In the vehicle-regulatory world (which I used to do legal work in) that isn't a motor vehicle. A tipoff is state vehicle codes where many laws applicable to motor vehicles, specifically motorcycles, don't apply. You can ride it in a bike lane, not on a freeway. Fed perspective too...they don't have DOT lamps, the rims aren't subject to the same standards, etc. I don't know if the IRS follows its own definition.