I'm swapping Ultegra parts from a GT to a Specialized Roubaix. Dumb me, I figured most downtubes would be the same diameter just for the sake of mounting FD's. Now I find there are two common sizes - 34.9 mm and 31.8 mm. ARGH! I don't want to buy a new FD.
Is it possible to shim it?
A rubber inner tube doesn't work - the FD squishes it under load and the whole FD twists. Obvious after I tried it.
Is it worth trying to fabricate a metal shim (1.5 mm thick)?