## 3 Results  
The first parameter analyzed was the width of the generated footprint. Figure 2 shows the values of this parameter for both tires used in the tests. Figure 2: The values of the width of the footprint with different inflation pressures and vertical loads. The highest values of the footprint width were found for a radial tire at a lower level of vertical load (19.6 kN). This tendency was observed at all three inflation pressure values--the range of the values was from 467 to 481 mm. In the case of a bias-ply tire with the same vertical load, a different tendency was found. Only at the lowest inflation pressure was the value of the width similar to that observed for the radial tire--it amounted to 477.6 mm, i.e., by approx. 3 mm (0.7%) less than in the case of a radial tire with the same operating parameters. At the other two inflation pressure values, the footprint widths of the bias-ply tire were smaller than those of the radial tire (by 3.7% and 3.8%, respectively). At a lower value of vertical load (11.8 kN), in most cases, the width of the footprint was smaller than at a higher load. The exception was in the case of a radial tire at the lowest inflation pressure--the width was then practically the same as at a higher load level (482 mm for loads 11.8 kN and 481 mm for a load of 19.6 kN). Analyzing the changes in the width of the footprint as a result of increasing the inflation pressure of the tires, it can be concluded that in most cases, the increase in pressure resulted in a decrease in width. However, the bias-ply tire was characterized by greater stability of these changes. It was probably caused by the greater stiffness of this tire's structure. With a higher vertical load, the first increase in inflation pressure in the bias-ply tire resulted in a decrease in width of 2% and a following increase of 4%. At lower vertical loads, these changes were approximately 1% and 3%, respectively. In the case of a radial tire with a lower vertical load, increasing the pressure resulted in a significant decrease in the width of the footprint (by 9% and 2%, respectively). However, with a higher vertical load, the changes in the footprint width were much smaller--they amounted to 1% and 2% for the first and second pressure increases, respectively. In order to demonstrate differences in the nature of changes in the footprint width value at different inflation pressures, a regression analysis was performed, the results of which are presented in Table 2.  
It can be concluded that the vertical load determined the differences in the course of changes in the width of the footprint as a result of increasing the tire inflation pressure. With a smaller vertical load, the changes in the footprint values were described using a second-degree polynomial function; meanwhile, with a larger vertical load, a linear function was used. Figure 3 shows the values of the length of the footprint generated by the tested tires at various values of inflation pressures and vertical loads. The longest footprints were found for a bias-ply tire with a vertical load of 19.6 kN and the lowest inflation pressure (the value was 518.9 mm). A very similar value was recorded for a radial tire with the same operating parameters (515.2 mm). The lowest values of the footprint length concerned the radial tire with the lowest vertical load and the highest inflation pressure--the length, in this case, was 373.0 mm. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Tire/Vertical Load** & **Regression Equation** & **Coefficient of Determination** &
\begin{tabular}{c} **Coefficient of Correlation** \\ **(r-Pearson)** \\ \end{tabular} \\ \hline Bias-ply, 11.8 kN & y = 890.6x\({}^{2}-375.5\)x + 478.2 & 0.98 & 0.99 \\ \hline Radial, 11.8 kN & y = 2631.2x\({}^{2}-1185.1\)x + 560.1 & 0.98 & 0.99 \\ \hline Bias-ply 19.6 kN & y = \(-168.4\)x + 489.3 & 0.95 & 0.97 \\ \hline Radial, 19.6 kN & y = \(-81.5\)x + 487.9 & 0.98 & 0.99 \\ \hline \hline \end{tabular}
\end{table}