Marin Four Corners 2 Gravel Road BikeMarin Four Corners 2 Gravel Road Bike Midweek commute, weekend adventures. No problem for the Marin Four Corners, the steel framed explorer that's as comfortable on the asphalt as in the dirt. A double butted 4130 CrMo steel frame and fork are designed to be comfortable on rough roads and handle fully loaded touring and bike packing. Marin have included six bottle mounts, rack and fender eyelets, lowrider fork mounts, generous tire clearance, and disc