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I have a carbon bike frame that has a seat post with diameter of 31.6mm and i got a clamp that said 34.9mm but it doesn't fit on the frame (too small). There is no manufacturer details outside of the below geometry. The frame was bought secondhand and it looks like its model number FR 905 from a Chinese company called FLYXii.

I am helping friend build it.

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I measured it with a ruler and it not perfect but it looks like its 35.5mm plus or minus .2mm

  • Could you just ask what the previous owner used? – ojs Oct 3 '16 at 21:28
  • no it was never built before. – nolawi Oct 4 '16 at 14:43
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First double check that it's not a simple case of it just being tight, which is actually pretty common. Remove the bolt completely and gently wedge it open just a little bit with a screwdriver, and see if it will fit on then. Do not force it - you're checking to see if it fits correctly albeit a bit snugly.

If that doesn't work, and/or if it's just clearly the wrong size completely, you need to measure both the clamp you've got and the outside diameter of the seat tube. Ideally use a caliper. You could almost get away with just using a ruler to see what size the frame is actually "trying to be", but the problem is that frames do exist just .5mm larger than 34.9, even though 34.9 (aka 35mm) is much more common.

There are some weird, semi-proprietary diameters in the 35-36mm range that have given rise to clamps like this and this. 36.4mm is also a standard size, but as far as I've observed it's not common for carbon road bikes.

  • i bought 2 different 34.9 clamps and it doesnt fit.. even messed them up trying to widen it. – nolawi Oct 4 '16 at 14:50
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You need to measure the actual seatpost diameter with a micrometer. There are over twenty standard size seatpost clamps. The difference in size can be as little as .1mm. It is possible that it is a 31.8mm or a 32.0mm seatpost diameter. A micrometer costs about $20. If you feel it is a tool you'll never need again, bring the frame to your local bike shop ask them to measure it and buy a clamp from them. Also pick up a tube of carbon paste while you are there. The paste decreases the clamping force required to keep the seatpost from slipping.

  • i did. it seems like its not standard size – nolawi Oct 4 '16 at 15:56

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