A tread assembly has front and rear main idler wheels supported by a frame. A continuous tread is provided around the idler wheels for supporting a cart on a bottom portion of the tread between the front and rear main idler wheels. An assembly is provided to permit the front idler wheels to pivot laterally as the cart encounters uneven ground, in order to improve weight distribution across the tread and the idler wheels. The rear idler wheel also has an assembly to permit the rear idler wheel to pivot laterally. An alignment feature is incorporated into the front idler wheel assembly to permit adjustment of the alignment of the front idler wheel relative to the rear idler wheel. A tensioning mechanism is provided at the rear idler wheel to maintain the tread in tension. A pivot is provided between the frame and the rear idler wheel, and located forward from the hub of the idler wheel, such that increased load on the frame tends to push the idler wheel rearward to further tension the tread.