the exhaust collecting duct 23 is positioned in the bottom air passage 52 during travel of the vehicle it will be apparent that air flows into the shell structure 3 through the air inlet opening 14 formed at the front end thereof and passing through the air passages 51 51 39 and 52 acts to cool the power unit p and exhaust duct 23 in an efficient manner finally flowing out of the shell structure through the air outlet opening 15 in the normal riding position of the driver d in which his feet are positioned on the steps 32 with the fuel tank t held between his knees as seen in fig1 his legs are both held concealed behind the shell structure 3 this together with the streamline configuration of the latter contributes much to reduction in air resistance of the vehicle during travel in the state of the power unit p fitted to the shell structure 3 when it is desired to separate the rear fork r f from the power unit p for maintenance work such as renewal of the drive chain 36 it is only required that an appropriate tool is inserted through the right and left tool windows 12 into the shell structure 3 in order to draw the pivot shafts 28 and 28 39 out of the respective bearing 29 and 29 39 to disconnect thereby the rear fork r f at its basal ends from the adapters 26 and 26 39 there is no need for removing.