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It looks like my saddle/seatpost is slowly sliding down into the frame, so every 2–3 months I have to move it up by 1–2cm. Yet I tighten the clamp pretty tight, so I can't move the saddle by hand.

Is this a common thing? Should I fix something?

I weigh about 55kg and cycle on the road, about 50km per week. I try to lift my weight off the saddle over bumps. The bike's a hardtail.

  • How heavy are you (roughly)? What terrain do you ride over? Do you take your weight off the saddle for rough bits? How much riding is that in terms of distance? – Chris H Jan 19 '18 at 12:51
  • I ask partly because mine does the same but only when it takes some hits, and I'm not light. – Chris H Jan 19 '18 at 12:54
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    Time to wheel out the torque wrench? – David Richerby Jan 19 '18 at 13:40
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    It's certainly not unheard of. But clarify what precisely is "sagging" -- is the seat post sliding down into the sat tube farther, is the seat sliding along on the "rails" that attach it to the post, is the pivot that adjusts seat angle slipping, or what? – Daniel R Hicks Jan 19 '18 at 13:44
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    Are you positive that the seatpost fitted is the right size for the frame? There are a dozen different sizes, some varying by only 0.2 mm. – Criggie Jan 19 '18 at 20:24
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It may seam counter-intuitive, but in the case like you describe a little grease may be the solution.

NOTE: If you have a carbon frame or seat-post, carbon specific grease and a torque wrench should be used. It's very easy to destroy a carbon frame or post by over-tightening when installing parts.

  1. Remove the seat post and clean it and the inside of the frame thoroughly with a little degreaser.
  2. Remove the seat post clamp if it has one (some frames have braze-ons through which the seat bolt is installed rather than a separate clamp) and clean the inside of the clamp and the outside of the frame.
  3. Apply a very thin coat of teflon or lithium grease to inside of the clamp, and to the threads of the clamp bolt. I generally do not apply any grease to the outside of the frame.
  4. Apply a very thin coat of teflon or lithium grease to the seat post and the inside of the seat tube - just enough to facilitate installation and inhibit corrosion.
  5. Install the post and adjust the height.
  6. Tighten the clamp to the manufacturers specification.

The cleaning should remove any grit or dirt that may have been working like tiny ball-bearings and allowing the post to slip. The small amount of grease on the clamp, frame and post should allow the parts to constrict and tighten evenly with out binding in one particular spot.

If, after cleaning, lubricating and reinstalling you still have a post that slips you may have a frame that has deformed, or a post that is slightly smaller in diameter than spec. You can try either replacing the collar/clamp with one that is a little "beefier" that can be tightened to a higher torque, or you use a shim like a piece of thin aluminum between the frame and the post.

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    I have it on good authority that PBR cans make the best shims. – Paul H Jan 19 '18 at 18:25
  • Not exactly grease it's a compound with "grain of sand" like stuff on it, will make post not slip that much. Also, the more length of post that you have inside the frame the better. (So if you have a 350 mm, you could change to a 400 ) – gaurwraith Jan 24 '18 at 15:09
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Ideally, the first thing to check is how good the fit is, or in other words whether tolerances are causing or contributing to the problem despite the nominal sizes matching. That way you have a better sense of what you're up against. If the post has a lot of slop with the collar loose, it's possible nothing will make it work, although that's an outlier.

Now that carbon grip is an everyday item, there's no reason not to use it in slipping post situations regardless of material. So start by cleaning everything, putting a generous amount of that on the post/frame interface, and making sure all the mating surfaces of the collar and bolt are clean and greased. That's about as much as you can do without changing parts.

If that doesn't work and it's a bolt-down collar as opposed to QR, the first thing to look at is how beefy the collar is, if applicable. The thicker and heavier, the harder it pinches the post and the less it itself flexes as the bolt is torqued. Slippage happening due to tolerances on frames with collar type clamps can often be obliterated by just putting on a stiffer collar.

If it's a QR collar, there are some bad ones of those in the world, and they can get worn/mangled with repeated hard re-tightening, so replacing them can be necessary. Bolt down ones clamp harder in general too, so if you don't care about QR then that can make sense. (The average case scenario of a slipping post in shops is it's a junky QR collar, and the fix is replace it with a bolt down one and slather with carbon grip. Usually fixes everything.)

If it's an integral clamp, or none of the above fixes it, and it's not an undersize post, then some of the more advanced, shop-tool-requiring fixes can come into play, such as reaming the seattube or knurling the post. Sometimes the seattube opening needs some reworking too.

  • Thank you for your answer, but that's perhaps not my case. My QR is in a good shape on aluminum frame. The parts are of a pretty quality ( not the best, but nice ones). – Alexander Jan 20 '18 at 18:36
  • Have you tried lubing or greasing the QR cam and seeing if that lets you clamp it down harder? – Nathan Knutson Jan 22 '18 at 17:22
  • I'm used to repair and assemble bikes, so I'm sure that it's well tightened. – Alexander Jan 22 '18 at 18:42
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Some stock seatpost clamps just do a poor job, too skimpy due to the manufacturer wanting to save a few bucks or a few ounces - I had a problem with the seatpost slipping on my otherwise excellent hybrid cruiser.

Beefy and strong generic aftermarket alloy seatpost clamps are a common upgrade, $7 USD or so on Amazon or Ebay in a variety of colors for metal frames, $20 USD if you want a name brand component compatible with carbon frames/posts.

  • Based on your usename, I've denoted the currency to be US Dollars. Problem with quoting prices is that its highly regional, and varies over time too. Example, for me, a budget BBB brand seatpost is $45 NZD ($34 USD) and a "brand name carbon seat post" could be $100-$300 NZD in early 2018. – Criggie Jan 24 '18 at 21:25

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