Abstract:  The in-wheel motor produces torque ripple，which exerts the wheel boss directly． It causes the longitudinal dynamic problems，such as fluctuation of tire slip rate and longitudinal force． Accordingly， the model characteristic of electric wheel is firstly analyzed and three indexes are proposed to evaluate the longitudinal performance of electric wheel． Secondly， the sensitivity of system parameter is analyzed，and the law is detected of longitudinal acceleration which is mainly affected by the mass of vehicle body and in-wheel motor． The error of slip ratio and fluctuation of load torque depend on mass parameters，circumferential rigidity of tire and tread rigidity． Finally， the parameter optimization determines the improvement of the parameter value of the three indexes and it is found that parameter optimization is able to cause each index to be radically reduced in maximum value under the condition of corresponding models． 

Key words: electric wheel system, longitudinal vibration, dynamic index, sensitivity, parameter optimization