Originally Posted by
CliffordK
As I understand it, 650b (584 BSD) was a relatively narrow 26" tire, but MTB riders have now adopted it as a mid sized 28" wheel/rim size. I suppose re-purposing an old rim size with new wider tires makes some sense, rather than introducing a new rim size.
Here's a forum post from Kirk Pacenti that talks about why 650b and not 650a:
Why not 650A instead of 650B?- Mtbr.com
He was taking a risk by funding a run of mountain-bike tires in an unproven size. He was after something in between the 26er and 29er sizes of the day. He needed for rims to be readily available. And it was important to him that conversions be possible on existing, 26er frames of the day. So 650b it was.