Note P. 6912: an anti-slip device for rear-wheel drive cars (1970)

In this document

This handwritten note, referenced P. 6912 and signed "P. Magès" on 1.6.70 p. 1, sets out a safety idea for rear-wheel drive cars.

The problem addressed

Magès starts from an observation: on these vehicles, the driving (rear) wheels can rotate faster than the front steering wheels when the engine torque exceeds the available grip, causing wheel spin and then "the vehicle to slide sideways" p. 1.

The proposed principle

The idea consists of constantly comparing the speed of the front wheels and that of the rear wheels, then, as soon as the rear tends to rotate faster, acting on the carburation or the ignition of the engine to reduce its power p. 1.

Two technical means mentioned

• A simple mechanical means: two speedometer cables (one at the front, one at the rear) activating an electrical contact as soon as the speed difference appears, this contact controlling an electromagnet on the carburettor or the ignition p. 1.
• A path considered more promising: reusing the electronic devices already envisaged for the "antiskid" when it is fitted to both axles, by electronically comparing the front/rear speeds to act on the carburettor, the ignition, or "even better" on a future electronic injection computer p. 1.

A footnote refers the author himself to "a patent from the 40s concerning a clutch synchronisation device" p. 1, as a possible technical antecedent for obtaining the comparison signal.

Context

In 1970, Paul Magès had long been deputy head of Super-Control at Citroën, where he had started as a self-taught engineer in 1925 (autobiographical notes); he would remain there until his retirement in September 1974 (photographs of retirement). At the turn of the 1970s, automotive on-board electronics were in their infancy: the first anti-lock systems (ancestors of the ABS) were only just beginning to be studied by manufacturers, and electronic injection was not yet widespread. Citroën was then renowned for its innovations in road holding, including the hydropneumatic suspension designed by Magès himself, introduced on the DS as early as 1955.

The principle described here — comparing front and rear wheel speeds to correct engine torque — anticipates, in its spirit, functions that would be taken up much later by electronic traction control or anti-slip systems, without this document allowing a direct link of filiation to be asserted.

Archivist's perspective

This single sheet, dated and signed, resembles an idea note rather than a completed project: Magès reasons aloud, envisaging successively a mechanical solution then an electronic solution, and himself cites an earlier reference as a line of thought p. 1.

It seems that this document bears witness to a personal watch on active safety devices, a field expanded in relation to his better-known work on suspension. The numbering "P. 6912" suggests its integration into a series of listed technical notes, of which other pieces in the archives could clarify the follow-up or the context.