Ride height correction project: parallel-to-road and pendulum diagrams (1944)

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Page 1 — Parallel-to-road correction

The plate presents an electro-hydraulic diagram of ride height correction applied to the four wheels. It includes a battery ("BAT."), terminals "R+A" for the front (left/right) and rear (left/right) wheels, as well as a Bimetallic Relay p. 1.

The circuit details, for the rear left wheel, a Lifting Electro and a Lowering Electro, supplied "From the Hydraulic accumulator" and discharging "To the reservoir", with a wheel contactor p. 1.

Two precise values are indicated: a "Max angular displacement" of 30° and a max wheel displacement of 200 mm p. 1.

The principle is explained as follows: "the car remains in a plane parallel to, and at a constant distance from, a plane passing through the point of impact, on the ground, of the three most heavily loaded wheels" p. 1. Sketches illustrate the behavior on a cambered road and in a turn, where "the ride height correction system tends to keep the car parallel to the ground" p. 1.

The plate is signed "Magès" and dated "11.1.44" p. 1.

Page 2 — Transverse pendulum correction

This second plate describes a pendulum correction device, also relying on a Bimetallic Relay, with terminals "R+A", "R.A" and "R", supplied from "From the Hydraulic Accumulator" to "To the reservoir" p. 2.

The handwritten text distinguishes two possible logics: a "complete" correction where the car's plane remains perpendicular "to the weight centrifugal force resultant" (with a limit on the car's max inclination relative to the ground), or an intermediate correction, choosing "a mean plane (between the plane ⟂ to the resultant & the plane // to the road)" p. 2.

A diagram mentions a "Right wheel Resistance control" p. 2. The sketches illustrate the behavior on a cambered road ("parallel-to-road" correction in the longitudinal direction) and in a turn, where "the correction system tends to make the car lean into the turn" p. 2.

The page bears an illegible signature and the date "11.1.44" p. 2.

Context

These plates date from 11 January 1944, one month before the plans for hydropneumatic suspension with differential interaction of 17 February 1944, and more than a decade before the commercial launch of this suspension on the DS Citroën, presented on 6 October 1955. They testify to an upstream research phase, where Paul Magès — then a technical writer in the design office, who joined Citroën in August 1925 — explored several ride height correction architectures — parallel and pendulum — before the chosen solutions equipped Citroën vehicles, ahead of the first tests on the 2 CV in May-July 1946 (Notebook of the first 2 CV hydropneumatic suspension tests (May-July 1946)) and then the test of the 15-Six H in April 1954 (Road test of the 15 Six H (ELV 412) by Paul Magès, 3-5 April 1954).

The distinction between "parallel-to-road" correction and "pendulum" correction (limiting roll in turns) evokes issues that would remain central to the history of hydropneumatic suspension, notably via the anti-roll systems developed later at Citroën.

Archivist's perspective

These two plates seem to constitute a pair of alternative proposals, studied on the same day p. 1-2, to solve the same problem: keeping the vehicle's stance stable despite road irregularities and cornering forces.

The systematic use of bimetallic relays, lifting/lowering electromagnets and an accumulator-reservoir circuit shows that the basic electro-hydraulic architecture was already conceived very early in Magès's thinking, as a working draft preparatory to the differential interaction suspension plans that would follow a few weeks later.

The precision of the numerical values given on page 1 (30° angle, 200 mm travel) suggests the beginning of concrete technical dimensioning, rather than a simple sketch of principle.