Hardest, toughest tread we offer, ideal for heavy loads, non-marking, and wear-resistant. This extra hard polyurethane has good water resistance but is not solvent resistant. It has excellent dynamic performance and very good physical properties. Wheels treaded in hard polyurethane can carry a heavier load than wheels of the same size treaded in standard polyurethane. The harder material makes an easier to roll but noiser wheel. (Shore 70D Hardness)