Originally Posted by
SkyDog75
All other properties being equal, larger diameter tubes are stiffer, which would be a pro.
Sure, it would be a pro if stiffer always = better. Sometimes stiffer just = stiffer.
Originally Posted by
SkyDog75
31.8 mm is the current standard for road bars, or at least it has been for the last two decades or so. The adoption of 31.8 mm handlebars coincides with threadless stems.
Nah. Threadless stems were around for at least ten years before 31.8mm became the "standard." It started with downhill mountain bikes, probably using motocross handlebars, hence the non-metric 1¼" size. No idea why it became the road standard. There's really no need for that much stiffness in a road handlebar. I'd say it coincides with the widespread use of CFRP for handlebars.
Plus, the aesthetics of the bar/stem combination are improved if both are about the same size tubing. Oversize-tube frames and forks with oversize steerers and aluminum and CFRP stems and bars kind of need each other.
Originally Posted by
SkyDog75
25.0 (French), 25.4, 26.0, and 26.4 (Cinelli) are past standards, typically used with quill stems.
Yes, because that's pretty much all there were. A few builders made "threadless" stems that fit onto an extension brazed into the steerer tube, but the headsets were still threaded.
Originally Posted by
SkyDog75
Since 31.8 mm bars and threadless came into general use at the same time, and threadless stems typically have removable faceplates, it is generally true that stems for 31.8 mm bars have removable faceplates.
BUT... Removable faceplates aren't unique to 31.8 mm handlebars. There are/were stems for 25.x and 26.x mm bars that also had removable faceplates. Both quill and threadless.
Truth.