Citroën’s old hydropneumatic cars have always sounded slightly fictional to people raised on ordinary springs and shock absorbers. The car rises after startup. It keeps itself level under load. The same high-pressure system can run brakes, steering, clutch functions and, in some models, the gear change. Citroenet explains how the weirdness actually works.
Read Citroenet’s hydraulic-system overview.
The core trick is simple enough to state: gas can be compressed, hydraulic fluid effectively cannot. Citroën trapped nitrogen behind a flexible diaphragm inside metal spheres, using the gas as the spring and pressurized fluid to transmit force, control damping and maintain ride height.
The system first appeared in production form on the rear of the Traction Avant 15CV Six H before becoming one of the defining features of the DS19 in 1955. Variants later appeared on the GS, SM, CX, BX, XM, Xantia, C5 and C6, along with experimental and lower-volume Citroëns.
Citroenet does a good job of explaining why the design was not simply French engineering theater. Soft suspension normally creates compromises: load changes ride height, geometry shifts and body control becomes difficult. Citroën’s self-levelling system corrects vehicle height as weight changes, allowing unusually soft springing without letting the car permanently sag under passengers or luggage.
Later Hydractive systems added electronic control. Sensors for vehicle speed, steering, braking and body motion could switch additional spheres in or out of the hydraulic circuit, changing the suspension between softer and firmer states. The Activa version went further and used hydraulic control to fight body roll.
The site also does not pretend the system was perfect. Early DS hydraulics suffered seal and fluid problems, and the pages discuss low-speed harshness, road noise, pitching and other quirks. The introduction of mineral-based LHM fluid in the 1960s greatly improved reliability.
That balance is what makes the page useful. It is enthusiast material, but it is not just worship. It explains both the engineering elegance and the ways real cars misbehaved.
The larger Citroenet site, maintained by Julian Marsh, contains model histories, brochures and technical material covering decades of Citroën experimentation. It is the kind of specialized reference that search engines can surface but rarely replace.
Explore the hydraulics section at Citroenet. If you have ever wondered why an old Citroën seems to stand up before driving away, this is where the magic trick stops being magic and becomes plumbing.