CITROEN DS series Technical Manual page 21

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The Citroën Guide
The steady connection between the sides requires an ex-
ternal management of body roll. Ideally, for any vertical
movement of the car body, the two sides of the suspension
should be connected, while for any movement that results
in different displacements of each wheel, they should ide-
ally be separate. This second movement can be viewed as a
rotation around the longitudinal or transversal axis.
For instance, if the front wheels run into a pothole and
the rear wheels go over a bump, the car will rotate around
its transversal axis. The angle of rotation remains relatively
small as the length of the car is its largest dimension; the
higher weights like the engine bay are far from the centre of
mass, resulting in a large inertial torque to counter outside
forces. If all suspension elements of the wheels were con-
nected hydraulically, the vehicle would absorb the bumps
very efficiently (the rear struts compressed by the bump
would deliver fluid into the front struts, resulting in immedi-
ate compensation: the rear would sink, the front would
rise, restoring the horizontal position of the car). Unfortu-
nately, this would also lead to slow transversal (dive and
squat) oscillations, made even worse by acceleration, decel-
eration and varying distribution of weight inside the cabin.
As the inertia of the car body around its transversal axis is
basically sufficient to counter the effect of longitudinal
bumps, the front and rear suspension circuits are sepa-
rated. The active height correction of the system acts as a
further a non-linear stabilizer both countering dive and
squat, and solving weight distribution problems.
On the other hand, if the bumps are transversal—for in-
stance, a pothole under the right wheel and a bump under
the left one—, the car will rotate around its longitudinal
axis. Being much less wide than long, the angle of rotation
will be higher and the inertial torque is considerably lower
to counter this kind of rotation. Completely independent
sides would result in very little damping of roll movements:
the low inertia provided by the body would find the reac-
tion of the suspension too stiff. Hence, the two sides in the
hydropneumatic suspension are interconnected, providing
a push-pull operation of the two sides. The interconnection
has special damping elements which react differently to dif-
ferent fluid movements between the sides: to quick suspen-
sion movements caused by potholes and bumps, or to
slower changes occuring when driving in a curve.
To counter body roll resulting from the second, an addi-
tional element, an anti-roll bar is also needed. The effects
of roll could be eliminated if the center of the roll could be
identical to the center of the mass. As this is not possible,
the opposite approach of moving the center of roll away
from the center of mass could also help overcome body roll
by increasing the opposing torque. This is the role of the
anti-roll bar: similarly to a bike leaning into a curve, it lifts
the inner side of the wheel, using the force on the outer
edge, and this moves the center of roll outwards. In other
words, the wheels and suspension elements do have roll,
the role of the anti-roll bar is to isolate this roll from the
body which should remain, ideally, horizontal. To accom-
plish this, the bar cannot be completely rigid (it has to ab-
sorb the road undulations without transfering them to the
body), a torsion spring is the usual solution.
Such anti-roll bars are used on conventional spring sus-
pension systems as well, however, there are substantial dif-
ferences in the way the bar interacts with the rest of the sus-
Suspension: Hydropneumatic Suspension
pension on Citroëns. In a spring system, there is a consider-
able amount of interaction, a significant flow of energy in
both directions between the suspension and the bar. The
shock absorbers have to provide the damping for the anti-
roll bar, introducing yet another interaction (in the hydrau-
lic setup this is catered for by the damping inside the con-
nection line between the sides).
much less interdependence and compromise between
damping, countering roll, squat and dive. In addition, it can
provide solutions which are simply unfeasible mechanically
in a conventional suspension. Cars with steel springs always
have roll, including diagonal one, induced by undulations
of the road—their anti-roll bar represent a constant me-
chanical connection between the sides, unable to differenti-
ate between bumps and curves. Citroëns, on the other
hand, have a varying interconnection depending on fluid
movement—this is very easy to accomplish with hydraulics
but extremely complicated with springs.
from the source of the disturbance, and due to the good
conductivity of sound via the hydraulic lines, this results in
slightly more noise. The same effect makes the hydropneu-
matic suspension somewhat noisier than a conventional
one. However, good sound insulation inside the cabin can
help overcome this small annoyance.
inflated or blown tires and cross-wind. Even with largely un-
even braking forces on the two sides the car will not pull to
either side.
integrated unit from a technical point of view, hydraulics
make it possible to place some hydraulic parts (for instance,
the center spheres on Hydractive systems) in different loca-
tions, reducing the amount of sprung mass. Conven-
tional springs have a considerable mass of their own while
the mass of the nitrogen in the spheres is practically negligi-
ble. Even adding the mass of the fluid moving around in the
system, the sum remains much below that of a steel spring.
Hydropneumatic struts can be kept relatively small by in-
creasing the operating pressure, which decreases the diam-
eter of the struts. The automatic height correction reduces
the mass further because the basic suspension mechanics
can be simpler, without requiring multilinks and similar
components.
This fluid boils at a very high temperature, therefore it pro-
vides great resistance to vapor lock. Due to the propor-
tional regulation a hydropneumatical Citroën can keep brak-
ing as long as there is anything left of the brake pad. Even if
the liquid starts to boil, there will be no vapor lock as the
pressure is automatically released and remains proportional
to the braking effort applied by the driver.
cated and prone to error, none of which accusations is true.
The suspension is actually quite simple when considering its
extra services in comparison to a conventional system and
experience shows that the whole system is very reliable. The
perfect functioning of the system relies mainly on the pre-
scribed cleaning of the system and the change of the hy-
draulic fluid—adhering to these simple prescriptions can
make the system very reliable.
Consequently, the hydropneumatic suspension has
The only disavantage is that damping occurs further
This suspension layout reduces the sensitivity to under-
Although the hydropneumatic spring-absorber unit is an
The brakes share the mineral fluid with the suspension.
This system is often criticized for being overly compli-
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