Monowheel Tank.

Another pipe dream. A concept that never was converted to reality and testing in the real world.

Might be useful to stimulate designers/engineers to come up with really useful ideas.

Not true, built many times Here is a simple example. I've also seen a version built around a H-D powerplant. Bearing inefficiencies mean heavy accelleration leaves driver rocking on his back waiting for the wheel to come around, and hard braking makes you glad you're wearing a seat belt. Tends to fall over until gyro effect stabilizies the contraption.