How it Works: Hydropneumatic / Hydraulic Suspension

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How It Works: Hydropneumatic Suspension

Hydropneumatic / Hydraulic Suspension Systems

The major manufacturer that developed and put into production the hydropneumatic suspension system was Citroen. Maintenance of this system requires removing both the engine and the gearbox followed by removal of the front axle. Citroen used this hydraulic suspension with great success from the early 1950s - a system so efficient at damping out road shocks that it enabled passengers to write legibly and to read fine print while the car charged at high speed over unmade, potholed roads.

At the rear the wheels were mounted on swinging arms joined to a stabilising torsion bar, which acted as a first spring. This was a radically different damping principle of the shock absorbers that gave the car its incredible stability. The shocks were filled with Lockheed-type hydraulic fluid, supplied under pressure from a seven-piston pump, belt-driven off the engine. Each of them was topped by a metal sphere that contained a rubber ball filled with inert gas and floating freely in the fluid. The shock-absorber piston was actuated by a lever at the pivoting end of the swinging arm (just under the stabilising bar). Its upstroke, acting through the fluid, compressed the gas-filled ball, damping out the initial shock; further damping was obtained by forcing the fluid through a series of calibrated openings.

In the absence of coil springs or other solid supports, the rear of the car would ride entirely on the combination of gas and liquid. This explained the softness of the ride. It also meant that the car's level and road clearance depended on the amount of pressure exerted on the fluid - but here uniformity was assured by a pressure regulator and an additional apparatus which automatically corrected any variations that may have occured. When the car remained stationary for some time, the driver turned a hand control on the dashboard; this locked up the pressure inside the suspension system and prevented the fluid returning to the reservoir via the recovery line, so that the car would not settle back on its haunches.

Another handy feature was that the system did away with the need to jack up the car to change a rear wheel. You merely operated a control inside the boot, which caused the body of the car to rise on the shocks; then you placed a specially supplied support under the appropriate jacking point and reversed the control. Since the car was unable to lower itself on the supported side, the wheel would rise instead. The system's efficiency amazed all who tried it and at launch many engineering experts acclaimed it as the greatest automobile engineering development since World War 2. It was initially fitted on the 15-horsepower Citroen.

KEY to diagram below:

  1. Reservoir of hydraulic fluid.
  2. Line to pump.
  3. Pump operating off the engine.
  4. Pressure regulator.
  5. Line taking pressurised fluid to rear suspension.
  6. Lines leading to the shock absorbers.
  7. Metal spheres fitted to the shocks, each containing a synthetic rubber ball fillet! with inert gas.
  8. Swinging arm to which wheel is fitted.
  9. Stabilising bar.
  10. Modified torsion bar of front suspension, longer and springier than those of previous production models.
Citroen Hydraulic Suspension
Hydropneumatic Suspension
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