Seat posts - that sinking feeling!

[Darkman]

Hello.

Went out on the bike today and noticed that my seat post seemed quite low. Pulled it back up and rode on, only for it to slowly sink again over the space of a couple of miles. Plodded on regardless, pulling it back up about half a dozen times over a 27 mile ride.

Frame is aluminium, dunno about the seatpost but it isn't carbon.

It does seem counterintuitive greasing something to make it stay put, but is that the answer? I'm 6ft3 and feel like a right berk riding a bike and almost kneeing myself in the face.

What's the right process? Do I degrease first?

Is there anything else I should do to stop this happening?

Ta!

[slowster]

I would say grease is neither the answer nor the solution. The post should be greased to prevent it seizing, and although anti-seize is probably usually a somewhat better product for that purpose, if your post is slipping it likely won't make much difference which you use.

Carbon paste however might make enough of a difference (carbon paste is grease with small particles of grit or similar added to increase the friction, and its use is not limited to carbon parts).

However, in my experience carbon paste is not as effective as stopping a seat post slipping as a much stronger seat clamp which permits more torque to be used on the bolt. I and others on this forum have used the Surly Constrictor seat clamp, which has an M8 bolt, allowing a lot more torque than most other clamps. Unfortunately it is only available in 30.0mm and 33.1mm seat tube sizes, and availability is currently poor in the UK, e.g. SJS only have a single 33.1mm in stock.

The JTek brand seat post clamp which SJS sell looks like another good option, but is only for 27.2mm seat posts in 31.8mm seat tubes.

Another option which has been suggested in previous threads on the subject is to fix a bottle mount clamp on the seat post just above the clamp.

https://www.sjscycles.co.uk/seat-posts/ ... amp-black/

https://www.sjscycles.co.uk/seat-posts/ ... to-318-mm/

https://www.sjscycles.co.uk/seat-posts/ ... age-mount/

[cycle tramp]

Yes, had it myself on several bikes my solution was to make a shim for the seat tube out of a long length of plastic cut from a plastic bottle - the shim then fitted between the seat post and the inside of the down tube, and then the seat bolt was tightened.

[thirdcrank]

Is this a new bike? If not, has something been changed which has caused it to happen?

[jb]

Hello.

Went out on the bike today and noticed that my seat post seemed quite low. Pulled it back up and rode on, only for it to slowly sink again over the space of a couple of miles. Plodded on regardless, pulling it back up about half a dozen times over a 27 mile ride.

Frame is aluminium, dunno about the seatpost but it isn't carbon.

It does seem counterintuitive greasing something to make it stay put, but is that the answer? I'm 6ft3 and feel like a right berk riding a bike and almost kneeing myself in the face.

What's the right process? Do I degrease first?

Is there anything else I should do to stop this happening?

Ta!

Has this just started happening? Sounds like the clamp has bottomed out and can't close up further. Is the clamp separate or part of the frame.
Assuming the post is the right size you may have to open the slot out.

[Darkman]

Yeah it's a new development - it's been fine for as long as I've had the bike (late 2019). Clamp (QR) is separate from the frame.

I've never regreased the seatpost, even though it was greased when I bought it - seems to be many arguments for and against use of grease on seatposts so other than periodically checking it wasn't seized, it's been left alone.

[jb]

Grease shouldn't make any difference to a properly clamped post. Check that the clamp doesn't have a hairline fracture causing it to stretch rather than tighten up, and that the quick release leaver or bolt is not seizing up just before full clamping force is applied. I had one once that felt tight but actually was just barely clamping causing a similar issue.

[mattsccm]

Clamp isn't "stretched" is it?

[Darkman]

Thanks for the replies.

Clamp all looks good - no cracks (that I can see), wouldn't say it's stretched. I might replace it anyway - not really a fan of QR clamps.

It was a bit grimy under there so I've had the seat post out, degreased it all, cleaned out the seat post tube (it felt a bit gritty, and there's some scratching to the seat post), tiniest amount of regreasing with Crystal Clear grease (claims suitable for seatposts) and retightened the clamp.

All looks and feels solid but then, it did before. Guess I won't know until I've given it a test ride.

[drossall]

Coke cans provide good shim material.

[Darkman]

Yeah but what if I lose it down the tube?

[drossall]

Why would that happen? If you do, just turn the bike upside down. It's not going to go into a sealed bearing unit anyway.

If it's so loose that it can slip down, Coke can is the wrong shim material!

[iandriver]

Lock rings like this are available like this https://www.google.com/search?q=M+Part+ ... aYCQ4:1716

I've not used this one but have had them supplied with carbon posts in the past. I use one on a Titanium frame that is almost impossible to tighten conventionally as it's so darned stiff.

[fausto99]

Assuming the clamp and seat tube are OK, why not try some of the gritty grease that they sell for use on carbon fibre framed bikes? e.g. Park tools SAC-2

[NUKe]

Yeah it's a new development - it's been fine for as long as I've had the bike (late 2019). Clamp (QR) is separate from the frame.

replace with a bolted collar