The study of these authors presented a positive and negative change in static overturning angle at a decrease and increase in the vertical coordinate of COG. The tyre contact patch was analysed according to the length of rubber lug \(n\) and length of contact patch \(u\), Figure 7c. The tyre footprints on the paper sheet showed the tyre contact patches of the tractor with various rear wheel ballast weights when the tractor was inclined at the static overturning angle. An example of the tyre contact patch of the tractor equipped with 122 kg of the rear wheel ballast weight (standard overall width on tyres \(o\) = 0.795 m) is shown in Figure 12.  
Experimental measurements confirmed the fact that the rear wheel ballast weight affects the tyre deformation. Inclining the tractor at the experimental static overturning \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Rear Wheel Ballast Weight, kg**} & \multicolumn{2}{c}{**Standard Overall Width on Tyres (0.795 m)**} & \multicolumn{2}{c}{**Extended Overall Width on Tyres (0.885 m)**} \\  & \multicolumn{2}{c}{**ISO 16231-2 Considering**} & \multicolumn{2}{c}{**ISO 16231-2 Considering**} \\  & \multicolumn{2}{c}{**ISO 16231-2**} & \multicolumn{2}{c}{**the Vertical Type**} & \multicolumn{2}{c}{**ISO 16231-2**} & \multicolumn{2}{c}{**the Vertical Type**} \\  & \multicolumn{2}{c}{**Deformation and Tyre Shift**} & & \multicolumn{2}{c}{**Deformation and Tyre Shift**} \\ \hline
0 & 2.57 & 0.95 & 2.40 & 0.91 \\
30.5 & 2.80 & 1.11 & 2.61 & 1.18 \\
61 & 3.04 & 1.29 & 3.11 & 1.69 \\
91.5 & 3.42 & 1.85 & 3.67 & 2.27 \\
122 & 3.88 & 2.42 & 3.99 & 2.57 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Percentage difference between calculated and experimental data. Figure 12: Experimental example for tyre contact patch. angle, the length of the tyre contact patch \(u\) (Table 8) depended on the rear wheel ballast weight due to the tyre deformation. The increase in the rear wheel ballast weight caused an increase in the length of contact patch (Figure 13a). Nonlinear regression equations pointed out that the tyre deformation is a complex problem. The coefficients of determination \(R^{2}\) higher than 0.98 and \(p\)-values lower than the significance level of 0.05 present a significant effect of tyre deformation due to the rear wheel ballast weight on the length of the contact patch. On the other hand, the results of experiments showed that the length of rubber lug \(n\) (Table 8) did not change at various rear wheel ballast weights, Figure 13b). The coefficients of determination \(R^{2}\) lower than 0.1 and \(p\)-value higher than 0.05 express a statistically insignificant relationship between the rear wheel ballast weight and the length of the rubber lug. The increase in the rear wheel ballast weight causes an increase in the static overturning angle which compensates the increase in the length of the tyre rubber lug due to tyre deformation. Considering the experimental results, the dimensions of the tyre contact patch did not affect the calculation of the static overturning angle because the length of the contact patch \(u\) did not affect the distance \(AA^{\prime}\) and the length of rubber lug \(n\) did not change. The tractor overturns around the rolling point when it reaches the limit value of the static overturning angle and loses stability (equilibrium). [6] present that the standard is intended for the calculation of static overturning angle in case of different tractor types with different agricultural tyres. Agricultural tyres are different in dimensions and elastic properties affecting the forces between the ground and tyre. The standard [12] defines the position of the force below the tyre in the rolling point of the fixed axle as three quarters (0.75) of the tyre width (Figure 4). If higher or lower value than 0.75 of tyre width \(p\) is considered, the calculated value of static overturning angle is higher or lower because the length of the base line of the stability triangle \(AA^{\prime}\) is changed.