Abstract: Since the lateral dynamics of a tractor semi-trailer vehicle is rather complex resulting from the large variations of the load and the centre of mass(CM) of the trailer, it becomes difficult to design the stability control system. The nonlinear dynamics and the lateral stability of the tractor semi-trailer vehicle is investigated. The relations of the lateral stability, equilibrium bifurcation and Hopf bifurcation with the load scenarios are analyzed in detail. Combining the means of understeer gradient and damping ratio, the lateral instability and mechanisms are discussed. The results reveal that nonlinear dynamical phenomena such as saddle-node bifurcation, Hopf bifurcation and homoclinic orbit are identified in critical cornering situations. For a typical tractor semi-trailer vehicle, jackknife caused by the sideslip of the rear axle of the tractor is the primary way to lead the vehicle to lateral instability. Trailer oscillation which is much related to Hopf bifurcation generally occurs when the load weight is considerably large and the trailer CM is much near to the rear axle of the trailer. Further, trailer oscillation is prone to further develop to jackknife and trailer swing often causes the whole vehicle including the tractor to sideslip with jackknife occurring simultaneously. Rather satisfying control performance is expected to be obtained by selecting the tractor yaw rate and trailer slip angle as the control variables to design vehicle stability control system.

Key words: Bifurcation, Handling and stability, Nonlinear dynamics, Tractor-semitrailer, Vehicle dynamics