First off, great idea Sanddrag.
I would actually like to experiment with this gearbox and see if I can design a complete drawing if you give me permission to do so. (I would have to pick up this project after the offseason that I am working on now.)
If I was to design this gearbox, I would make it much smaller as you did point out before. Maybe the output (sprockets) can be placed inside the gearbox. Now in order to do this design I would like inputs and ideas about how to shift (please share exactly how you would do it). Thanks in advance.
