A wheel assembly is disclosed, the wheel assembly including a rigid wheel and an airless tire mounted on the wheel. A portion of the tire is separated radially from the wheel by a space and is configured to flex inwardly toward the wheel when subject to ground engaging pressure. A spacer is interposed between the wheel and the portion of the tire separated radially from the wheel by a space and is configured to limit the inward flexing of the tire. The spacer may be permanently affixed to the wheel assembly or may be removably attached to the wheel assembly. The spacer may be adjustable between a first size and a second size.