## 3 Results
Moreover, it can be stated that for a bias-ply tire, an increase in vertical load resulted in an increase in the value of the analyzed parameter, while for a radial tire, this tendency occurred only at extreme values of inflation pressure. The lowest values were shown for a radial tire at the lowest inflation pressure. In this case, the influence of the vertical load was small--with a lower value (11.8 kN), the depth was 9.7 mm, and with a higher load (19.6 kN), the depth was exactly 1 mm greater. The highest footprint depth values were found at the highest inflation pressure for a bias-ply tire--they were 31.9 and 41.8 for the lower and higher vertical load levels, respectively. In analyzing changes in depth due to changes in inflation pressure, it can be seen that at a higher level of vertical load, the changes had a more linear character than in the case of a lower load. For a bias-ply tire, the first increase in inflation pressure resulted in an increase in the footprint depth of over 13 mm, which was as much as 71% compared to the initial value of 18.6 mm. The next increase in inflation pressure resulted in an increase of 10 mm, which was 31% compared to the value of 31.8 mm. In the case of a radial tire with a higher vertical load, the first increase in inflation pressure resulted in an increase in the footprint depth by over 7 mm, which was 68%, and the subsequent increase in pressure resulted in an increase in the value of the analyzed parameter by 10 mm (an increase of 56% compared to the value corresponding to the medium pressure). Different types of changes in the depth of footprint were observed at lower vertical loads. Both for radial and bias-ply tires, the largest increases in the footprint depth were observed after the first increase in inflation pressure--they amounted to 15.8 mm for the bias-ply tire (118%) and 11.7 mm for the radial tire (119%). In both cases, a further increase in inflation pressure resulted in a slight increase or even a decrease in the footprint depth value--for the bias-ply tire, an  
Figure 4: The values of the depth of the footprint with different inflation pressures and vertical loads. increase of only 2.7 mm (less than 9%) was observed, while for the radial tire, there was a decrease in the footprint depth of 0.7 mm, i.e., 3.2%. In both cases, the second increase in inflation pressure resulted in a change in the character of the increase in the analyzed parameter (the function was no longer linear). When assessing the impact of changes in the vertical load of a radial tire on the depth value, some tendency can be noticed--the higher the inflation pressure, the higher the increase in the footprint depth as a result of the increase in load. At the lowest pressure, the difference in depth values between loads 11.8 kN and 19.6 kN is 1 mm (9.9%), and at the highest inflation pressure, it is 7.2 mm (34.8%). In the case of a bias-ply tire, load changes had a slightly different impact on changes in the footprint depth. The largest increase in the analyzed parameter as a result of increasing the load (by 5.2 mm, which means 38.7%) was observed at the lowest inflation pressure. At a pressure of 0.16 MPa, the increase in load resulted in an increase in depth of only 2.6 mm (9.0%), while at the highest pressure, an increase of 9.9 mm (31.2%) was achieved. The results of the regression analysis carried out for the footprint depth as a function of inflation pressure are presented in Table 4. As in the case of the first analyzed parameter (footprint width), it can be seen that at a lower load level, the nature of the footprint depth changes was different than at a higher load level. This tendency was observed for both tires and is confirmed by the different course of the function described with the regression (at a load of 11.8 kN, there is a second-degree polynomial function, and at a load of 19.6 kN, there is a linear function). One of the parameters that may affect the compaction of the ground by the tractor wheel is the contact pressure.