Abstract The in-wheel-motor (IWM) drive system has some interesting features, such as the vibration of this structure at low velocity. An explanation of this phenomenon is given in this paper by considering the dynamics performance of the in-wheel motor drive system under small slip ratio conditions. Firstly, a frequency response function (FRF) is deduced for the drive system that is composed of a dynamic tire model and a simplified motor model. Furthermore, an equation between the resonance velocity with the parameters of the drive system is obtained by combining the resonance frequency of this drive system with the fundamental frequency of the motor. The correctness of the equation is demonstrated through simulations and experimental tests on different road surfaces. The impact of different parameters on the vibration can be explained by this equation, which can give the engineer some instructions to design a control method to avoid this feature. Introduction Nowadays the electric vehicle is becoming the mainstream of future clean automotive technology with broad prospect, because of the depletion of fossil fuel resources and environment contamination problems caused by massive using of traditional vehicles equipped with internal combustion engine. Among the various configurations of the electric vehicle (including fuel cell vehicle), undoubtedly the in-wheel-motor (IWM) drive system will be an important key technology in the near future.[ 1] Besides the advantages that allow to flexible layout and roomy interior spaces by eliminating the mechanical parts connection of the driveline, the direct electric drive system also occupies the potentials of enhancing vehicle dynamics performances due to IWMs have the capability of quick and accurate torque vectoring control under various driving conditions. Therefore, development of in-wheel motor systems has become a popular topic of research in recent years. [ 2, 3] With further in-depth studies, some special phenomena are observed, such as the vibration of this structure at low velocity. [4] To explain these phenomena, studies on the modeling of in-wheel motor system should be made. Since the motor and tire in wheel drive system are highly electromechanically coupled, many published researches regarding the in-wheel-motored EV control based on the simplified rigid wheel dynamics models. [ 5] As a result, the different dynamic properties caused by different parameters of the wheel and tire cannot be sufficiently captured when the rigid wheel assumption is adopted. Particularly in case when the vehicle starts from stop, deviation introduced by transient tire dynamics may significantly affect the wheel performance, due to the fundamental frequency of the motor sweeping from the low to high level. [ 6] In this paper, an equation was proposed to explain the rolling speed vibration phenomenon of the in-wheel-motor, and the dynamic characteristics about resonance frequency and resonance velocity were illustrated. Firstly a relaxation-length-based dynamic tire model and a simplified motor model were introduced to establish the drive system. Its frequency response function (FRF) and the resonance frequency (FR) were deduced to analyze the wheel dynamics in the frequency domains. Combining the FR with the fundamental frequency of the motor, the equation of the resonance velocity was obtained, which can explain the impact of different parameters on the vibration. This proposed equation was verified by simulation. Also, the impact of longitudinal stiffness on the vibration is demonstrated by real vehicle test, which showed the correctness of the equation.An Explanation of the In-Wheel Motor Drive System’s Vibration at Low Velocity Using Motor-Wheel Frequency Characteristics2016-01-1673 Published 04/05/2016 Long Chen, Shuwei Zhang, and Mingyuan Bian Tsinghua Univ. Yugong Luo Tsinghua Univ., CICEV Keqiang Li Tsinghua Univ. CITATION: Chen, L., Zhang, S., Bian,