Choose this to upgrade any standard 7' wide tandem 3500 lb leaf spring axle model (7k GVWR) to heaviest duty tandem 7000 lb leaf spring axles (14k GVWR) and larger 16" 8-lug wheels/tires. Also requires separately purchasing the upgrade to 6" I-beam main frame on any 7' wide model up to 14' long. (7x16 comes standard with I-beam frame).