Hydro-pneumatic suspension, the story of Citroën’s best hit

Paul Magès was an average student in France, fresh out of school at just 17 years old he sent his resume to Citroën, where he first started working as a maintenance mechanic, until he took over more artistic duties, such as technical sketches. In 1942, Pierre Joule Boulanger, CEO of Citroën, appointed him to the […]

Paul Magès was an average student in France, fresh out of school at just 17 years old he sent his resume to Citroën, where he first started working as a maintenance mechanic, until he took over more artistic duties, such as technical sketches.

In 1942, Pierre Joule Boulanger, CEO of Citroën, appointed him to the company’s Development Department, where he was entrusted with a rather particular task: solving the problems associated with the brakes and suspension of future Citroën models, including the 2CV, the Type H and the DS.

Sadly, World War II delayed all Citroën developments until a few good years later, when the 2CV began to roll out and the Traction Avant was taking its leave. And while the 2CV had an unusual suspension scheme, the idea was to look for something more sophisticated. The truth was that in France the quality of the roads was not the best, less after the war, so the development of a comfortable suspension became a priority.

This is how Magès, reading, realized that the problem with the excessively comfortable suspension was that it did not provide correct performance. Little prepared, but far from being inept, he began to experiment with hydraulic elements and pressurized nitrogen in secret from the Nazis, who did not want anyone to shade the Volkswagen Beetle.

The hydro-pneumatic suspension could only be tested in 1954, on the rear axle of a Traction Avant, and then officially debuted in one of the most iconic Citroën cars, the DS. The most important thing about this advance is not only the suspension, but the ability of the system to also be able to operate the brakes, the steering and many other elements.

In 1962, the then President of France, Charles De Gaulle, was the victim of an attempted attack, in which they tried to intercept his Citroën DS Presidential, in which he was traveling with his wife. The hail of bullets destroyed the tires but thanks to the self-leveling hydro-pneumatic suspension system, the car was still able to maintain stability, dodging death.

How does it work?

This scheme is quite complex, but with a basic principle of operation. It consists of an auxiliary pump strapped to the engine and four independent damping cylinders for each wheel. These cylinders have a metallic sphere on their upper part, which connects to these cylinders directly with a pressure valve.

Inside each sphere is a rubber membrane that separates the pressurized nitrogen from the buffer liquid. There is also a fifth receptacle, which acts as the main reserve. This system is highly pressurized, but not hermetically sealed, because the main liquid reservoir must have a breathing method so that the fluid can rise or fall from the reservoir. This is how the pump maintains system pressure.

When a wheel passes through an obstacle, this wheel moves the piston of the damping cylinder upwards, propelling the fluid towards the spheres, being injected by the damping valve and where the nitrogen that supports the rubber membrane absorbs as it is, as a spring road imperfections, without the mechanical fatigue or annoying mass transfers associated with a completely solid and mechanical scheme. Nitrogen has a greater resistance to flexibility than that of any suspension, which led to the installation of a self-leveling system, with an internal circuit between wheels.

Thus, by detecting differences in weight, the car could be automatically adjusted after five seconds at a suitable ride height, regardless of the amount of load you were carrying. In any case, there is also a lever that allows you to preset some height levels.

In addition, the brake, steering and clutch system was operated by the same pump and the same fluid, allowing for a fully integrated and very smooth system of operation. This innovation marked a before and after in the history of Citroën, marking a revolution in driving and comfort. In fact, companies like Mercedes-Benz and Rolls-Royce borrowed the system with licenses for their cars.

Citroën BX 19 GTi 1993 test

It is no use talking about history, without having an element to illustrate this note. For the same reason, courtesy of Roberto Pizarro, we put ourselves in command of a Citroën BX 19 GTi, the perfect car to comment on the behavior of this system, since it is not as old as the DS, but it is not excessively sophisticated to lose. the north on the basic operation of this system.

The Citroën BX 19 GTi is a firmer model than its peers and thanks to the fact that it was a product developed in conjunction with Peugeot, it boasts of a very light body (platform shared with the 405) and in this case, an XU9 1.9 L engine injected, 8-valve, with about 120 hp and 150 Nm of torque, associated with a manual transmission of five changes, short.

Being a second-generation model, there is a noticeable improvement in its quality, but on the other hand, more traditional elements are beginning to be seen in its instruments, as well as in the arrangement of its controls. Its very angular exterior design is the work of Marcello Gandini, who worked at Bertone for these times and who we know from the Lamborghini Countach and many other European models.

Getting in command of the BX 19 is not complicated, except for some classic Citroën quirks, such as its interior handles or the controls behind the steering wheel in the shape of cams. Its seats are very comfortable, something that the brand has wanted to perpetuate in its new models, especially the new C4 Cactus. A lever between the seats regulates the four preset levels of height. Level 0 is basically when the engine is off and the car is stopped, giving the impression of being very close to the ground. The highest level of suspension can achieve up to 250mm clearance and is recommended exclusively for changing a wheel or for facing a specific obstacle at very low speed.


The motor is awake and quickly lets us see its lightness. His delivery is progressive, but giving away that he has more background to run. In fact, this engine achieves 100 km / h in about 9s. Something that draws attention is that as the brakes are part of the hydraulic circuit. They are excellent, although very sensitive and of a much higher level of stability. The pedal tends to “pump” lightly as we step on it, a sensation one gets used to.

But for those who want to really check if it is like a “magic carpet”, the answer is a resounding “yes”. Being a GTi, the suspension configuration is firmer, so do not expect that when going through a pothole you do not feel it, but the car controls inertia movements very well and on normal roads, it “balances” very subtly, like wanting to float.

And then?

Later, this system was refined. The main fluid was changed to a new one, mineral, hydrophobic, which allowed it to resist humidity. Over the years, other models added more functionality, such as the SM, that Gran Turismo created together with Maserati, which used the system to adjust the main lights, as well as the DIRAVI power steering system with variable gear ratio and automatic steering wheel return.

The hydro-pneumatic scheme extended throughout the range, including the GS, the CX and the already discussed BX, to name a few models. The arrival of electronics allowed generating even more sophisticated systems, such as the Citroën XM, which could electronically regulate the desired height for the car. In addition, thanks to electronic sensors, the car could adjust the height of both axles in case of differences in acceleration.

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