Sondors Original Fat Tire Bikes
I would recommend starting with this video:
The Sondors bikes use a rather narrow motor axle (as far as fat bikes go) on the single speed bikes. It's approximately 160mm, whereas most fat bike motors have an axle width of 175mm or 190mm. This means the room to add gears is tight, however not impossible. If you wish to add gears to your S...