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I noticed that that my saddle started moving down when riding (maybe 1cm/hour). When cleaning the inside of the seattube I noticed that the part at the height of the clamp became smooth, compared to the rest that has a texture designed to prevent the seat post from sliding down (see photo).

The seat clamp is a quick release one, installed by the shop. The height of the collar is 15mm.

Besides adding seatpost and seattube cleaning to the maintenance routine, and applying an "assembly paste", are there other techniques to increase the friction at this interface so that the seatpost remains in position?

worn seat tube

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  • What seat clamp? Do you have any way to check the diameter of both the seat tube and seatpost? That smooth section appears to be the contact area, which looks to be too small for a properly matched tube and post.
    – MaplePanda
    Dec 31, 2021 at 9:53
  • @MaplePanda Updated the question for the seat clamp (quick release, 15mm). The smooth section is indeed the contact area. Given the phenomenon is recent, I would think it's wear.
    – Rеnаud
    Dec 31, 2021 at 10:09
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    Use a normal seatpost clamp with a bolt and tighten to ~5Nm (or even more, if you have only metal components). There are also clamps which tighten to the seatpost directly (in addition to compressing the seattube): bike-components.de/en/KCNC/…
    – Michael
    Dec 31, 2021 at 11:10
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    If the fit is loose, see if you can wrap a piece of aluminium can around the seatpost.
    – Michael
    Dec 31, 2021 at 11:13
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    The striations inside the top of the seat tube are just the marks where the frame has been reamed - this is necessary because the tube distorts where it's welded to the top tube. The usual reason for a slipping seat post is either that it's chromed steel, or it's undersized.
    – bertie
    Dec 31, 2021 at 22:21

2 Answers 2

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Two possible remedies:

The easiest is to wipe off all of the grease and degrease with some solvent. Then instead of mounting the post with grease use carbon mounting paste. The gritty particles in said paste will make slippage more unlikely.

or

Use a double clamp with two clamping screws, although they may be hard to come by these days The top one grips the seatpost and the lower one sits on the seat-tube where the standard clamp used to be. It was the solution that worked on my CX-bike that also suffered similarly from a slipping seatpost (frame Al - post C).

Considering the recent difficulties in the market of bicycle parts, I'd go for the first solution first.

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Visually inspect the seatpost clamp, specifically the bolt and threads. If it is stretched or thinner at any point, discard and replace.

I've seen clamps that have been overtightened, where the bolt is distorted and will stretch under riding conditions. This reduces clamping pressure and allows the seatpost to drop slowly.

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    I doubt a QR clamp would be able to tighten enough to exceed the bolts yield point, but a cheap clamp may have a bolt with a low yield point or elasticity. With these kinds of problems, unless I know the history, I presume its been tightened beyond yield point and replace the bolt (or clamp). I also use carbon paste on seat posts as a matter of course nowadays for this very reason.
    – mattnz
    Dec 31, 2021 at 23:14

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