Comparative load/deflection chart: metal spring vs. air cushion (4 to 8 kg/cm²)

A handwritten technical study sheet compares, via a graph on graph paper, the load/deflection behavior of a metal spring and an air cushion under increasing pressures — a possible milestone in research into hydropneumatic suspension.

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Understanding this document

In this document

The studied assembly

A technical sketch at the top of the sheet represents a helical metal spring associated with an air pressure gauge ("Mandmètre / Air / p= Kg/cm²"). The spring dimensions are specified: wire of φ 12 mm, diameter Dm 120 mm, height H 460, pitch of 75, with diameters φ105 and φ195 EXT recorded in static and maximum position p. 1.

The comparative graph

The heart of the document is a handwritten graph on graph paper, bearing on the ordinate the "CHARGES EN Kgs" (from 0 to 3500) and on the abscissa three superimposed scales: the deflection in mm (0 to 230), a percentage (0 to 40%), and the "Hauteur H stabilisée" (from 460 to 240) p. 1.

Two curves are plotted there:

a low curve, annotated « pression 0 Kg/cm² », starting from the origin and rising gradually — probably the response of the metal spring alone p. 1;
a high curve, starting towards 1300 kg, associated with several points of increasing pressure on what appears to be a sphere or an air cushion:

Pressure	φ EXT (mm)	Page
4 Kg/cm²	220	p. 1
4.5 Kg/cm²	224	p. 1
5.5 Kg/cm²	228	p. 1
6 Kg/cm²	230	p. 1
7 Kg/cm²	234	p. 1
8 Kg/cm²	238	p. 1

A small circular dial at the bottom left bears the mention « TARE » with a needle p. 1. A circular stamp is visible at the top right, but remains illegible; vertical inscriptions in the right margin, partially masked, cannot be exploited p. 1.

Context

This type of comparison between a classic metal spring and a pressurized air cushion evokes, in a general way, the research conducted at Citroën on hydropneumatic suspension, a system that would notably equip the DS from 1955. The principle consists of replacing the spring with a sphere containing a compressed gas (nitrogen) separated from a liquid, offering progressive elasticity and better comfort. Paul Magès joined the Citroën design office during this period and sketched plans for hydropneumatic suspension with differential interaction as early as 17 February 1944, before the first tests on 2 CV in 1946; this comparative sheet could be situated in this study phase, without it being possible to confirm it.

This perspective is based on general historical knowledge and is not stated as such in the document, which is limited to measurements and a plot of curves.

Archivist's view

This sheet seems to constitute a comparative study milestone, probably intended to demonstrate, with figures to support it, the interest of the pneumatic solution compared to the classic metal spring: at equal load, the sphere of air curve seems to offer a different behavior from that of the spring p. 1.

The absence of a date, author's name, or explicit legend, however, prevents this document from being precisely situated in the chronology of Paul Magès' work, who is otherwise known to have joined Citroën in August 1925 and spent the bulk of his career there working on hydropneumatic suspension until his retirement in September 1974 (Photographs of Paul Magès' retirement (1974)). It could be linked to other technical pieces in the same file, such as the dimensioned plans in Italian of a SACA suspension element or the operating diagram of suspension with lightening no. 6046, which could clarify the technical context of this comparative study.