Do they exist and would they do me any good if they did? I participated in a trail ride with my Boy Scout Troop this weekend. I was riding my vintage 10-speed road bike with road tires. This was on the C&O Canal (for those unfamiliar with this ride, the part of the canal that we were on parallels the Potomac, and you ride on what used to be the path that mules walked on to pull canal boats). Flat and smooth for the most part, but many tree roots and sticks. I was the only one I saw the entire time with a bike like mine. Almost everyone else had mountain bikes. One of the other adults had what could best be described as a "first generation" mountain bike with a standard road-type frame with no suspension and 700C knobby tires. I was able to do the ride, but every tree root was jarring, and I nearly lost my balance in muddy spots. Even if I could find 27" knobby tires, would they do me any good?