Hydro pneumatic suspensions have been available on cars for over 50 years. The concept was to provide a much smoother ride than was available from other forms of suspension.
It was an engineer at Citroen in France who developed the original version back in the mid to late 1940s and it hit the showrooms on the Traction Avant model in 1954. The car was unique in its day having front wheel drive, a monocoque construction and the gearbox ahead of the engine. At the time Citroen was owned by the Michelin tyre company and the suspension was fitted to the 1954 model in preparation for the ground breaking DS model that was launched the following year. Incidentally, that car was built in France, Belgium, Germany and the UK.
Hydro pneumatic suspension was a very successful and clever form of independent suspension. Each wheel had a separate spherical unit that contained nitrogen in the top half and hydraulic oil at the bottom. The oil came from the car’s hydraulic systems that controlled the gearbox, steering etc. There were other spheres in the car to help maintain the self levelling component of the system by pumping hydraulic oil to the braking system thus enabling the designers to manage the braking effect with the suspension working in tandem.
Basically the nitrogen acted as a cushion for a piston driven by the hydraulic oil. The motion of the wheel was absorbed by this two stage cushioning and the hydraulic fluid was pressurised when the engine started and this raised the cars body ready for use. I remember seeing this for the first time when I was a kid on holiday in Europe, for a young lad it was fascinating!
Originally the car used a vegetable oil for its hydraulics. Like many hydraulic systems, if the engine lost power so did the ancillaries like braking, steering and in this case the suspension. The pumps also took away some of the power that should have been sent to the wheels.
In 1962, the Morris Motor Company in Britain introduced their own version called Hydrolastic, which was supposed to be a simpler fluid driven suspension unit. The Hydrolastic system used displaced fluids under pressure to provide support for a rubber spring. The front and rear wheels were connected so that the fluids could move around the system.
Ultimately British Leyland, as Morris became, replaced the Hydrolastic system with a new nitrogen gas based system called Hydragas, seen on the infamous Austin Allegro and Princess models.
In the mid 1960s, Rolls Royce licenced the Citroen technology for its cars, which by now had replaced the oil with a mineral based one after trying out a synthetic oil. The switch was due to the fact that mineral oils don’t absorb water like the vegetable or synthetic based ones. The first rule of using a hydraulic system is not to get water in it as you lose pressure!
Later Citroens connected the suspension units at the front and also the rear. This had the effect of spreading the load when the car hit a bump in the road, such that the body didn’t move. The nitrogen in the system proved to be a very good absorber of loads and when one side of the car compressed the nitrogen, the opposite side uncompressed to the same degree causing a smooth transition of fluids without affecting the body.
In 1990, Citroen introduced a new version called Hydractive, where they added in more electronic sensors as well as the ability for the driver to select the type of suspension to be used, i.e. sports or comfort. The XM and Xantia models used Hydractive 1 and the replacement, Hydractive 2.
For the 2001 model C5, Citroen launched a newer version, spookily called Hydractive 3, and this maintained the body height after the hydraulic system was stopped by using electronics to keep the car up. The C6 model took this further with Hydractive 3+.
Other manufacturers that have used a form of hydro pneumatic suspension include Mercedes-Benz, BMW and even Maserati, although this is not surprising as they were owned by Citroen for a while in the early 1970s before Michelin sold the bankrupt Citroen to Peugeot who onsold Maserati to De Tomaso.