Microcellular urethane jounce bumpers are used in many automotive suspension systems. This study experimentally determined the dynamic force, acceleration, displacement, dynamic stiffness, and natural frequencies of constant-cross-section bumper samples made with microcellular urethane materials of three different densities. Four impact energy levels were used. Dynamic responses were analyzed in both time and frequency domains. The dynamic displacement, acceleration, and force responses are nonlinear and the dynamic stiffness exhibits a hysteresis loop. A mathematical model for dynamic stiffness and damping is developed. Average dynamic stiffness proved to be approximately twice that of the static stiffness for the same materials as independently measured over the same range of deflection by the bumper manufacturer. Natural frequencies of the sample bumpers were measured by two methods with close correlation of the results which ranged from 320 to 420 Hz for the samples tested. The materials are highly under-damped with damping factors much less than unity. The nonlinear dynamic stiffness characteristics of these bumpers can be modeled as quadratic functions for design purposes.