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I recently got a new ride (Rossin track frame with Miche seatpost) and after the first ride I noticed that the seat post had lowered a bit. I thought fine, the clamp was a bit loose - but it wasn't. When I tried to tighten it more it wouldn't budge - then the bolt simply broke. Fair enough - I'll get a new bolt, but my question is why the tube would have moved if everything was really tight?
 

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check seatpost size first

Be sure the seatpost is the correct size for the seat tube. I think an LBS will have a tool that easily measures the seat tube on the frame, which will give the seatpost size you need.

If the seatpost is the correct size, I recommend a "Coke can" shim. Cut a section of aluminum from the can and place between post and frame. This has worked for me on my steel bike with a new post that wouldn't hold. Start with a section that almost wraps the seatpost completely. Then trim it as necessary to get it to fit in the frame.

stratoshark
 

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With the seatpost clamped in place, take a look at the slot in the seat tube. If the seatpost is the right size, the sides of the slot will be nearly parallel. If the sides of the slot aren't parallel, you might well benefit from a slightly larger seatpost.
 

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It is also possible that the seat collar bolt was beginning to seize so that it was getting very hard to turn from galling, and not from coming up to torque by pulling the seat collar together and therefore not putting insufficient pressure on your post. Just a thought FWIW.
 
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