Features of the Suspenz Caster for free Standing Racks Allows you to freely roll your boats while mounted on the free-Standing Racks 8 heavy duty casters that easily Fit into the base of the free-Standing racks 4 of the 8 casters Are locking to avoid run away racks Indoor or Outdoor use After adding these casters to a 3-Boat Rack, the post height is: 80.5in.. The maximum suspension arm height from floor with Casters is 91.5in. To move fully loaded free-Standing Racks with Roll-On Casters, the Suspenz Cross-Bar Kit is required (sold separately) Double sided racks will need 4 casters to be attached to center of rack 8 Heavy Duty, 3in. Diameter Casters; 4 Locking, 4 Standard