Citroën 1958 Patent: servo-control device for vehicle steering

In this document

Context and problem posed

The memorandum describes a patent filed by Société Anonyme André Citroën, under provisional no. 659,310, dated 3 December 1958 p. 1. The text starts from a critical observation regarding existing power steering systems: they require "special precautions" to function correctly at all speeds p. 1, and the driver no longer "feels" the resistance of the wheels but only that of the mechanism p. 1-2. Another defect noted: in known systems, it is the displacement of the mechanical transmission components that controls the distribution valves, which generates significant friction and impairs sensitivity p. 2.

The proposed device

The invention introduces, between the steering shaft (7) and the pinion (6), an elastic rubber ring (8) which only flexes beyond a certain effort p. 2-4. This angular slip is captured by a system of crossed milling and rollers mounted on an axis (11), itself connected by a rod (13) to the spool (14) of a hydraulic distributor (15) p. 4.

The hydraulic circuit includes a reservoir (17), a pump (20), an accumulator (19), and a control cylinder (2) acting on the rack (5) via a piston (3) p. 4. A tap (24) with two positions allows the servo-motor to be isolated from the circuit at high speeds; its control can be manual or automatic, for example linked to the gearbox or to a speed threshold "less than 25 km/h for example" p. 4-5.

The text specifies that the elastic organ intervenes "practically not at all" in normal driving, its torsion being limited to "2 or 3 degrees of amplitude" by stops p. 5. The effort required to move the spool is given as "of the order of 500 g" p. 5.

Summary of claims

The document ends with a summary in six points (1 to f) covering the elements: elastic connecting element, angular elastic sleeve, notches and rollers, complete hydraulic circuit, automatic or manual shut-off tap p. 6-7.

Context

This patent is part of the history of Citroën power steering in the 1950s, a period when the brand was developing its hydropneumatic suspension in parallel, the first plans for which date from 17 February 1944 and which would be applied in particular to the future DS presented on 6 October 1955 (Hydropneumatic suspension plans with differential interaction). The use of high-pressure hydraulic circuits (pump, accumulator) to control mechanical functions of the vehicle is characteristic of Citroën's technical approach at that time, where Paul Magès — who joined Citroën in August 1925 and was then deputy head of Super-Control (Draft letter from Paul Magès to Mr. Sarre) — played a central role in hydraulic developments. This information is part of general historical knowledge and is not stated in the document itself.

Archivist's perspective

This descriptive memorandum is a key document for understanding the engineering of power steering at Citroën in the late 1950s. It testifies to a search for simplicity and mechanical sensitivity: rather than placing the control effort on the transmission components themselves, the invention isolates this effort thanks to a dedicated elastic element, which reduces parasitic friction (p. 2, p. 6).

It seems that this text was designed to be filed with a set of figures, since the description explicitly refers to diagrams not reproduced here p. 3 — see the Patent drawing sheet, dated the same day, which precisely illustrates the parts mentioned (pinion, elastic ring, spool, tap).

The precision of the speed threshold ("25 km/h for example", p. 4) and the control effort ("500 g", p. 5) gives this patent a very concrete dimension, probably derived from real tests conducted within the Citroën design offices, although the document does not explicitly mention it.