Mechanically speaking, the monowheel is a one-wheeled (obviously), single-track vehicle. Unlike a unicycle, the person driving will sit inside the wheel as opposed to on top. The wheel itself is a rim, commonly driven by smaller wheels pressing against the inside of the rim.
Now, there are some very good reasons why you don't see people riding these. I've compiled a list of questions that someone should ask themselves before purchasing one of these:
1. First of all, how do you steer? Being a single-track vehicle, you can lean, but that makes your turning radius very low. Another option for steering is to provide friction on either side of the vehicle, but that will inevitably slow the vehicle down making it less efficient than it already is. Lack of steering = FAIL.
2. With so many other methods of transportation available, why would you choose something so stupid and impractical? You can purchase a bike, a skateboard, a long-board (which is my preference, second to driving of course), a scooter, and the list goes on.
3. Do you hate your tail bone? If not, the monocycle may not be for you. Even with suspension and a padded seat, if you're taking a long ride with this thing, you're bound to do some damage down there.
4. Is gravity your friend? Unfortunately not. It is the only thing keeping you at the bottom of the ring, so if you brake too hard, the force could overcome gravity and a "hamster" effect can occur which spirals you around the ring...like a hamster on its wheel!
A close-up of the drive-chain for the self-powered monocycle:
|Only one gear on this model|
|As you can see, balancing can be a slight issue|
A video of a monocycle powered by a V8 engine! Just a tip, skip to 3:00 because it doesn't end well: