Cycling UK Forum

I have an odd-sized seat tube, and having sourced a correctly sized seat post, I find it to be too loose, and no matter how tight I fasten the bolt, the seat sinks inexorably downwards, even though it's fastened tight enough that it won't turn to the side.

I had this problem when I first got the bike, but then ended up with a narrower seat post fitted with a factory-made shim and that was fine. Now I'm back to the 'correct' size of seat post and I obviously need a thin shim - presumably thinner than anything I can buy off the shelf?

So what should I use for a seat post shim? A beans can is made of metal (steel?) that is too thick to go all the way round the post, so perhaps an ally drinks can? Or a PET bottle?

I have a strong feeling the ally drinks can is the answer, but I'd be interested to hear of any other options (not least because the frame is steel, but then again, there are plenty of places on a bike where ally and steel meet and nothing drastic happens).

Thanks for suggestions.

If seatpost is alloy could be knurled.

Galactic wrote: ↑28 Dec 2023, 12:39pm I have a strong feeling the ally drinks can is the answer,

That's what I've used, on I bike I had for around six years. Seatpost removed every year or so as part of a service, no issues.

if you undo the pinch bolt and "rock" the saddle front to back, does it move?
If it does move, then it probably rocks when you are riding, and inevitably if it rocks it will work its way down.
If you clamp the bottom of the seatpost in a big vice and "squash" it a bit side to side, this might stop the rocking; if you stop the rocking, you will (probably!) stop the sinking.

531colin wrote: ↑28 Dec 2023, 1:55pm if you undo the pinch bolt and "rock" the saddle front to back, does it move?
If it does move, then it probably rocks when you are riding, and inevitably if it rocks it will work its way down.
If you clamp the bottom of the seatpost in a big vice and "squash" it a bit side to side, this might stop the rocking; if you stop the rocking, you will (probably!) stop the sinking.

What a brilliant idea. I wish I'd thought of that when I had a similar problem with slipping Al seatpost in a Ti frame. I fixed it by using carbon grip paste on the post instead of copper grease.

Cyclothesist wrote: ↑28 Dec 2023, 2:06pm

531colin wrote: ↑28 Dec 2023, 1:55pm if you undo the pinch bolt and "rock" the saddle front to back, does it move?
If it does move, then it probably rocks when you are riding, and inevitably if it rocks it will work its way down.
If you clamp the bottom of the seatpost in a big vice and "squash" it a bit side to side, this might stop the rocking; if you stop the rocking, you will (probably!) stop the sinking.

What a brilliant idea. I wish I'd thought of that when I had a similar problem with slipping Al seatpost in a Ti frame. I fixed it by using carbon grip paste on the post instead of copper grease.

Its Brucey's idea, not mine!
Carbon grip paste could also work; and some Ti frames have an oversize seat tube with a short sleeve at the top for a 27.2 seatpost, so ovalising the bottom of the seatpost won't work!
As far as I know, dissimilar metal corrosion occurs with ali and steel, but not ali and Ti.

531colin wrote: ↑28 Dec 2023, 1:55pm if you undo the pinch bolt and "rock" the saddle front to back, does it move?

Yes, just tested this - a surprising amount of play there.

531colin wrote: ↑28 Dec 2023, 1:55pm If you clamp the bottom of the seatpost in a big vice and "squash" it a bit side to side, this might stop the rocking; if you stop the rocking, you will (probably!) stop the sinking.

OK, will have a try at clamping it in a vice (if a bloke on the street had come up with this then I would have ignored the suggestion, but since it's from 531Colin I'm going to give it a go). The post is pretty solid ally (Canecreek Thudbuster Mk II) with thicker bits (aft and fore) at the bottom, so presumably I should try to deform the tube a couple of inches above that (the post has about 5 inches in the frame)?

Valbrona wrote: ↑28 Dec 2023, 12:48pm If seatpost is alloy could be knurled.

Now this would be the most elegant solution, but I don't have tools for or experience in knurling, so I think I'm going to stick with either a shim or 531Colin's vice technique.

The fore and aft thick bits of the tube go all the way up....the front and back of the tube are the bits which are under tension and compression (respectively) as you ride.
(similarly, front forks are ovalised to handle stresses of riding and braking)

531colin wrote: ↑28 Dec 2023, 2:29pm The fore and aft thick bits of the tube go all the way up.

Ah yes, it looked like they were just at the bottom, but sticking my finger in there I can feel what you mean.

So the plan is to press the sides to get the reinforced front and back of the tube to bulge out a wee bit. I'll give this a go in the morning and report back on my inevitable success.

If ovalising does not work, assuming there is some bare seat post above the seat lug, you could fit a seat post clamp* at the bottom of the visible/exposed seat post. The extra grip from that should/probably would stop any slippage.
*ie. Seat clamp collar used on lugless steel ftames and a lot/most alloy frames and carbon frames.

Try a coke can shim. Can is generally 0.1mm thick and can be cut to size with scissors. A 2nd clamp on the seat pin itself above the frame clamp may also be beneficial. Even a light bracket may be sufficient. It's a shame to squeeze an expensive seat post unless unless other solutions have proved to be unsuitable.

These are quite neat, and the 28.6 size closes enough to nip up on a 27.2 seatpost......
https://www.merlincycles.com/dmr-hinged ... ieshopping

Sardine can lids used to be my favourite shim. They are pre-curved once removed!

"The Zen Shim Question":
viewtopic.php?p=1806731#p1806731

Happy Christmas

Jonathan

rjb wrote: ↑28 Dec 2023, 3:30pm Try a coke can shim. Can is generally 0.1mm thick and can be cut to size with scissors. A 2nd clamp on the seat pin itself above the frame clamp may also be beneficial. Even a light bracket may be sufficient. It's a shame to squeeze an expensive seat post unless unless other solutions have proved to be unsuitable.

..I'm currently using a coke can shim... and to echo RJB, another forum member (deepest apologies, age & memory means I've forgotten their name) suggested using a hose clamp (universal clamp) as a 2nd clamp... however in the past, I've gotten away with using a bit of old inner tube and a cateye led seat post clamp.