Originally Posted by
cango
See the graphic - is there a reason that they are different? Or is it just because?
In the grip part road is 23.8 and mountain is 22.2.
Makes it difficult to move parts around from mountain to road and vice versa.
It's a good question.
Maybe someone will come along with better history on this than I have, but I think you should look at the early days of mountain biking and those clunkers of Marin County and so on.
Remember their big gnarly brake levers? Huge alloy jobs that held all your fingers with a curlique knob to spare? I'm guessing those were borrowed from motorcycles, which have a common 7/8" bar size. Of course, 7/8" is 22.2 mm.
So if you were Gary Fisher or whoever, and you wanted to start making mountain bikes, having a handlebar that fit in the quill stems available that could take motorcyle style brake levers, you ended up with a 25.4 mm to 22.2 mm handlebar. Tooling up, or contracting out, to do that would be a lot easier than launching a new component, like a new brake lever line.
I'd be happy to read a better history, as this is just a semi-informed guess.