Front derailleur clamp too big!
 

I just bought a SRAM Rival front derailleur, stupidly assuming that the 31.8 clamp would fit my seat tube. It's too big. Is there any kind of shim I can put in there? Or do I just need to buy a new front derailleur?

Just found a shim on Universal Cycles. I guess that answers my question.

I just got a Shimano XTR in 34.9mm for my bike. It came with adapters to allow it to fit both the 28.6mm and the 31.8mm. So the objects do exist. Maybe ask your LBS if they have some of these. They might give you one.

The FD is one of the cheapest components. Consider buying the correct size.

Just cut up some old inner tube and wrap it 'round and 'round until it's the right diameter. Not.

go get some metal shim stock. should be able to get it locally.

I like P7's suggestion. Seems like all the FD's I've bought have come with an adapter or two, which I just toss in my probably-never-use box.

If all else fails, Schedule 40 PVC pipe has a 1/16" wall thickness. It makes a perfect shim for a front derailleur that's one size too big. A 1 1/2" long piece works perfectly.

I bought a wrong-size front derailleur (doh!) and used a PVC shim to make it work. It's still there, working perfectly, 6 years later.