Vehicles, such as automobiles and trucks for example, include a rear drive module (RDM) that is connected to the vehicle engine by a prop-shaft. The prop-shaft transmits rotational energy (torque) developed by the vehicle engine to the rear drive module, which in turn transmits the rotational energy to the wheels. In a rear-wheel drive vehicle, the prop-shaft directly couples the RDM to the vehicle's transmission. In an all-wheel or four-wheel drive vehicle, additional components may also be included, such as a power take-off unit for example.
It should be appreciated that the transmission of rotational energy from the propshaft to the RDM, and from the RDM to the wheels generates reaction forces within the RDM to counter the transmitted torque. These reaction forces generally may be characterizes as a “roll” type and a “pitch” type movement. The roll type movement is a rotation about a longitudinal axis passing through the RDM. A roll type movement may cause the axles to flex with respect to the RDM and cause undesired noise and vibrations. A pitch type of movement is a rotation about the lateral axis the RDM due to a reaction to the drive torque at the wheels. Articulation of the RDM due to pitch also results in undesirable noise and vibration, and may also reduce the operating life of the prop-shaft.
Traditionally, to counter the reaction torques placed on the RDM, a mounting system was used that securely coupled the RDM to the vehicle structure, such as directly to the vehicle frame, or to an intermediary cross-member or cradle-member. Typically, these systems used some type of three-point mount that included isolation bushings that reduced the transmission of vibration from the RDM to the structure. It should be appreciated that these vibrations may have been due to the operation of the RDM and by the operation of the engine as well.
Traditionally, the vehicle engine was an internal combustion engine having cylinders that are alternately fired to produce the rotational energy. Due to a need to improve fuel efficiency, alternate control schemes for the vehicle engine have been developed that selectively deactivate cylinders. Under certain circumstances, when a cylinder is deactivated, no fuel is combusted and fuel efficiency is increased. However, it has been found that the deactivation of cylinders results in low frequency vibrations being transmitted to the RDM via the prop-shaft that were not previous experienced in traditional engine control configurations. Further, it has been found that in some circumstances, existing RDM mounting arrangements were inadequate to counter the excitation forces generated at these low frequencies.
Accordingly, it is desirable to provide an RDM and RDM mounting arrangement that provides a desired level of performance when subjected to low frequency vibrations from the vehicle engine.