Originally Posted by
Nessism
I've built frames where one had a heavy/thick down tube, and another that was similar in most regards except for a fairly thin down tube, and the thick tube frame most definitely was stiffer. The friend I gave the frame to commented about how stiff it was when sprinting.
Agreed, when wall thickness is changed the ride will change. But that's not what we're talking about - he says 'grade'. Uno 858 tube will ride like a Zero 858 tube all else equal. Zero is stronger (higher psi) which allows it to be drawn with thinner walls and maintain the same strength but then it will lose some stiffness so, we increase the diameter to regain the stiffness but then we give up some of the weight savings. My point is now we're deep into the framebuilder's experience, expertise and fab skills and that's not something, I don't believe, the OP can 2nd guess from a post in a framebuilding forum.
In defense of the framebuilder, Uno might not be available in the really thin walls that give the desired ride feel or frame weight. Conversely, Zero might not be available in the thick walls necessary to support a heavier rider or for a bike that will see hard use as a commuter with racks. It doesn't make sense to put higher grade thin-walled tubing of a larger diameter on a commuter - it will dent easier. The frame builder is designing around use cases and grade of tubing is secondary to the tubing dimensions.