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Can I fit a 34.9mm front Shimano derailleur on my steel frame? I understand that steel frames are narrower. However, is there some sort of adapter that I can use, or can I even wrap the frame around there with some inner tubes to "fatten" it up? Will this be sufficiently stable?

Any thoughts will be appreciated, thanks.

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Try it. With the proper spacer it should work. – Daniel R Hicks Sep 6 '12 at 15:35
My $0.02 on derailleur shims: I don't like them. They can make it difficult to get everything tuned and they slip in use. If it's a $$ issue, you gotta do what you gotta do...if not, spend a few bucks on the right size derailleur. – Ken Hiatt Sep 6 '12 at 21:35

You need a derailleur shim

The one linked adapts a 34.9 mm derailleur clamp to a 31.8 mm seat tube. I haven't seen other sizes but I'm sure it could be augmented/replaced with something homemade.

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