Mount Stiffness
The bodies connected by the bushing are flexible and may deflect under the load being transmitted. This phenomenon is modeled with the Mount Stiffness feature. Mount stiffness models the structural stiffness of the bodies, thus mounting the bushing as a linear spring and damper in series with the bushing in each direction.
Modeling Mount Stiffness
The mount or structure surrounding a bushing deflects under the force transmitted by the bushing. The local stiffness models this deflection. If the IBody and JBody connected by the bushing are modeled as rigidbodies, then you must estimate structural deflection separately and add that value to the bushingrubber deflection. Mount stiffness is NOT required if the two bodies are modeled as flexible bodies.
To include local structural stiffness in the bushing model, you enter the combined local stiffness of the IBody and JBody, and then set the local stiffness flag to active.
When one or both bodies is flexible, you specify that the mount stiffness is conditionally active only when the IBody and JBody are both rigid.
Let:
 ${d}_{k}$
 be the bushing deformation in the k^{th} direction.
 ${s}_{k}$
 be the net local structural deflection in the k^{th} direction.
 ${s}_{ik}$
 be the local structural deflection of the Ibody in the k^{th} direction.
 ${s}_{jk}$
 be the local structural deflection of the Jbody in the k^{th} direction.
 ${K}_{sk}$

be the local structural stiffness in the k^{th} direction.
 ${K}_{ik}$
 be the local structural stiffness of the IBody in the k^{th} direction.
 ${K}_{jk}$
 be the local structural stiffness of the JBody in the k^{th} direction.
 ${C}_{sk}$
 be the local structural damping in the k^{th} direction.
 ${C}_{ik}$
 be the local structural damping IBody in the k^{th} direction.
 ${C}_{jk}$
 be the local structural damping of the JBody in the k^{th} direction.
 ${F}_{k}$
 be the bushing force in the k^{th} direction.
Then the net structural deflection for the kth direction is:
Since the Ibody structural stiffness, the Jbody structural stiffness and the bushing act in series, the forces in k^{th} direction in the bushing, the Ibody and Jbody are all equal.
Through similar logic, one can conclude:
The structure and the bushing act in series. Therefore they must generate the same force:
The above equation is integrated to yield ${s}_{k}$ , where the structural deformation is known and accounted for.