*2.1. Simplified Vehicle Road Dynamic Model*

According to different research needs, traditional vehicle models can come in three different styles. They are the quarter two-degree-of-freedom vehicle model, the half fourdegree-of-freedom vehicle model, and the whole vehicle model, respectively. The first two models are widely used due to their simple calculation, but the parameters of vibration are not fully considered and the calculation accuracy is not high. Based on the traditional model, a 7-DOF vehicle model is established in this paper, as shown in Figure 1. In this model, the car body and wheels are treated as rigid bodies. A vehicle consisting of a single body and several wheels passes through the line at a uniform speed; for the body itself, the freedom of nod vibration, deflection vibration, and vertical vibration will be considered [20], along with the vibration of the four wheels, for a total of seven degrees of freedom. Furthermore, it is assumed that the wheels of the vehicle are in close contact with the road surface. In this paper, we mainly study the vibration of the vehicle–road system caused by the excitation of road roughness. In particular, this paper studies the effects on vehicle ride performance, including comfort, safety, and irregularity. To simplify the analysis process, it was simplified into the 7-DOF vehicle structure model shown in Figure 1.

**Figure 1.** 7-DOF vehicle structure model.
