Originally Posted by uoficowboy
Wow - I had assumed the 25.4 vs 26 thing was some rounding thing or something. It's insane that they'd make two sizes so close to each other. Is there a good way to confirm which I have? Do I just measure the diameter at the point that the stem connects to the handlebars?
Yes that's how you'd measure it, but you'll need a measuring caliper and know how to use it to get an accurate measurement if your question is really between 25.4 and 26.0 (that's less than 1/32 inch). I don't believe you'd be able to tell the difference by eyeballing it with a ruler.
But the difference between 26.0 and 31.8 is obvious.
As for why.... I think it may be that 25.4, being a MTB standard was established in the US because it's exactly 1 inch. I have no idea why the road bike standard was/is 26.0.
As for 31.8 (which is actually 31.75 and is sometimes called 31.7), that's exactly 1 1/4 inch.