## 2 Theoretical Premises  
In order to draw up a design scheme, we will consider an option where the following external forces and movements act upon the modular draft device in the process of its movement with a semitrailer (Figure 2): \(\overline{T}_{a}\), \(\overline{T}_{b}\), \(\overline{T}_{c}\) (kN); \(\overline{P}_{fa}\), \(\overline{P}_{fb}\), \(\overline{P}_{fc}\) (kN); \(M_{a}\), \(M_{b}\) and \(M_{c}\) (kN\(\cdot\)m) are the tractive forces, as well as the forces and movements of the rolling resistance of the front, rear and technological axes of the MDD, respectively; \(\overline{G}_{b}\) is the front axle ballast weight force MDD, kN; \(\overline{N}_{a}\), \(\overline{N}_{b}\) and \(\overline{N}_{c}\) are the surface reaction forces, kN; \(\overline{G}_{t}\) and \(\overline{G}_{m}\) are the forces of the weight of the power and technological modules of the MDD, respectively, kN; \(\overline{X}\) and \(\overline{Y}\) are the reactions at the point \(O\), kN; \(\overline{R}_{g}\) and \(\overline{R}_{v}\) are the horizontal and vertical components of the tractive resistance of the semi-mounted trailer, kN. Three of these movements and forces are unknown: \(N_{a}\), \(N_{b}\) and \(N_{c}\). For their definition, it is necessary, as it is known, to have three independent MDD equilibrium equations in the longitudinal-vertical plane. In reality, we have only two such equations, namely: (i) the sum of the forces on the \(Y\) axis; and (ii) the sum of the movements relating to some point. Since there are three unknown forces, and two independent equations of equilibrium, we obtain a statically indeterminate system. To solve this, it is proposed to consider the balance of the power and technological modules of the MDD separately. At first, we will do this with the technology module. We will replace the influence of the PM on the TM in the longitudinal-vertical plane with reactions \(X\) and \(Y\), concentrated in point \(O\) (Figure 2). This is formed as a result of the intersection of the directions of the central and the lower links of the rear-mounted EM mechanism, and it represents the instantaneous center of rotation of this mechanism in the longitudinal-vertical plane. As a result of this approach, we obtain a state of the TM in which three unknown forces act upon it: \(X\), \(Y\) and \(N_{c}\). However, now, three well-known equilibrium equations of the system under consideration are sufficient to determine them. The impact of the TM upon the PM is expressed by the same modulus, yet opposite in direction and the already known reactions \(X\) and \(Y\) applied at the same point \(O\) and indicated in Figure 2 with a dotted line. In this case, only two unknown forces act upon the PM: \(N_{a}\) and \(N_{b}\). To determine these, two equilibrium equations of the given system are sufficient. Namely: (i) the sum of the forces on the \(Y\) axis; and (ii) the sum of the movements in relation to \(A\) (Figure 2). Figure 2: Scheme of forces acting upon the transport. As a result, the equilibrium of the entire MDD in the longitudinal-vertical plane is described by the following system of five equations:  
\[ \begin{array}{l}Y=G_{m}+R_{v}-N_{c},\\ X=N_{c}\cdot(\varphi_{t}-f)-R_{g},\\ N_{a}=G_{t}+G_{b}+Y-N_{b},\\ N_{b}=\frac{G_{t}\cdot(L-b_{1})-G_{b}+A_{b}\cdot f_{a}+Y\cdot(L+d_{1}-Z_{x})-X \cdot Z_{y}}{L-f\cdot f_{b}},\\ N_{c}=\frac{G_{m}\cdot(Z_{x}+d_{2}-b_{2})+R_{v}\cdot(Z_{x}+d_{2}+z)+R_{v}\cdot(h -Z_{y})}{Z_{x}+d_{2}-f\cdot r_{c}}.\end{array}  \tag{1}\]  
In the system of Equation (1), the following designations are accepted: \(\varphi_{t}\)--adhesion coefficient the tire to the soil; \(f\)--coefficient of rolling resistance; \(r_{a}\), \(r_{b}\), \(r_{c}\)--rolling radii of the front \(b_{2}\) and rear \(d_{2}\) wheels of the power module, as well as the wheels of the technological module (m); \(a\), \(d_{1}\), \(Z_{x}\), \(Z_{y}\), \(z\), \(h\) (m)--design parameters (see Figure 2).