The low yawing natural frequency of vehicle system and/or the coupled natural frequency of driver-vehicle system is considered as a reason for vehicle fluctuation. In the case of passenger cars, it is well known that the yaw-gain-peak of vehicle system exists at 1–2 Hz and that of driver-vehicle system exists at 0.3–0.4Hz. In the case of large sized vehicle systems, I, Momiyama, reported in SAE No.892519 that the yaw-gain exists at 0.3–0.4 Hz. As this value is so close to the value of driver-vehicle system, sometimes the discussion regarding the vehicle dynamics of large sized vehicles seems to be confused. The purpose of this paper is to confirm the yawing behavior and to clarify the reason for the low yawing frequency of vehicle system. Firstly verification procedure and a 7DOF vehicle model considering the lateral and rotational motion of sprung mass, front/rear un-sprung mass and rotational motion of steering system are introduced. Secondly the lateral fluctuation running under fixed steering wheel is shown. We named this behavior “tottering”. Thirdly the yaw response test results by random steering input based upon ISO/TR8726 are shown. From the above testing and calculating analysis the existence of a low yaw-eigen value is confirmed. From the vector analysis of the low yaw-eigen value it is known the main factor of the low frequency is the low rigidity of steering with power steering system. The low rigidity is caused by the low spring constant of power steering servo-valve that has the dead zone at on-center.

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