## 4 Conclusions  
Four levels of the rear wheel ballast weight and extended overall width on tyres were applied to analyse the agricultural tractor lateral stability. The ballast weight placed in the rear wheels decreased the vertical coordinate of COG. The extended overall width on tyres widened the base line of the stability triangle of the tractor. The experiments confirmed the improvement of the tractor stability using the rear wheel ballast weight or increase in the overall width on tyres. The statistically significant relationship between the ballast weight and the height of COG can be used to predict the tractor lateral stability. The tests performed to characterize the tractor stability check the static overturning angle and are helpful for real operation conditions. Comparing the static overturning angle calculated according to [12] with the experimental data, the difference was observed. The reference reached the values of 2.27%, 2.8%, 3.04%, 3.42% and 3.88% in the case of standard overall width on tyres and 2.14%, 2.61%, 3.11%, 3.67 and 3.99% in the case of extended overall width on tyres, at 0, 30.5, 61, 91.5 and 122 kg of the rear wheel ballast weight. Considering the measurement instruments with adequate precisions and statistical tools to decrease the standard measurement errors, the influence of the tyre deformation on the difference between [12] and experimental data were analysed. When the tractor was tilted, the vertical tyre deformation increased the static overturning angle due to the vertical shift of COG. This parameter is a component of the tyre deformation causing the improvement of the tractor stability. On the other hand, the lateral shift of the tyre decreased the static overturning angle due to the decrease in overall width on tyres. The tyre lateral shift is a component of the tyre deformation worsening the tractor stability and safety of the tractor operation. Both parameters act together resulting from the tyre deformation. Combination of the vertical tyre deformation and the lateral shift of the tyre decreased the difference between the experimental data and data calculated according to [12]. The difference reached the value of 1.81%, 1.33%, 0.98%, 0.73% and 0.22% in the case of standard overall width on tyres and 1.95%, 1.73%, 1.28%, 0.74 and 0.34% in the case of extended overall width on tyres at 0, 30.5, 61, 91.5 and 122 kg of the rear wheel ballast weight. The tyre contact patch was also analysed. The increase in the rear wheel ballast weight caused an increase only in the length of the contact patch, but this parameter does not affect the tractor lateral stability. The length of the rubber lug of the tyre tread pattern did not change because the tractor inclination corrected the influence of the weight increase on the tyre deformation. The result showed that the changes of the tyre contact patch due to tyre deformation did not affect the rolling point below the tyre and the static overturning angle. Experiments showed the increase in the difference between experimental data and data calculated according to the standard [12], considering different constants of the regression equations of models. This problem results in the deformation characteristics of the tyre which can be described in detail by the complex analysis of the tyre deformation. Considering the minimal influence of this problem on the tractor lateral stability, the experiments defining the complex tyre deformation was not the aim of this article. The static overturning angle calculated according to [12] is lower in comparison with the experimental data at both overall width on tyres and all rear wheel ballast weights. Based on results relating the tyre deformation analysis, it can be concluded that the difference is caused by the different tyre deformation when the front axle is lifted during the measurement of the COG coordinates in comparison with the tractor tilted at the static overturning angle during the experimental measurements.