Handwritten note of 1 June 1970 on limiting rear wheel spin

In this document

This handwritten sheet, dated 1 June 1970 p. 23, presents a reflection by Paul Magès on the safety of rear-wheel drive cars.

The principle

Magès poses the problem in terms of safety: "for safety purposes, it should be prevented that the rear drive wheels can turn faster than the front steering wheels" p. 23. The objective is clear: "to avoid wheel spin […] in the event that the transmitted engine torque is too high in relation to the grip" and thus avoid "the vehicle skidding sideways" p. 23.

Two solutions considered

Magès imagines two ways to compare the speeds of the front and rear wheels and act accordingly on engine power (carburation or ignition):

• A mechanical means, inspired by speedometer cables ("one on the front, one on the rear"), which would trigger an electrical contact as soon as the rear cable turned faster than the front one p. 23.

• An electronic combination, considered more promising by the author ("it seems that a happy combination could be made"), relying on the devices already planned for "anti skid" when it is fitted to both axles. The signal comparing the rotation speeds could then act "either on the carburettor or on the ignition or even better on the brain of the electronic injection" p. 23.

A footnote, added in a different handwriting, refers to "an old patent (years 4[0/8])", without further details p. 23.

Context

The "anti skid" mentioned here corresponds to the anti-lock braking systems that were developing at the time in aeronautics and were beginning to interest the automotive industry — mass-market ABS would not appear at Citroën and its competitors until several years later. Electronic injection, still in its infancy in 1970, was only fitted to rare high-end models at the time.

This note is part of a period when Citroën was working in particular on the SM, a rear-wheel drive vehicle with a Maserati engine, which makes Magès' concern for the traction and road holding of rear-wheel drive vehicles particularly consistent with the brand's current events.

Archivist's perspective

This document is valuable because it shows Magès in the position of a generalist inventor, going beyond the sole field of hydropneumatic suspension to tackle a problem of traction and active safety — which would later become traction control.

It seems that this reflection, contemporary with the road holding tests of the SM, may have nourished subsequent discussions on regulations, as suggested by the Magès note of 7 November 1970 on road holding regulations written the same year.

The reference to an "old patent" mentioned in the margin p. 23 also testifies to Magès' technical culture, always attentive to situating his ideas in relation to prior art — a lead that remains, as it stands, not precisely identified.