Handwritten comparative study: DS hydropneumatic suspension vs Cadillac Eldorado pneumatic suspension

In this document

Citroën / Cadillac Eldorado comparison (p. 9)

The document opens with a comparison between the high-pressure "Citroën-type" hydropneumatic suspension and a "100% pneumatic, low-pressure, Cadillac Eldorado-type" solution p. 9. The author states that "comfort increases with the increase" of flexibility and that, "contrary to a well-established opinion, flexibility improves road holding" p. 9. On the DS, the usable flexibility is said to be limited by the pitching movement during acceleration/deceleration p. 9.

A table of figures calculates the dimensions of "GMC-type air springs" that would provide the same flexibility as the current DS spheres:

Isochronism and study by Mr. Maurice A. Julien (p. 10)

Page 10 mentions a "Compensated suspension with constant volume and variable air mass" and then the combination of "Solid" and "Pneumatic" springs (Pirelli/Paulstra solution) p. 10. The author reports that at the SIA, Maurice A. Julien sought to demonstrate that the "constant air mass" suspension (Citroën) is "not isochronous" like the "variable air mass" one (GMC), and that a solid spring + air spring combination would allow for "perfect isochronism" within 4% p. 10. The author judges this study "theoretically perfect for the gallery" and believes that the body periodicity is of no importance as soon as a "practically aperiodic" system is obtained p. 10.

Comparative calculations of pistons and pumps (p. 11)

Page 11 compares DS piston sections (∅ = 35, section 9.6 cm²) and "GMC" (required section ~214 cm², ∅ 165 mm) p. 11. It evaluates the volumes of oil and air to be injected to increase the pressure: "375 cm³ of oil" to go from 60 to 120 Kg/cm² on a DS sphere, against "45 litres of air" for the GMC sphere p. 11. A box notes that "GMC Cadillac announces 9.8 l. per minute: 7 Kg. (450 cm³?)", deemed "probably erroneous" p. 11.

Conclusion on the air-solid combination (p. 12)

Page 12 concludes that in an "air-solid" combination, the rigidities "in parallel" add up, therefore "the rigidity cannot be lower than the rigidity of the solid spring". The author judges this solution "not applicable on DS or on any car where comfort is a No. 1 problem" p. 12.

Context

Historically, the Citroën DS (1955) uses the high-pressure hydropneumatic suspension invented by Paul Magès, while the Cadillac Eldorado of the same era offered a low-pressure pneumatic suspension, which was highly commented on in the United States at the time. The reference to the SIA (Société des Ingénieurs de l'Automobile) places this debate within the French technical circle of the 1950s, where American (GMC) and French approaches were readily compared.

In 1957, Paul Magès, who joined Citroën in August 1925, had been working on the hydropneumatic suspension for over ten years: his initial plans date from February 1944 (Hydropneumatic suspension plans with differential interaction), followed by the first 2 CV tests in 1946 (Notebook of the first 2 CV tests) and the presentation of the DS on 6 October 1955.

Archivist's perspective

This document appears to be an internal working study, probably by Paul Magès himself, intended to objectively compare — with supporting figures — the DS solution to that of the Cadillac Eldorado and to the theoretical theses presented at the SIA p. 9-11.

The numerous crossings-out and crosses placed on the calculations suggest a rough draft, reworked, where the author tests and then abandons certain calculation paths.

The very firm conclusion on page 12 — judging the air-solid solution inapplicable "on DS or on any car where comfort is a No. 1 problem" — seems to confirm the doctrine defended by Magès in favour of maximum flexibility. This document is likely part of the same reflection as the Reflections on SAE report No. 28/78 and the Note to Mr. Cadron on pneumatic suspension classes, both from 1957.