hobkirk

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)?

Thanks.

mechBgon

Shims will work. In fact, your LBS may have some commercially-made shims for that purpose. The metal pieces from a set like this would do the job (do NOT overtorque the clamp bolt on your Roubaix, however): https://www.jensonusa.com/store/produ...leur+Shim.aspx

abdon 

I have a folded playing card shimming the front derailleur on my latest build (A magic the gathering card, can't vouch for any other type of playing card )

The derailleur is 31.8, the tube is 28.-something. It is on my to-do list to buy a real shim for it, but because this is one of those parts whose markup is ***tarded ($4 for 2 cents worth of aluminum, plus another $4 for shipping) I'm just waiting to find a cheap set. With a bit of luck one of these days I'll buy something at a LBS and they will let me have a used shim for a buck or two.

operator

I shimmed a 31.8 derailleur clamp to 28.6 when I bought the wrong size years ago, with a couple layers of a heneiken can. Worked great. Looked hack as **** though.

FastJake

You can shim with anything as long as it holds and doesn't F up your paint (if you care about that.)

By the way, I think you mean seat tube A FD on a downtube would be quite the sight.