French Patent # 934,933 - Spirax

French Patent 934,933 - Le Spirax main image

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.