Among important dynamic problems of the tractor-trailer unit are transverse vibrations of the trailer. The problem is so important because it concerns not only trailers of tractors, but also other units which can move quicker, and therefore, the vibrations can be very dangerous. The important task of this analysis was the estimation of the effects of parameter variations.This investigation showed that stability of the system is determined mostly by the damping of the coupling. The roots lines of the equations for many various dampings and velocities were therefore plotted. The optimum damping was found. At the end of this paper, as a result of investigation, the active control system was introduced. In the solution of this problem, the damping was changing according to the traveling speed. The block diagram of this follow-up control was presented.