Technical note "Variable geometry steering" (ref. D 12 54, 26 Dec. 1954)

In this document

The principle: a geometry that depends on speed

The note immediately states the theoretical observation: "for a given turn, the steering geometry must be different depending on the speed at which the turn is taken" p. 1. At zero speed, the Jeantaud geometry (noted as "Janteau" in the text) remains valid, but as soon as the car is moving, "it is not the plane of the wheels that counts, but the drift plane" p. 1.

Physical reasoning

The author follows with a series of observations: drift depends on the load on the wheel, the transverse force, and the inclination (camber, leaning of the car in a turn). Under the effect of the transverse force, the outer wheel is unloaded while the inner one is overloaded, which increases the drift of one and decreases that of the other. To avoid "inner tyre scrubbing", it is necessary to be able to provide "toe-in" p. 1.

Envisaged devices

Three solutions are outlined to vary the length of the track rod according to the difference in load between the front wheels:

• an anti-roll bar whose torsion controls a hydraulic distributor p. 1;
• a pendulum acted upon by centrifugal force p. 1;
• in the case of an hydropneumatic suspension with separate right/left circuits, the direct sending of pressure from the suspension cylinders to cylinders modifying the connecting rod p. 1.

Sizing (page 2)

Page 2 specifies that it is necessary to calculate "the stiffness and initial tension of the springs (R) as well as the sections of the cylinders and the stroke of the pistons [...] to obtain the correction law for a given car" p. 2. A variant is envisaged for cars with two half-connecting rods: varying only one of them, or modifying the setting angle of the levers of a relay, hydraulically controlled by a "follower device" sensitive to the right/left suspension pressure difference p. 2. A technical diagram illustrates this mechanism (wheels, levers, springs "R", suspension cylinders) p. 2.

The document is signed and dated "26.12.54" p. 2, with a later annotation in red ink: "exact but it was going a bit far" p. 2.

Context

This note is part of the development, at Citroën, of the hydropneumatic suspension, of which Paul Magès is the recognised inventor and which will be launched on the DS in 1955. The Jeantaud geometry, mentioned as a classic reference, is a geometric principle of steering dating from the 19th century, used to orient the front wheels differently in a turn. The idea of an "active" steering linked to the dynamic load of the wheels anticipates concepts that will only be industrialised decades later on certain vehicles with variable power steering.

Archivist's perspective

This two-page document, very dense and written without frills, clearly illustrates Magès's working method: starting from a simple physical observation (variation of drift according to load) to derive, almost immediately, a concrete mechanical-hydraulic architecture p. 1-2.

The late handwritten annotation — "exact but it was going a bit far" — suggests that this is probably a critical review, perhaps hierarchical, judging the idea correct in theory but premature or too ambitious for immediate application p. 2. The explicit link with the hydropneumatic suspension with separate circuits p. 1 shows that this reflection on steering is directly articulated with Magès's other work on suspension, without it being possible to state, as it stands, that it led to an industrial implementation. PMREFnumberEND