From Practical Classics – October 2017
This is a Concorde moment. Once upon a time, we could sip champagne in comfort whilst travelling at 24 miles a minute. Years before that, mankind was dancing about on the moon and planning excursions to the outer reaches of the galaxy. And there was once a time when motorists could glide across the earth on a bed of nitrogen gas a in vehicle that could maintain a constant ride height irrespective of load, untroubled by undulations and broken tarmac. 
The hydropneumatic Citroën was an engineering masterpiece of comfort, safety and control efficiency, unveiled to a world enduring suspension made up of medieval leaf springs and 19th century coil spring technology. As Citroën drivers wafted about serenely, the rest of the world failed to appreciate the benefits.
And now, for successive years, we’ve been told we want sporty cars. That means even MPVs and SUVs have stiff suspension springs and low profile tyres, vulnerable to our appalling road surfaces. See how occupants of modern cars jiggle about in their sports seats on the way to work, enormous alloys thumping into potholes. And for what? To live out the fantasy that you might one day tackle the Nurburgring?  
In 2017, we took a great leap backwards. Yes, I know the manufacturing cost was as prohibitive as supersonic flight was to most of the airlines but it doesn’t mean Concorde wasn’t dynamically superior to a lumbering Jumbo. As a regular user of cars with both springs and hydropneumatics, I am convinced the latter should have been the future. It’s so typical of the human race. Mankind pulls a blinder and then throws it all away.

How it works
Following in the footsteps of André Citroën, visionary Citroën CEO Pierre Boulanger instigated the next chapter of Citroën’s long history of innovation with the line: ‘Study all possibilities. Even the impossible.’ He assigned designer Paul Magès to Citroën’s development department in 1942, who was introduced to aeronautical engineer André Lefebvre and sculptor Flaminio Bertoni (the uncompromising stylist responsible for the Traction Avant, 2CV, H Van, DS and Ami 6). Paul’s suspension design brief was to enable ‘fast travel on poor road surfaces.’
What he came up was a high pressure gas and oil system (or ‘Oléopneumatique’), designed to enable suspension, steering, brakes and gearshift to work in harmony with each other, with the entire system operating from an engine driven pump.

There’s a metal sphere containing nitrogen gas at each wheel, replacing the conventional spring and damper. Julian Marsh – creator of this site – says it’s simpler than you might think: ‘The gas acts as the spring and is housed in the metal spheres, which contain a flexible diaphragm. When a wheel hits a bump it rises and via a mechanical link it pushes the suspension piston back and this squeezes fluid through a tiny hole (effectively a damper valve) in the sphere to let the gas absorb the energy of the bump. Once the car is over the bump, the gas pushes the diaphragm back out, pushing the fluid down and thereby pushing the wheel down to the ground.’
There’s a fifth sphere, known as the accumulator, which evens out the pressure caused by whichever part of the system is being used at that time – be it suspension, brakes, steering or gears. It also acts as a reserve of pressure in case engine power is lost. All of this is made possible by the bright green Liquide Hydraulique Minéral (LHM) fluid - the lifeblood of any hydro-Citroën. It was developed by the company and unlike standard brake fluid, it’s non-corrosive and hydrophobic. Additional features of the system include automatic ride height. Height corrector valves respond to the road surface or load by letting more fluid into the suspension cylinders. Both the Citroën SM and CX (plus, some XM models) got fully powered (not power assisted) self-centring steering which varied in weight, depending on speed. As with adaptive lights and intelligent suspension, Citroën got there decades before the rest with technology now in widespread use across the industry. The impossible became possible.

Developments
With no natural roll stiffness in the system, Paul Magès sought to eliminate body movement with active ride as early as the 1940s. Despite experiments with an SM prototype in 1973, active ride wouldn’t debut in a production Citroën until 1989.
The XM’s ‘Hydractive’ suspension was fed by electronic sensors on the steering, brakes and throttle pedal. To combat roll and deliver better handling, depending on driving style and road conditions, the computer was able to switch an extra pair of spheres in or out of the circuit.
Arguably the greatest leap forward in suspension technology occurred in 1994 with the introduction of the Xantia Activa. Engineers added a computer and hydraulic rams which completely eliminated roll, yet retained a limousine ride quality. Despite recent challenges from the Porsche 911 GT3 RS and brand new McLaren 675LT, the 1997 Xantia Activa is still the fastest car to negotiate the Swedish ‘’elk test’ slalom – and by a significant margin.
Cost implications (and possibly the fact owners kept binning them into ditches) meant the Activa’s complex set-up was canned in favour of Hydractive 3, as fitted to the Xantia’s replacement. The 2001 C5 could adapt instantly to the road surface and detect driving style, although purists bemoaned the fact it made do with conventional brakes and steering.