This is the first in a series where I'm going to try to summarize what I'm learning into concise notes. The idea is to isolate the variables involved in making a choice or performing a task.
Smaller is more flexible and thus better suited for lighter riders and those who don't need/want stiffness.
The 29er MTBs I'm working on lately are 38 mm DT, 36 mm HT, 31.8 mm TT and ST.
Thinner is generally more flexible, but may require better technique to join. See also below.
Chris is able to do S3 frames without using a ST sleeve or an externally-butted ST. I had thought that was impossible without excessive distortion.
Air-hardening alloys (e.g. OX Platinum, S3, 853), though often used for the thinnest tubes, are stronger/stiffer/harder after welding, all else being equal.
We talked a little about the engineering specs that are sometimes provided with tubes, i.e. yield strength, ultimate tensile strength, and elongation. Yield strength is the amount (or range of amounts) of force the tube can recover from without deformation. UTS is the breaking point in a pull-apart test.
The most interesting number is elongation. It's a way of describing how flexible a tube is. Two tubes can be physically identical but will flex to different degrees if they have different elongation.
Pipe Dream Cycles has a great tubing comparison chart listing a lot of these specs. Columbus seems to be the only tube maker publishing all three engineering specs on their website, and even there it's confusing...what do "Rm", "Rp0.2", and "Ap5" stand for?
Brazing results in a more flexible ride than welding, all else being equal. See also above.
Chris says brazing the air-hardening alloy tubes is a waste. Thus it seems Columbus, Deda, and Tange are better choices for high-end, lightweight, flexible frames.