March 24, 2009

What Lies Beneath: Citroën's Hydropneumatique suspension.

Mention the word "Citroën" to someone in the United States and you'll most often get one of two responses: 1) "a what now?" or 2) "are those the cars with the suspension you can raise?" Option two is right - sometimes. In the U.S., only the DS (and what little gray-market CXs found their way here) were sold with the Hydropneumatique suspension that, to quote a Grantsville, Utah resident, "you can raise." This system is so smooth that it can induce motion sickness in certain passengers as it makes the car feel like its floating. For the rest of us, it makes a sometimes fifty year old car more comfortable than a lot of new cars.




The basic system that is found on early cars consists of a pump that is driven by the motor and that creates about 170 bars of pressure. An accumulator then distributes the pressure around the hydraulic system. This obviously includes the suspension but on other models (notably the DS), it also includes the power steering, power brakes and a hydraulic clutch, all significant advances for its time. There are four suspension spheres (pictured below), each of them is hooked up to a suspension arm via a piston that moves up and down as the suspension arm does. The piston pushes liquid into the sphere which contains compressed air and is separated from the liquid by a membrane. The whole lot of it swallows road imperfections effortlessly.


Cars equipped with this system will automatically lower very close to the ground when parked for a long time. This means that when you start them, you have to wait for them to rise up before you can drive them. This system eliminates the need for a conventional jack- if you get a flat tire, set the car to the highest level, put the jack under it and then set the car to a lower level.

Like everything it has its ups and downs (pun not intended.. or was it?) Since the system auto-adjusts the height of the car, you can tow your trailer or load up your trunk and the ground clearance will stay even. The comfort and the road handling are both outstanding. On the other hand, it's obviously a lot more maintenance intensive than a standard suspension system: the spheres can leak, the membrane inside the spheres can break and if the whole system goes haywire (i.e. the pump gives out), the car is undrivable.

It should be worth nothing that later cars such as the current C5 and C6 use a variation of this called Hydractive. While the basics are the same (pump/etc), it's an infinitely more complicated system that relies heavily on computers.

A 1983 GSA at the highest setting:




A 1973 DS at the lowest setting:

2 comments:

James Mackintosh said...

they even spell it in that sexy french way!

As much as I love my 900 Turbo, hydropneumatic suspension is something I'm glad I don't have to deal with. old saabs are weird enough.

jashugita@gmail.com said...

Some thoughts from a european guy ;)

The backside tends to get to the floor first because there spheres aren´t linked with the acumulator, also, the rear brakes takes pressures from the rear spheres so if you brake with the engine off the back lowers. This is true until 1994 xantias, these adds a rear acumulator sphere.
The hidractive just adds one sphere to each axis in order to give a more confy ride, these spheres are blocked in sport mode giving a firmer ride.
The xantia active adds one more sphere and a hidraulic actuator to each axis to control body roll, in these cars the antiroll bars are much wider than usual, they are softened by the extra spheres until roll is to occur then the spheres are blocked in order to avoid body roll. The hidraulic jacks compensates extreme body roll.
The roll is detected by body sensors and there where sensors in the steering wheel and accelerator in order to anticipate hard manouvers.
Maximun lateral acceleration was near of contenporary Ferraris, but tires wear very fast if intensively driven.
This system have a total of 10 spheres of three different types, is easy to see that it havent been carried to next models due to its complexity.
The system in the C5 and C6 is simpler because it doesnt include power steering and brakes (the other one that doesnt include they is the ID, a cheaper versions of the DS) also the sphere position is redesigned, in earlier models most notably the XM the spheres can leap from their attachment and go throught the hood.
Hydropneumatical cars doesnt had power brakes, inteads they used high pressure hidraulics brakes, in the DS there were not a brake pedal but a floor button, latter models where given a conventional pedal but with very short traverse. C5 and C6 uses conventional power brakes.
The spheres last much less than a conventional shock absorvers, and the car is undrivable when they are worn. But they where very cheap and easy to replace compared to shock absorvers. C5 and C6 have redesigned spheres of much longer duration.
C6 is designed to give a magic carpet ride using body sensors.