Activa Suspension - CITROEN DS series Technical Manual

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The Citroën Guide

Activa Suspension

The Activa suspension—used only on some Xantia
models—creates mixed feelings. Drivers requiring
sporty handling and roadholding praise it be-
cause this car turns into curves without turning
a hair: it stays completely horizontal and neutral.
However, this comes at the expense of ride
comfort.
The Activa system operates in two distinct steps. The first
one is controlled mechanically by a roll corrector (the com-
ponent is identical to the height correctors used in the sus-
pension, see the details on page 23).
The corrector is connected by an L-shaped spring to the
bottom wishbone. When the car takes a sharp left turn, its
front left wheel will be forced down by the body roll caused
by centrifugal force. As the wheel moves down, so does the
end of its wishbone, pulling the linkage to the corrector.
The piston inside the roll corrector moves upwards, open-
ing the pressure feed into the stabilizing cylinders. These
two cylinders are attached to the wheel suspension differ-
ently: in the front, the piston pushes the left wheel upwards
while in the rear, the right wheel will be forced downwards.
This diagonal correction counteracts the roll of the body.
Turning to the other side result in an inverse operation:
the roll corrector opens the connection from the stabilizing
cylinders back to the reservoir. The front left wheel moves
downwards, the rear right one upwards, once again coun-
tering the effect of body roll.
STRAIGHT-AHEAD
RIGHT TURN
SHARP RIGHT TURN
Suspension: Activa Suspension
Activa sphere
with control block
1
2
reservoir
rear
stabilizing cylinder
An additional Activa sphere in the front acts as an extra
accumulator but the rear sphere can be connected or de-
coupled electrically. Depending on the position of the pis-
ton inside the electro-valve, the high pressure feed is ei-
ther allowed to reach the piston 2 inside the control block,
pushing it up and connecting the sphere 1 to the rest of
the circuit (dashed line on the illustration), or the residual
pressure in the sphere moves the piston 2 down, isolating
the sphere 1.
When the Activa sphere is open to the rest of the system,
roll correction is applied through a spring element formed
by the accumulator and the Activa sphere. The supply side
of the stabilizing cylinder pistons have half the area of
the other side, connected to the Activa sphere 1 with the
valve 2 open. Changes in the length of the linkage is there-
fore not transmitted directly to the roll bar. Upon the influ-
ence of external forces like body roll, the movement of the
piston compresses the gas content in one sphere and at the
same time, expands it in the other.
The stabilizing cylinder works as a spring with asymmetri-
cal characteristics: its effective hardness is smaller around
the corrected position, but it hardens progressively as the
piston is forced out of that position.
The Activa system has two operating modes, depending
on the position of the electro-valve 2. In the first mode roll
correction is always active because the roll corrector is up-
set. The resulting flow of fluid will tend to move the active
linkage upsetting the balance of presssure in the two extra
ECU
vehicle speed
steering wheel angle
steering wheel speed
electro-
valve
pressure feed
feed pressure
work pressure
leakage return
front
stabilizing cylinder
anti-roll
bar
Activa
sphere
roll
corrector
33