Features of the Suspenz Caster Set for Back Leg 4 non-locking heavy duty casters Fit onto the center of the Double-sided Rack - required for center load Support Allows you to freely roll your boats while mounted on the Double-sided free-Standing Racks Indoor or Outdoor use To move fully loaded free-Standing Racks with Roll-On Casters, the Suspenz Cross-Bar and 8 Roll-On Casters Are required (sold separately) 4 Heavy Duty, 3in. Diameter Casters; 4 non-locking