unterhausen

I suggested to Reed that he could try ultrasonics, but I didn't know how to do the measurements. The relatively cheap ultrasonic thickness measuring devices only go down to about .1mm, so not useful for most bike tubing. But he didn't give up on the idea. Now that he found a company that makes a device that will measure tubing thicknesses less than that, I understand the technique and feel a bit slow for not realizing how it could be done. I haven't done it with any of the equipment I have available to me yet. I should try it. I'm not sure how well his equipment works in all instances, but he has checked it against measuring with calipers/hole gauges and it works well in the instances where he has compared the two.

My experience building frames tells me there is a lot of variation in tubing. I used to really get annoyed with 531, for example. The OD was all over the place, sometimes too big, sometimes too small. It's not surprising to me that they would have large variations in thickness.

mstateglfr 

Top of seat tube measurements dont look correct on some. An example would be the 1.16mm top of seat tube on a black mountain cycles frame. Its a 28.6mm seat tube and uses a 27.2mm seatpost. The math doesnt work.

Cl904

The seat tubes were probably single butted with a 0.6 mm unbutted section. Pretty sure Reynolds and Columbus seat tubes were made like that. Tange we’re usually double butted, but my pro tour 15 has a 27.0 seatpost, and probably should take a 27.2 in champion no 2 tubing, which is the same 0.9-0.6-0.9 butting for double butted tubes.

i have heard of production bikes with seat tubes brazed upside down, where a British 531 db tubeset had a 26.6 seat tube or something, but they are anomalies.

mstateglfr 

A 1.16mm thick tube on a 28.6mm tube means a seatpost that is 26.2 or 26.4(accounting for variation).
But the seat post is actually 27.2mm.

So the measurement of tubing thickness is wrong.
...or I am misreading the data.