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How much shim is TOO much?

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How much shim is TOO much?

Old 07-26-10, 04:36 PM
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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?
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Old 07-26-10, 05:08 PM
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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.
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Old 07-27-10, 12:39 AM
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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.
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Old 07-27-10, 03:27 AM
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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.
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Old 07-27-10, 02:31 PM
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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.
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Old 08-02-10, 05:57 AM
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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......
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Old 08-02-10, 08:59 PM
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It may look 'ungainly' at rest, but when you het it up to 20mph, nobody will notice!
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