This paper extends the model developed previously by the authors,1 for studying dynamic stability of mobile manipulators. The extended model not only takes into account the dynamics of the base that can potentially rock back-and-forth, but also accounts for the flexibility of the contact between the base and the ground. A novel method of virtual links is used to reformulate the problem of modeling the non-fixed base manipulator in terms of a fixed-base manipulator with one degree of freedom joints. The resulting model is then used to simulate planar movements of a Caterpillar excavator-based log-loader. The results clearly demonstrate the relationship between movement of the implement and the stability of the machine. The flexible contact between the base and the ground is also shown to influence the stability. The results are also compared with those obtained by the previous model, which assumed rigid contact between the base and the ground. It is demonstrated that the assumption of rigid contact tends to overestimate the stability. Consequently, flexibility in the contact bet- ween the base and the ground must be considered to more accurately predict the stability of mobile manipulators.