# Idle Vibration Reduction of a Diesel Sport Utility Vehicle
## 2 Experimental Analysis
Furthermore, the bending deformation dominates over the torsional deformation. Notably, the ODS of the body floor suggests flexible-body motion, whereas the ODSs of the powertrain, radiator, and intercooler indicate rigid-body motions. Comparing the vibration amplitudes of the ODSs for these four components, the amplitude of the powertrain is the largest, followed by that of the intercooler, radiator, and body floor. Modal tests were performed to obtain the natural frequencies and mode shapes of the powertrain, radiator, intercooler, and body floor under the condition that all the components are assembled together. The modal tests were conducted on the diesel SUV with all these components assembled; an impact hammer (B&K 8206) was used to apply input excitation, and accelerometers (PCB 356A15) were used to measure the output acceleration signals. Considering that the idle vibrations at the driver's seat occurred mainly in the vertical direction, vertical excitation was imposed on each component using the impact hammer to extract the natural frequencies and mode shapes related to vertical vibrations. The impact and measurement points for performing modal tests of each component are illustrated in Figure 6, wherein the red squares and circles represent the impact hammer excitation points and the accelerometer attachment positions, respectively. After obtaining a series of the frequency responses functions for each component using the sound and vibration analyzer, the natural frequencies and associated mode shapes were determined. In this study, the natural frequencies and mode shapes of the components were chiefly analyzed in the low-frequency region. The vibration modes in the low-frequency region are closely related to the vibration in the idle state. The mode shapes of the powertrain, radiator, and intercooler in the low-frequency region are presented in Figure 7. The powertrain of the SUV employed in this study is supported by the engine, transmission, and roll mounts. Because these mounts act as springs and dampers and support the powertrain, the powertrain exhibits six rigid-body vibration modes. The powertrain possesses the rigid-body modes corresponding to the natural frequencies of 4.8, 13.0, and 18.5 Hz. The mode for the natural frequencies of 4.8, 13.0, and 18.5 Hz are the bounce (refer to Figure 7a), pitch (refer to Figure 7b), and roll modes (refer to Figure 7c). In the low-frequency region during the idle state, three natural vibration modes of the radiator and intercooler exist, which are related to the ride comfort. These modes comprise the bounce mode of the radiator with a natural frequency of 20.0 Hz (refer to Figure 7d), the roll mode of the radiator with a natural frequency of 30.3 Hz (refer to Figure 7e), and the bounce mode of the intercooler with a natural frequency of 29.3 Hz (refer to Figure 7f). Figure 6: Impact and measurement points for the experimental modal tests: (**a**) powertrain; (**b**) radiator; (**c**) intercooler; and (**d**) body floor. In the low-frequency region, unlike the powertrain, radiator, and intercooler, which solely exhibit rigid-body modes, the body floor possesses not only rigid-body modes but also deformed modes. The mode shapes for the four lowest natural frequencies of the body floor are depicted in Figure 8. The modes depicted in Figure 8a,b are rigid-body modes of the body, whereas the modes represented in Figure 8c,d are deformed modes. In particular, the mode with a natural frequency of 3.2 Hz is the bounce mode of the vehicle body (refer to Figure 8a), and the mode with a natural frequency of 3.7 Hz is the pitch mode (refer to Figure 8b). Conversely, the mode with the natural frequency of 23.8 Hz is the bending mode of the vehicle (refer to Figure 8c), and the mode with the natural frequency of 28.4 Hz is the torsional mode (refer to Figure 8d). These flexible modes are expected to possess larger natural frequencies than the rigid-body modes.