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In this paper, straight running stability without controlling behavior of the rider is analyzed with regard to a mathematical model of 12 degrees of freedom. This model has 2 degrees of freedom concerning leaning movement of upper body and lateral movement of lower body of the rider as well as 4 degrees of freedom of sideslip, yawing, rolling of motorcycle, and rotation of steering handlebar, and 6 degrees of freedom of frame rigidities. Vibration characteristics of the rider's body influence both weave and wobble modes. Theoretical calculation results by using a mathematical model of 12 degrees of freedom are relatively and qualitatively similar to the results obtained by full-scale running experiments. The extent of quantitative coincidence between theory and experiment, however, varies with types of motorcycles. Parameters concerning vibration characteristics of rider's upper body influence weave mode, and those of rider's lower body influence mainly wobble mode.