# Angular momentum of uni wheels

James.Gifford@maths.anu.edu.au (James.Gifford) wrote:

>once you get the knack, and it struck me that the important thing is that the
>unicycle wheel develops angular momentum when rolling. My guess is that this
>angular momentum is effectively what makes it easy to avoid sideways topples
>(like on a bike). Presumably this is why is is harder to ride slowly.

I never took physics, so I may also be totally wrong, but I did study unicycling
for 15 years. I think the angular momentum gained by pedaling a unicycle wheel
has little or no effect on balance. You can perform this test yourself:

Hang your unicycle by hooking one or two fingers under the edge of the seat.
Spin the wheel fast. What does it do? The rotation of the pedals and crank arms
makes the wheel wobble all over the place. I would guess that this force,
combined with the weight of your legs while pedaling, would counteract most of
or any stabilizing effect that is gained by the rotating wheel. Most unicyclists
wobble to some degree, especially when riding fast. Another test: ride through a
puddle at high speed, and then observe the wet trail left by your wheel on the
dry pavement. Is it a wavy line?

>Okay, nothing world-shattering here. But then I remembered seeing pictures of
>giraffes with two wheels, where the pedals drive the top wheel which drives the
>bottom wheel. I had assumed that the reversal of the pedal direction and the
>sludginess of the drive would make these things hard to ride.

Also, by stacking the wheels higher and higher, it makes the gyroscopic force
they may provide that much weaker in the area of side to side balance. A two
wheeler seems to me to balance about the same as a 3 or a 5 wheeler.

>It also occurs to me that you could go one step further and make the bottom
>wheel light and the top wheel heavy (so the total angular momentum was
>opposite to what you expect). My gut feeling tells me that such a device
>would be really weird to ride. I would even go as far as to suggest that you
>might have to lean out around corners (!)

It would be an interesting experiment. As for leaning out around corners, I’d
like to see that!

John Foss, the Uni-Cyclone unicycle@aol.com