For a tractor-semitrailer system with four degrees of freedom the influence of variations of parameters on the driving behavior for steady state and braking motions is investigated. As parameters the driving speed (for steady state motion), the steering angle, nonsymmetric loading and the braking force distribution are considered. The goal of the paper is not only to give stability boundaries but also to obtain at least qualitative information about the behavior of the system after the transition from the stable to the unstable regime. To achieve this goal a nonlinear mechanical model must be used, the nonlinearity of which is mainly due to the relationship between the tire side force and the tire slip angle. Results obtained in bifurcation theory during the last decade are used to give a complete description and classification of the driving behavior if the parameter values pass through critical values.
For the steady state motion for practically important cases divergence bifurcation occured, which is extensively discussed for a symmetric and a nonsymmetric model. In the motion during braking besides divergence bifurcation also Hopf-bifurcation was found. This means that an oscillatory motion sets in after the parameter values pass through a critical value. Which type of bifurcation is found is heavily dependent on the braking force distribution to the different axles and also the arrangement of the loading on the trailer.