Effect of Local Stiffness Coupling on the Modes of a Subframe-Bushing System

The elastomeric joints (bushings or mounts) in vehicle structural frames are usually described as uncoupled springs (only with diagonal terms) in large scale system models. The off-diagonal terms of an elastomeric joint have been previously ignored as they are often unknown since their properties cannot be measured in a uniaxial elastomer test system. This paper overcomes this deficiency via a scientific study of a laboratory frame that is designed to maintain a high fidelity with real-world vehicle body subframes in terms of natural modes under free boundaries. The steel beam construction of the laboratory frame, with four elastomeric mounts at the corners, permits the development of a highly accurate, yet simple, beam finite element model. This allows for a correlation study between the experiment and model that helps shed light upon the underlying physical phenomenon. In particular, the effect of local stiffness coupling of elastomeric bushings or mounts is demonstrated through computational modeling and experimental validation. It is seen that the joint stiffness matrices strongly influence the modal properties of a laboratory subframe-mount system. For instance, a strong correlation between the rigid body modes (r = 1 to 6) and the first three elastic modes (r = 7 to 9) is only possible when the coupling (non-diagonal) terms are included in the bushing model.