That's my hand holding one of the side wheels that has been cut away to show its construction. When the aluminum hub is surrounded by a urethane liner that is chemically bonded to the metal hub. The wheel reaches the end of its service life, usually in less than one operating season, when a specified amount of its liner has worn off. When this occurs, the wheel liner is stripped off and a new urethane liner is placed on the wheel. A durometer is used to measure the hardness of the new coating before it can be used on the coaster.