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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.
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.
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.
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.
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.
