French Patent # 849,729 - Spirax

French Patent 849,729 - Spirax main image

This is the second patent for a Spirax derailleur. It’s a development of the design featuring in French patent # 796,583, again the cage of the single pulley travels along a spiral groove. Again this spiral moves the pulley backwards and upwards as it moves inwards. And again this is counter-intuitive as it would maintain a more consistent chain gap if it moved the pulley cage downwards and forward. However by 1939 Spirax had at least added a shaped pusher plate that effectively keeps the chain gap more even.

Other notable features of this patent are:

  • Raymond Bon’s address is now given as La Garenne, Paris.
  • The gear hanger is now conventional.
  • The tyre shown in the drawings is notably narrow - possibly 25mm wide?

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