I've got a friend who has a 29er mountain bike with Bontrager rims and disc compatible hubs. I'm not sure what the width of the rims are, but the bike came with 29"x 2.2" tires, and she's ridden thousands of road miles with Conti 700 x 28c Ultra Gatorskins installed on the rims with no problems. Pre-built wheelsets intended for 29" mountain bikes will have 135mm hub spacing.