The 1912 Le Chemineau derailleur was developed by Joanny Panel, based on a prototype built by Paul de Vivie. In many ways it defined many of the attributes of a modern derailleur. It has a guide pulley that moves in and out, moving the chain across a sprocket cluster, it has a sprung arm that provides chain tension with a tension pulley on it, and it was controlled by a single cable, working in opposition to a spring.
Eerily the centre to centre distance on the pulley arm is 75mm - a measurement commonly found today. The tension pulley has 10 teeth - another recurring theme.
Unfamiliar to us today are the ideas of mounting the derailleur on the underside of the chainstay, and of using a toggle chain to pull the derailleur along a rod - but both these ideas would have been totally mainstream as late as 1960.
This pleasantly weathered example is one of three in this collection. I think that this example dates from between the other two. Its pulley arm is round in cross section and has a 'D' shaped loop at the top to take the spring. However its other parts are very similar to my first example.
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