Citroën’s Hydropneumatic Suspension Made Rides Smoother (With A Catch)

French carmaker Citroën never reached the same level of global success as its rivals in neighboring Germany and Italy, and it may not be a familiar name to American readers. Perhaps it’s not a surprise, then, that its most innovative invention never caught on in other cars across the world. And yet, it’s still in use around the globe, and you may have even been on something that uses it, without realizing it.

This technology is hydropneumatic suspension, which discards traditional leaf springs and coil struts in favor of fluid dynamics. In brief, the system makes use of the fact that a gas can be compressed, but a liquid can’t. With each wheel connected to a liquid-filled piston, each bump in the road would push that liquid up into a gas-filled sphere. While the gas would compress, its pressure would increase as it did so, pushing the liquid back down again and, thus, the wheel.

That made for a legendarily smooth ride, vastly exceeding anything else on the market in terms of sheer comfort. Even drives along rocky terrain felt like sailing on a cloud. But a few critical flaws dragged the system back down to earth. In the early days, the liquid had a tendency to rot, which isn’t ideal for, well, anything. Later liquid mixtures solved that issue, but even then, the system was complicated, which made it expensive – one of a business’s least favorite things.

The heart of the system is its spheres, which hold the liquid and the gas. The engine is used to pump in the liquid to the amount necessary to achieve the desired pressure. There’s one sphere for each wheel, one to be a holding tank for the liquid, and (in some cars) one to be a dedicated holding tank for the brakes. Why would you need one for the brakes? Well, in addition to being the suspension, this system could also be used to operate the brakes as well as the clutch, gear change, and even steering.

Because the liquid was being actively pumped in and out (to keep the pressure from the compressed gas at the desired amount), the hydropneumatic system allowed for a wildly larger range of suspension firmness than any other system to date. Perfectly soft at low speed, much stiffer at high speed. The car could also self-level, even if under load or on an uneven or sloped surface. You could raise or lower the car at will. It truly is a remarkable system, brought down by its sheer complexity and expense.

Hydropneumatic suspension was the brainchild of Paul Mages, who developed prototypes for Citroën throughout the 1940s and ’50s. It debuted for sale as the rear suspension in the 1954 Citroën Traction Avant, and a year later as the sole suspension in the Citroën DS, a car that made it cool to be weird. In the latter’s public unveiling, the man behind the wheel was Mages, enjoying his, and his invention’s, moment in the spotlight.

Over the following decades, the fluid suspension system would be a signature of Citroën and a major marketing point for the company. Other companies, such as Mercedes and Toyota, would eventually showcase their own versions. Rolls-Royce, ever in search of a more luxurious experience for its drivers and their passengers, even licensed Citroën’s system for the Silver Shadow.

Most of that has faded away with time, however, and nearly all cars use coil struts these days (though some SUVs and most trucks use leaf springs). Even Citroën is abandoning its own achievement: The Citroën C5, which ended production a few years ago, was the last model the French carmaker produced with the technology.

In the early days, Citroen used a fluid that would rot the whole system if water got into it. By the 1960s, however, an improved chemical mixture eliminated this issue. Still, because the liquid was being pumped by the engine, that meant horsepower that could otherwise be used for speed was instead being used by the suspension, a problem that coil springs don’t have.

But the biggest issue with hydropneumatic was simply that it was expensive. The complexity of a machine that automatically pumps liquid in the right amounts is much higher than, say, a simple steel coil. In later decades, Citroen was installing the system on fewer cars anyway, so few that economies of scale were being lost which, again, increased costs. 

By 2015, Citroen threw in the towel, announcing that it would be leaving its signature invention behind. However, even if Citroen doesn’t use the system anymore, it’s not gone completely.

Despite the end of its popularity in passenger cars, hydropneumatic suspension is actually still in wide use today. In fact, if you’ve ever flown on an airplane, you’ve experienced it yourself. That’s because nearly all aircraft use oleo struts, a form of hydropneumatic, on their landing gear.

The same can be said for military tanks, which are just about the heaviest type of vehicle you can roll along the ground. The crew needs a suspension system that can keep them from breaking their bones even when powering through the planet’s roughest terrain. Just as important is the system’s self-leveling ability, which can keep the cannon steady even when on the move over uneven surfaces. Since a tank’s main job is to, you know, aim at things, that’s a pretty important feature.

And if you absolutely must have this system for yourself, it is actually still available for consumers today. Mercedes’ modern-day E-Active Body Control suspension is a hydropneumatic system, currently available in upper trims of the S-Class sedan and the GLS580 and GLS600 Maybach SUVs. Just so you know, Mercedes does warn: “System cannot overcome the laws of physics, nor is it a substitute for being attentive to driving, traffic and conditions.” Ah well.