Based on root locus comparisons between a 3 degree-of-freedom coupled model and approximate models (plane model, cross section model, simple roll model), effects of vehicle parameters on coupled dynamics, such as cornering stiffness using to determine the plane motion characteristics and roll stiffness using to determine the roll motion characteristics, were confirmed. Moreover, effects of vehicle parameter interactions on coupled dynamics were also explored. It is shown that the natural frequency ratio ω_n2/ω_n1 between a simple roll model and a plane model has a dominant influence on coupled characteristics. Additionally, effects of inclinations of principal inertia and roll axes on motion characteristics were also confirmed using the root locus analysis.