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The steering on the SM really broke new ground in 1970. Citroëns
hydraulic wizard, Paul Magés had the system ready for 10 years,
but it first appeared in production form in the SM. The DERAVI (direction
à rappel asservi) has a vary fast ratio requiring only two turns
from lock to lock. The variable power assistance gives maximum assistance
for parking and maneuvering and very little or none at all when the car
is driven at speed. Another new idea was the self centering action of
the steering. When there is no driver applied effort on the steering wheel
the wheels will come back to the straight ahead position. The variable
assistance is regulated by a regulator driven from the gearbox. A centrifugal
mechanism operates hydraulic valves to increase or reduce assistance in
accordance with vehicle speed.
Although this system requires some getting used to (typical first time drives will be oversteering the car swerving from side to side with sweaty hands) after about 10 minutes you start to appreciate the systems relaxed, precise and very safe qualities and after another 10 minutes you're wondering who not all cars are equipped this way. With the use of electronics in modern cars some manufacturers are now putting similar systems into production
Rack-and-pinion steering with servo-assistance varying according to vehicle speed and angle of deflection, with power-assisted return to center (automatic return to straight-line position. even at rest). A centrifugal regulator coupled with the gearbox acts on a hydraulic valve which determines the effort to be applied to the steering wheel. so that this effort varies with the vehicle's speed. Gearing ratio: 1: 9.4. Turning diameter between curbstones: 34's 1/2'