When I bought a new bike my LBS put grease on the part of the seatpost that inserts into the frame.

Why is this? Surely it can't seize?

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    Surely it can seize. – Stephen Touset Sep 7 '12 at 9:16
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    They did it because they were being thorough. – Daniel R Hicks Sep 7 '12 at 9:36
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    @StephenTouset I should explained further that my seat post is coated in something black and also the frame is painted. If they were both bare metal it would be understandable – Mark W Sep 7 '12 at 13:51
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    The inside of the seat tube is almost certainly not painted, and any substance your seatpost is likely to be coated in to make it black will almost certainly not prevent seizure. – Stephen Touset Sep 7 '12 at 21:47
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    The black coating on an aluminum seatpost is probably black anodizing, which is only atoms thick. – Daniel R Hicks Sep 8 '12 at 0:40

On old steel bikes a good reason would be to prevent the seatpost from rusting onto the frame, thus preventing saddle height adjustment.

More recently I understand from my LBS that you do it to prevent water from seeping into the frame along the (imperfectly sealed) seatpost. Depending on your frame you could end up carrying one or more kilo's of water with you if you have a lot of rainy rides.

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    Aluminium alloy seatposts can seize too. I'm not sure whether it only happens if the surface oxide layer is damaged during insertion, but it definitely happens. – Useless Sep 7 '12 at 12:46
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    Now the water point is a good one. I often had water inside the frame on my old bike. – Mark W Sep 7 '12 at 13:52
  • Wouldn't it be more effective an apropriate seatpost collar cover? – Jahaziel Sep 7 '12 at 14:18
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    The metals oxidize at different rates and can create a very strong bond with the newly formed metal compound created by the freshly oxidized molecules. – Matt Adams Sep 7 '12 at 16:40
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    en.wikipedia.org/wiki/Galvanic_corrosion --- corrosion is likely at seams between aluminum and steel (as well as between different aluminum alloys). – WTHarper Sep 7 '12 at 19:16

I present you with evidence that seatposts can, in fact, seize!

Alloy and steel seatposts are both perfectly capable of seizing to both alloy and steel frames.

IIRC, carbon seatpost/frame combinations may need carbon assembly paste for almost the opposite reason: to increase friction and reduce the torque required for a good grip.

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  • A good answer, but im afraid the other covered more reasons. – Mark W Sep 7 '12 at 13:53
  • I followed the link expecting something better than a generic list of search results. – amcnabb Sep 7 '12 at 19:17
  • If you have a better link to illustrate how commonly this happens (and is discussed), I'll happily edit it in. – Useless Sep 7 '12 at 23:59

In addition to the seizure problem that others have mentioned, greasing the seatpost also prevents another problem: creaks.

I once spent weeks chasing down a creak that I initially thought was coming from the crank or bottom bracket. After disassembling and reassembling all that stuff, then tightening almost every single bolt on the bike, someone suggested that the creak could be coming from the seatpost rubbing inside of the seat tube and I should grease it. Bingo! Problem solved.

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  • Frame and post material matters. With metal, bonding or rust can occur
  • Carbon posts can need the friction because they tend to be a smoother material
  • There can be imperfections that are too small for us to notice, so the grease can help to fill the gap for the seatpost retention to work
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That bit of grease on the seat post into the seat tube will help "seal" the seat tube from getting any moisture into the bike frame. It may also keep the seat post from seizing in the seat tube. It does happen if the seat isn't moved for years.

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  • It also will make the seat a lot easier to adjust in the near term, while "fine tuning" the bike. – Daniel R Hicks Sep 8 '12 at 0:39

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