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 in vehicle that
could maintain a constant ride height irrespective of load, untroubled
by undulations and broken tarmac.
The hydropneumatic CitroŽn 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 CitroŽn 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
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 Andrť CitroŽn, visionary CitroŽn CEO
Pierre Boulanger instigated the next chapter of CitroŽn’s long history
of innovation with the line: ‘Study all possibilities. Even the
impossible.’ He assigned designer Paul MagŤs to CitroŽn’s development
department in 1942, who was introduced to aeronautical engineer Andrť
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
What he came up was a high pressure gas and oil system (or
‘Olťopneumatique’), 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 Minťral (LHM) fluid -
the lifeblood of any hydro-CitroŽn. 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 CitroŽn 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, CitroŽn got there decades before the
rest with technology now in widespread use across the industry. The
impossible became possible.
With no natural roll stiffness in the system, Paul MagŤs 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 CitroŽn 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
|© 2017 CitroŽnŽt/James Walshe/Practical Classics