Rubber Bibax vs hydraulic suspension: the 1946 Citroën-Michelin correspondence

In this document

This set brings together three pieces of the same epistolary exchange concerning the Bibax suspension E-331.000 (also referenced as "E 531.000" on the copies, p. 54-55), a rubber part studied as a suspension alternative.

The letter from Citroën to Michelin (29 October 1946, p. 56)

Citroën informs Michelin that the Bibax received have been mounted on a vehicle and that "the first tests are extremely encouraging" p. 56. Two problems remain identified: the "fine-tuning of the suspension quality", which Citroën is taking charge of, and the "endurance of the Bibax, regarding fatigue, over time, and weather conditions", for which Citroën states it has no data and "relies entirely" on Michelin p. 56. The letter urges Michelin to respond to allow for a decision on a potential mass-production release.

The response from Michelin (4 November 1946, p. 54-55, in two copies)

Michelin (dictated by Mr. Saint Frison) replies that the Bibax "tested by us on the bench, gave very encouraging results", but acknowledges the lack of direct road experience p. 55. A test had been planned for "a few months" on a Citroën 11 CV, where the Bibax would have replaced the rear torsion bars, but delays (including the purchase of a vehicle) prevented it; it was to be launched "during this month" p. 55.

Michelin poses a technical reservation: for the results of this test to be transferable to the suspension studied by Citroën, "the static working rate, the amplitudes of normal dynamic deformations, their frequency and the maximum amplitudes" would need to be comparable between the two cases p. 55.

Regarding durability, Michelin considers aging "not to be feared" if the Bibax are exposed "neither to the sun, nor... to contact with oil or grease", and deems mass production achievable "with the same guarantee" as the company's other rubber parts p. 55. The letter concludes that if the launch deadlines for the T.P.V. do not allow for sufficient road tests, it will be necessary to "give up on the Bibax or take a leap of faith" p. 55.

The two copies of the Michelin response bear the handwritten note "D. to: M. Magès" dated "7 NOV. 1946" p. 54-55, attesting to their transmission to the engineer. A grid paper superimposed on page 54 bears subsequent filing notes: "1946 to 1953", "Rubber suspension / Hydraulic suspension", "Vehicles TPV - PV - PVL6", "Correspondence with Michelin for rubber", "Comparative tests of the 2 solutions", "Laboratory report" p. 54.

Context

The T.P.V. (Toute Petite Voiture) refers to the project that would give birth to the Citroën 2 CV, commercially launched in 1948. During this development period, several suspension solutions were being studied in parallel, including rubber systems — the subject of this correspondence — and hydraulic or hydropneumatic solutions, the latter being the field in which Paul Magès would become the recognized inventor, with the hydropneumatic suspension finally adopted on the DS in 1955.

The Michelin brand, then the majority shareholder of Citroën since the 1930s, played an important technical and industrial support role in the brand's choices, which explains the direct and confident nature of this technical exchange.

Archivist's perspective

This correspondence sheds light on a probably transitional stage in the suspension research for the future 2 CV: the rubber Bibax appears here as a seriously considered path, in competition with the hydraulic solutions that Paul Magès was developing elsewhere, as evidenced by other pieces from the same collection such as the plans for hydropneumatic suspension with differential interaction.

The transmission of these letters to Magès as early as 7 November 1946 p. 54-55 suggests that he was following, or was consulted on, all the suspension options under study for the T.P.V., and not only on the hydropneumatic solutions that would make him famous. The late filing notes p. 54 confirm that this file was kept precisely to document the comparison between the two paths — rubber and hydraulic —, a useful milestone for understanding why the latter was finally chosen. The summary of folder 41-59 also places this exchange within a broader documentary set devoted to these developments.