The wheel shown in Fig. 5 shall consist of a steel disk with an outer layer of chlorobutyl rubber molded to its periphery. Uncured rubber shall be bonded to the rim and fully cured in a steel mold. The optimum hardness of the cured rubber is Durometer A-60. A range from A58 to 62 is acceptable. At least four hardness readings shall be taken on the rubber approximately 90° apart around the periphery of the wheel using a Shore A Durometer tester in accordance with Test Method D 2240. The gage readings shall be taken after a dwell time of 5 s. The recommended composition of the rubber and a qualified molding source is noted in Table 1. (See 9.9 for preparation and care of the rubber wheel before and after use and see Fig. 2 and Fig. 5.)