DIRAVI manual: assistance, hardening and power-assisted return of the SM steering

In this document

This training manual body details the operation of the DIRAVI (direction à rappel asservi - power-assisted return steering) fitted to the Citroën SM. The text specifies from page 3 that this "new design" aims to "improve safety when driving fast and at low speeds", to "increase the speed of evolution" and to "increase driving comfort by eliminating reactions" p. 3.

The steering is described as a "hydraulically assisted rack and pinion" system comprising three functions: I - Assistance, II - Steering hardening according to speed, III - Return servo-control p. 3, based on three hydraulic components: the rack hydraulic control, the pinion-shaft-distributor-variable flow regulator assembly, and the centrifugal regulator p. 3.

I. Assistance function

The assistance piston is subjected to two opposing forces: chamber A permanently receives pressure from the main accumulator on a surface twice as small as chamber B, so that a pressure of P/2 in chamber B is sufficient for equilibrium p. 5. Any imbalance (chamber B above or below P/2) moves the rack, drives the pinion and then the distributor spool, which restores equilibrium p. 5, 7. The document highlights the safety benefit: the steering angle always corresponds to a precise position of the steering wheel, insensitive to asymmetrical braking, a tyre blowout or an obstacle p. 7.

II. Hardening according to speed

A pinion integral with a cam, on which a roller piston presses, applies an increasing torque according to the pressure of the centrifugal regulator and the rotation angle of the control shaft p. 6-7. In a straight line, the contact point remains on the O-O' axis; when turning, it moves away from it, creating a torque that opposes the driver and stiffens the steering with speed p. 7. The diagrams illustrate the "neutral 50 km/h" and "neutral 200 km/h" positions p. 6.

III. Return servo-control

With the steering wheel released, the cam torque acts on the control shaft to return the steering to a straight line via the assistance function p. 8. To avoid too abrupt a return, a variable flow regulator brakes the fluid; its pressure is "always equal to that" of the centrifugal regulator p. 8. It includes a body, a return piston, a calibrated hole sleeve, a spring R and a valve p. 8. Pages 9-10 detail the three states — neutral, turning, return — with chambers A, B, C: at rest the spring is without tension and the orifices closed; when turning the piston compresses the spring; on return, the flow through the calibrated hole decreases progressively, slowing the return of the steering wheel until the straight line p. 9-10.

Context

The DIRAVI, developed by Paul Magès and his team at Citroën, is a hydraulic power steering system without direct mechanical linkage between the steering wheel and the wheels, introduced on the SM (1970) and then extended to other models such as the CX. It differs from conventional power steering by its automatic return to the neutral point and its progressive hardening with speed, two characteristics that have marked the brand's reputation for comfort and driving precision. The Citroën SM, a high-end coupé powered by a Maserati V6, served as a showcase for this innovation.

Archivist's view

This document is likely an internal educational aid intended to train Citroën technicians or sales staff in the operation of the DIRAVI, as indicated by the cover of the same file dated June 1970.

It is of definite didactic interest: the logical progression — assistance, hardening, return — allows one to grasp how the same mechanical mechanism (cam/roller torque) serves both to stiffen the steering and to return it to a straight line p. 7-8, which seems to be the keystone of Magès' invention. The cross-sectional diagrams on pages 4, 6 and 9 usefully supplement a text that is otherwise very clear and educational for the time.