Best material for a shim for seat post?

[rjb]

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.

Hose clamp, sometimes known as a Jubilee clip.

[cycle tramp]

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.

Hose clamp, sometimes known as a Jubilee clip.

Thanks - That's the badger - Blimey, vascular dementia, here I come.

[Brucey]

IME most 'factory' frames have not been reamed and are fitted with a seat pin that is actually about 0.2mm undersize. The problem is often that the seat lug region is/has been distorted even though the seat tube is a little bit wider lower down.
The best thing you can do is to ream the frame so it will accept a larger seat pin (and should you ever sell it.this is what the frame needs to sold with/as), and in the meantime use a shim.
If you use a shim and you don't want it to become tiresome I suggest bonding the shim to the seat pin.. You can have all manner of ruses to achieve this, including double sided tape or epoxy resin eg on a perforated shim. It is also possible to build the seat pin up slightly,eg by using a thin layer of fibreglass or by winding a layer of thin cotton thread, which is then coated with epoxy resin. In either case the seat pin will have to be sanded to size. This is best done if you can spin the seat pin eg. in an electric drill, using an expanding mandrel with a rubber component. I think you should easily be able to make a suitable expanding mandrel using a length of studding, washers and nuts to suit (SGd to size if required) and rubber washers cut from an old inner tube.

[rogerzilla]

According to Tony Oliver's book, they're usually reamed but are previously distorted (out of round) by the welding or brazing process, so the reamer allows a 27.2 post (or whatever) to go in but it may well have gaps around it. Lots of grease should keep the water out. Oliver used to silver-braze everything and found a 27.4mm post would go into 531DB/531C. His maths makes sense - the internal diameter of a new, unbrazed, seat tube is correct for a 27.4 post with enough clearance to allow it to be inserted.

The worst-fitting seatposts I've come across are on 1960s Moultons, particularly those built by white goods factory workers in Kirkby. 25.4mm posts are normally fitted and slip insufferably, especially as they are chromed steel. I ream them to 26.0mm and use a 26.0mm post, then there's no slippage.

*not always reliable

[Galactic]

Update on the seat post situation

Have distorted the seat post in a vice as suggested by 531Colin (shove bottom of post in vice, tighten a 1/4 turn, check, tighten again etc). It now has a satisfyingly solid feel when feeding the post into the frame, and the rocking is no longer noticeable when the pinch bolt is loosened.

However, it still felt like I was having to tighten the pinch bolt a little too much to get the post fixed (no, I don't have one of them torque-gadgets), so I may yet have to resort to a shim. I won't have time today to take my distorted post for a spin, so will report back tomorrow on whether I'm still sliding downwards while in the saddle.

Thanks for all the suggestions thus far - this is what I like about this place: come along with what you think is a simple question and get a dozen possible ways forward. I guess some might find this frustrating (those looking for the one 'right' answer?), but I learn something from every thread I read

[Cugel]

Update on the seat post situation

Have distorted the seat post in a vice as suggested by 531Colin (shove bottom of post in vice, tighten a 1/4 turn, check, tighten again etc). It now has a satisfyingly solid feel when feeding the post into the frame, and the rocking is no longer noticeable when the pinch bolt is loosened.

However, it still felt like I was having to tighten the pinch bolt a little too much to get the post fixed (no, I don't have one of them torque-gadgets), so I may yet have to resort to a shim. I won't have time today to take my distorted post for a spin, so will report back tomorrow on whether I'm still sliding downwards while in the saddle.

Thanks for all the suggestions thus far - this is what I like about this place: come along with what you think is a simple question and get a dozen possible ways forward. I guess some might find this frustrating (those looking for the one 'right' answer?), but I learn something from every thread I read

As I recall, there are stepped seat post retainers that wrap around the seat post - seat tube joint, tightening on the seatpin but with an overlap of the seat tube top as well, so the retainer tightened on the seat post stops the seat post from slipping any further down the seat tube.

The retainer bolt tightens the retainer on both the seat post and the top of the seat tube, so the seat post isn't loose in the frame. It's a sort of belt and braces arrangement.

To use such a retainer, the top of your seat tube would need to be able to accept such a retainer. If your frame has the old fashioned pinch bolt acting on the seat tube itself, via a short split and a lug for the pinch bolt, this may not be possible.

[maanderx]

For 10 years (2013 - 2023) I had a similar problem (of a slipping seatpost) on my Spa Audax
frame. After using a 'coke can' shim and jubilee clip to keep things stable for so long, I decided
that things had to change.

After concluding that I needed a 27.4 seatpost my LBS suggested using a Seatpost Shim Converter Seat Tube Adapter (25.4 to 27.4)* and then fit a 25.4 seat post, as he had
problems sourcing one sized 27.4.

After a bit of searching I found a nice UNO Kalloy post, size 27.4. It fits very snuggly indeed.

*This one


Still unable to log in as De Sisti

[Brucey]

According to Tony Oliver's book, they're usually reamed......

it is a nice enough book but it is probably well out of date regarding what you can actually expect to buy. At one time, reaming was commonplace but alas no more.....I've seen numerous instances of persistent slippage in recent years in.'factory' frames and not once have I seen any evidence of reaming.

[Brucey]

What a brilliant idea....

acttually I think that might have been my idea, several years ago....it is just about the only way you can have a seat post enter a hole that tapers slightly outwards,whilst it remaining a good fit (not rocking) at the bottom. The other 'acceptable' (ie. not relying on a additional clamp) solutions all rely on the bore of the seat tube at least beng parallel.

[cycle tramp]

Of course back in the 1970s Richard Ballitine suggested drilling a hole some distance under the seat clamp and passing a 4mm bolt through both the seat post and seat tube to stop the seat post slipping....
Does anyone know if someone actually took that advice?

[asinus]

All depends on the relevant dimensions. To get a good fit I'd use a Vernier caliper (not that expensive) to measure seatpost and seat tube a try and find something of half the difference in thickness. Thus, when I wanted to fit a 25.8 (!) mm seatpost into a 27 mm seat tube, some stainless sheet 0.6 mm thick from an old fire surround was just the job. Alternative, if you have the exact dimensions, you might be able to get a shim off the peg.

[TheBomber]

It is possible to buy adhesive aluminium tape in diy stores. It will be somewhere near the Cellotex/Kingspan insulation boards as it is used for edging and joining them. I’ve never tried it on seatposts but it looks like it should be suitable. You’ll have a lot left over but you may come across other uses for it too.

[rogerzilla]

Beware of used seatposts that have been polished to get the scratches out of them. Some of those pretty Campag 27.2 aero seatposts are 27.0 now!

[531colin]

What a brilliant idea....

acttually I think that might have been my idea, several years ago....it is just about the only way you can have a seat post enter a hole that tapers slightly outwards,whilst it remaining a good fit (not rocking) at the bottom. The other 'acceptable' (ie. not relying on a additional clamp) solutions all rely on the bore of the seat tube at least beng parallel.

Yes Brucey, one of yours, I did attribute it!

[Galactic]

Dawn ride this morning to test my distorted seat post (à les 531Colin et Brucey), but after ten minutes my saddle was an inch lower. Today I'll fashion a shim and try that. If that doesn't work I'll do the hose/jubilee clip trick (presumably together with shim).

there are stepped seat post retainers that wrap around the seat post - seat tube joint, tightening on the seatpin but with an overlap of the seat tube top as well

This is what I've got on at the moment. Brilliant little thing.

It is possible to buy adhesive aluminium tape in diy stores.

I have some of this somewhere, but am going with the shim idea first. Shims have the advantage that they can be easily removed, unlike scraping ali film off an ali post. If I were out there on tour then the tape idea would be my first port of call. Although buying a can of beer and a pair of sacrificial scissors would be doable on tour too. Still, I've put the tape on the list just below shim and jubilee clip.

Thanks again for all the ideas.