onewheeldave wrote:

Secondly, I've been wondering about the characteristic 'wheel wobble' of
unis. With a 20"-er it's pretty high and noticibly less on a muni. Even
less so on my 29-er with short cranks, the wheel tends to stay in the
direction it's pointed and roll steadily ahead.

Cokers, I hear, are even better for a good steady wobble free ride.

So what is the most important factor in low 'wheel wobble'?

My gut feeling is that it's the shortness of the cranks, but I'm not
sure of the process by which shorter cranks would lead to less wobble.

Wheel wobble is caused by rotating masses (or masses in simple harmonic
motion [SHM] in phase with wheel rotation) that are not in the plane of
the wheel. These masses are the pedals, cranks and a significant
fraction of the leg ( foot is 100% and upper thigh is much smaller
fraction do to a much smaller path of simple harmonic motion). Please
note that the leg motion is clearly more complex than simple harmonic
motion, but it is still the cause of wheel wobble primarily.

Could wheel mass be a factor, like I said the muni rides straighter even
though its cranks are longer than the 20"-ers.

Yes, wheel mass is a huge factor. Especially the mass of the rim and
tire which counter wheel wobble. The Coker and other big wheels are
good examples where the large rotating masses in the plane of the wheel
counter balance the masses that are not in the plane.

Or is it not just the crank length, but the ratio of crank to wheel
diameter.

Yes, the ratio of the crank to wheel is a factor. The masses of all
rotating and SHM objects must be considered and their effect depends on
the distance from the axle which reduces to the ratio. However, the
masses of both in plane and out of plane objects are an independent
variable of the equation as well (Consider more or less mass in the tire
and rim and how that would affect wheel wobble; consider also larger or
smaller legs and how that would affect wheel wobble).

Sincerely,

Ken Fuchs