The static stability factor (SSF) of a vehicle is mainly dependent on the lateral and vertical location of the vehicle's centre of gravity (CG) and this point is, in turn, influenced by many factors associated with, for instance, the vehicle suspension, tyres and the fifth-wheel among others. These systems allow the movement of the vehicle body along with its CG, which modifies substantially the SSF calculation. This paper describes the modelling of a leaf spring suspension system and the tyres using mechanism theory and subsequent kinematic analysis using the Davies method. The proposed model is planar and allows relative motion between the sprung and unsprung masses of the vehicle. This paper also compares the classic vehicle stability analysis with the proposed model, and the influence of the suspension and tyres in relation to the vehicle stability is evaluated.