Survey of the mean pressure exerted by a wide range of tractors on the soil

In order to predict the mean pressure exerted by a vehicle tyre on the soil, in 1984 Plackett suggested measuring the contact area of a tyre on a hard surface. The model proposed by Hallonborg in 1996, based on the “super ellipse theory”, provided the best results in terms of prediction of the tyre-soil contact area. In this study 82 4WD tractors with different power, mass and age of manufacture were surveyed. Relying on their technical features, the contact area of the front and rear tyres of each tractor and, therefore, their pressure on the soil was calculated, in order to assess the attention paid by manufacturers to soil compaction. The results of this survey showed that the mean pressure exerted by the front tyres of a tractor is higher than that applied by the rear ones. In addition the pressure of each tractor tyre was compared to the threshold value of 100 kPa, defined by Vermeulen and Perdok (1994) as the maximum pressure that a tyre could exert on the ground to prevent soil compaction and, relying on this value, the “threshold allowable load” was calculated. Manufacturers seem to pay scarce attention to soil compaction caused by the tyres, especially when implements are fitted to the rear and/or front power lift.