How much shim is TOO much?
 

I picked up a nos SRAM Force front derailleur with a 35mm tube bracket (at $27 too good to pass on) with the intention of installing it on my old Raleigh frame which has a 28mm diameter seat tube. 7 mm doesn't sound like a lot until you attempt to match up a 35mm ring on a 28mm tube....urgh! Is it logical to attempt to set the fd in place with a shim to make up the difference? I have the ability with available raw materials along with my Dremel tool to manufacture a shim, but am concerned re: appearance and functionality. Seller will accept it back on my nickle for shipping (which is fair), but doesn't have any Sram with 28mm bracket to replace it with. This is a nice unit, hate to send it back... On the other hand, I suppose I could list it on ebay or local CL. Suggestions?

I have seen seat post shims to go from 25.4 to 34.9, so if 9.5mm is OK for a seat post (which seems to have more force applied with rider weight), I would think 7mm should not be too much for a FD. The shim needs only be 3.5mm, since it wraps around the seat tube, and with both sides used, will make up the 7mm.

28.6mm clamp derailleurs are becoming less and less common anyway. I don't think SRAM makes any. It's very common for derailleurs to come with 34.9mm clamp and a couple of shims to adapt to 28.6 and 31.8 seat tubes. a little ugly but it works fine.

You can use a slice of 1" schedule 40 PVC pipe for the shim, should be about the right thickness.

There is very little load on the front derailleur, unlike the rear. Shimming in general causes no structural problems with most anything on or off a bike. Older bikes had 1" steer tubes. We shim them out to 1 1/8" and use the 1 1/8" stems. Keep in mind that on most carbon bikes the cage is held on by 2 rivets.

On an old Raleigh it's probably not a issue, but make sure you have rear wheel clearance of an extra 3.5mm at your derailleur mounting height, particularly when removing/replacing the rear wheel for service.

Thanks for the information; I was fairly certain that it wasn't going to be an issue structurally, but I'm still not convinced that big 35 clamp is not going to look a little 'ungainly' on the 28 frame......

It may look 'ungainly' at rest, but when you het it up to 20mph, nobody will notice!