A sole has forward and rearward ends and lateral side edges. The sole has an upper and a lower surface with a periphery. A forward extent and a rearward extent each constitute a minority of the length of the sole. A central extent is provided between the forward and rearward extents. An upwardly extending tube has a free upper end and a lower end coupled to upper surface of the sole. The tube has a threaded aperture. A bolt is threadedly received within the threaded aperture. Strengthening ribs couple to the upper surface of the sole.