A need to vary the wheel base during operation exists for some mobile equipment. This need can be caused by legal considerations or the geometry of the haul road. To meet this need, we consider a hydraulically-operated device used as a fifth wheel. This type of unit is being built and installed on some tractor trailer combinations.Since the apparatus can be operated with the vehicle in motion, the system needs to be analyzed under various dynamic conditions. System pressure peaks and component operation is examined under a variety of conditions involving road surface irregularities and the effect of striking obstructions such as loading docks. Some recommendations for design improvement are included.The analysis was carried out with the use of the IBM-CSMP (Continuous System Modeling Program) (1).* The purpose of the work was to gain preliminary knowledge regarding probable operation of the apparatus.