I have rode my first 500 miles on my ssr sandviper a 500 watt fat tire bike. No problems what so ever. One bolt fell out of the rack attachment. I am on the heavy side so I could use more power, but that is not the bike's fault. Great bike.
I was riding up a hill at the end of a very hilly 23 mile ride with a 500 watt motor with a 10.5 battery. On this last hill the motor quit with one bar of voltage showing. I turned off the motor and kept pedaling up the hill with great difficulty. After a while I tried the motor with level 1...
Harbor Freight sells a 400lb motorcycle rack for a 2inch reciever which I welded a upright steel bar to which I installed two rings. You have to use straps, but it holds all weights and tire sizes for $125.