I designed and built a backyard carnival swing ride for my kids. It's entirely made of steel, and welded strong enough for adults to ride too. The starting point was a 9-foot axle made for manufactured homes. It's mounted on a trailer, with stabilizing and leveling jacks on each corner, which also allows it to be tilted for a "high side, low side" ride. It's driven by a quiet 24v DC drive motor with sprockets, a chain, and two car batteries. It comes apart for transport.