Hi all, i was just wanting to know if there is any possible way to adapt a normal crank (39/53 chainrings) to take a smaller inner chainring (34). Is there any company or seller that makes an adaptor that can allow this to happen.
I don't see how it would be possible to decrease the bolt circle diameter without impacting the chain line and/or the spacing between chain rings. The smallest chain ring possible on a 130mm bolt circle is 38.
It might be possible (but unlikely) to have your existing crank machined, but it would probably cost more than buying a compact crank (and possibly new BB if needed).
If you are using a vintage bike, a few older cranks had 110 mm bolt circles, the same as modern compact cranks and if that is the case you just need to buy the appropriate chain rings... that i what I did with an old Sugino crank.
I am sure you can find a more complete answer than this on Sheldon Brown's site.