CITROEN DS series Technical Manual page 22

U
The Citroën Guide
A typical example: the BX
rear strut & sphere
height
corrector
rear strut & sphere
Finally, there are no forces in the suspension when the cir-
cuit is depressurised, allowing very easy and safe servic-
ing of the relevant suspension and transmission parts.
Modern spring suspension systems are in fact capable of
achieving some of these results. For instance, variable diam-
eter or pitch springs coupled with hydraulic shock absorb-
ers (incidentally, with a similar internal geometry as the
damper elements used in Citroën spheres) behave similarly
to these hydropneumatic units. The main difference is that
even if these elements would be practically identical, all
other functionality that comes either for free or at a small
additional cost in Citroën systems—constant height, anti-
dive, brake force regulation and so on—, require complex
and expensive additional systems.
The illustration shows the basic layout of the suspension
(differences on models fitted with power steering or ABS
will be described in the corresponding chapters). Most com-
ponents have an output line to collect leakage (which is in-
tentional to keep the elements lubricated) and return it to
the reservoir—although the outputs are indicated, the lines
themselves are omitted for the sake of clarity. In reality, they
are grouped together and go back to the reservoir.
The high pressure supply subsystem consists of a five-pis-
ton volumetric high pressure pump drawing the mineral
suspension liquid called LHM from the reservoir. The fluid
under pressure is stored in the main accumulator. It is the
ON
5
1 2
pump
6
flow
distr*
3 4
Suspension: Hydropneumatic Suspension
rear brake
LHM feed
front suspension
rear suspension
front brakes
rear brakes
operational return
leakage return
rear brake
TRANSITION
5
1
2
feed
rtrn
3 4
brake valve
security valve
task of a pressure regulator—built into the same unit
with the accumulator—to admit fluid into the accumulator
as soon as the pressure drops below the minimum value of
145 bar; as soon as the pressure reaches 170 bar, the regula-
tor closes and the fluid continues its idle circulation from
the pump, immediately back to the reservoir.
OFF
5
1
pump
flow
distr*
3
The spring below the piston 1 is calibrated so that it will
collapse only when pushed down with a pressure exceed-
ing the cut-in threshold (145 bar). While the pressure in the
main accumulator remains inferior, the piston stays in the
upper position, allowing the pump to deliver fluid into the
accumulator through the ball valve 5: the unit is switched
on. The piston 2 also remains in the upper position (its
spring is calibrated to the cut-out pressure, 170 bar), letting
the entering fluid fill up the chamber 3 as well. This, in
turn, ensures that the piston 1 stays in the upper position:
the fluid pressure in this chamber plus the force of the
spring counters the downward pressing force even if the
pressure in the accumulator rises well above 145 bar.
The fluid supplied by the pump raises the pressure in the
accumulator; as soon as it reaches 170 bar, its pressing
force will exceed the retaining force of the spring under the
6
piston 2, forcing it to the lower position. In this moment,
the high pressure line coming from the another piston will
be cut off and the fluid from the chamber 3 can escape
back to the reservoir (yellow in the illustration).
front brake front strut & sphere
main accumulator &
pressure regulator
HP pump
height
corrector
reservoir
front brake front strut & sphere
On simpler models the out-
put marked with an asterisk
is omitted and it goes to the
return ouput inside the regu-
lator unit instead, as shown
by the dashed line. On mod-
2
feed els fitted with power assisted
6
steering (DIRASS) this inter-
connecting line is missing
rtrn
and both outputs are used in-
4
dependently.
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