Brake & Tire

Power Assisted Steering

The PAS steering (DIRASS, Direction Assistée) used on Citroëns is not radically different from similar systems on other cars. Naturally, having a high pressure hydraulic system at disposal influences the layout.

The fluid requirements of the various hydraulics subsystems differ significantly: while the brakes require only a very little amount of LHM and the suspension somewhat more, the power steering cannot work without large amounts of mineral fluid provided at a moment’s notice. A flow distributor built into the first hydraulic circuit—that of the hydraulic pump, the main accumulator and the pressure regulator—controls the hydraulic pressure between the steering circuit and the suspension-brake circuits on PAS cars.

The rest is rather simple. A hydraulic ram cylinder is mounted on the rack of a traditional rack-and-pinion steering gear unit. The pressure of the hydraulic fluid supplied to assist the driver in turning the steering wheel is controlled by the flow distributor and a control valve. The flow distributor has the following components:

DIRAVI Steering

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.

Anti-lock Braking System

Models with higher performance level came fitted with ABS.

The principle of operation is the same as on cars with conventional braking systems but the layout is much simpler as all we need to control the operating pressure of the brakes are a few electro-valves.