I run 38mm tires on my fixed gear road bike. It's pretty fun off road and I've taken on the local mountain bike trail a few times. You've got to pay more attention to your lines and can't just bomb over everything like a real mountain bike. That said, it's a blast... A fun way to experience off road riding. One thing you may want to consider in the future is converting to 700c wheels. That will allow you to run a larger tire. You'll need longer reach brake calipers, but it's something to think about.
Last edited by croscoe; 03-17-09 at 08:37 AM.
Reason: I suck.