Page 1 of 3

Seat pin slippage

Posted: 5 Sep 2020, 1:30pm
by terry_on_a_bike
How do I stop my seat pin from slowly going down during a ride?
I've got a Mercian 531 frame, the aluminium seat pin is the correct size (27.2), it's good quality (Thomson) and it's a nice firm sliding fit in the seat tube.
I have de-greased and cleaned the seat tube and the pin, there's no grease or other contamination on it.
I've tightened the clamp as much as I think is sensible, and the clamp isn't bottoming out.
It still goes down about a millimetre or so on every ride.
I can't think of anything else to do, in desperation I'm thinking about Loctite, a fellow cyclist has suggested carbon assembly paste.
Any ideas please?

Re: Seat pin slippage

Posted: 5 Sep 2020, 1:42pm
by Steve
Maybe just be a bit more assertive in tightening the bolt. The worst thing that could happen if over-tightened, would most likely be the bolt shearing.

Re: Seat pin slippage

Posted: 5 Sep 2020, 1:53pm
by Jdsk
Could you add a photo showing the bits unassembled, and another with them assembled and fitted and showing the gap.

Jonathan

Re: Seat pin slippage

Posted: 5 Sep 2020, 1:56pm
by chris_suffolk
Mine used to do the same, despite being all parts supplied as new with the bike.

I ended up making a tiny shim from an old Coke can, so slip under the clamp / collar as it seemed (despite being fully tight) that the collar was just that tiny bit too big.

I also have to tighten way beyond what I think it should be. I've not got a torque wrench, but it's marked up as 15nm, which is a lot.

Re: Seat pin slippage

Posted: 5 Sep 2020, 3:11pm
by cromo
I had the same problem and found it very annoying! I did not want to use excessive force on the seatpost bolt. So, I purchased a seatpost clamp (30.0 mm) and a seatpost shim (27.2 to 30.0mm) and cut the shim to fit inside the seatclamp. This extra clamp was then tightened around my existing seatpost so that it was in contact with the top of the bike seat tube. Problem solved!

Re: Seat pin slippage

Posted: 5 Sep 2020, 3:12pm
by rogerzilla
I also have a slightly undersized seatpost (because a previous owner polished the scratches out of it, making it a 27.05 instead of a 27.2). But it's a nice Campag aero post, so it stays.

I second the recommendation for a Coke can shim, which will barely show (see photo). Don't muller the seatbolt. You can cause far more expensive damage. Framebuilders often have to replace warped or chewed seat lugs, and it always requires a respray.

Re: Seat pin slippage

Posted: 5 Sep 2020, 3:14pm
by Jdsk
I believe that it's conventional at this point to refer to The Zen Shim Question:
http://www.hilarygallo.com/the-zen-shim-question/

Jonathan

Re: Seat pin slippage

Posted: 5 Sep 2020, 3:18pm
by Marcus Aurelius
Hairspray is surprisingly effective.

Re: Seat pin slippage

Posted: 5 Sep 2020, 3:19pm
by tim-b
Hi
Be careful because you could end up promoting corrosion if you degrease and leave it that way. If the seat tube is at the high end of tolerance and the the seat post is at the low end then you could have a problem, can you get them measured?
I'd try a carbon assembly paste which will hopefully prevent corrosion while promoting stiction, Motorex do a smooth one if you'd rather not use a gritty paste
If that fails then either try a separate collar around the seat post (a Jubilee clip will do to test the theory) or try a couple of different seat posts
Tightening the bolt correctly is fine, but I'd avoid over doing it
(EDIT two others posted as I was writing)
Regards
tim-b

Re: Seat pin slippage

Posted: 5 Sep 2020, 5:35pm
by peetee
If you are sure that the post is a good match for the seat tube then try a bit of carbon assembly grease. It is gritty and designed to give a bit of extra assistance to carbon interfaces to prevent over tightening and damage.
Incidentally, because seat clusters can distort during the brazing process frames are normally reamed out to 27.2mm but this is not universal. Some may accept 27.4mm seat pins. Good luck finding one of those to try.

Re: Seat pin slippage

Posted: 5 Sep 2020, 6:03pm
by De Sisti
A jubilee clip around the seatpost, so the the clip rests on the top of the seat tube.

Re: Seat pin slippage

Posted: 5 Sep 2020, 6:32pm
by fausto99
De Sisti wrote:A jubilee clip around the seatpost, so the the clip rests on the top of the seat tube.

Yuk!!!!! :shock:
Go with the gritty grease designed for carbon seat pins.

Re: Seat pin slippage

Posted: 5 Sep 2020, 10:47pm
by martinn
I had the same issue with a 725 tube, I cleaned the seat post, de-greased, then tightened the nut up as hard as i could,
Still slipped, But on trying to tighten it further to get home,i managed to round the hex bolt,
I then cut and drilled out the remains of the bolt, cut a new bolt to the right length, tried it in the seat post collar first, greased the threads well, reassembled, with a small amount of carbon assembly paste, and its not slipped since.

Martin

Re: Seat pin slippage

Posted: 6 Sep 2020, 5:38am
by Valbrona
Get some carbon jelly.

Re: Seat pin slippage

Posted: 6 Sep 2020, 5:51am
by pwa
Quick question.

Two things go wrong with seatpost insertions. One is that they keep slipping. The second is that due to corrosion the seatpost freezes solid and won't come out. Carbon paste sounds good for preventing slippage. But does it offer any protection against corrosion if used to stop a metal post slipping in a metal tube? If not, it is no use. It will just swap one problem for another that is even worse.