The invention relates to a flat-track unit for motor-vehicle test beds in which a roadway-simulating belt is guided over two rollers lying in parallel. The upper test side of the belt lies roughly at ground level and accommodates a vehicle wheel and can be excited by a supporting shoe below the belt via an excitation cylinder to develop vertical oscillations which are to simulate roadway vibrations. The two belt-guiding rollers are in turn mounted in a frame which in turn is guided so as to be vertically movable and can be excited by a further excitation cylinder to develop vertical oscillations. In the process, the supporting shoe oscillations are limited to short-stroke and high-frequency oscillation portions of simulated driving conditions whereas the frame is excited merely by the long-stroke and low-frequency oscillation portions. This offers the advantage that long-stroke vertical oscillations can also be reproduced in a manner close to reality, but nonetheless, the vertical oscillations of the test side, relative to the belt guide rollers, are only relatively small. By the use of hollow pistons, the required excitation energy is kept within acceptable limits and over all weight is reduced.