Article by Maurice A. Julien on compensated pneumatic suspensions (1956)

In this document

This technical offprint is signed by Maurice A. Julien, Civil Engineer of Mines, Director of Research at the Paulstra factories, presented as president of the 2nd S.I.A. Technical Section "Engine-Transmission" p. 18.

Introduction and issues

The author announces his intention to theoretically study the family of "compensated" pneumatic suspensions, in reaction to the "recent appearance in mass production of two solutions": the Firestone — General Motors device in America and the Citroën device in France p. 18.

Generalities and terminology

Julien distinguishes between "solid" suspension and pneumatic/hydropneumatic suspension, then introduces the notion of "compensated suspension," static or dynamic, and cites as examples automatic level correctors at Citroën and Packard, or non-automatic ones on the Mercedes 300 p. 18-19. He then develops the notion of isochronism: an isochronous suspension maintains a constant oscillation period regardless of the load p. 19.

The elementary pneumatic spring

The author sets out the equations for the restoring force law of a pneumatic spring (fig. 2), distinguishing between isothermal (σ≈1), adiabatic (σ≈1.43 for air), and "intermediate" (σ = 1.3, a value adopted "in the vicinity of usual suspension frequencies") transformations p. 20.

Two families of compensated suspensions

Julien identifies and names two processes p. 21-22:

He demonstrates that the first type is isochronous p. 21, while the second, oleopneumatic, is not: its pulsation varies according to the square root of the load ratio p. 22.

Mixed suspension

Finally, he proposes a "solid + compensated oleopneumatic" combination (fig. 6) where a coil spring and a pneumatic bellows are combined in equal parts, providing almost perfect isochronism over a wide range of loads p. 22-23.

Conclusions

The author draws up a balance sheet of advantages/disadvantages (level regulation, comfort, damping, isochronism / complication, price, risk of failure) and cautiously concludes on a future "triumph" of compensated pneumatic or hydropneumatic suspensions p. 23. The bibliography notably cites an earlier article by Julien himself in the Journal S.I.A. of 1954 p. 23.

Context

This text is part of the context of the release of the DS 19 in 1955-1956, for which the hydropneumatic suspension designed by Paul Magès constitutes the major innovation. The comparison with the Firestone-General Motors system, which then equipped certain high-end American models, was a topic of technical interest in the specialized press of the time.

The Paulstra factories, specializing in rubber and automotive elastic elements, were a classic industrial partner of French automotive engineering, which explains Julien's interest in these suspension issues, including outside the purely pneumatic field.

Archivist's perspective

This text offers an external and scholarly insight into the very principle that Paul Magès developed: it allows for measuring, through a third-party and theoretical perspective, the physical relevance of the Citroën choice (constant gas mass, variable volume) against the American solution, while honestly highlighting its lack of isochronism compared to the variable pressure solution p. 21-22.

It is probable that this document was kept by Magès or his team as an external theoretical reference, offering a rigorous conceptual framework (isochronism, action height) applicable to internal reflections on suspension sizing, of which contemporary traces can be found in the handwritten notes of 1959 on capacity per side or per axle.

The proposal for a "solid + oleopneumatic" mixed suspension p. 22-23 also echoes, without a direct documentary link being attested here, the internal debates found in the minutes of the meeting of 17 February 1960 on the Bougeard patent, devoted to alternative suspension devices studied at Citroën.

The main interest of this archive therefore lies less in a direct action by Magès than in the proof of an early external scientific recognition, as early as 1956, of the hydropneumatic principle he had just brought to fruition.