Dynamic analysis of the resonant beam machine is presented. Equations typically used to determine the dynamic properties of a rubber product are derived from a simplified model. A complex mathematical model representing a more realistic parametric system identification is formulated. Using the complex model as standard, the errors in the results from the simplified model are identified quantitatively, and it is shown they can be considerable. The sensitivity of the errors to the system parameter variations is also shown. These machine errors may be virtually eliminated by measuring the transmitted force and displacement across the sample.