[Brad Voracek]

In all actuality, if you attach the arm to the sprocket then you -don't- have to attach the sprocket to the axle, as it can just be free spinning and still work...


As you might be able to see in that picture, that's how they did it, and their sprocket looks free spinning on that axle although it is hard to tell.


If ultimately you do want to attach the axle to the sprocket, then something like that hub is really all I can think of... Sorry I'm not of more help to what you are asking. (and sometimes testing outweighs hard numbers. there can always be something you are forgetting, overcompensating for, etc...)