Talking of confusion, is there an accepted way to measure seatpost setback? Engineers usually measure stuff centre to centre, but if you measure seatpost setback from the centre of the post to the centre of the clamp, then I think that will vary depending on the seat tube angle, and you could have 2 posts with 25mm setback, but the one with a 20mm wide clamp will put the saddle 10mm further back than the one with a 40mm wide clamp....??
crepello wrote:A related question. Is there any way of accurately working out seat post layback? If the measurement is merely taken seatpost centre to clamp centre, it ignores the width of the clamp (looking sideways on). So a wide clamp like on a Campag post (40mm IIRC) gives poor layback - the clamp hits the limit of the saddle rails very easily. Whereas the narrow clamp on the Grand Cru (29mm) ensures what is already a good c-c layback. The shape/position of the front of the clamp is critical for determining layback.
So what's the most reliable way of making the measurement? I can compare most of my seatposts which are all 27.2mm, by taking the measurement from the front of the post to the front of the clamp. But it's not a universal way of getting the measurement. Measure a wide seatpost and the figure given will be false in relation to the 27.2, because of the extra thickness of the seatpost.
SNAP! From viewtopic.php?f=5&t=75623&start=15
As you say, too many threads running. Funny, this question has been going round my head for ages - I never read your post until now!
I've always measured layback from the front of the seatpost to the front of the clamp.Its the only way that makes any sense to my mind.
As you say any other method can be variable.
It's not quite foolproof though - the measurement is going to be different between 25 and a 31.6 posts of the same design. But for getting the best measurement of the post I've got in front of me/on the bike, it's the better measurement to take.