Citroën's Amazing Hydropneumatic Suspension
You may not have heard of French car marker Citroën, but they pioneered an unusual and pretty amazing dynamic suspension system that offered sublime ride quality. The company created 'Hydropneumatic' suspension in 1954 and the principles behind it are still in use today on automobiles, plane landing gear and even tanks.
What is it?
Citroën completely re-invented the concept of traditional spring based suspension. A hydropneumatic suspension system doesn't have a spring of any sort. Instead, the shock absorbing spring is replaced with a sphere that has an internal diaphragm creating 2 halves. On one side, you have hydraulic fluid, on the other, a gas such as Nitrogen.
The sphere is connected to a swing arm that has the hub and wheel on one end. As this arm moves up and down, it operates the suspension.
The gas in the suspension sphere is compressible and so when hitting bumps, absorbs the initial shock. The hydraulic fluid is not compressible, taking up the traditional dampers role of controlling suspension oscillations.
The system is powered by a hydraulic pump then runs off the engine via a pulley and belt. The hydraulic system also runs the power steering, brakes, and transmission.
Benefits of Hydropneumatic Damping
This design was one of the most technologically advanced marvels of its day. It was approximately 6 times more sensitive than standard suspension of the time and offered a supple ride that was extremely comfortable. The first production car with all around Hydropneumatic, or "oléopneumatique" damping was the 1955 DS19.
The French road network in the 1950s was said to be of poor quality, with asphalt breaking up and huge potholes forming. Having such flexible suspension not only gave an impeccable ride, it meant drivers could go faster.
The system is also entirely self-leveling and has adjustable ride height. The car body-shell can be lifted up several inches for increased ground clearance on rough farm tracks.
Not only did the car raise and lower at command, but it had 'assisted jacking'. The body raises to its maximum height, at which point a stand is placed under jacking rails. When the suspension is lowered again, both wheels on the raised side retract, making tire changes extremely easy! And it's all thanks to the hydropneumatic suspension system! Plus, the rims only have one wheel nut, making for super fast changes!
This video shows just how much smoother the suspension is even when compared with modern vehicles.
The car could even be driven with one of the rear wheels removed! What a marvel!
Later developments in the hydropnuematic design incoprated anti roll systems too, making for incredibly stable handling and a car that would destroy the moose test.
What happened to Hydropnuematic Suspension?
Now you're wondering, "Hey, why doesn't my car have this suspension set-up?". There's a few reasons, first, Citroen had patent rights to the concept, secondly it was perceived to be quite complicated.
This lead other manufacturers, like Mercedes-Benz and Lincoln to create less complex alternatives using compressed air suspension. In a way, modern vehicles with air suspension can thank Citroen for that innovation. It also didn't help that only garages with specialist tools could repair the DS19's hydraulic suspension.
Sadly in 2015, Citroen themselves killed off the hydropneumatic suspension system when it announced they would no longer produce cars with it installed. It's such a shame that they pulled the plug after 60 years of development, but we still want to thank Citroen for innovating and creating such a technological wonder!