A payload displacement in mobile cranes is performed via an adequate composition of working movements. Hoists are used for lifting and lowering of a payload. The following devices are used for displacement of a payload from initial point to another one: a mechanism for generation of crane slewing motion and a mechanism of reach variation. Displacement of heavy payloads by mobile cranes in some conditions can lead to a loss of stability. In this paper authors propose assessment of a stability surplus via: monitoring of values of stress forces of the supports on the ground, calculation of the vertical resultant force of these stress forces and determination of the acting point of the resultant force. The mathematical model of the control system (regulating chassis slewing motion) integrated in the intervention system of a jib reach is presented. Finally, some representative results of numerical simulations are provided.