6 is the front wheelbase a when the three wheeled vehicle 8 is performing a station turn of gyration as the front wheel turns 90 degrees the turning radius r 8 is the wheelbase l 8 of the front and rear wheels as the front wheel 81 is the driving wheel and the rear wheels can freely roll comparatively since the front and rear wheelbase l 6 of the rhombus four wheeled vehicle 6 is equal to the front and rear wheelbase l 8 of the three wheeled vehicle 8 the turning radius r 6 of the rhombus four wheeled vehicle 6 is half of the turning radius r 8 of the three wheeled vehicle 8 thereby greatly reducing space for turning further referring to fig5 and fig5 a in the preferred embodiment the front wheel unit 2 has the driving force and the steering operation device 13 is operated to steer the front wheel unit 2 through the switching mechanism 14 the right and left side wheel units 3 4 and the rear wheel unit 5 can freely roll it must be noted that besides the above arrangement the front wheel unit 2 may be configured to steer only the right and left wheel units 3 4 are configured as driving wheels having motors 34 and 44 disposed there on respectively and the rear wheel unit 5 is configured to steer freely or the front wheel unit 2 the right and left side wheel units.