Authors carried out a theoretical study of the force impact during emergency braking on the strength of the frame of a truck tractor as a part of a road train with a semi-trailer in order to build a model of the dependence of the safety factor index of the frame on the geometric and weight parameters of the car, semi-trailer, transported cargo and the type of the pavement. Using D'Alembert's principle, a generalized calculation scheme of the frame of the semi-trailer and the truck unit after reaching a steady deceleration during emergency braking while maintaining straight-line movement and adhesion of all wheels to pavement was made. Through a static calculation, the dependences for the unknown reactions of the car frame constraints were obtained, its dangerous cross-section was established, and the condition for the static strength of the longeron was determined. Using this condition and the obtained ratios for active and reactive loads, a model of the dependence of the safety margin coefficient of the car frame on the road train parameters was built. By the example of a road train consisting of a KrAZ-6446 truck tractor and a VARZ-6006 semi-trailer the application of the constructed model for evaluating the strength of the car frame and determining the permissible safe zone location of the center of gravity of different weights cargo in the plane of the road train is demonstrated. The method proposed in this work and the built model can be used to study the strength of the frames of truck tractors and semi-trailers of other manufacturers and types and to establish safe schemes for the placement and fastening of loads to ensure the operational reliability of the vehicle and the safety and security of cargo transportation.