Paul Magès' notes on the Dowty hydraulic suspension (February 1946)

In this document

References and origin of the study

The note is dated by the handwritten bibliographic references at the top of the first page: Automobile Engineer, February 1946, p. 79, and Autocar, February 1946, p. 103 p. 1. It is these two British journals that seem to have inspired Paul Magès to examine the Dowty hydraulic suspension, of which he reproduces a sketch (articulated arm connected to an elongated cylinder) under the title "Suspension Hydraulique Dowty" p. 1-2.

Magès' calculation

Magès starts with a cylinder volume of 5600 (mm³ or cm³, unit not specified) and compares this volume, once the rod is retracted, to a reduced value of approximately 4820, which according to him corresponds to 14% necessary elasticity p. 2.

To obtain this volume variation with the liquid used, he notes that, according to an elasticity graph, it would be necessary to go from a pressure of 0 to 2500 kg/cm² p. 2.

With a rod section of 5 cm², he deduces an effort at the end of the stroke of 5 × 2500 = 12,500 kg p. 2.

Technical conclusion

Magès notes that the Dowty device does not take into account the inherent elasticity of the cylinder, which according to him must "contribute significantly to the flexibility" p. 2, as well as the elasticity of the attachments, given the forces involved p. 2. He also mentions, without developing them, several critical points: seal resistance at these pressures, friction, shear forces, joints p. 2.

He concludes: "Irrational solution for the automobile" p. 2, while qualifying it on page 3: "It is possible that the solution is acceptable for aviation where the problem is primarily a damping problem" p. 3.

A later annotation, dated 6 March 1961, explicitly links this document to a comparative reflection between "liquid spring" and "gas spring" p. 1.

Context

The Dowty suspension mentioned here is a British liquid spring system, developed in particular for aeronautics (landing gear). In the post-war period, several engineers explored the elastic properties of liquids under high pressure as an alternative to conventional metal springs.

At Citroën, this research intersected with the work that would lead to the hydropneumatic suspension, presented in 1954 and then integrated into the DS in 1955: a system combining compressed gas (nitrogen) and liquid, precisely to avoid the pitfalls that Magès identifies here in the Dowty system (excessive forces and pressures on a liquid alone).

Archivist's perspective

This 1946 document is valuable: it shows Paul Magès in technological monitoring long before the development of the Citroën suspension, analyzing a competing solution by calculation to evaluate its relevance — and rejecting it for the automobile p. 2.

The precise quantification (14% elasticity, 2500 kg/cm², 12,500 kg of effort) illustrates a rigorous engineering approach, seeking to quantify before judging. It seems that this note was taken up later, as suggested by the 1961 annotation on the superimposed sheet, as part of a broader comparison between liquid springs and gas springs — a theme found in the gas spring / liquid spring comparison note of 6 March 1961 and in the associated handwritten drafts.

The nuance provided on page 3 regarding aviation shows that Magès does not reject the principle of the liquid spring per se, but its application as conceived by Dowty for the automobile — a distinction that sheds light on the continuation of his research.