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Old 07-21-10, 04:59 PM   #1
Seijun
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Getting a 31.8mm clamp to fit a 29.8mm seat tube

So I need a 29.8mm seat tube clamp with braze-ons for a bike rack. Only one company makes such a thing in that diameter, but the clamp itself looks rather flimsy. Two or three other companies make much sturdier-looking clamps but only in 31.8mm and 34.9mm (2 or 5 mm too big). I found a seat tube shim to increase the outer tube diameter from 29.8 to 34.9, but I can't find any to convert from 29.8 to 31.8. I have two options:
1) Buy the bigger 34.9mm clamp and the 5mm thick shim to go with it
2) Buy the smaller 31.8mm clamp and try to find a 2mm thick piece of metal to make my own shim.

Is either one of these a better idea than the other?
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Old 07-21-10, 06:16 PM   #2
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Quote:
Originally Posted by Seijun View Post
So I need a 29.8mm seat tube clamp with braze-ons for a bike rack. Only one company makes such a thing in that diameter, but the clamp itself looks rather flimsy. Two or three other companies make much sturdier-looking clamps but only in 31.8mm and 34.9mm (2 or 5 mm too big). I found a seat tube shim to increase the outer tube diameter from 29.8 to 34.9, but I can't find any to convert from 29.8 to 31.8. I have two options:
1) Buy the bigger 34.9mm clamp and the 5mm thick shim to go with it
2) Buy the smaller 31.8mm clamp and try to find a 2mm thick piece of metal to make my own shim.

Is either one of these a better idea than the other?
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Old 07-21-10, 06:26 PM   #3
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I prefer a smaller shim whenever possible. It shouldn't be THAT hard to find someone with a lathe to machine up a nice spacer for you to use that has the required 2mm or .078 walll thickness. Heck, I'd offer to do it myself but I'm in the middle of moving house and for various reasons related to the new house priorities it'll be about 3 months before my lathe is able to turn in anger again.
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Old 07-21-10, 08:16 PM   #4
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I think you need to re-check your calculations. A 31.8mm clamp is 2mm larger than a 29.8mm clamp, but a shim to work will be 1mm thick, as we are talking about the diameter of clamp, so the shim will be around both sides.

I have a 26.8mm seatpost that I wanted to use with a 27.2mm seat tube. .4mm shim, right? WRONG! I got some .2mm shim stock and made a shim that works perfectly. Again, half the required difference, since both sides are in play.

To use a 34.9mm clamp, the difference is 5.1mm, so shim should be 2.55mm thick. I think you will have better luck finding 1mm shim stock.

Last edited by badamsjr; 07-21-10 at 08:19 PM.
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Old 07-21-10, 08:48 PM   #5
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Oh, you're right-duh!
However, the smallest seat clamp shim I could find was still 29.8 to 34.9, so 2..5mm thick. Couldn't find any as thin as 1mm.
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Old 07-21-10, 11:22 PM   #6
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and P clamps will go around the seat stay tubes to mount the rack on.

Get a Vernier caliper that measures to .0001"/0.04mm and you will be able to measure
things like beer cans to see if they are usable as shims
there are shims of brass made in many thicknesses AKA shim stock.
If you cannot find any .. you just haven't looked hard enough.

Last edited by fietsbob; 07-21-10 at 11:29 PM.
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Old 07-21-10, 11:58 PM   #7
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ahh, so that's what you mean by "shim stock".
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Old 07-22-10, 12:19 AM   #8
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Shim stock is available from online sellers, but you may not need it in the large quantities they normally carry. You could try a good auto repair shop to see if they might have the size you need (thickness). I bought a 20ft roll (by 4") to have on hand for future use, but not everyone will want to do that.
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Old 07-22-10, 04:23 PM   #9
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A good source of small pieces of shim stock are feeler gauges. Pick up cheap ones from someplace like Harbor Freight (or your local equivalent) and take the gauge apart. Find the appropriate sized feeler and use snips to cut as appropriate. The feeler should be pretty close to the right size, so not much trimming should be necessary.
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Old 07-22-10, 04:35 PM   #10
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My boss has offered to take my existing too-small clamp and bore it to the correct size. Will see if that works.
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Old 07-22-10, 04:36 PM   #11
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myrridin: +1 Good tip.
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