I have an old Fuji road bike that I broke the rear derailer while on a ride last week (it was old, I am big, and I was trying to get out of the way of a car).
I took the broken derailer off, shortened the chain a few links to see if I could run 'single speed' for a bit; worked but the chain was jumping around under heavy load. Tried a few different cogs along with changing the chain length but it still jumped around...
Took the functioning derailer off my old nonused Trek mtn bike, and after a few test fits, realized it wouldn't work... unmodified. Removed the upper cog wheel in the derailer, bent the mounting tab on frame out a bit, along with a few metal tabs on the derailer so they wouldn't hit the spokes (oh god...), and hooked up the cable so I could adjust it a bit.
Lucky me it ends up with chain right in the middle of the cassete, I can't change the rear gear cause the derailer is totally on a different curve or something but the front derailer still functions beautifully, and the whole set up purrs like a kitten. So now I got a two speed bike I guess, went for a quick 2 mile test run around the block and to my surprise it runs and shifts (front derailers that is) great.
Moneys tight and I just bought new brakes and tires for this second usable bike of mine, so the fact I didn't spend any money is a definite plus. Hopefully it'll last for a few months or at least till I can convert to a single speed and/or get a real chain tensioner...
2003 KHS F20-Westwood folding & enough parts to make several more bikes...
Haha right on man! I'd totally do that if I had to. +1 on street cred'.
Just a thought - depending on where you live, there may be a bike co-op in your city. The local co-op where I volunteer sells used derailleurs for as little as $5.