I don’t know much about them, other than these custom bikes look pretty cool.  Found here and here.

This last one is properly called a Monowheel. It was built by David Southall. He says: 

The wheel itself is a 5 foot diameter hoop of 2 inch tube. It was a bit beyond what I can do so was made for me by The Angle Ring Company . The pulleys on which it runs were custom-made by AED rollers. Everything else was cut, bent and welded in my shed!  The engine is a 90cc four stroke from a Chinese quad bike. I was initially going to use the engine/wheel unit from a 50cc scooter but the wheel was too big and the two stroke power delivery can be a bit peaky! The quad engine is fully auto with electric start. The friction drive wheel is the rear wheel from a mini moto. It has a cable-operated disc brake which must be used with extreme caution. The tyre on the outer wheel is three 24 inch mountain bike tyres cut up and pop riveted on. 

There are plenty more from a whole web site devoted to monowheels.