## 3 Results and Discussion  
### Experimental Validation of the Proposed Model  
For the purpose of validating the proposed theoretical model, several experiments were conducted to measure the oil pressure of the hydraulic cylinders with the load located in various positions on a modular trailer. This validation process is used to determine the reliability of a theoretical model, and is usually applied in the same way in many fields of study [30; 31; 32; 33]. The greater the number and variety of experiments, the more reliable is the validation. However, experiments are generally very expensive. Therefore, it is very important to adopt the correct experimental procedures in order to obtain a successful validation. The number of experiments depends on the number of inputs. The model considers load weight, CoG location, load shape, trailer modules, groups' definition, road inclinations, speed, acceleration, turning radius, and wind speed. Testing some of these inputs is very difficult. For example, measuring the acceleration and the platform inclination is very complicated, because the trailer is in motion. Dynamic and aerodynamic inputs were not considered in designing a feasible set of experiments in this study. Further, it is assumed that the stability of the trailer depends mainly on the load's location on the platform. In order to simplify the experiments, only the longitudinal and transverse locations of the load were considered. All of the experiments were carried out under the following conditions:  
* Trailer: SPMT 6-axle: weight 23.5 and maximum capacity 216.3 Tn. * The oil pressure is supplied to the hydraulic cylinders at the axles by a Power Unit (PPU). * Even surface: camber and slope equal zero. In addition, the load used in the experiments was a nacelle wind turbine. The cover housing contained the generator and gearbox, drive train, brake assembly, and all other generating components of the wind turbine. The properties of the nacelle wind turbine are:  
* Weight: 42,500 kg. * Dimensions: 5133 \(\times\) 2650 \(\times\) 2975 mm. * CoG position: 97.5 \(\times\) 0 \(\times\)\(-\)180 mm. Figure 7 shows the elements used in the experiments to validate the proposed model. Figure 6: Forces in a 3-point suspension system: (**a**) 3D coordinate systems with X, Y and Z axes (**b**) 2D coordinate systems with X and Y axes. Design of experiments (DoE) is a set of methods to determine relationships between the inputs and outputs of processes [34; 35; 36]. These techniques establish the order of variables to determine cause and effect relationships with confidence. DoE is used here to define the combination of inputs to determine the accuracy of the model. There are many DoE techniques. However, all involve the construction of a table, in which each row corresponds to a single experiment and each column to a factor. The input variables are called factors, and each of the values are called levels. However, factors can be controllable or uncontrollable, also called noise. The uncontrollable factors are due to environmental changes or small variations in components properties. Their effects can be minimized by randomizing and grouping the experiments. Nevertheless, experiments always have uncontrollable effects, called experimental errors. These can be determined by repeating the same experiment to measure the dispersion of the responses. In this case, there are two inputs with which to create the set of experiments. Table 1 shows the inputs and their symbols, units, and levels (design matrix), whereas Figure 8 clarifies the physical meaning of the inputs. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Inputs**} & \multirow{2}{*}{**Symbol**} & \multirow{2}{*}{**Unit**} & \multicolumn{3}{c}{**Levels**} \\ \cline{4-5}  & & & \(-\)**1** & **0** & **+1** \\ \hline Longitudinal location of the load & X & mm & 0 & 675 & 1350 \\ Transversal location of the load & Y & mm & 0 & 175 & 350 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design of experiments: inputs, units, and levels.