Modelling effects of tyre inflation pressure on the stress distribution near the soil–tyre interface

Several investigations have shown that the distribution of vertical stress in soil just below a loaded tyre is not uniform. The stress distribution and the size and form of the tyre–soil interface are decisive for the stress propagation in the soil profile. We measured the distribution of vertical stress in the contact area for two radial-ply agricultural trailer tyres (650/65R30.5 and 800/50R34) loaded with ∼60 kN. The study took place on a sandy soil at a water content slightly less than field capacity. We tested the effect of three different inflation pressures (50, 100 and 240 kPa) in a randomised block design with three replicates. The vertical stress was measured with load cells located in 0.1 m soil depth. The vertical stress data were used also for identifying the soil area in contact with the tyre, i.e. the tyre footprint. A model (named FRIDA) is proposed that describes the tyre footprint by a super ellipse and the stress distribution by a combined exponential (perpendicular to the driving direction) and power-law (along the driving direction) function. The contact area doubled when the inflation pressure was reduced from 240 to 50 kPa. For both tyres, the measured peak stress increased significantly with tyre inflation pressure and was generally about 90 kPa higher than tyre inflation pressure. The model-fitted maximum stress was about 50 kPa higher than the inflation pressure. The 650/65R30.5 tyre displayed a longer footprint and a more uniform stress distribution in the driving direction and performed better at non-recommended inflation pressures than the 800/50R34. At the recommended inflation pressure, both tyres displayed a stress distribution across the width of the wheel that could be evaluated as optimal with regard to a minimised topsoil compaction. The suggested model seems very well suited for describing real stress distributions at the soil–tyre interface, but should be validated also with other tyres, wheel loads, and soil conditions. It has the potential to improve soil compaction models considerably and also for use to evaluate important features of tyres with scope to improve future designs.