mechanical contarction of length

i am desgning a tank like structure, that is based on 6 wheels, 3 wheels at each side, and the central wheel is driver and of 4 inches diameter, while the remaining are driven and of 1.5 inch diameter, there are two belts, one on each side, just like the chain of the tank.
the total size of this assembly is about 34 inches. size can't be reduced as required, but i want variable size, so that the object can be expanded or contracted as needed.
here is a youtube link, the shape will be demonstrated. kindly provide the simple solutions

<a href="https://www.youtube.com/watch?v=woMgCoHu_x0" target="_blank" rel="nofollow noopener noreferrer">Southwestern Community College Mechatronics Surveillance Robot For Cops - YouTube</a>

[video=youtube;woMgCoHu_x0]https://www.youtube.com/watch?v=woMgCoHu_x0[/video]