The occupants on board are being exposed to an environment in noise and vibration. Among them, the tire-road noise has come to relatively conspicuous because of improvement of silence. To improve comfort for occupants, reduction about the environmental noise and vibration is strongly expected. In this paper, we aim at knowing the vibration characteristics of tires related to tire-road noise of automobiles. Taking two kinds of tires, we carried out the impact tests. One has standard specification and the other has comfort specification absorbing vibration. Setting non-contact and contact to the ground, accelerances are obtained for single fixed input and multi output by impacting tests about tire treads and side-walls. To identify natural angular frequencies, modal damping ratios and modal amplitudes, the real and imaginary parts of accelerance are used in the error function of the optimized problem, which is solved by the down-hill simplex method. Both the natural angular frequencies and modes are identical for two kinds of tires each other. However, it is found that the modal damping ratio of the tire considered to be superior is not much different from the other. It is also found that when the tires contact with the ground, some natural frequencies could increase and other could decrease. Concerning about mode shapes, some axial mode shapes of tread might contradict with those of side-wall.

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