During operation of some types of trucks, permanent flexural deformations of propshafts between an engine and a gearbox appear. This paper theoretically explains and exemplifies causes of these deformations. Propshafts are in the steady state stressed by the dynamic exciter bending torques of harmonic course and their vectors are perpendicular to the rotating plane of an appropriate fork of Hook´s joint. The mentioned exciter torques cause relative lateral oscillations of the propshaft in its rotating area. When formulating a mathematical model, therefore isn´t justified assumption of a fixed inclination of the rotating shaft´s line deflection, which is generally accepted in the literature. It is necessary to assume formation of the relative transverse spatial oscillation in the system of the shaft, which makes rotation around Hook´s joints´ centers connector in the static equilibrium position.