## 2 Materials and Methods  
### The Test Bench  
The tests were carried out in controlled conditions using a specialized stand for generating vertical load on the soil placed in a soil box with side lengths of 1000 mm and a height of 600 mm. The experiment was conducted in static conditions--the wheel with tested tires did not roll. Soil samples taken from an arable field were used for the tests. This soil was classified as sandy clay (according to [44]), its moisture content was 25% and its compactness was 0.9 MPa (these values were measured using a Penetrologger device according to [45]). As a first step, the soil compaction was measured in the field. In several repetitions, the values were in the range of 0.84 to 0.96 MPa and the arithmetic mean was 0.90 MPa. Then, the soil moisture was measured--the mean was 25%. These means were used as reference values in experiments in controlled conditions (in the laboratory). Each time the case was filled with the soil, the compaction was measured. When its value was too low, the soil was compacted using the plate loaded with the hydraulic jack. Otherwise, when the compaction was too high, the soil was loosened with the spade. The soil moisture was also controlled--when it was too low, the soil was sprinkled; otherwise, it was set aside to dry. After this preparation, the footprint was made with the wheel, using a special test stand. A view of the test stand is shown in Figure 1.  
In the first stage, the box (4) with intact soil was placed in the lower part of the stand, and then the wheel (1) was moved down using the mechanism (7). Then, in the upper part of the internal frame (3), a hydraulic jack with a load capacity of 5000 kg and a TecSis inductive foremeter with a measuring range of 0--100 kN and an accuracy of 50 N were installed. Using the hydraulic jack, pressure was exerted on the internal part of the frame together with the wheel. In the initial part of this step, the value of the vertical load was read--two levels were used: 11.8 kN and 19.6 kN. The reference point for the chosen values of the vertical load was the load index formulated by the tire manufacturer. Both for bias-ply and radial tires, it was determined to be 145, which meant the load was 2900 kg. The first level of the vertical load in the experiment was equal to 11.8 kN--after calculations, the mass was found to be 1200 kg--in turn, it was about 40% of the maximum vertical load. In practice, what is recommended is the lower limit of the exploitation of agricultural tires (when the lower loads are often used in exploitation, the farmer should buy the tires with a lower load index). The second level of the vertical load was 19.8 kN, which provides a mass of 2000 kg, which was about 70% of the maximum load. In agricultural practice, it is the upper limit of the load that is recommended for the continuous operation of the tractor. Other (intermediate) levels of the vertical load will be the subject of further research. The selection of inflation pressures was realized based on the recommendation of tire manufacturers. They determined the maximum inflation pressures--for both tires, it  
Figure 1: View of the test stand used in the tests: 1—tested wheel, 2—main frame, 3—internal frame with wheel mounting system, 4—box with tested soil, 5—scanner, 6—computer with scanner software, 7—lifting mechanism wheels, and 8—installation site of the inductive foremeter and hydraulic actuator. was equal to 0.26 MPa but this value is not recommended for continuous operation (it is used just at the assembly of the tire on the rim). For this reason, the maximum value of the inflation pressure was determined to be 0.24 MPa--this value is sometimes used for continuous work at very high vertical loads of tractors, especially in transport operations. Sometimes, this high pressure value is used in field operations by farmers who do not know the principles of inflation pressure selection.