INTRODUCTION The rapid growth of car ownership has brought serious problems such as energy shortages, environmental pollution, and so on. Developing eco-friendly electric vehicles has become an effective way to mitigate these problems. Among numerous structural forms of electric vehicles, in-wheel motor driven type has gained great attention due to its obvious advantages in structural layout, integrated chassis control and manipulation. As a critical part of in-wheel motor driven vehicles, electric wheel system becomes the focus of research [ 1, 2, 3]. The electric wheel assembly is a multi-physical and multi-body system, which includes tire/wheel, in-wheel motor and controller. Interaction between electromagnetic excitation of in-wheel motor and structural dynamics of tire results in the emergence of dynamic problems in electric wheel system. From the perspective of electromagnetic excitation, due to non-ideal factors such as non-sinusoidal magnetic field distribution, cogging torque and current harmonics, the electromagnetic torque of motor will inevitably fluctuate [ 4]. In addition, the driving torque of in-wheel motor changes rapidly in unsteady conditions such as starting and acceleration [ 5]. The driving torque will act as main source exciting vibration of electric wheel system. From the perspective of structural dynamic features, direct connection between in-wheel motor and wheel without buffer in the driveline makes the electric wheel system a weak damping system [ 6]. As a result, abrupt change of driving torque in combination with torque ripple will cause transient vibration problem of electric wheel system in starting condition. As the tire is coupled in longitudinal and circumferential directions through tire/road interface, driving torque will easily cause longitudinal vibration of electric wheel. Therefore, unlike the extensively studied vertical vibration of traditional suspension tire system caused by road roughness, longitudinal vibration in electric wheel system is worthy of attention. Scholars have made some researches with respect to longitudinal dynamic problems of electric wheel system. It is demonstrated through bench test that longitudinal vibration of electric wheel system is noticeable. Subsequent analysis indicates that this longitudinal vibration is caused by torque ripple of in-wheel motor [ 7]. Furthermore, the influence mechanism of electromechanical coupling on torque ripple and longitudinal vibration performance are studied in [8]. In order to attenuate longitudinal vibration of electric wheel system, flexible connection between rim and in-wheel motor is adopted in [ 9] to modify the inherent characteristics of electric wheel and decrease longitudinal vibration in particular frequency bands. The above studies analyze electromechanical coupling of electric wheel system in steady conditions and neglect transient behavior of electric wheel system in unsteady conditions. In [ 10] and [11 ] the acceleration condition of in-wheel motor driven vehicle is simulated and transient dynamics of electric wheel system are analyzed. Slip ratio of tire fluctuates violently because of instantaneous change of driving torque. As a result, longitudinal dynamic characteristics of Longitudinal Vibration Analysis of Electric Wheel System in Starting Condition Yu Mao, Shuguang Zuo, and Xudong Wu Tongji University ABSTRACT Due to coupling of in-wheel motor and wheel/tire, the electric wheel system of in-wheel motor driven vehicle is different from tire suspension system of internal combustion engine vehicle both in the excitation source and structural dynamics. Therefore emerging dynamic issues of electric wheel arouse attention. Longitudinal vibration problem of electric wheel system in starting condition is studied in this paper. Vector control system of permanent magnet synchronous hub motor considering dead-time effect of the inverter is primarily built. Then coupled longitudinal-torsional vibration model