Originally Posted by
Werkin
It is a way to run wider tires with a similar rolling diameter as most road bikes were designed for. Not all mountain bikes can fit a larger rim also. I have an MTB with plenty of room for width, but the front suspension fork brace and the top pull front dérailleur prevent use of a 650b rim with a tire wider than 32mm/1.25 inch mounted. Overall diameter increases when tire width is increased.
...not to mention that most MTBs ride (and handle) like a truck on pavement. There's a lot to be said for a light, responsive bike with cush tires. Especially here in the land of chipseal.
SP
Bend, OR