## 1 Introduction
The drag equation below is generally used to calculate the aerodynamic forces:  
\[F_{D}=\frac{1}{2}\cdot\rho\cdot v_{w}^{2}\cdot A\cdot C_{d} \tag{2}\]  
with \(F_{D}\) as the drag force, \(\rho\) as the fluid density, \(v_{w}\) as the velocity of the air relative to the truck, \(A\) as the area that is vertical to the relative motion and \(C_{d}\) as the drag coefficient. The drag coefficient depends on the relative position and geometry of the payload. However, the equation is accurate only for isolated objects. It is normally employed for basic load shapes. The actual truck-trailer configuration can increase the aerodynamic forces. The greatest contributor to the longitudinal aerodynamic force acting in commercial tractor-trailer configurations is the space between tractor and semitrailer [22; 23]. The space is expressed by \(b/\sqrt{A}\), a non-dimensional form, in which \(b\) is the width of the space (gap) and \(A\) is the area. The drag force that the gap produces is zero for small gaps. It increases suddenly when \(b/\sqrt{A}\approx 0.5\), but becomes stable at 0.7 [22; 23]. The other parameter that raises the drag force is the yaw angle, especially in trucks that have sharp-edged cabs. Crosswinds on exposed highways and bridges can cause difficulties in driving in tall and long vehicles and the risk of a serious accident. The force of the crosswind on a conventional truck-trailer in North America as it crosses a bridge was examined by using Computational Fluid Dynamics (CFD) [24]. The authors validated the model with experimental data that previous studies had provided [25; 26]. The conclusion was that wind pressure can be sufficient to overturn the truck-semitrailer at the wind speeds that were studied. This paper describes a novel theoretical model that can determine the stability of special transport. The proposed model determines the load on each axle, the tipping angle, and the suspension system's oil pressure. It was based on modular trailers models that appear in the literature, as well as those in common use within the heavy transport sector [27]. The model was tested experimentally by a full scale test [28] in which oil pressure data and output from the model were compared. To conduct the test, a nacelle wind turbine that weighed 42,500 kg and was 5133 \(\times\) 2650 \(\times\) 2975 mm in length, width, and height, respectively, was placed at different points within a modular trailer. With the validated theoretical model, a proposed optimization algorithm is capable of determining the optimal CoG of the load, as well as the number of trailers, number of axles, oil pressures, and hydraulic configuration. The theoretical model and optimization algorithm were tested with a cylindrical tank that weighed 108,500 kg and had dimensions of 19,500 \(\times\) 3200 \(\times\) 2500 mm. The results showed that the optimization algorithm could safely optimize the configuration of the hydraulic suspension of modular trailers in special road transport, increase the accuracy and reliability of the calculation of the load configuration, save time, simplify the calculation process, and be easily implemented. The theoretical model and the optimization algorithm proposed are able to (1) optimize safely the configuration of modular trailers' hydraulic suspension in special road transport, (2) improve the reliability and accuracy of load configuration calculations, (3) saves time, (4) simplify the calculation process, and (5) be easily implemented.