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The Citroën Guide Suspension: Hydropneumatic Suspension 23
With the back pressure now vanished from behind the
piston 1, the pressing force of the accumulator fluid drives
it down at once: the regulator is switched off now. The fluid
supplied by the pump returns back immediately: on PAS-
equipped cars, to the flow distributor, on other vehicles,
straight back to the LHM reservoir through the internal con
-
nection (dashed line).
Shortly, as the suspension and braking circuits start to
use up the pressure in the main accumulator, the piston 2
will return to its original position. Once there, the regulator
is ready to start a new cycle.
The characteristic ticking which can be heard in Citroëns
is the sound of the regulator pistons quickly moving one af
-
ter the other, in quick succession: 2 down, 1 down, 2 up.
The opposite tick—1 up, when the regulator is switched on
to replenish the accumulator—is much softer.
The interconnection 6 is normally closed. Opening it lets
all the fluid stored under pressure return back to the LHM
reservoir—this is the way the system is depressurized when
any of the suspension elements need servicing.
The liquid—supplied to the rest of
the system from the main accumula
-
tor—passes through a security valve
whose task is to ensure safety by feed
-
ing the brake circuits first. The front
brake circuit is always open but the
other two outputs are blocked by a pis
-
ton. If the pressure in the main circuit exceeds 100 bar, the
fluid pushes the piston back against the force of the spring,
opening up the suspension outputs as well. The electrical
switch for the low hydraulic pressure warning lamp on the
dashboard is built into this valve as well. This way, a sudden
failure of the pump or the belt driving it will not leave the
car without sufficient braking power.
The second circuit fed from the security valve is the front sus
-
pension. The fluid goes to the front height corrector.
When the vehicle height is stabilized, the piston inside the
corrector blocks the inlet of fluid, isolating the struts from
the rest of the suspension. Body roll is limited by the damp
-
ing effect of the restrictors built into the sphere supports
and by forcing the fluid to run from the left to the right
strut through a connection line. If the movement of the
front anti-roll bar dictates that the front of the vehicle
should be raised, the connecting linkage moves the piston
upward, opening the inlet and letting additional fluid enter
the front struts. When an opposite movement is required,
the piston moves downward, letting the fluid at residual
pressure flow back from the struts to the LHM reservoir.
Both directions of flow are stopped and blocked when the
height corrector piston resumes its middle position.
The mechanical connection between the anti-roll bar
and the height corrector is not a rigid linkage but has some
free play. Just before the height corrector, the connecting
rod coming from the anti-roll bar hooks into a small win
-
dow on the corrector side. Small movements of the control
rod do not change the position of the height corrector, only
those are large enough to exceed this free play. In addition,
the corrector has its internal (albeit low) resistance, besides,
all rods are somewhat elastic, so in the end, all these factors
make the height correction system filter out the higher fre
-
quency components of the suspension movement.
Observing an initial threshold which has to be crossed be
-
fore any correction occurs not only reduces the strain and
wear on the correctors but also prevents the system from
developing self-oscillation. A powered system provides am
-
plification and any feedback mechanism with a delay—
such as the height correction—could potentially result in os
-
cillations. The initial threshold ensures that there is no feed
-
back, and consequently, no oscillation when the required
correction is too small.
The next circuit is the rear suspension. Its layout and op
-
eration is identical to the front one, having its own height
corrector.
The first circuit, as already mentioned, feeds the front
brakes. The liquid under pressure flows into the brake
compensator valve, operated by the brake pedal. In its
neutral position, the brake circuits are connected to the re
-
turn lines to ensure that the brakes are not under pressure.
When the driver pushes on the pedal, this moves the first
piston, closing the return output and opening up the outlet
going to the front brake cylinders.
This piston and a spring behind it pushes the second pis
-
ton which works similarly for the rear brakes, although
those are not fed directly from the security valve but receive
their supply from the rear suspension (later brake valves
have three pistons but their method of operation is practi-
cally the same). In consequence, the braking force at the
rear depends on the load: the more the back of the car is
loaded, the stronger the rear brakes work. Actually, on a
Citroën mostly used to carry only its driver, without much
load in the trunk, the rear brake pads and disks wear much
slower than those in the front.
The damping elements in the
sphere supports consist of a central
hole which is always open and addi
-
tional small holes closed and opened
by a spring as the flow of the hydraulic
liquid dictates. Slower suspension
movements like body roll, squat or
dive result in a slower flow of the liquid and the smaller dy
-
namic pressure differences are not sufficient to bend the
spring cover open over the additional holes. The damping
effect is therefore only determined by the diameter of the
center hole.
The abrupt jolts caused by road irregularities, in contrast,
cause faster flow. With the increasing pressure difference
the fluid will open the spring cover and use the additional
holes as well. This increased cross section results in a lower
damping effect.
The additional holes are located in a circle around the
center hole. There are two spring covers, one on each side,
but they do not cover all the holes equally. Half of the holes
(actually, every second one) are slightly enlarged on one
side, the remaining half on the other side. By carefully ad
-
justing the size of the holes, the designers could fine tune
the damping factors independently for both directions of
strut travel.
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