Hydropneumatic Suspension

As we saw, the ideal suspension would require elasticity decreasing with the load, constant ground clearance, shock absorbers integrated into the suspension—all these beyond the obvious independent suspension for all wheels. And this is exactly what Citroën’s unique hydropneumatic suspension offers.

According to the Boyle–Mariotte formula defined in the 17th century, the pressure and the volume of a mass of gas are inversely proportional at a constant temperature. Therefore, by keeping the mass of the gas constant and changing the volume of its container, its pressure can be controlled (the usual pneumatic suspensions operate on the opposite principle: air is admitted or withdrawn from the system by compressors and exhaust valves, modifying its mass while keeping the volume constant).

The volume changes are controlled by hydraulics, a technology in widespread use in every branch of the industry. As liquids are non-compressible, any amount of liquid introduced at one end of a hydraulic line will appear immediately at the other end (this phenomenon was first formulated by Blaise Pascal). Using this principle, motion can be transmitted, multiplied or divided (according to the relative sizes of the operation cylinders), with velocity increased or decreased (using varying cross sections in the tubing), to any distance desired, over lines routed freely.

Hydraulics are immensely useful, very efficient, reliable, simple to use, and—due to their widespread deployment—relatively cheap. It is no wonder that it is used for many purposes even in the most conventional vehicles: shock absorbers, brake circuit and power assisted steering being the most trivial examples; however, Citroën is the only one to use it for the suspension.

A Suspension Primer

From the early days of the automobile—and even before, in the time of horse-drawn carts—it was already well known that the body of the car, housing both the passengers and the load, has to be decoupled from the unevenness of the road surface.

This isolation is much more than a question of comfort. The vertical force of the jolts caused by the repeating bumps and holes of the road surface are proportional to the square of the vehicle speed. With the high speeds we drive at today, this would result in unbearable shock to both people and the mechanical parts of the car. Jolts in the body also make it more difficult to control the vehicle.

Consequently, there has to be an elastic medium between the body and the wheels, however, the elasticity and other features of this suspension medium are governed by many, mostly contradicting factors.

Hard suspension

If you came from other Citroëns, most notably CX or DS, you might be disappointed. Citroën—to save room in the engine compartment but also for economical reasons, to share some components with Peugeots—redesigned the front suspension. The traditional layout had a triangle trailing arm (the suspension cylinders and the wheels attached at opposite corners of the triangle) while the new setup uses McPherson struts (the cylinders and the wheels attached at the same point).

General troubleshooting

Any work on the hydraulic parts below the car should be carried out with the car safely supported on axle stands or standing over an inspection pit. Never work or allow anybody to work on the car (not even for a small adjustment or even just inspection) when it's only the high setting of the hydraulic system that keeps the car up.