CITROEN DS series Technical Manual page 41

U
The Citroën Guide
Thus, the lower mechanical link, the feedback from the
roadwheels does not extend beyond the hydraulic unit. Ev-
erything the driver feels is generated artificially. One draw-
back for uninitiated drivers is the lack of noticeable feed-
back indicating that the wheels are skidding or driving in a
ditch. The driver has to learn to feel the behavior of the car
via other sensory means and this is probably the main rea-
son why anyone not prepared for a period of learning will
immediately dislike DIRAVI. But once accustomed to the sys-
tem, it is more ergonomic and stress-free than any other
steering system.
The DIRAVI system uses four main components:
The steering rack and hydraulic ram cylinder with a
piston inside. The areas on which the pressure acts on the
left and right sides of the piston are different—the left one
is twice as large as the right one—, thus to keep the piston
in neutral position, the right hand side must have twice as
much hydraulic pressure than the left hand side. As this side
is fed from the high pressure of the hydraulic system, a con-
trol unit manipulates the pressure on the other side.
This steering control unit is connected to the steering
column. It has a coupling 4 inside which is very loosely con-
nected, with a significant amount of free play (nearly 30 de-
grees). Under normal circumstances this coupling stays in
the middle, so the free play is irrelevant but it serves as a me-
chanical backup for safety if there would be any failure in
the hydraulic system. In this case, the car can be steered me-
chanically, although much heavier and with a large free
play on the steering wheel.
TO LEFT
L
HP
rack
reservoir
The main illustration shows the steering system with the
steering wheel in the straight-ahead position. When the
driver rotates the steering wheel, the steering column turns
the gear 1 inside the control unit. The set of levers 9 at-
tached to this wheel transform the relative rotation (relative
to the previous hydraulically stabilized steering wheel posi-
tion) of the steering wheel into a horizontal motion: turn-
ing the steering wheel to the left pulls the slide valve 3, let-
ting the high pressure fluid enter the left chamber of the cyl-
inder. The right chamber is constantly at this same pressure,
however, the area on the left side of the piston is twice as
large as on the other side, thus the resulting higher force
will move the steering rack to the right, turning the road-
wheels to the left.
If the driver rotates the steering wheel to the right, the le-
vers 9 push the slide valve 3, draining the LHM from the
left chamber of the cylinder back to the reservoir. As the
right chamber is still under the constant pressure, the result-
ing force moves the rack to the left, thus the car starts to
turn to the right.
As we have already mentioned, the moving steering rack
rotates the pinion and—through the steering feedback—
the cogwheel 2. The levers linking this gear to the valve 3
now work in the opposite direction, returning the valve to
its neutral position, cutting off the LHM supply to the steer-
ing rack. The roadwheels stay in the angled position corre-
Steering: DIRAVI Steering
TO RIGHT
R
HP
rack
reservoir
sponding to the position of the steering wheel; due to the
closed valve 3, the steering gear and the roadwheels are hy-
draulically locked, resulting in high turning stability.
To make the steering progressively heavier as the speed
of the vehicle increases, the steering centering pressure
regulator—a centrifugal device—is driven by a cable from
the gearbox. Its spinning weights open up a slide valve 8 ad-
mitting some fluid from the high pressure circuit into the
centering device, or closes it to drain the extra fluid back to
the reservoir.
The faster the car runs, the bigger is this hydraulic pres-
sure sent to the steering wheel centering device. This
consists of an eccentric cam 5 geared to the steering wheel
side of the unit, with a ratio making it turn less than a full
turn while the steering wheel is rotated from lock to lock. A
piston 6 forced down by the mentioned hydraulic pressure
pushes a roller 7 against this cam. Being eccentric, the only
stable position is when the cam is centered. The centering
force can be regulated by changing the hydraulic pressure
behind the piston.
The hydraulic pressure behind the piston 6—being de-
pendent on the vehicle speed—represents the progressive
counter-force needed to make the steering gradually
heavier at highway speeds. In addition, it returns the steer-
ing gear to the neutral, straight-ahead position when the
driver releases the steering wheel. While the wheels of a
DIRASS car return to the center themselves, forcing the rack
and steering wheel as well, on DIRAVI the opposite is true:
the force on the angled wheels is attenuated infinitely, hav-
ing no influence whatsoever on the steering wheel. This ad-
ditional device returns the steering wheel to the center in-
stead, just as if you have turned it back yourself.
During the rotation of the steering wheel, the lower pis-
ton was pushed up by the roller 7
and the eccentric cam 5. The fluid
leaves the chamber through the ball
valve now opened. While this piston
moves upwards, it compresses the
spring, which in turn pushes the up-
per piston slightly up, freeing the cali-
brated bore Û.
As soon as the driver releases the
steering wheel, the opposite of the
previous operation takes place. The
ball valve will be closed by the enter-
ing fluid, thus the LHM has to go
through the center bore of the upper
piston, leaving via the calibrated
bore Û. Due to this resistance, it car-
ries the upper piston down slightly,
compressing the spring. This downward force pushes the
lower piston together with the roller 7 down, and the
torque exerted on the eccentric cam 5 forces that to rotate
back into its neutral position, returning the complete steer-
ing gear to the straight-ahead position. At the end, the
spring will return the upper piston to its original position in-
side the centering device. The restriction of the bore Û
keeps the steering wheel from returning to the center posi-
tion too fast.
The last component is an adjustment cam allowing the
adjustment of the pinion relative to the disk on the pinion
end of the steering column.
41
STEERING
RETURNING