Rigid Body Modes

These modes exist for all objects and are the modes of vibration where the object itself remains rigid and are the rotation or translational modes of the object on its mounting system.

A powertrain subsystem would be expected to develop six rigid body modes on its mounts. Ideally, all six of these modes would occur below the suspension modes, below approximately 11Hz. However, the reality of vehicle and powertrain mount system design dictates that some of these modes will occur at slightly higher frequencies. Therefore, it is critical to maintain control of certain powertrain modes within this region of the frequency domain. The bounce and roll modes of a tranversely mounted, front wheel drive powertrain may couple with front suspension vertical modes (hop and tramp). Therefore, they should have separation within the frequency spectrum. It is desirable to develop a lower frequency powertrain pitch mode for optimal idle vibration isolation. The other powertrain modes may occur at higher frequencies as their respective mode shapes limit their ability to couple with suspension resonances, even if they occur at similar frequencies. A similar philosophy should be applied to other types of powertrain systems.

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Subjects: Noise & Vibration