In general, long combination vehicles (LCVs) show poor performance with regard to stability, and this has been the focus of many studies around the world. Some characteristics such as the suspension topology, tyres, chassis and fifth wheel have been analysed, separately, to determine their influence on the vehicle stability calculation. Most vehicle stability models are developed in two dimensions, and they do not consider the longitudinal aspects of the vehicle and the road such as the stiffness of the chassis, the gravity centre location and the longitudinal slope angle of the road. In this context, the aim of this study was to develop a three-dimensional mechanism model of the trailer to represent all of these characteristics and their influence on the lateral stability.