Hydropneumatic Suspension - CITROEN DS series Technical Manual

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The Citroën Guide

Hydropneumatic Suspension

As we saw, the ideal suspension would require
elasticity decreasing with the load, constant
ground clearance, shock absorbers integrated
into the suspension—all these beyond the obvi-
ous independent suspension for all wheels. And
this is exactly what Citroën's unique hydropneu-
matic suspension offers.
According to the Boyle–Mariotte formula defined in the
17th century, the pressure and the volume of a mass of gas
are inversely proportional at a constant temperature. There-
fore, by keeping the mass of the gas constant and changing
the volume of its container, its pressure can be controlled
(the usual pneumatic suspensions operate on the opposite
principle: air is admitted or withdrawn from the system by
compressors and exhaust valves, modifying its mass while
keeping the volume constant).
The First Embodiment
The Citroën DS, introduced at the 1955 Paris Motor Show,
was radically different from any of its competitors on the
market at that time: suspension, running gear, steering,
brakes, clutch, body, aerodynamics were all unique, not
only in details but in the main operating principles as well.
The hydropneumatic spring-absorber unit uses an inert
gas, nitrogen (colored blue on the illustrations) as its spring
medium, resulting in very soft springing. The flexibility of
the gas decreases as the increasing load compresses the sus-
pension pistons, reducing the vol-
ume of the gas and adding to its
pressure. The damping effect is ob-
tained by forcing the fluid (colored
in green) pass through a two-way
restrictor unit between the cylinder
and the sphere. This effect provides
a very sensitive, fast and progres-
sive damping to reduce any un-
wanted oscillations.
There are many great advantages to this hydropneu-
matic suspension. First, by adding or removing fluid from
the suspension units (practically, by adjusting the length of
the hydraulic strut), ground clearance can be kept con-
stant under any load variations. Although this might
not seem very important at first sight, it means that the sus-
pension geometry is also constant—in other words, the
handling of the car does not depend on the load.
The compressed gas has a variable spring effect, becom-
ing harder as the load increases. This compensation for the
increasing load keeps the resonance frequency of the sus-
pension nearly constant. As a consequence, the same excita-
tion in the suspension moves the same amount of fluid
through the dampers regardless of load (which is not the
case with conventional springs). The working range of the
Suspension: Hydropneumatic Suspension
nology in widespread use in every branch of the industry. As
liquids are non-compressible, any amount of liquid intro-
duced at one end of a hydraulic line will appear immedi-
ately at the other end (this phenomenon was first formu-
lated by Blaise Pascal). Using this principle, motion can be
transmitted, multiplied or divided (according to the relative
sizes of the operation cylinders), with velocity increased or
decreased (using varying cross sections in the tubing), to
any distance desired, over lines routed freely.
simple to use, and—due to their widespread deployment—
relatively cheap. It is no wonder that it is used for many pur-
poses even in the most conventional vehicles: shock absorb-
ers, brake circuit and power assisted steering being the
most trivial examples; however, Citroën is the only one to
use it for the suspension.
dampers becomes much smaller and this fact makes the
use of a simple damper element very effective.
also contributes to the consistent behaviour independent
of the load. In essence, it ensures that both the road con-
tact and the feeling transmitted to the driver remains al-
ways the same. This is something absolutely unique: all con-
ventional suspensions have an optimum point around aver-
age load; when carrying more or fewer passengers or load
usual, hence the braking force is distributed more evenly.
Manufacturers of cars with conventional suspension and
braking only start to add brake force distributors to their ve-
hicles these days. The first DS did have a force distributor
but Citroën later realized that the suspension, with the addi-
tion of a single pipe, can fulfill its role entirely.
tween the left and right side of the suspension has another
important implication: lower difference in forces on the
wheels. Coupled with variable damping this keeps the
wheels in contact with the road at all times, which in
turn maximizes the tractive forces on the tires—braking
while turning still leaves the vehicle with the grip of all four
wheels: this is essential for security in low adherence condi-
tions, such as ice, snow, rain, mud.
The volume changes are controlled by hydraulics, a tech-
Hydraulics are immensely useful, very efficient, reliable,
This basically constant suspension resonance frequency
than this average value, the han-
dling characteristics change, not sel-
dom so radically that the car be-
comes utterly dangerous to drive.
Another advantage is the limited
but very useful anti-dive behav-
ior: this is essential for efficient
braking with a basically very soft
suspension. The center of mass of
the car moves much less than
The height correction and the constant connection be-
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