I've rode the trail on an old 10-speed with 27 x 1-1/4 tires and also 26 x 2" street tires on my MB. They both did fine....as long as it was dry. One time a buddy and I were on it when a tornado came thru. It rained and turned very cold and the high winds hit Old St. Charles hard. A number of trees came down across the trail and much of the St. Charles Trailhead lost power. My buddy was on a Trek hybrid with 27 x 1.50 tires I think and I was on my MB with 26 x 2" tires. It was a real struggle trying to ride as messy as it was but I was a lot better of than my buddy. He almost had to stop and walk while I could still get along in fair condition. I'll be back to ride the whole thing this September but this time I'll be on 26 x 1.5 tires. Hopefully it will be dry.