I have been trying to figure out how to build a mid drive for a bicycle that is shiftable between 2 or 3 speed gears for steep hill climbing. Mostly because the bike will weigh up to 500lbs or more, fully loaded with cargo. I will probably need to use a 409mm rim pulley on the drive wheel, hopefully a 24 inch wheel/tire,at maybe 10mph on a 9% grade. I though of using a Sturmy-Archer 3 speed hub. But that is just not strong enough to take the amount of torque needed. So I ham hoping that some one can direct me to a small gear box that will be strong enough, and tell me how to rig a clutch if needed. Is there such a thing as a centrifugal expansion clutch; probably not good for slow rpm??? Is there a gear box that doesn't need a clutch? With planetary gears?