Another gem of engineering, the DIRAVI steering, made its debut on the SM, excelled in many CXs and the flagship, V6 XMs (left hand only, the small amount sold in the UK never justified the expenses of the conversion to RHD).
The DIRAVI (Direction Rappel Asservi, Steering with Limiting Counterforce) steering is as unique as the hydropneumatic suspension—it was never used by any other manufacturer, although its excellence over conventional power assisted systems speaks for itself.
As usual, it has some quirks confusing the average driver during their first meeting. First of all, it is geared very high: it only took two turns of the steering wheel from lock to lock (one turn for each side) to steer on the SM. Later models, the CX and the XM retained this feature although the number of turns was larger (2.5 and 3.3). The gear ratio could have been much higher, the engineers themselves insisted on a single turn lock to lock for the SM (which would, interestingly, void the need for a circular steering wheel completely). The final solution was a compromise to reduce the initial strangeness of the steering for the drivers already accustomed to traditional systems.
Certainly, making the gearing so high is not a complicated feat in itself but a conventional (even power assisted) system with such rapid a response would be unusable. As the car obviously has to have a similar turning circle as other cars, too responsive a steering would mean that even the slightest movement of the steering wheel would induce excessive deviation of the car from the straight line. To avoid this, it uses an opposing force, increasing with the vehicle speed. With this setup, in spite of the very high gearing, it is very easy to use it during parking, yet it offers exceptional stability at high speeds: it actually runs like a train on its rails, requiring a sensible amount of force on the steering wheel to deviate it from the straight line. And an additional feature: the steering wheel (and the roadwheels, naturally) center themselves even if the car is stationary.
Second, there is no feeling of feedback from the road through the steering wheel. Other steering systems have a constant mechanical connection between the steering wheel and the roadwheels, the DIRASS only adds some force to the one exerted by the driver. DIRAVI is different: simply put, the usual path between the steering wheel and the steering rack is divided into two halves, with a hydraulic unit in the middle. When the driver turns the steering wheel, this only operates the gears and valves in the hydraulic unit. The hydraulic pressure then moves the steering cylinder and the roadwheels. The lower half of the mechanics works in the opposite direction, as a negative feedback, returning the hydraulic system to the neutral position as soon as the wheels reach the required direction. The hydraulic cylinder and the wheels become locked, no bump or pothole can deviate them from their determined direction. Note that this neutral position is not always the straight-ahead direction, the hydraulics return to neutral whenever the steering wheel is held at a given angle for any longer period of time. Letting the steering wheel rotate back or turning it further in the previous direction will initiate a new mechanical-hydraulical cycle as described above.
Thus, the lower mechanical link, the feedback from the roadwheels does not extend beyond the hydraulic unit. Everything the driver feels is generated artificially. One drawback for uninitiated drivers is the lack of noticeable feedback 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 reason why anyone not prepared for a period of learning will immediately dislike DIRAVI. But once accustomed to the system, 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 control 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 connected, with a significant amount of free play (nearly 30 degrees). Under normal circumstances this coupling stays in the middle, so the free play is irrelevant but it serves as a mechanical backup for safety if there would be any failure in the hydraulic system. In this case, the car can be steered mechanically, although much heavier and with a large free play on the steering wheel.
The 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 attached to this wheel transform the relative rotation (relative to the previous hydraulically stabilized steering wheel position) of the steering wheel into a horizontal motion: turning the steering wheel to the left pulls the slide valve 3, letting the high pressure fluid enter the left chamber of the cylinder. 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 roadwheels to the left.
If the driver rotates the steering wheel to the right, the levers 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 resulting 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 steering rack. The roadwheels stay in the angled position corresponding to the position of the steering wheel; due to the closed valve 3, the steering gear and the roadwheels are hydraulically 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 admitting 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 pressure 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 dependent on the vehicle speed—represents the progressive counter-force needed to make the steering gradually heavier at highway speeds. In addition, it returns the steering 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, having no influence whatsoever on the steering wheel. This additional device returns the steering wheel to the center instead, just as if you have turned it back yourself.
During the rotation of the steering wheel, the lower piston 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 upper piston slightly up, freeing the calibrated bore 10.
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 entering fluid, thus the LHM has to go through the center bore of the upper piston, leaving via the calibrated bore 10. Due to this resistance, it carries 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 steering gear to the straight-ahead position. At the end, the spring will return the upper piston to its original position inside the centering device. The restriction of the bore 10 keeps the steering wheel from returning to the center position 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.