# Analysis of Operating Safety of Tractor-Trailer under Crosswind in Cold Mountainous Areas
## 2 Construction of Tractor-Trailer Operation Scenario  
### Model of the Tractor-Trailer  
The tractor model and the trailer four-axle model were built on Trucksim software which integrated speed control, steering, braking, variable speed, tires, suspension, aerodynamics, and other subsystems, shown in Figure 1. The tractor has one steering axle and one solid axle, the trailer has two solid axles, and the box payload is used instead of the cargo in the trailer. The parameters of the four-axle complete vehicle model are shown in Table 1.  
The vehicle is affected by the air force while driving on the road. According to the SAE Road Vehicle Aerodynamics Committee standard (Vehicle Aerodynamics Terminology J1594_201007), the location of the aerodynamic reference point for a tractor-trailer is shown in Figure 2, and the equations for the aerodynamic forces and aerodynamic moments to which it is subjected under the action of side winds are as follows.  
\[\left\{\begin{array}{l}F_{x}=-C_{F_{x}}(\beta)AQ=-\frac{1}{2}C_{F_{x}}(\beta) A\rho v^{2}\\ F_{y}=-C_{F_{y}}(\beta)AQ=-\frac{1}{2}C_{F_{y}}(\beta)A\rho v^{2}\\ F_{z}=-C_{F_{z}}(\beta)AQ=-\frac{1}{2}C_{F_{z}}(\beta)A\rho v^{2}\\ M_{x}=-C_{F_{x}}(\beta)ALQ=-\frac{1}{2}C_{F_{x}}(\beta)AL\rho v^{2}\\ M_{y}=-C_{F_{y}}(\beta)ALQ=-\frac{1}{2}C_{F_{y}}(\beta)AL\rho v^{2}\\ M_{z}=-C_{F_{z}}(\beta)ALQ=-\frac{1}{2}C_{F_{z}}(\beta)AL\rho v^{2}\\ \end{array}\right. \tag{1}\]  
where, \(\rho\) is the air density, this paper takes 1.206 kg/m\({}_{3}\); \(v\) is crosswind speed, m/s; \(F_{x}\) is the aerodynamic drag, \(F_{y}\) is the side force, and \(F_{z}\) is the aerodynamic lift, N; \(M_{x}\)roll moment, \(M_{y}\)  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Value** & **Unit** \\ \hline Lead unit sprung mass & 4455 & kg \\ Tractor size & 6750 \(\times\) 2438 \(\times\) 3030 & mm \\ Tractor wheelbase & 4000 & mm \\ Trailer sprung mass & 5925 & kg \\ Trailer size & 10,000 \(\times\) 2800 \(\times\) 3200 & kg \\ Trailer wheelbase & 1300 & mm \\ Load mass & 21,340 & kg \\ Load size & 9000 \(\times\) 2500 \(\times\) 3000 & mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Four axle box trailer and trailer vehicle parameters.  
Figure 1: Model of (**a**) four-axle FL box tractor-trailer and (**b**) box payloadFL.  
Figure 2: Tractor-trailer aerodynamic reference point and aerodynamic slip angle.  
is the pitch moment and \(M_{\rm z}\) is the yaw moment, N/m; \(C_{\rm F_{x}}\) is aerodynamic drag coefficient, \(C_{\rm F_{y}}\) is lateral force coefficient and \(C_{\rm F_{x}}\) is lift coefficient; \(\beta\) is the angle between the forward direction of the vehicle and the crosswind direction, that is, the aerodynamic slip angle, deg; \(A\) is the projected area of the vehicle, m\({}^{2}\); \(L\) is the vehicle length, m.  
Because of the structure of the tractor-trailer, compared with ordinary vehicles, the impact of the crosswind is more obvious, and its aerodynamic coefficient is also very different from that of ordinary vehicles, so it is necessary to measure its aerodynamic coefficient before analysing the impact of the crosswind. TruckSim provides the aerodynamic force coefficients and aerodynamic moment coefficients for tractor-trailer, as shown in Figure 3.