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.That's an interesting option, thanks.
I have measured the posts tried so far and the 27.4 is a true 27.4, the alloy 27.2 varied between 27.1 and 2, the 27.2 carbon post was 27.2/3.
Basically I think this means I need a seat tube size that was never made/sold.
My vernier only have 1 decimal place so can't tell the exact size.
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!
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.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 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.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.
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.
Well, it's certainly going to help if the top of the tube HASN'T been distorted by clamping the wrong size of post!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.
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.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.