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Building up a new frame with new parts. I have a new alloy frame and offered up the seatpost (Cannodale Hollogram Carbon) to check all OK. Seatpost slid into place really easily and has some movement in the frame (if moved back/forth or side to side, this puzzled me as I'm used to posts being a pretty snug fit (though most of the recent frames I've had are carbon). I've offered up a couple of old spare posts to the frame, one (an old budget carbon model) is a pretty snug fit, the other (an alloy one) is a similar fit to the Cannodale post.
I've checked my old alloy frame (which I'm breaking down to build the new bike), I've found the fit with each of the posts similar to what I've outlined above, to add to my confusion the alloy post tested is the one that came with my old frame when it was in its original complete form (so I'd assume the right fit), and the old carbon post is the one I'd swapped it out for several years ago. I've not added any carbon paste to the new post yet (don't want to get it mucky if I need to return).
My current assumption is that the posts are slightly different diameters (they should all be 27.2), I've ordered a cheap set of calipers to make further checks, in the meantime what's the STW hive mind think? My main concern is slippy seatpost or damage to the frame. Worst case scenario I guess it to get one of the USE shims that seem to come in ultra thin versions.
I can't comment on the variability in post diameters but unless they've changed them in recent years I think that the USE shims only come in specific sizes, like 25.4 to 27.2 or 30.9 to 31.6 rather than generic 0.2mm or whatever thicknesses that it sounds like you would need.
Your best option may be using a coke can (other soft drinks are available) to shim the seat post. I'd be careful using cheap calipers as they may not be 100% accurate either. They would give you an idea but not necessarily be spot on.
On sizing a budget set of calipers might not help that much, there is all to often too much flex in the jaws and the repeatability will suffer.
A micrometer (25-50mm) is more likely to help. A used reasonable brand analogue one from fleabay would be my suggestion.
I can't comment on the variability in post diameters but unless they've changed them in recent years I think that the USE shims only come in specific sizes, like 25.4 to 27.2 or 30.9 to 31.6 rather than generic 0.2mm or whatever thicknesses that it sounds like you would need.
Your best option may be using a coke can (other soft drinks are available) to shim the seat post. I'd be careful using cheap calipers as they may not be 100% accurate either. They would give you an idea but not necessarily be spot on.
I've already checked, they do a 27.2 / 27.8 and a 27.2 / 28 so should be good. I know I could use a drinks can but for a tenner would rather have a purpose made shim.
If you want to try the shimming option i have some stainless shimming steel i could send you?
I had to buy some to sort out a seat tube that was slightly oversize and have loads of it left doing nowt.
FWIW a Coke can is 0.2mm thick and typically, you’d want a gap of between 0.1-0.2mm between a seatpost and seattube for a good fit. Any more and you can get problems with creaking, fretting and slippage.
27.0 and 27.2 are both standard sizes, 27.2 being much, much more common. I wonder if there's a 27.0 in there somewhere, which was close enough to work on the old bike but has now just been noticed?
My current assumption is that the posts are slightly different diameters (they should all be 27.2)
Seat posts are fairly easy to make to tolerance, seat tubes less so, but either could be wrong. A few manufacturers make 27.4mm posts, Thomson Elite is one example
It's also more difficult for the average cyclist to measure a seat tube beyond a few mm deep with cheap kit, a local engineer will probably do that for a packet of biscuits
A touch patronising. An engineer time served it uni educated doing something for a packet of biscuits
A touch patronising. An engineer time served it uni educated doing something for a packet of biscuits
It isn't protected in the UK AFAIK 🤣
I'm curious why you think he needs to measure deep down? The reamer goes in from the top, so it isn't going to cut a hole low down that is bigger than where it entered. That is my experience of making the odd frame from a pile of tubes and owning a reamer for 27.2 posts.
If the frame is meant to suit a 27.2 post, then it would have to be reamed way way oversize to take the 27.8mm machined shim mentioned. Any smaller size difference is the territory of shim steel, coke can or a slightly different sized post that actually fits.
The OP already has a post that is described as a snug fit (and two that are slack). Any vaguely decent measuring device will give an idea of the external size difference between them. A vernier scale Clarke CM100 is £12 and the few I have all compare well with the proper calibrated stuff at work (and more reliable than a cheap digital one).
And if the OP has a decent local bike shop, a visit with the calipers and snug post might yield a nicer post of comparable size.
I'm curious why you think he needs to measure deep down? The reamer goes in from the top, so it isn't going to cut a hole low down that is bigger than where it entered. That is my experience of making the odd frame from a pile of tubes and owning a reamer for 27.2 posts.
As a frame builder you'll have way more experience than me, but I'd want to look for all of the possible variables in case there's a tube-forming problem on the new frame, which does happen.
Take that out of the equation and it can only be the seat post. I take your point about the "pretty snug fit" on one post out of three
Quick update. Callipers arrived and measure the new seat post at 27.1 / 27.2, harder to accurately measure seat tube as discussed in the thread, but it also seems OK. Knowing I can get a very thin shim I’ve fitted the new post, one the carbon paste was on it the fit is much more snug. Bike not fully built yet so not had a chance to test.