Magès' note on the safety of Bibax rubber silentblocks (8 January 1947)

In this document

The internal debate

Paul Magès settles a technical debate on the reliability of bonded Bibax silentblocks, advocated by Mr. Chataigne. He recalls that the laboratory tests presented by Chataigne had initially convinced the team, before Mr. Boussu "cooled our enthusiasm by raising objections about the bonding" — Boussu being unaware of Chataigne's tests at the time p. 57.

The Michelin Laboratory report

At the request of Mr. Cadiou, the Michelin Laboratory transmitted at the end of December a report written by Mr. J. Hauvette on 13/12/46 p. 57. This report explains that the 3% shrinkage of rubber during molding creates internal stresses near the bonding surfaces, stresses which remain low if the reinforcements can move closer together freely, but become considerable otherwise p. 57.

Magès draws a key conclusion from this: if one forces this approach of the reinforcements to put the rubber under slight compression, one can "totally eliminate residual internal stresses" p. 57 — this is precisely the principle adopted by Chataigne for his Bibax, placed under compression during assembly p. 57.

Arguments and numerical data

The Michelin report cites the example of cylindrical silentblocks: a thickness ratio (R2-R1) of less than a quarter or fifth of the generatrix causes stresses to appear, which are the start of debonding. Magès notes that the front silentblocks of the PV, with a ratio of 1/9, "all debond sooner or later" p. 58.

Magès finally proposes a solution to a molding problem noted by the "rubber manufacturers" (entrainment of "skims" by rubber eddies): pre-molding the rubber in an intermediate mold p. 58. He concludes: "one has the right to be more optimistic" p. 58.

Context

The Bibax appears here linked to the rubber suspension envisaged for the TPV, an automotive project that would lead to the Citroën 2 CV. The Michelin Laboratory, then a majority shareholder of Citroën, played a technical advisory role on materials, particularly rubber — knowledge external to the document, not detailed in the transcribed pages.

At the time of this note, Magès is a technical writer in the design office, having joined Citroën in August 1925 (autobiographical notes); he has already been working, since February 1944, on his own hydropneumatic suspension designs (hydropneumatic suspension designs), while the 2 CV tests with this same hydropneumatic suspension would only begin in May-July 1946 (notebook of the first 2 CV tests): this January 1947 note therefore clearly relates to a rubber path still being explored in parallel, and not to an already abandoned hydropneumatic path.

Archivist's perspective

This note, signed and dated "8.1.47" p. 57, illustrates Magès' method: settling a technical dispute by confronting the points of view of engineers (Chataigne, Boussu) with rigorous external expertise (Michelin). It testifies to a phase where rubber suspension solutions were still being explored in parallel with the hydropneumatic path, which Magès had already been developing since 1944 and which would subsequently prevail.

The handwritten annotation "1 copy given to Mr. Cadiou" p. 57 confirms the controlled distribution of this type of technical summary within the team. This document dialogues with other pieces in the same file, such as the Summary of folder 41-59, which precisely mentions rubber among the paths explored, or the BEA 1945 work programs, dealing with suspension tests.