I don't have an FX, but a good friend has an FX 4 and regularly travels rail-trails. He recently spent 4 days riding the GAP rail-trail. He still has the stock tires on his FX4. When I first started mtn.biking, over mtn bike trails with rock and roots, the bike did not have suspension, other than mtn bike tires which I ran "soft", so you should be fine w/o susp. It didn't have disc brakes either. Had a blast on that bike-the lack of susp probably helped me with picking good lines on mtn bike trails. You should be fine with tires a little wider, and don't inflate to the max (most tires are not at their best at max pressure anyway). You'll be fine--enjoy!!