This is the first of the patents relating to the definitive Spirax design (see also French patent # 1,020,378). The derailleur is driven inwards by the cable rotating a collar with a pin running in a spiral in much the same way as in the classic Le Cyclo design. However the derailleur only needs a single cable, as a large flat clock spring (marked 4 above) both drives the derailleur towards the top gear position and provides the chain tension.

The derailleur is shown here mounted on a bracket brazed onto the chainstay.

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• Publisher: L’Institut national de la propriété industrielle
• Inventor: Raymond-Eugène Bon
• Country: France
• Date: French application October 1946
• Derailleur brands: Spirax
• Derailleurs: Le Spirax Cyclo-Tourisme