CITROEN DS series Technical Manual page 25

U
The Citroën Guide
SOFT MODE
suspension pressure
system feed pressure
strut
return
rear
5
security
valve
While the computer keeps the suspension in soft mode,
the electro-valve 5 is energized by the ECU, opening the
feed pressure onto the isolation valve piston 2 and by mov-
ing it, connecting the center sphere 1 to the rest of the sus-
pension. The fluid in the suspension has to pass through
two damping elements 4 (one for each strut connection).
When both struts move in unison, the center sphere be-
haves as a standard sphere with a damper hole twice as
large as a single damper element, but when the car starts to
roll, the fluid has to move from one strut to the other, pass-
ing through both damper elements consecutively. In addi-
tion to this double damping, the sphere 1 itself acts as a
damping string, absorbing quick changes in pressure be-
tween the two dampers. This dampens the body roll to
some extent even in soft mode.
HARD MODE
suspension pressure
system feed pressure
residual pressure
strut
return
rear
5
security
valve
Whenever the computer feels it necessary to switch to
hard mode, it closes the electro-valve 5, not allowing the
main feed pressure to move the isolation piston 2. The pres-
sure inside the center sphere 1, always higher than that of
the return path under normal operating conditions, will
move the control piston into a position which closes off the
center sphere completely. The remaining pressure in this
sphere remains unknown but as the main circuit pressure
might change while the suspension is in hard mode (due to
either the dynamics of the suspension—acceleration, brak-
Suspension: Hydractive I
1
2
4 4
3
control from
computer
height
corrector
1
2
4 4
3
control from
computer
height
corrector
ing, movement due to uneven surface—or the vehicle
height altered by the driver), the computer equalizes the
pressure periodically by enabling the control block to as-
sume the soft position for a short period of time.
Hard mode serves three reasons. First, it provides higher
resistance to body roll. The cross-flow of LHM from one
strut to the other has to pass through both damper blocks
as in soft mode, but it is additionally limited using the pis-
ton and ball valve 3, now switched into the hydraulic circuit
strut
between the damper elements instead of the center sphere.
The ball is positioned in the fluid so that any cross-flow
moves the ball and thus limits the flow, dampening the
body roll as well.
Second, it limits dive and squat by helping out the height
correctors. A stiffer suspension damps the vertical motion
and therefore reduces the amount of correction required.
Third, hard mode not only limits the suspension travel be-
tween the body to the road but between the suspension ele-
ments and the body. Its aim is to reduce suspension move-
ment at the cost of comfort but to gain safety, limiting the
influence of the body movement to
steering, very important in extreme sit-
uations like a flat tire.
When the vehicle is making a sharp
left turn, tending to roll to the right,
the right strut will be compressed and
the left one expanded. The fluid is
then forced from the compressed strut to the expanded
one, moving the ball in the valve towards the outlet of the
left strut; as soon as it reaches and covers the outlet orifice,
it closes off any further cross-flow. The corner spheres are
now isolated and has to provide all the damping them-
selves.
At the same time when the body roll is present, the car
might need to change the ground clearance as well: for in-
strut
height
corrector
strut
pushes the piston, which in turn dislodges the ball and the
pressure will raise equally in both struts (without dislodging
the ball, only one of the struts would receive the fluid, result-
ing in incorrect operation).
If the body has to be lowered, the
higher pressure in the struts will dis-
lodge the ball again, opening the pis-
ton towards the return line ad the
fluid will escape from both struts, low-
ering the vehicle.
Sensory perceptions
The computer of the suspension system takes its input sig-
nals from the various sensors and based on a set of rules, dy-
namically activates the electric valve.
There are eleven inputs to the ECU. First, the Comfort/
Sport switch on the dashboard, enabling the driver to
choose between the two settings. The status light on the in-
stance, when braking in a curve. The
valve 3 therefore has an additional pis-
strut
ton which lets the LHM flow between
the circuits of the struts and of the
height corrector. If the body has to be
raised, the pressure in the height
correctors will be higher than that in
the suspension. This higher pressure
25
strut strut
height
corrector
strut strut
height
corrector