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Downloaded Fthen used to create equivalent foundation models. An alternative method to include the dynamics of the support structure is to calculate polynomial transfer functions from the measured FRF’s. These transfer functions can be used for unbal-ance response and stability analysis ~@13,14#!. A research work was initiated to answer the question of how much the dynamics of the support structure affects the rotor bear-ing system and how accurately one can predict this effect. To that end, a flexible rotor supported by fluid film bearings on flexible supports was tested for unbalance response and stability. The re-speeds of the system are located at 2550 rpm and 4130 rpm, respectively. A one-horsepower motor drives the rotor through a flat belt and flexible coupling. Any rotor speed up to 11000 rpm can be obtained. Figure 1 shows the experimental setup. Two identical three lobe bearings were used in this work. Tabl