A SHORT HISTORY OF SUSPENSION  

Part Eight

HYDROPNEUMATIC SUSPENSION

On the Citroën DS 19 or ID 19, each of the independent wheels is connected to the body by a lever arm, integral with a piston. This piston acts, in a cylinder, on the hydraulic fluid, which compresses, to a varying degree, following the action of the piston, a constant mass of gas in a suspension sphere located above the cylinder and fixed on the body. These spheres do not contain ordinary air as in the majority of other so-called pneumatic suspensions, but an inert nitrogen-gas. A perfectly tight rubber diaphragm separates the nitrogen from the hydraulic fluid, while at the same time allowing considerable changes in volume, owing to its elasticity. Any vertical movement of the wheel acts on the piston, which discharges the transmission fluid into the cylinder and the lower portion of the sphere.  It thus acts on the separator diaphragm and decreases the volume of gas to a varying degree following the initial movement of the wheel. The gas then reacts in accordance with the inherent properties of pneumatic springs, i.e. with great flexibility. 
 

The hydropneumatic suspension is extremely soft. It is understandable that if the car is loaded, the suspension sphere will move down with the body to which it is fixed and the piston will compress the gas in the spheres, by means of the fluid.

If the gas volume is decreased by half in this way, its pressure is doubled. If it is divided by four, the pressure quadruples. It can be seen that the flexibility of the nitrogen sphere decreases very sharply with a reduction in volume.

The hydropneumatic suspension is of the decreasing flexibility type (decrease in flexibility with an increase in load).

Also, the pressure can be controlled by varying the quantity of non-compressible liquid.

The central organ of the system is a high-pressure pump which maintains a controlled circulation in a network comprising a buffer tank, an accumulator and the suspension assemblies, with a return line to the tank.

The high pressure pump takes the fluid from the tank and discharges it into the accumulator, via a connector-disconnector (which maintains the accumulator at the correct pressure by opening or closing, as the case may be, the pressurized liquid line from the pump). This assembly constitutes a real hydraulic plant. Why are this plant and this circuit necessary?  Because we shall need them to keep the body at all times at the same distance from the ground.

The front wheel support arms are connected by a stabilizer bar, as are the rear wheels. In addition to its normal function as a stabilizer, following any changes in load this bar operates a slide valve which immediately reestablishes the balance of the suspension, by adjusting the volume of fluid, either by injecting liquid from the accumulator (slide in intake position in the height corrector) or by discharging to the tank (slide in exhaust position).

In other words, if four people get into the car, the weight increase makes the stabilizer bar rotate and, by means of a control rod, opens the sliding valve in the height corrector, which at once routes the necessary pressure (additional liquid) to the suspension cylinders, to make the car rise to its normal height. The sliding valve is then closed by the stabilizer bar.

Any change in the ground clearance of the body (load variation) operates an automatic ground clearance adjustment device, which, by increasing or decreasing the volume of liquid between the piston and the gas, reestablishes the car's normal ground clearance (16 cm).

The suspension cylinders are isolated. When the four people get out of the car, the opposite will occur: the excess liquid being routed back to the tank. The height corrector automatically maintains the car at a constant height above ground level - the reason for Citroën's constant level ride.

The damping action is obtained by having the fluid passage through a two-way restrictor valve located between the cylinder and the sphere. The lamination of the fluid reduces the rate of movement and the result is an almost viscous action, thus extremely sensitive, continuous and progressive, always proportional to the speed of fluid displacement. The Citroën hydropneumatic suspension is thus a self-damping suspension. From the triple standpoint of safety, comfort and appearance, a suspension can
only approach perfection, as we have seen, if it offers:
 

 

• maximum but controlled flexibility; 

• constant ground clearance, i.e. a height corrector; 

• continuous shock absorption, remaining proportional at all times. 

The Citroën's hydropneumatic suspension possesses all these criteria of the ideal suspension. On account of its permanent and automatic adaptation to circumstances, it achieves this by conciliating an amazing flexibility in a position close to the position of equilibrium, with an ever-efficient shock absorber system. It should be added that the height corrector has been provided with a manual control to enable the driver to increase the clearance at will, on rough ground or to drive over deep snow, for example.

This very convenient device also serves to change a wheel and is an excellent substitute for a jack. (The car is placed in its high position, a crutch is placed on the side of the wheel to be changed to maintain this side upraised when the car is lowered to its normal position. The wheel is then changed and the car is placed again in high position to remove the crutch.)

In the present state of technical progress, the hydropneumatic suspension is undoubtedly the most perfect. It ensures both the comfort and the complete safety of the passengers. But it has an additional and typically Citroën characteristic: it is better and it is more than a mere suspension: it is a coherent hydraulic network that ensures the overall safety of lD19 and DS19 and is perhaps a pointer to what the car of the future will be.

In 1933, the 8 HP Baby Rosalie Citroën model broke the world record over 300 000 km (approximately 187,500 miles) at Montlhéry at an average of mre than 93 kph (58 mph).  It ran nonstop for 133 days.