Alpengeist

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.