## 2 Frame Model and Calculation Method
### Boundary Conditions  
This paper mainly carried out simulation experiments on the four working conditions of the frame: full load bending, full load torsion, emergency turning, and emergency braking. The schematic diagram of each working condition is shown in Figure 3. In the simulation process, it was necessary to define the contact types of various components of the frame. The contact types were divided into bonded, no separation, frictionless, rough, and friction types. Due to the welding fixation of various parts of the frame, the bonded type was used in the simulation process, and the difference in contact types is shown in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name** & **Material** & **Density/(g\(\cdot\)cm\({}^{-3}\))** & \begin{tabular}{c} **Poisson/s** \\ **Ratio** \\ \end{tabular} & \
\begin{tabular}{c} **Elastic** \\ **Modulus/GPa** \\ \end{tabular} &
\begin{tabular}{c} **Yield** \\ **Strength/MPa** \\ \end{tabular} \\ \hline Beam & Q345 & 7.85 & 0.2 & 206 & 345 \\ Axles, axle & & & & & \\ supports, support & 40 Cr & 7.85 & 0.3 & 211 & 785 \\ plates & Rubber & 1.2 & 0.47 & \(7.8\times 10^{-3}\) & - \\ Other parts & Q235 & 7.85 & 0.3 & 210 & 235 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Frame material parameters. Figure 3: Schematic diagram of each working condition. (**a**) Bending conditions, (**b**) torsional conditions, (**c**) turning conditions, and (**d**) braking conditions. The full load bending condition is the operating condition when the vehicle is driven on a flat road. Under this condition, the uniform and stable contact between the wheels and the road is maintained. At this time, the force of the car body in the vertical direction is mainly considered, and the influence of other factors such as lateral wind and inertial force is ignored. When setting the boundary conditions, the translational degrees of freedom in the UX, UY, and UZ directions of the four wheels before and after the car body are constrained, and the rotational degrees of freedom in the ROTX, ROTY, and ROTZ directions are constrained (transverse: X, vertical: Y, longitudinal: Z; U is the translational degree of freedom, ROT is the rotational degree of freedom, and the rest of the working conditions are the same). The full load torsion condition is the condition in which a wheel of a vehicle is suspended under severe road conditions. The unevenness of the road surface causes asymmetric support of the frame, which results in torsion. Under the condition of full load torsion, the wheel cannot maintain uniform and stable contact with the road surface, and so the torsion condition mainly simulates a situation whereby the left front wheel suddenly hangs in the air during the driving process. When setting the boundary conditions, all of the degrees of freedom of the left front wheel are released and all of the degrees of freedom of the right front wheel are constrained. At the same time, the vertical degrees of freedom in the UY direction of the two rear wheels are constrained and all of the other degrees of freedom are released. The emergency turning condition means that when the vehicle changes its driving direction, each part of the frame will cause displacement and stress changes due to centripetal acceleration. In order to simulate the state change in the frame under turning conditions, a gravitational acceleration of 1 g and a left-handed acceleration of 0.4 g were applied to the frame in this paper [21]. When setting the boundary conditions, the three translational degrees of freedom of the front and rear four wheels UX, UY, and UZ were constrained. At the same time, the rotational degrees of freedom of the four wheels in the ROTX, ROTY, and ROTZ directions were released. Under emergency braking conditions, the frame will be subjected to a longitudinal inertia force load, resulting in displacement and stress changes.