When used for industrial purposes, vulcanized rubber products must often meet an extraordinarily complex set of requirements. Not only are they subjected to shearing stresses, but they must withstand various compression, tensile and bending stresses, such as are encountered in V-belts, conveyor belts, and diaphragms. In many cases these stresses are very rapid, and consequently a knowledge of the dynamic stiffness of a product may often be of technical interest and value. As will be shown, this dynamic stiffness cannot be estimated directly in terms of static hardness, for it depends also on the character of the fillers or pigments in the rubber.