Paul Magès' reflections on SAE report 28: Cadillac Eldorado air suspension

In this document

Critical analysis of the SAE report

On 5 March 1957, Paul Magès wrote his "Reflections on SAE REPORT 28" dedicated to the air suspension of the Cadillac Eldorado, signed by engineers V.D. Polhemus and L.J. Kehoe Jr. He notes that they started from the idea of a "suspension without questionable compromise" p. 3, by moving towards a piston-diaphragm combination to approach an "ideal" load-deflection curve p. 3.

Magès points out a contradiction: this ideal curve was supposedly chosen primarily to avoid an excessively large chamber volume, as a 50% reduction in stiffness would otherwise lead to a 250% increase in volume p. 3. The result, according to him: stiffness is only reduced by 10% compared to a conventional spring, for chambers of 7 litres at the front and 5 litres at the rear p. 3.

Comparison of devices

He comments on the level corrector p. 4, noting that the GMC system uses air springs to fight against banking in corners, whereas the DS entrusts this role to "roll" bars p. 4. He deduces a risk of oversteer linked to the distribution of the overturning torque on the rear axle p. 4. He also notes a seal p. 15 "having a slight family resemblance to ours" p. 4, and is surprised by the choice of an electric compressor when it can only function while the engine is running p. 4.

Regarding sealing, he notes the Freon gas checks supplemented by a "Soap water super-check" p. 5, and judges air leaks to be more difficult to detect than oil leaks p. 5.

Comparative calculations

A numerical table p. 6 compares the DS hydropneumatic spheres and an equivalent dimensioning in a Cadillac solution:

He also calculates that the necessary compressor would need to have a displacement 112 times greater than that of the DS pump for equivalent speed p. 7, and compares the energies expended (60 Kgm against 168 Kgm) p. 7.

Magès' conclusion

He concludes p. 7 that "a low-pressure air suspension is not feasible on a passenger car if one wants to obtain DS quality", deeming it incompatible with end-of-stroke damping and hydraulic anti-roll. On page 8, he criticises the SAE report's method for calculating the oscillation period, judging it too simplified, particularly regarding the distinction between roll, pumping and pitching.

Context

The SAE (Society of Automotive Engineers) report mentioned here documents, as general historical knowledge confirms, the research of General Motors/Cadillac on an air suspension applied to the Eldorado, a prestigious American car of the 1950s. These compressed air systems, alternatives to conventional metal springs, would experience limited commercial success and reliability problems with several American manufacturers of the time.

At Citroën, the DS, presented in 1955, relies on the hydropneumatic suspension developed by Paul Magès, combining gas spheres and high-pressure liquid — an architecture radically different from the pure American pneumatic approach.

Archivist's perspective

This document illustrates Magès' working method: rigorous international technical monitoring, coupled with a numerical demonstration intended to defend the technical choices of the DS against competing solutions. The tone is that of an engineer confident in his system, sometimes ironic towards the authors of the SAE report (p. 3, p. 8).

It seems that this study is part of a broader reflection on classes of air suspensions conducted by Magès during the same period, as evidenced by the Handwritten note from Paul Magès to Mr. Cadron on classes of air suspension. The document also extends a more detailed comparison kept under the title Citroën hydropneumatic vs Cadillac pneumatic suspension: handwritten comparative study, suggesting a working file followed over several weeks around this same subject.