Abstract: In order to discover the effect of driving torque on the dynamic characteristics of vehicle system,firstly,a planar-motion nonlinear dynamic model of the vehicle system with five degrees of freedom (longitudinal velocity,lateral velocity，yaw rate，front-wheel rotational velocity and rear-wheel rotational velocity) is established on the basis of Magic tyre formula.Then,by taking the rear-wheel driving system as an example,the phase space analysis of multiple initial values,the time domain analysis of single initial value as well as the corresponding tyre-force dis- tribution feature analysis is carried out,and the effect of constant driving torque on the dynamic characteristics of vehicle without front-wheel steering angle input is discussed.Finally,through a further analysis of the numerical calculation with constant driving torque，the bifurcation diagrams of system state variables are plotted.The results show that,under the condition of zero steering angle input,bifurcation phenomenon of the vehicle dynamic system with 5 degrees of freedom may occur with the increase of driving torque; and that driving torque has an obvious effect on the dynamic characteristics of vehicles.

Key words: ground vehicles, dynamics, driving torque, phase space analysis, bifurcation analysis