“Fundamental characteristics of compensated pneumatic suspensions” — annotated copy

In this document

A theoretical offprint on compensated pneumatic suspensions

This document is an article signed by Maurice A. Julien, civil engineer of Mines, Director of Research at S.A. Usines Paulstra, and president of a technical section at the S.I.A. p. 24. The author announces his intention to compare, on a theoretical level, two families of suspensions that appeared “in mass production”: the Firestone — General Motors solution in the United States and the Citroën solution in France p. 24.

Terminology and principles

Julien distinguishes “solid” suspension (metal spring, rubber, wood) from pneumatic or hydropneumatic suspension, based on a “gas spring” whose pressure can be adjusted p. 24. He introduces the notion of “compensated suspension”, static or dynamic, the latter being able to treat roll and pitch separately thanks to the “Broulhiet theorem” p. 25.

He then develops the theory of isochronism: a suspension is said to be isochronous if its natural frequency remains constant regardless of the load, which assumes an exponential restoring law p. 25.

Two families of compensated suspensions

Starting from an “elementary pneumatic spring” (fig. 2, p. 26), the author distinguishes:

• the constant volume and variable air mass suspension, illustrated by the Firestone-GM device p. 26-27, which he mathematically proves to be isochronous p. 27;
• the constant air mass and variable volume suspension (oleopneumatic solution), illustrated by the Citroën device, which he demonstrates is not isochronous, its frequency varying as the square root of the load ratio p. 27-28.

Julien then proposes a “solid and compensated oleopneumatic” mixed suspension combining equal parts of a linear solid spring and a compensated pneumatic spring, whose respective isochronism defects compensate for each other (fig. 6, p. 29). He concludes by drawing up a balance sheet of the advantages/disadvantages of compensated suspensions, judging that “we are now approaching the inversion point” in favour of pneumatic/hydropneumatic p. 29.

The handwritten annotation

At the bottom of page 29, a note in red ink, partly uncertain, reports a memory: during the discussion following the presentation, a speaker (name uncertain) allegedly objected that, if the suspension is aperiodic, “talking about isochronism is a matter of mathematical aesthetics”.

Context

The Citroën hydropneumatic suspension, developed in particular by Paul Magès, equips the DS presented in 1955, then the 2 CV and other models. It is based on a principle of constant gas mass and variable liquid volume, exactly the second type described by Julien.

The American competitor mentioned, Firestone-General Motors, had developed at the same time a compressed air pneumatic system, fitted in particular to certain Cadillac models in the 1950s — a solution of the first type theorised by the author.

Paulstra, the company for which Julien works, was a supplier of rubber components and suspensions for the French automotive industry, in technical and commercial contact with the manufacturers of the time, including Citroën.

Archivist's perspective

This text occupies a special place in the Magès archives: it is not an internal note but an external theoretical offprint, probably distributed during a technical conference (S.I.A.), and kept for its favourable — albeit nuanced — conclusions regarding the Citroën solution.

The handwritten annotation in red is valuable: it attests that a discussion followed the oral presentation of the paper, where the very relevance of the notion of isochronism was allegedly called into question p. 29. The link with contemporary exchanges on the subject, such as the minutes of the meeting of 17 February 1960 on the Bougeard patent or the Magès note of 19 June 1961 on the Bongrand affair, suggests that this document probably circulated within the Citroën technical team as a piece of comparison and theoretical justification for the choices made for the hydropneumatic suspension.