The clutch is a centrifical clutch. The weight of the clutch "pads" are forced out against the "drum" by the high RPM's of the engine.. There is enough friction/pressure to turn the drum/ sprocket assembly.
This sprocket must have a small number of teeth for this project to work. Too many teeth on the sprocket and the engine will just stall, and you will go nowhere.
Also, the engine at 5000RPM with a gear ratio 6-1 still will turn your tire 833 times a minute.
A 26 inch tire is approx 80 inches around (R=PIxD)
so you would travel 5500 ft a minute,or 60 miles an hour (Approx). More teeth on the clutch theoretically would be faster, if the engine has the HP to drive the machine..
I intend to mount a chainsaw engine driving a 3 speed hub to easily change the ratio on the fly...