I'm almost positive that the reason many of you lurk on here is to get info about the history of bikes. Well I admit it is one of my own reason's for trawling the post's. I came across a picture of Pierre Gallien cycling through the Alps on Flickr. I took a closer look and upon inspection saw that there seems to be some sort of chain tensioning going on? Can anyone shed any light on this.
1937PierreGallien&.jpg
The Zoomed in image:
1937PierreGallien&.jpg

Many of the early derailleur systems used chain tensioners that that extended from the bottom bracket shell. You can just discern the pulley immediately behind the chainring.

hi T-Mar, thanks for the quick reply. I was wondering if you had more info or what should i search for. i have read about these pulley's and seen a picture or two along the way but not sure where to go look again. The photo is of the 1937 Tour so that might give more specific's about what was around at the time.

After Velocio showed up Desgrange in '03, the Tour man got a real bug against variable gears.

'37 was the first year variable gears were allowed back in the ToF, and IIRC all riders were required to use Super Champion derailleurs.

Derailleurs came late (last!) to the Tour, and had been used prior to this in the classics and other grand tours. Over in the UK riders were burning up the roads (and besting derailleur equipped riders) with 3- and 4-speed Sturmey hubs which were banned from French racing.

If you're interested in this sort of thing, you need to read Mr. Berto's The Dancing Chain.

Google Vittoria Margherita derailleur, Osgear derailleur, Cervino derailleur.