EN
The work presents the analysis of the work of field aggregates on slopes along and transversely to contour lines. As the result of the analysis of a force system influencing the aggregate which is working along contour lines, it can be assumed that the work of a tractor which is on tyres, is characterized by a lateral skid of wheels caused by lateral elasticity of tyres and lateral shift of wheels caused by plastic soil deformation. At the same time decrease of tractor towing power is observed. With the aggregate goind up an increase of rolling resistance is so big that significant part of traction force must be spent on its overcoming. As the result a tractor can be loaded on a catch with very small towing power. A downhill dreve is, in fact a situation in which favourable proportons appear between forces characterisitc for a movement, except for breaking forces. It is the main reason of accidents, very frequently tragic ones.