Heavy Duty Mold-On Rubber on Iron Core Wheel. Heavy Duty cushion rubber treads are vulcanized to cast iron cores. The harder rubber tread still provides a cushion effect to the load and excellent floor protection. These wheels are highly shock absorbing and provide quiet operation, while dampening vibration. Wheel face: Moderate crown. Finish: Black tread on iron core. Temperature Range: -40 degrees up to +180 degrees Fahrenheit. Hardness: 90-95 Shore A.