Algodoo

This is really awesome! I love the details. What's keeping it stabilized?

It uses thrusters. Towards the front there's a box that assigns its angle to a variable, then it controls the force of those thrusters based on the angle of the box. So if it starts to lean backward, the thruster at the front (They're flipped upside down, so they push down, not up. I found if I had them facing up it created some interesting physics when airborne) is activated, then cuts out again once the thing gets stabilized. It's also proportional, so if it leans more, it will apply more force.

I originally wanted to do a PID-style system like s_noonan did, but I couldn't get it to work very well

Awesome!
Another way of doing this is putting a box with inertiaMultiplier +inf, and hinging it with bend or springs. If you fix it the monocycle wouldn't rotate at all

Don't use high density spawned balls on the light bikes or cars, it causes massive vibration. Also the engine is very unstable.
There's something wrong with that engine, he shouldn't shake that much, it would if you would use single cylinder engine.

vaidas, you should read the description first!

But yeah, if you want the engine not to vibrate, you should make underpowered 2cyls

umm, it already has 2 cylinders!