windows! Over time, the system became more sophisticated, moving towards an active system (as we have mentioned previously, there'll be more on those in a later section). As this section is concerned with suspension alone, I haven't covered these here.

  The reputation that these suspension systems have for unreliability isn't entirely justified: The principles are sound, and it's mostly poor maintenance that causes issues, though French build quality and raw material choice can often be relied upon to provide a certain amount of entertainment (I don't know whether this opinion is classified as rascism or xenophobia. Either way, I'm sure that it's somehow illegal, and I await further input from the Brussels Thought Police and/or Social Realignment Officers*). Stripped of add-ons and embellishments, the basics of a Hydropneumatic system are no more complicated than either a Hydragas or air spring system.

  For a more detailed look at Citroën technology, I would recommend this PDF File by Željko Nastasić & Gábor Deák Jahn.

Back to Suspension Basics 8 - Air Springs

Forward to Suspension Basics 10 - Undamped Suspension

*I bet the Americans reading this think I'm joking...