Hydropneumatic Suspension - Manufacturing

Manufacturing

The whole high pressure part of the system is manufactured from steel tubing of small diameter, connected to valve control units by Lockheed type pipe unions with special seals made from desmopan rubber, a type of rubber compatible with the LHM fluid. The moving parts of the system (e.g., suspension strut or steering ram) are sealed by contact seals between the cylinder and piston for tightness under pressure. The other plastic/rubber parts are return tubes from valves such as the brake control or height corrector valves, also catching seeping fluid around the suspension push-rods. Height corrector, brake master valve and steering valve spools, and hydraulic pump pistons have extremely small clearances (1–3 micrometres) with their cylinders, permitting only a very low leakage rate. The metal and alloy parts of the system rarely fail even after excessively high mileages but the rubber components (especially those exposed to the air) can harden and leak, typical failure points for the system.

Spheres are not subject to mechanical wear, but suffer pressure loss, due to the pressurised nitrogen diffusing through the membrane. They can, however, be recharged which is cheaper than replacing them. When Citroën designed their Hydractive 3 suspension they re-designed the spheres with new nylon membranes, which greatly slow the rate of deflation. These are recognisable by their grey colouring.

Older, green coloured, suspension spheres typically last between 60,000 and 100,000 km. Spheres once had a threaded plug on top for recharging. Newer spheres do not have this plug, but it can be retrofitted enabling them to be re-gassed. The sphere membrane has an indefinite life unless run at low pressure, which leads to rupture. Timely recharging, approximately every 3 years, is thus vital. A ruptured membrane means suspension loss at the attached wheel, however, ride height is unaffected. With no springing other than the (slight) flexibility of tyres, hitting a pothole with a flat sphere can bend the suspension parts or dent a wheel rim. In the case of main accumulator sphere failure, the high pressure pump is the only source of braking pressure for the front wheels. Some older cars had a separate front brake accumulator on power steering models.

The old LHS and LHS2 (coloured red) cars used a different rubber in the diaphragms and seals that is not compatible with green LHM. The orange LDS fluid in Hydractive cars is also incompatible with other fluids.