## 4 Discussion  
In accordance with the assumed aim of the research, differences in the values of the footprint parameters generated by tires of different internal structures, at different inflation pressures, and vertical loads were assessed. The research was carried out in controlled conditions using a 3D scanning technique. In this way, the deformation characteristics of both tires due to inflation pressure changes were determined. When comparing the method used with other techniques described in the literature, some analogies can be found. Very similar techniques were used in studies [21, 37]--in these works, a three-dimensional model of the footprint was obtained in static conditions on soil placed in a soil bin. In turn, in works [22, 25, 26], a 3D image of the footprint was also obtained but this took place in dynamic conditions while rolling the wheel on the soil in long soil bins. In works [12, 40], the parameters of the footprint were also assessed but this took place in field conditions. One of the relationships observed in the research was the fact that a tire with a higher inflation pressure is characterized by higher stiffness and, therefore, a potentially more negative impact on agricultural soil. This tendency is also confirmed by works [12, 46]. In the first of these references, the authors concluded that the tire with dimensions similar to the tires used in the discussed results can obtain a width of footprint in the range of 0.38-0.54 m while in the presented results, this range was from 425 mm to 481 mm. However, the tire structure and its possible impact on the tire's deformability at various pressures and vertical loads should also be taken into account. The relationship most often described in the literature is that radial tires are more sensitive to pressure changes than their bias-ply tires [42, 47, 48]; however, in the studies performed, the obtained relationships also depended on the operational parameters. For a bias-ply tire, it was shown that, with low vertical load, the footprint area increases with increasing pressure faster than the horizontal cross-section area of the tire. This increase, however, was caused by an equally dynamic increase in the depth of the footprint--this situation is not favorable at all, because the total surface of the footprint also includes its vertical parts, which do not carry the loads resulting from the weight of the vehicle. With a higher vertical load, a similar tendency was shown for the bias-ply tire, but it was most visible after the second increase in inflation pressure (from 0.16 MPa to 0.24 MPa). In this case, it was clearly demonstrated that the increase in the actual surface of the footprint was correlated with the increase in the footprint depth--this is consistent with the results obtained in [49]. In this work, the difference in footprint depth generated by the radial tire at the minimum and maximum inflation pressures was about 21%, while in current research, this difference was 29.4%. Moreover, for the bias-ply tires, there were small changes in the width and length of the footprint as a result of changing the inflation pressure, which only further confirms the hypothesis that the inflation pressure has a large impact on increasing the stiffness of the tire and, consequently, on its deeper impact. Slightly different relationships were demonstrated in work [27], where a bias-ply tire was inflated to three pressures (0.10, 0.16, and 0.32 MPa), and with the increase in pressure, a decrease in the actual footprint area was observed each time. In this case, the maximum difference in values of the actual footprint area at extreme inflation pressures (0.100 MPa vs. 0.325 MPa) was 86%, while in the current results, this difference does not exceed 44%. However, in those studies, tires with a less aggressive tread design were used, so its in-depth impact was probably smaller. In turn, work [28] showed some analogies to the results obtained in this study--the lowest contact area values were achieved at the middle value of the inflation pressure.