Magès note of 19 June 1961: assessment of the Bongrand affair and comparison of suspensions

In this document

The initial observation: the Bongrand affair

This handwritten note, dated 19 June 1961 and addressed to Messrs. Cadiou, Delerive, Falque, Henry and Roubaud p. 14, summarizes the study conducted on the suspension system of Mr. Bongrand, tested on a 4 CV and then on a 2 CV. Magès notes that the good results obtained on the 4 CV, "relatively easy", were "not as" replicated on the 2 CV, "except for a very [...] damping due to friction" p. 14.

The identified technical problem

Magès explains that the Bongrand principle makes it difficult to obtain identical comfort on both axles when the front/rear load ratios vary significantly: adjusting the pressure as close as possible to the balance of the most heavily loaded axle reduces the comfort of the other axle, "the pressure [...] adjusting itself automatically in the reaction chamber" p. 14. He deduces that it would be necessary to "create a suspension in which compressed air [...] is sent into each cylinder until the desired height is reached" — a solution he compares to "classic pneumatic suspensions" p. 14.

Comparison with the "Bourgeau" solution

Magès judges the flexibility obtained to be "even greater" in the Bourgeau solution thanks to the reaction chamber, but points out that its compressor must be "all the larger as the flexibility is great" p. 15. He quantifies the friction problem: "If one has 20 kg of friction on a suspension having a flexibility of 100 mm per 100 kg, the balance [...] can only be obtained to within ± 20 mm" p. 15. He also recalls the test of the "17 Tournants", where the flexibility of the Bourgeau car made it necessary to be "on the bump stops" p. 15.

American pneumatic suspensions

Magès mentions American tests on pneumatic suspensions "with bellows or membranes", affected by "condensation [...] icing troubles"; he fears that the problem is "even more difficult to solve" with metal pistons, and concludes that no cost price gain appears possible "according to the various projects" p. 16.

Table of comparative tests (p. 17)

The 2 CV, handled by "Darcotte", was the subject of research on displacements with variable volumes; the first tests at high flexibility were "stopped immediately by friction" p. 17. The seal friction measurements give "± 10%", a minimum of "± 5%", against "20% normal" in industrial pneumatic equipment p. 17. A definition of volumes "by Mr. Harny" indicates that at equal flexibility, the total volume is "4 times larger" than a suspension with metal springs p. 17.

Context

Let us recall, outside the document, that Paul Magès is the inventor of the hydropneumatic suspension equipping the Citroën DS (1955): this general context explains why Citroën, in the years 1959-1961, systematically evaluated competing systems — including Bongrand's — to verify the superiority of its own solution. The vehicles cited (4 CV, 2 CV, Vedette) are iconic models of the post-war French automotive industry, used here as comparative test benches.

Archivist's perspective

This synthetic document seems to constitute a progress report intended for management or fellow engineers, after several years of testing on the Bongrand system p. 14-17. It illustrates Magès' method: rigorous, numerical comparison between competing solutions and the hydropneumatic principle, with a constant concern for friction and attitude stability.

The reference to the "letter 5 Nov 59" p. 17 likely links it to the Letter from A. Bougeard on the 2 CV suspension tests, while the theoretical comparison developed here likely extends the reflections recorded in the B.E.A. meeting record of 17 January 1961.