A 4-DoF spatial mechanical model of trailers is used to analyze the snaking, rocking and roll-over motions. The derivation of the governing equations is presented. The non-smooth characteristics of the tire forces are taken into account together with other geometrical non-linearities. Beyond the linear stability analysis the nonlinear vibrations are investigated via numerical continuation and simulations. Linear stability charts and bifurcation diagrams are constructed to identify the parameter ranges, where large amplitude vibration can occur. The domains of the snaking and the rocking motion are detected. Numerical simulations verify the existence of periodic solutions predicted by numerical bifurcation analysis. A small scale experimental rig is presented and the nonlinear vibration of the trailer is investigated.