Abstract A proper design of a tractor-trailer system depends on fulfilment of the 2 main conditions : (a) safety and stability of the system as a whole; (b) prevention of excessive loads on the frame and tyres of the driving unit. The analysis presented in this work suggests a method which enables an appropriate choice of a system in 2 different ways: (a) adaptation of an existing trailer, with known geometry and permissible load, to a proper tractor; (b) design of the geometry and permissible load of trailers in order to meet the standard specifications of tractors.