Seatpost shims.....

[neilob]

Advice please.....I have an aluminium Colnago frame that takes a 31.8mm seatpost. I have a 27.2mm 3T carbon seatpost I'd like to use in it. Is it Ok to use a shim between alu and carbon....and what material? And where would I get one?

[tyred]

Cut up a Coke tin and use the material as a shim.

[CREPELLO]

Personally, I wouldn't use home made alu shim, because you wouldn't want any sharp edges cutting into the carbon. USE make a range of good quality plastic shims which I think would be ideal.

[Steve Kish]

I've used a Coke tin to shim 27.0 pin into a 27.2 frame but 27.2 to 31.8 would need an oil drum!

[neilob]

Cut up a Coke tin and use the material as a shim.

On a Colnago????

[gbnz]

SJS Cycles were stocking shims a couple of years ago.

Inserting a carbon seat post into an aluminium shim, shouldn't be any different than inserting it into an alumnium seat tube, subject to the shim being a suitable length

[hubgearfreak]

i've used a bit of 22mm copper water pipe on one of my bikes. make a slit so it opens up and burr over the top so that it neither drops into the frame nor scatches your seatpost. this is only any use if you've already got a bit in the garage and a vernier to check

[Vorpal]

Cut up a Coke tin and use the material as a shim.

Personally, I wouldn't use home made alu shim, because you wouldn't want any sharp edges cutting into the carbon. USE make a range of good quality plastic shims which I think would be ideal.

Cut up some plastic yogurt or cream pots to make shims. Or maybe a milk jug.

[Phil_Lee]

i've used a bit of 22mm copper water pipe on one of my bikes. make a slit so it opens up and burr over the top so that it neither drops into the frame nor scatches your seatpost. this is only any use if you've already got a bit in the garage and a vernier to check

Would that work for that amount of difference though?
31.8-27.2=4.6mm, or 2.3mm thickness.

I'd get a made-to-measure one.
SJS have them (in that size) from £3.99 (Post Moderne aluminium) or £6.99 USE Thermoplastic.

[hubgearfreak]

Would that work for that amount of difference though?

this is only any use if you've ..... a vernier to check

[Brucey]

there are two worries here;

1) will it stay put?

2) will anything get damaged?

#2 requires that you don't deform the frame very much when the clamp is tightened. This in turn means that the shim should be both rigid and accurate in size.

Fortunately 27.2 is a common size for suspension seat posts that are fitted to mountain and commuter bikes, so aluminium shims are available. Any of the big online vendors will have them. 'Post moderne' make cheap ones IIRC and others do posher ones. If there is a concern re the sharp edge bearing on the carbon, a little filing and sanding can produce a suitable radius on the aluminium. Cost; less than a fiver.

I've used all kinds of awful lash-ups in the past myself, and frankly it is not worth it, not even on a bike that you are happy to throw away; its a complete waste of time for want of one phone call and one five pound note.

Should you need to obtain a non-standard shim (25.4 and 27.2 centre bore are standard, other sizes are rare) then my advice would be to find a different seat pin (the correct size for the frame or 25.4 or 27.2 so that shim can be used) or find someone with a lathe; a slightly fatter alu pin can be turned down or a special shim can be made. I've done both myself, many times, and (say) managed to make a seat pin that will stay put in an MTB frame without having to ream it out.

hth

cheers