Seat post tolerance in a frame
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onyerbike531
Retro Guru
I know it's a road bike and this is a mtb thread but is this acceptable?
This is on my Bob Jackson Audax, that's a 27.2 Kalloy Uno seatpost, no wobbling and seems to fit nicely, had problems with the original clamp bolt.. needed quite a bit of tightening, LBS had to fit a new bolt because the original wasn't tightening up very well, this one has the little keyed bit that goes in the slot. Seems fine now.
Whilst on the subject you should have seen the diameter of the post originally fitted to my Raleigh Yukon when I found it, it was two sizes too small and they'd overtightened the clamp to the point the seat tube had split beneath the slot. I had to drill out a hole beneath where the crack had propagated and extend the slot down to that. Fitted the correct diameter post and can report no further issues.
Poor bike.
This is on my Bob Jackson Audax, that's a 27.2 Kalloy Uno seatpost, no wobbling and seems to fit nicely, had problems with the original clamp bolt.. needed quite a bit of tightening, LBS had to fit a new bolt because the original wasn't tightening up very well, this one has the little keyed bit that goes in the slot. Seems fine now.
Whilst on the subject you should have seen the diameter of the post originally fitted to my Raleigh Yukon when I found it, it was two sizes too small and they'd overtightened the clamp to the point the seat tube had split beneath the slot. I had to drill out a hole beneath where the crack had propagated and extend the slot down to that. Fitted the correct diameter post and can report no further issues.
Poor bike.
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Guys, FFS, talk about the blind leading the blind! This is a real downside of forums, where all sorts can post irrespective of their knowledge of cycle mechanics. You end up with lots of "advice" that isn't worth a cup of cold tea, and the odd well-informed post getting lost in the noise.
No one, absolutely no one, has mentioned that simple and effective piece of kit ... A SEAT POST GAUGE. It costs about £20 from the likes of SJS and will tell you in 5 seconds what the correct seat post size is for your frame.
The seat cluster lug (assuming you have a lugged frame) or top of the seat tube (for a lugless frame) is almost never round because of heat distortion during manufacture. (Sometimes a diligent frame builder will ream it to be nicely round after building, but that's unusual.) So stick the gauge in, let it find its natural position, and read off the required set post diameter. Now you know what the correct seat post size is supposed to be. But be aware that no manufactured article is ever completely round and there are always manufacturing tolerances. This is why measuring with a Vernier is effing hopeless ... it will only measure the diameter at one point on the diameter but the bloody thing ain't round! Never mind, a decent manufacturer can be reasonably relied upon to get this more or less right. So get a seat post of reasonable quality in the nominal, measured size and get on cycling and enjoying life.
Phew!
I think you missed the point that even if you know exactly the correct size that the Seat tube is it doesn’t guarantee that the seatposts are true to size. I’ve had 3 x 27.2 Thomson’s and they all slightly different.
Anyway - a Lightning is 27.4 if has no shim and 27.2 if it has a shim. Finding a true 27.4 is the issue.
old_coyote_pedaller
MacRetro Rider
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How is that going to help? One of those seat-post gauges will only go so far in to seat-tube so will only measure the top 20mm or so of seat-tube and there's no guarantee that top of seat-tube isn't distorted by clamping wrong size of post. You'll have no idea what diameter rest of seat-tube is.
I don't think I'm missing the point at all. You have to know both ... the seat tube diameter and the seatpost diameter. You are groping in the dark if you don't know the seat tube diameter.
Of course seat posts are never exactly round and of exactly the nominal diameter. There are always manufacturing tolerances. But the start point is to know what it ought to be.
Incidentally, my experience is that the cheaper seatposts tend to have the most problems with sloppy tolerances. I guess that's only to be expected.
1. I currently own a un-shimmed Lightning. Which is fitted with the standard 27.4 post.
2. Last month I owned a shimmed Lightning which had a 27.2 post.
3. The standard cheap 27.4 post measures exactly 27.4.
4. I purchased a brand new Thomson 27.4 but it measured slightly over so wouldn’t fit.
Well, it's certainly going to help if the top of the tube HASN'T been distorted by clamping the wrong size of post!
It's always a good idea to start out simple and assume you don't have that problem. (I think Einstein had something to say on this.) If your gauge says you need a 27.4mm seatpost and you've fitted a 27.4mm post but still have problems, then you think again.
The cure for the difficulty that you mention (top of tube distorted) is to ream the tube out to the next size down from that indicated on the gauge. (So you need the gauge.) This isn't really a job for an enthusiastic amateur as you need the seatpost reamer and a bit of facility in working with metal.
OK, so that's how a seatpost gauge is going to help.
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Quite often a good clean/scrub/scrape/hack of the seatube inner with a rough object sand paper round a rod and a tube brush to finish off etc work wonders on old bikes when post don't fit as they should.
The amount of cack they can accumulate through a seemingly tight clamped surface is insane.
The seatube sizes are an excellent idea, but here I think they knew the size needed.
Also trial and error is so much fun :-D
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