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In this paper, a simple vehicle model was investigated for the study of vehicle behavior in the straight-ahead motion. The vehicle model has two degrees of freedom without any suspension effect included. Through the analyses of the model with the various tire properties, we can explain how force and moment characteristics of the tires can cause lateral drift in the straight- ahead motion in free control mode. In the second part of the paper, a commercially available car was selected for the validity of the analyses. This car was modeled in detail with one of the multi-body dynamics simulation programs. The simulation results show that the amount of vehicle deviation from the intended straight path is directly related to a characteristic of the front axle tires, known as ATSP (Aligning Torque Static Phase).

In the dynamic simulation of vehicle straight ahead motion, the vehicle model usually drifts from its intended straight path. It is quite difficult to have a vehicle model run along the intended straight path unless a control algorithm is applied for this purpose. This is particularly true when a tire model based on experimental data is used. It is often assumed that this drift is due to errors accumulated during numerical integration. In this paper, effect of tire characteristics is studied on the dynamic simulation of vehicle straight ahead motion. One of the cars commercially available is selected for the study. A multibody dynamic simulation program, known as ADAMS(Automated Dynamic Analysis of Mechanical Systems), is used for modeling and simulation. The vehicle model used in this study has 137 degrees of freedom. The possible tire characteristics affecting vehicle straight ahead motion are identified from the simulation.