Carbon Paste: aluminum frame + aluminum seatpost

ghammer · #1 02-24-2021, 04:53 PM

My son has one of those popular SE bikes (basically an adult size bmx). However, the seatpost does not stay put. It's not necessarily slipping, but it moves side to side. Question: is carbon paste ok to prevent aluminum seatposts from moving?

Thanks,

G

prototoast · #2 02-24-2021, 04:58 PM

Yeah, it's fine. It's basically just grease + sand. Adds some friction to the assembly.

jimoots · #3 02-24-2021, 05:03 PM

When you say the post moves from side to side... that makes me wonder if the frame or seatpost is out of spec?

I'd take back to the shop, if an option.

If bought second hand I'd try making a DIY shim from a coke/beer can and seeing if that makes the difference.

As above no issue with carbon paste in non-carbon applicatins.

Charles M · #4 02-24-2021, 05:06 PM

Quote:

Originally Posted by ghammer

My son has one of those popular SE bikes (basically an adult size bmx). However, the seatpost does not stay put. It's not necessarily slipping, but it moves side to side. Question: is carbon paste ok to prevent aluminum seatposts from moving?

Thanks,

G

Physics wise this doesnt make sense. if by side to side you mean twisting (the saddle nose moves side to side) it should also go down when weighted (unless it's bottoming out on something).

There should be no tilting (side to side across the frames center line).

All in all, this is not something I would fix with assembly paste.

ghammer · #5 02-24-2021, 05:09 PM

No tilting. Twisting.

Charles M · #6 02-24-2021, 05:14 PM

Don't try and fix that with paste...

Have someone take some measures (and make sure it's not the seat post to saddle clamp twisting.

Nothing should twist and not go downward while riding. there's little twisting force and all downward force at play. something doesnt add up.

dddd · #7 02-25-2021, 01:43 AM

I suspect that the saddle with post is twisting in response to lateral impacts, since there is considerable leverage due to the length of the saddle.
I've had this occur even when the post didn't tend to slide down, it just ended up twisting off-center every so often due to the saddle getting bumped.

This is also a common problem with the saddle clamp slipping on a cheap style "plain" seatpost, only the post itself isn't twisting just the saddle and clamp.

Lastly, seatposts with heads that plug into the post can loosen, sometimes the bonding adhesive fails, and can be caused by cracks which form at the top of the post itself.