[zyphlar]

"Most people have seen a gyroscope in action, so the stability of a rapidly rotating wheel should be fairly intuitive, making this a focus from the start. People have built bicycles with counter-rotating wheels and found that they still remain upright, so that can't be all of the story."

Um, counter-rotating wheels are still gyroscopes. In fact they're extremely stable gyroscopes in that they won't impart rotational velocity on the frame. So by having the wheels on a bike rotate oppositely, you're actually making the gyroscopic effect even stronger.

I think Ars is pulling from this article, which isn't about staying upright but is about the self-correcting steering of a bike wheel (i.e. the fact that you can ride hands-free.) http://www.sciencemag.org/content/332/6027/339.abstract -- in which case the conclusion is correct and likely due to the geometry of the wheel. For example tractors have convex pulley systems that allow leather belts to self-center despite not being perfectly aligned. It's counterintuitive but it works.

[ScottBurson]

> Um, counter-rotating wheels are still gyroscopes.

Um, no. A system consisting of two identical wheels mounted on the same axle and spinning at the same speed in opposite directions has a total angular momentum of zero. It will behave like a solid object of the same mass.

[rwmj]

I'm still a bit confused about how a bicycle with counter-rotating wheels can move.

[chad_oliver]

The counter-rotating wheels are not the same as the two that the bicycle rolls on. So there's four wheels, two of which only exist to cancel the rotational inertia of the 'normal' wheels.

[pbhjpbhj]

This. However I wonder what it would be like to ride a bike on two rolling-roads so that the front and back wheels moved in opposite directions. Or, indeed, a trike/quad with large coaxial wheel separation and contra-rotating rolling-roads.

[usaar333]

The counter-rotating wheels probably don't touch the ground.

[nazgulnarsil]

no, they didn't build it so it has gyroscopic effects in the other direction, they built it so the reverse rotating part would cancel out the forward rotating wheel effect and you wind up with a gyroscopic-ally neutral design.