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I have a Cannondale Synapse (2015 model), and have swapped the stock Cannondale seat-post for a Thomson Elite inline seat-post. However, I am experiencing problems with the seatpost slipping, even when tightening to 5Nm, as specified on the Cannondale frame. The seatpost clamping mechanism is an internal two-wedge system.

The frame is carbon, and the seatpost is aluminium. I am using carbon paste, but this does not seem to have truly solved the problem.

I am considering adding a Thomson seat post collar directly onto the seat post, but am nervous about adding extra downward stress onto the carbon frame.

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You would be best served by first going to you dealer for advise. I am unsure why you would change the seatpost, as this may provide more salient information regarding your problem.

That said, if you really want to use the seatpost mentioned, it sounds undersized. You can buy seatpost specific shims. I again would recommend going to a LBS as they have the ability to best gauge the correct size shim, and order it for you. CF frames do not handle stresses that they weren't designed for well.

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  • I changed the seat post because the Cannondale one has a set back, but I need an inline one to get the correct fore/aft for my figure.
    – ocharles
    Commented Aug 10, 2016 at 16:11
  • Definitely go to the shop where you bought the bike. You can change out the seatpost, and they often will give you in store credit for items that you may want or need..
    – user26705
    Commented Aug 10, 2016 at 16:59
  • Seat post shims are usually not recommended on carbon frames.
    – mattnz
    Commented Aug 10, 2016 at 21:18
  • There is no engineering reason that one could not use a properly sized shim between cf and alloy surface. Overtightening an undersized seatpost to a cf frame is a real danger. Using a shim, be it made of alloy or carbon fiber, spreads the compressive forces equally from a seat post collar.
    – user26705
    Commented Aug 12, 2016 at 5:41

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