This little wonder was invented by CitroŽn in the late 50's and has been fitted
to many of their cars since. I've had to separate it into it's
own category because it is quite different from any other type of suspension
Since the early fifties CitroŽn have been running a fundamentally different
system to the rest of the auto industry. They call it hydropneumatic and it
encompasses features as diverse as brakes, suspension & steering. As its
name may suggest, its core technology and mainstay of its functionality is
hydraulics. Superbly smooth suspension is provided by the fluid's
interaction with a presurised gas.
The system is powered by a large hydraulic pump operated directly by the engine
in much the same way as an alternator or an air conditioner is, and provides
fluid to an 'accumulator' at presure, where it is stored ready to be
delivered to servo a system.
Because this page is all about suspension, for clarity we'll look at the
simplified version of this as installed in the 'BX' model. The
CitroŽn BX was a major turning point in the company's history as it was the
first car to be produced under the company's new Peugeot management, following
the 1970s take-over of CitroŽn by Peugeot. As a direct consequence of the
Peugeot influence, the car is somewhat more conventional than its larger
sibling designed earlier - the CX. This Peugeot-enforced 'normalisaiton'
of the design makes it easy ennough to examine as an illustration of how
hydropneumatic suspension works.
There are two main components you need to familairise yourself with and to
understand. The spheres are like the springs on the car, and the struts are the
hydraulic components that make the fluid act like a spring.
Lets start with the sphere. The spring in this
suspension system is provided by a hydraulic component called an accumulator,
which is gas under pressure in a bottle contained within a diaphragm,
effectively a balloon which allows pressurised fluid to compress the gas, and
then as pressure drops the gas pushes the fluid back to keep the system's
pressure up. As you can see in the drawing above the pink gas (nitrogen) is
compressed when the pressure in the green fluid (LHM) overcomes the gas
pressure, and pushes back the diaphragm which compresses the gas. Then as the
pressure in the fluid reduces, the gas pushes back the diaphragm and as the gas
overcomes the fluid, it expells the fluid from the sphere, returning gas and
fluid to equlibrium. This is the hydropneumatic equivalent to the spring
getting compressed (bound) and getting depressed, ie springing back (rebound).
Still with me? We can keep going
How can a gas, a diaphragm and a hydraulic fluid compressing, form a spring?
Simple(ish): The pressure of the gas is the equivalent to the spring weight.
The inlet hole at the bottom of the sphere restricts the flow of the fluid and
provides an element of damping. By replacing the sphere for ones of different
specs, it is possbile to adjust the ride characteristics with these cars.
Rumour even has it that a racing team in Anglesey
is customising their car by pressurising their own spheres to custom pressures
to make an exact match for the circuit the are on.
Before we go any further it is pretty important that you understand where the
fluid acting on the diaphragm in the sphere gets its force from, and to do that
we are going to have to look at the operation of the other key component in the
CitroŽn system - the strut.
As you can see in this diagram, the strut has a sphere on top of it and the
strut in itself acts like a syringe to inject fluid into the sphere. When the
wheel hits a bump it rises, pushes the piston of the
strut back and this sqeezes fluid through the tiny hole in the sphere to let
the gas spring absorb the energy of the bump. Then when the car is over the
bump and its time to let the wheel back down, the gas pushes the diaphragm back
out, pushing the fluid down to the strut, pushing the wheel down to the ground.
Some interesting possibilities were opened up by the company deciding to use
this system to spring their cars. One or two of the more obvious ones are that
since the system is hydraulic, the ride height can easily be altered, a trend
low riders are now following on with in California, nearly fifty years later.
Also, they could link the four corners together to make a system that prepared
the car for the bump to keep it even and offer the passengers a smoother ride.
Basically they put fancy valves called height correctors on the anti-roll bar.
These were mounted in such a way that as the suspension twisted, this operated
the valves that controlled the transfer of fluid to the struts. It was possible
to isolate the front and rear systems and have the front suspension set at a
height which required 'x' litres. So when the front nearside wheel takes a
knock compressing its sphere, x/2 L is lost in the sphere, then the height
correctors allow another x/2 L in, to inflate the offside strut by that much.
This keeps the front of the car level in a horizontal plane.
As the car clears the bump, the reverse happens; the sphere displaces that
fluid, the strut returns to its own height pulling the anti roll bar back true
with it which in turn tells the height corrector to lose that extra x/2 litres
of fluid from the other side. As one side extends its strut in reaction to
clearing the bump, the other is retracting by the same amount to return the car
to its set height above the road. Neat huh?
A further mechanical advantage of hydraulic suspension is that the car is able
to link its braking effort to the weight on the wheels. In the CitroŽn BX, the
rear braking effort comes from the pressure exerted on the LHM fluid by the
weight on those struts. This means that as the weight travels forward under
braking, there is less pressure on the back suspension. The suspension is the
able to exert less pressure on its fluid, and as weight and grip diminish on
the wheels, so does the braking effort, thus the hydropneumatic system prevents
rear wheel lock ups.
In addition to these benefits, CitroŽn pioneered computer controlled suspension
in the early nineties by inserting a computer to take readings from the cars'
chassis and control systems and let the computer make informed decisions about
how to handle the cars suspension. The computer could then effect these
decisions by things like servo valves, and offered benefits like soft
suspension for cruising, but stiffer, sportier suspension for faster harder driving,
allowing the driver to cruise in comfort and still enjoy a responsive car. It
also moves substantially towards eliminating body roll and if used for a
sportier driver will save tyre wear as well (they claim).