In the Traction Avant, the system first dreamed up by Citroen employee Paul Mages during World War II was only used by the rear suspension. But the design showed true potential, and it was soon launched onto a mass production car, one we know today as a design icon. In 1955, Citroen launched the DS (the DS19). It was advertised as a rocketship, and they really did go all-out.
But how did they do this? Every wheel was attached to a control arm. The movement of the swingarm pushes the hydraulic fluid which in term compresses gas within a sphere. The two are separated from one another by a flexible membrane. The major advantage of this system is that the nitrogen gas has non-linear force-deflection characteristics. To put is simply, if a normal suspension had only springs, it would bounce around for a long time after every bump. That’s what the dampers on our cars are there for, but Citroen’s system does away with this need.
Cars equipped with the hydropneumatic suspension system have five or six spheres, one for each corner, a main accumulator and sometimes another sphere for the braking system, which needs to be independent for safety’s sake. However, up to nine of ten cylinder can be used. For example, the Citroen Xanta from the ‘90s had an extra sphere for each axle for
While cornering, cars also have a tendency to lean on one corner. This makes driving a bit uncomfortable, buy the hydropneumatic suspension system can actually keep everything on the straight and level as well. It pumps the hydraulic liquid under pressure to whichever corner is being pressed down, filling the reservoir and lifting the suspension back up. These corrections are controlled by valves on older cars and electronic sensors on modern ones. The adjustments are made at a delay in order not to correct the regular suspension travel.
On the so-called elk test, a simple slalom to you and me, the system provides just the right body control, while cars with simple steel springs sometimes violently swing and crash, leaning in too hard.
The system, innovative as it might have been, gained a reputation for unreliability because it was actually released before it was really ready. The LHS hydraulic fluid used by the system had a tendency to absorb water, which in turn resulted in corrosion. The problem was intensified by the fact that the reservoir had to be vented to allow the fluid level to change when it was pumped around.
Citroen solves this problem with a mineral oil called the LHM in 1967 for the DS. This meant the pipes would not corrode and to keep dust in the air from destroying the system over time they introduced a filter and you also had to change the oil and regular intervals. Only specialist mechanics did this, and Citroen thus became known as a somewhat “special” car brand, basically because you could’t service it everywhere.
Despite its faults, the system was packed with advantages which attracted other automakers. Rolls Royce licensed the system from Citroen in 1965. Mercedes Benz tried its hand with air suspension, which used air pumps to increase the stiffness of the suspension, but in 1974 it introduced the 450SEL 6.9 with hydropneumatic suspension. Peugeot also used the system on the 405 in 1990.
It might have its faults, and some people say it makes them sick, but it’s the single biggest effort in suspension design. It was, for the guys at Citroen, like going forwards on a magic carpet of hydraulic fluid. “Creative Technologie” indeed!