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At Citroen, to improve the maneuverability of the vehicle is not the
objective. The French manufacturer confers on the aft wheels direct only one
virtue: to ensure a better stability of the car in turn. When it falls under
a turn, any car undergoes a lateral thrust. Then, the wheels must resist the
force of support. With the back, they do not resist so that that, because
they direct (very, very slightly, of course) in the direction opposed to that
of the nose gear wheels: the axle takes full whip the lateral thrust. Blow,
the car has a tendency to skid of the back. This phenomenon, baptized
"survirage", is especially corrected by the suspensions: today, the
oversteers do not exist practically any more. However, to direct the aft
wheels in the direction of the nose gear wheels mitigates the effect of
rotation which the car records, returning it more balanced still a little in
curve.
In fact, the mission of the axle postpones autodirectionnel is to be used
auxiliary for the nose gear (whose castor angle was reduced), in order to
more frankly register the car in the turn. Autodirectionnel wants to thus say
that it is guided all alone... A self-evident truth, certainly, but which
hiding place a technological innovation that Citroen takes care well not to
reveal. Because, contrary to the Japanese systems, nothing does not connect
the rear axle to the direction of the vehicle. At each end and on both sides
of this axle, two very small members support each one an elastic rubber hold.
These are the four holds which will make swivel the framework of axle
horizontally, according to the pressure which they will undergo when the car
is in support in turn.
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That appears very stupid. Citroen however jealously maintains towards oneself a small secrecy: on is each side of the axle (in front of and behind), which the most flexible hold or on the contrary firmest? The question has its importance because it is necessary well to determine which of both will engage the effect of swivelling of the axle. One can suppose that it is most flexible, firmest correcting the push, and that it is located in front. Another mystery that no Sherlock Holmes of the car is ready to solve: the difference in angle between the radius turning of the nose gear wheels and that of the aft wheels. Because it is this difference which determines all the system effectiveness: too much angle, the car leaves out of crab; not enough, the device is not used for nothing. The answer is surely in a numbered trunk of a Swiss bank...
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Damage, moreover, that Citroen
does not make benefit his/her small comrades from its clever lucky finds.
Because, on the road, the system reveals astonishing faculties. What we
checked by carrying out some turns on a sinuous circuit at the wheel of a ZX
1.8. The extreme promptness of this car is always astonishing, sometimes
diverting. The ZX engages frankly in the turns, holds well the cord without
"attacking in the tires" and in fate while going back perfectly on
line in the axis chosen by the driver. The direction seems very light and,
especially, the wheel hardly should be turned. A concept with keeping well at
the head, bus if one turns too extremely, station... Some blows of wheel in
full turn also made it possible to note that one can correct the trajectory
without risking the head with tail. Other note: even at important speeds, one
can keep his line perfectly.
The effectiveness of the axle postpones autodirectionnel is undeniable and
fact of the control of the ZX, which in addition is not free from defects, a
true treat. Let us wish only one thing: that it spreads. With, however, a
small flat. Like any supposed system to improve safety, that it is passive or
active, one should not "draw on the cord". The rear wheel-axle unit
estampillé Citroen will not prevent the exit of road if one returns too
quickly in a turn. No technical artifice will replace the conscience of the
driver.