Amir Avitzur wrote.
Were single-sided dual derailleur levers such as Huret's Louison Bobet
model
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The small outer lever that came with the Huret Louison Bobet derailleur was used to control the tension of the chain. The larger lever selected the ratio as normal. The idea was that you could tighten the chain when riding over cobbles and loosen it when the road was smooth. By detensioning the chain there was far less friction in the transmission. Those of us that ride single freewheel will apreciate how much less resistance there is when you are not turning around those derailleur pulleys. The Simplex double RH lever was used to control front and rear derailleurs. The first Simplex front deralleur to be controlled this way was the converted Simplex 'Competition' rod operated model for which Simplex made a conversion kit that made it cable operated. I think that this was introduced in about 1950. Subsequently introduced a cable operated front derailleur (Juy 53?) which was controlled by the small RH lever. Simplex also had a detensioning devise incororated in their Juy '51 and Juy 543 rear derailleurs. However although the derailleur had two cables for changing and detensioning the cables were joined so that both were pulled by the normal RH lever. The idea was that as the chain moved to a larger sprocket the tension on the spring would be reduced keeping the tension of the chain more constant. It is an idea that I think should be incorporated into modern derailleurs. There is quite a noticable increase in resistance as the chain goes to a larger sprocket. Campagnolo addressed the problem, not too successfully in their ill fated Croce d'Aune model. Other early derailleurs had seperate levers for changing and chain tension, notably the Osgear, (Super Champion, France and Constrictor, England), the Vittoria Marguerita (Italy) and the Cervino (Italy).
Mike Barry.
Fiddling with derailleurs in Toronto.