Seatpost slip.

[tim-b]

Hi

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.

An aluminium seat post will corrode quite nicely inside a CF frame for the same reason as it will inside a steel frame. I haven't experienced that using Motorex CF paste (link)
"Schützt wirksam gegen Kontaktkorrosion" Effective protection against contact corrosion
"Erleichtert die Demontage schlüssiger Verbindungen" Facilitates the dismantling of coherent connections
(Thanks Google)
Regards
tim-b

[terry_on_a_bike]

Thanks everyone for all your advice. I especially enjoyed reading 'The Shim Question'. What 'The Shim Question' misses though is that there is something actually wrong with my machine, and I really want to know what that is and how to fix it properly. That is classical thinking I reckon. Although the shim would solve the problem I regard that as papering over the real issue, even if I used an actual calibrated piece of brass shim instead of a coke can, it's still, at bottom, a bodge. So I'm going to try the carbon paste because it is, for me, the most aesthetically pleasing solution. I'll let you know how I get on.

[Brucey]

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've tightened the clamp as much as I think is sensible, and the clamp isn't bottoming out...…...

Thomson seat posts are anodised, and the anodising is harder and more slippery than boring old polished aluminium would be.

Depending on the seat lug design, much of the torque can be lost in friction within the assembly etc rather than tightening on the seat pin.

FWIW the 'slippage' is often actually 'micro wobbling', i.e. the problem is actually caused by the end of the seat pin being a loose fit in the seat tube lower down, so the seat pin can 'micro-wobble' (mostly in the fore and aft direction) and thus work downwards. If the seat pin is built light and is rather flexible, this may help it flex and move within the clamp too. If the seat pin pokes out the top of the frame a long way, this is also non-ideal from the POV of the seat binder on a traditional frame.

By all means try the carbon paste, but depending on the nature of the problem (some seat tubes are actually 27.4mm ID even if the clamp is only reamed to 27.2mm; these can allow micro-wobbling) a shim might be a better solution, and a custom-machined seat pin might be the best solution of all.

cheers

[iandusud]

Thanks everyone for all your advice. I especially enjoyed reading 'The Shim Question'. What 'The Shim Question' misses though is that there is something actually wrong with my machine, and I really want to know what that is and how to fix it properly.

The answer to that question is, I believe, that something is out of tolerance. Either the seat post is undersized or the seat tube is oversized (or is could be a combination of both). Brucey has also indicated that an anodised seat post is more prone to slippage which agree with. Fitting a shim, whilst at first might appear to be a bodge, is a proper engineering solution to adjusting the fit of parts which run at close tolerances and is probably the answer in your case although carbon assembly paste might well have the same effect and have the advantage of improving the fit along the whole length of the seat post.

BTW I also enjoyed reading the "The Shim Question" and I think that you need to be careful not to get hung up on the "romantic" view point.

[terry_on_a_bike]

Thanks Brucey - loads of good info there, I've learned a lot. I've just taken the post out and had a good measure with my vernier guage.
The post and seat tube are both 27.2.
The post is quite a tight sliding fit, so I don't think a shim is called for, so I'm going ahead with the carbon paste.
My favourite theory is that the post is hard anodised, and so relatively slippery.
I also notice is has fine ridges cut into it, presumably they are aware of this problem and have tried to improve it.
Nice try but no banana.

[gregoryoftours]

I've also had this problem recently on a steel bike with Thomson seatpost. For me what worked was exchanging the cheap post clamp (just a bolted one with no steel or brass barrel to thread the bolt into) for a better one, in this case a qr clamp but one with a strong brass bushing on the cam. The slot in the top of the seat tube is sloped in a bit so it's a slightly looser fit than ideal, but it's not too bad and it stopped the slippage.

With a more traditional style seatpin binder bolt to fit a lugged seatpin cluster and the same problem I agree with Brucey that much of the torque applied when fastening doesn't translate to clamping force. This would be especially true if the slot in the seat tube slopes inward noticeably toward the top as more energy is lost in bending the bolt as well as poor surface contact in the clamping area.

If using carbon paste doesn't work I'd be tempted to try an M6 nut, bolt and washers in place of the binder bolt, tightening the nut onto the bolt rather than vice versa. If the tabs on the lugs are quite out of parallel I'd even add v-brake cup and dome washers to the arrangement. It's certainly a bodge but worth a try if other attempts fail.

Of course you could always ditch the Thomson seatpost! I think that they are unusually slippery, and in this case I think that the ridged surface actually contributes to this due to there being less surface area in contact with the seat tube. I've seen a disproportionally high number of black anodised Thomson posts where the ano/ridged surface had completely worn away to silver, I am guessing through repeated slippage.

[Jdsk]

Yes.

A couple of photos might help.

Jonathan

[rogerzilla]

I had a Thomson Elite post on my old On-One Inbred. That had a separate seat collar and it needed to be tight (a shim wouldn't have fitted). So yes, they are slippery.

I have an Elite 30.9mm post in the Moulton TSR which doesn't slip. That actually has two shims: an off the shelf shim and a rolled piece of 100mm x 100mm aluminium sheet. There was no ready-made shim of the correct thickness.

[bgnukem]

I had/have a similar issue with a new Spa Cycles Aubisque frame. Tried several nominal 27.2mm diameter seat posts (which actually varied from 27.05 - 27.15mm in diameter) but all were loose enough to rock sideways inside the seat tube (with seat clamp left undone).

In the end had to get a 27.4mm seatpost machined to fit the frame. The 27.4mm diameter is no easy to find though.

[gxaustin]

I believe that it's conventional at this point to refer to The Zen Shim Question:
http://www.hilarygallo.com/the-zen-shim-question/

Thank you for relieving me of the need ever to find out what that book is all about

[gxaustin]

I found a seatpost clamp which both clamped the frame and the seat pin. OK it had two bolts but did the trick.
https://www.ebay.co.uk/itm/AUTHOR-bicyc ... Sw9oFckfuI

Like this but I found one in UK without the hefty carriage charge

[Chris Jeggo]

I believe that it's conventional at this point to refer to The Zen Shim Question:
http://www.hilarygallo.com/the-zen-shim-question/

Thank you for relieving me of the need ever to find out what that book is all about

I found 'that book' fascinating, one of the few I have read three times. It has several strands: a motorcycle trip across the US; a man rebuilding his memory after mental illness and brain surgery; a little philosophy; not much Zen; and even some motorcycle maintenance that is well within the grasp of most readers of this forum board. Recommended. Incidentally, I prefer Pirsig to Gallo.

[tenbikes]

I found a seatpost clamp which both clamped the frame and the seat pin. OK it had two bolts but did the trick.
https://www.ebay.co.uk/itm/AUTHOR-bicyc ... Sw9oFckfuI

Like this but I found one in UK without the hefty carriage charge

Where in the UK please?

[De Sisti]

Could this be the one?

From here?
https://www.amazon.co.uk/gp/product/B00 ... 1OLE&psc=1

[gxaustin]

Could this be the one?

Where in the UK please?

Mine is a little different - more compact vertically- but its the same principle.