Paul Magès' drafts: gas spring versus liquid spring, comparative calculations for the DS

In this document

These handwritten work sheets develop a comparative demonstration between the gas spring and the liquid spring, based on the technical data of the DS.

The comparative calculation based on the DS

Magès starts from the operating pressure of the DS front, "110 Kg/cm² at full load" p. 29, and calculates the volume of gas necessary to vary the pressure from 100 to 110 Kg/cm²: "40 cm³" for a piston section of "9.6 cm²", i.e., a displacement of "4.2 cm" p. 29.

He then transposes this calculation to a purely liquid spring. By using the compressibility modulus of "motor oil" ("approximately 11000 Kg/cm²") and that of special "Hydra Spring" oils (around "5000" to "8000" below 200 Kg/cm²), he obtains a necessary oil volume of "20,000 cm³" or "20 litres", "instead of 0.750 on DS" p. 29-30.

By doubling the operating pressure (200 Kg/cm² instead of 100), the necessary volume drops to "8 litres", which remains, according to him, "a good weight and a good price" compared to the 0.750 litre DS sphere p. 30. He notes, however, that this volume "can be reduced if one takes into account the elasticity of the casing" p. 30.

The distinction between the two uses of a spring

Magès then theorises the notion of a spring as an "energy accumulator" (Kg.m). He distinguishes two cases:

• high force and short displacement → material with a high modulus of elasticity, illustrated by the example of side press springs, where a "liquid-spring" could replace "40 coil springs" mounted "in parallel" according to "fig 19 of document ②" p. 31;
• low force and long displacement, the case of "automotive suspensions" requiring "great flexibility": here, "nothing can replace gases", and "a liquid spring is inconceivable" p. 31.

He considers an indirect assembly via a "lever system having a ratio of 40" for a "series" assembly, while acknowledging that "many reasons" may oppose it p. 31.

Page 33 resumes this demonstration in a reworked form (numerous crossed-out passages), with a numerical example: "0.350" litres of air at "100 Kg/cm²" would give a suspension quality equivalent to "9 litres of air" working at "5 Kg/cm²" (sphere of approximately "400 mm ø") p. 33. Page 32 contains only a summary sentence: "The choice of a spring depends on the use one wants to make of it".

Context

The DS, launched by Citroën in 1955, owes its hydropneumatic suspension to Paul Magès: a gas (nitrogen) compressed in a sphere acts as an elastic element, with the oil serving only as an incompressible pressure transmitter. In the 1950s, several American manufacturers (including the company Wales-Strippit, with its "Hydra Springs", and Tayco Developments) proposed purely liquid springs, exploiting the slight compressibility of certain special oils as an alternative to classic metal springs.

Archivist's perspective

This document illustrates, through a rigorous comparative calculation, the line of defence that Magès likely opposed to the competing proposals for liquid springs from the United States. By demonstrating that the most compressible oil would require a volume 25 to 60 times greater than that of gas for the same flexibility p. 29-30, he justifies the fundamental technical choice of the Citroën hydropneumatic suspension.

These drafts seem to respond directly, or at least echo, the exchanges surrounding the letter from Paul H. Taylor on the Liquid Spring suspension and the comparison note between gas spring and liquid spring, dated 6 March 1961, which likely offers a typed and finished version of it.