Though it is well known that the vibration and the noise of agricultural machinery during running on a road are strongly influenced by the tire lug form, few studies have been carried out to clarify this mechanism, while many studies were done on the characteristics of automobile tire. In the previous studies for automobile tire, the tire is typically modeled as a thin, cylindrical beam on a continuously distributed elastic foundation representing the sidewall stiffness. However, this model cannot be directly applied to agricultural tire, because of non-uniform stiffness effect caused by tire lugs. In this research, natural frequencies and mode shapes of a non-rotating agricultural tire are assessed by the experimental modal analysis method for two boundary conditions: without the ground contact and with the ground contact. Then, the tire is modeled as a thin, cylindrical beam with multiple radial spring attachments, which represent tire lugs with non-uniform radial stiffness. A calculation procedure based on the receptance method for obtaining the natural frequencies of tire is formulated, then applied to the said two boundary conditions. Furthermore, the fundamental parameters of tire are identified from the measured natural frequencies using Downhill Simplex method. As a result, the influence of boundary conditions on the parameter is clarified.

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