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enter image description hereWhen I last got on my Specialized Stumpjumper the seat wanted to spin freely. The seat post was firmly clamped in the frame. With further inspection the seat is spinning where the seat clamp goes into the seat post. Is that a connection that is just glued together? If so, what do I use to re-glue? It believe its a carbon fiber post.

The image below is not from my bike, but does show that junction where it spins. Any help would be appreciated.

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    I would guess that it's swaged or glued, and that the bond has been broken by the seat getting bopped somehow. You might be able to glue it, or it might (depending on the materials involved) be possible to drill a small hole through the tube and partway into the clamp base, then insert a small self-tapping screw. – Daniel R Hicks Apr 11 at 0:37
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    Of course, if you spent the thousands of dollars that a Stumpjumper goes for, it's maybe worth it to just buy a replacement part. And Specialized is a fairly responsible and responsive operation, so you may want to reach out to them. – Daniel R Hicks Apr 11 at 0:40
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    Imagine, you are shredding your favorite down hill trail, just past the point of no return for that super scary jump you love so much..... the seat falls off, leaving just the seat post, the rim of the open ended tube pointing up .... Once your sphincter has relaxed, go buy a new seat post... (Check warranty details first ) – mattnz Apr 11 at 1:39
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    Just replace the seatpost - its a critical structural element and is not supposed to turn. – Criggie Apr 11 at 4:45
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    Can you remove the aluminium part from the carbon tube to properly inspect it? Any signs of a crack in the carbon tube? Signs of insufficient epoxy glue? – Michael Apr 11 at 7:29
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Any time that different materials are joined together, there is a small potential for failure. I emphasize that if the manufacturer knows what they’re doing, the probability is small. However, if I understand the OP correctly, they may be one of the unlucky few.

The arrow in the photo on the original question seems to point to an aluminum structure that contains the clamping mechanism. Most likely, this structure was bonded (I.e. attached with industrial strength glue) to the rest of the carbon seatpost. Other methods of construction exist for carbon posts that don’t rely on that large an aluminum structure being bonded to a carbon tube. However, carbon to metal bonds or metal to different metal bonds are pretty commonplace. For example, many current generation cranksets have a steel or aluminum spindle bonded to aluminum or carbon crankarms. Carbon frames sometimes have aluminum bottom bracket inserts bonded to them. The list could probably go on, and I’m not a bike engineer (or any sort of engineer), so I’ve probably left a few off.

Nevertheless, if I understood the question correctly, the OP probably has a seatpost where an aluminum insert has un-bonded from a carbon tube. This was probably glued together. However, it’s almost surely not with glue you can just buy from Home Depot, Lowes, or another home repair store. This would be a warranty issue. If the OP isn’t the original owner, they should just buy another seatpost. To emphasize, the aluminum part was originally bonded with industrial-strength glue. Not consumer-strength glue. Remember that the person sitting on top almost always has at least 100lbs (45kg) of weight, plus this is a mountain bike that is being operated on rough terrain. The glue needs to be very, very strong. This is almost surely beyond the average consumer’s capability to repair. If any bicycle engineers want to correct me, please do so.

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  • Isn’t “industrial strength glue” just some good epoxy? – Michael Apr 11 at 7:27
  • Industrial supplies aren't sold in consumer outlets. Some "good epoxy" is almost sure that is less ambitious than an industrial supply. – djnavas Apr 11 at 7:50
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    @Michael it could be. But i would not want to take the chance that the epoxy I got isn’t strong enough, nor would I want to take the chance that I hadn’t properly prepped the surfaces. Also, that carbon section could now be damaged from the aluminum head rattling around in it. – Weiwen Ng Apr 11 at 12:11
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    Seat posts aren't that expensive, except maybe for custom ones linked to one particular and expensive bike. They are also much too safety relevant for some DIY repair. Get a new one. – Carel Apr 11 at 13:57

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