i have heard that gearing up a 20" unicycle would be a lot less efficient for riding long distances than just using a bigger wheel. it had something to do with the gyroscopic effects of the bigger wheel…is this true?
by “less efficient” i mean it takes more energy out of you to ride the same distance
Gearing up?
2 obvious ways of doing it: a sun and planet gear, or a chain drive.
Both methods introduce some extra friction, so there is some small loss of efficiency.
A chain-drive would normally mean a giraffe unicycle, which means a taller machine, and more air resistance. On the other hand, there would be less air resistance from the smaller wheel.
The gyroscopic effect would have little influence on efficiency, except for making the unicycle directionally stable, and reducing weave.
The biggest bonus of a large wheel is that it is less affected by bumps and hollows in the road.
The flywheel effect (which is not the same as the gyroscopic effect) of a larger wheel will also tend to keep progress smooth.
i see. THANK YOU:D
I think the real effect is that the large wheel rolls over bumps better. The flywheel effect changes the feel of the ride, but it takes more energy to manage the heavier wheel than the flywheel saves you by being smoother. (Ask the guys who put 29er tubes in their Cokers).
ok cool 
This depends on the nature of the ride. Apart from minor changes in the amount of the friction in the system, the effect of increased mass is only noticeable when accelerating or decelerating, or ascending a hill, or braking on a descent.
On a flat straight road, once the wheel is rolling, if anything, a slightly heavier wheel will be easier to ride as it will even out the very minor fluctuations caused by small bumps and changes in the surface.
On the other hand, if your ride includes lots of speeding up, slowing down, steering, climbing and so on, then the lighter the better.
Years ago, when I was a bicyclist, I remember reading details of a study in which a tiam trials bicyclist did a (?) 50 mile ride, and then repeated the ride in similar conditions a few days later, but with several pounds of lead strapped to the bike. The second ride (with the lead) was only a few seconds slower. On the other hand, if it had been a hill climb, or a ride through city traffic, it would have made a huge difference.
And yes, the effect of extra mass on the wheel rim is more significant than extra mass on the frame, but the principle is the same.
Except that a unicycle wheel, unlike a bicycle wheel, is under constant acceleration as it wobbles back and forth on every pedal revolution.
I don’t think a geared 20" would be much fun for riding long distances. It would take tons more mental energy to stay on it. A larger wheel enables you to be more mindless, and it gives you more time to make corrections. A geared 20" would give you very little time to make corrections before it dumps you. Technically speaking, a non-geared system should be more efficient, but only under ideal operating parameters.
<<begin threadjack>>
Tom, if you were willing to walk a block over, you could try riding a geared coker
<<end threadjack>>
But less if it’s heavier, as the gyroscopic forces are greater.
And less, the better you get at riding.
…and a geared wheel has less wobble over the same distance than a non-geared wheel of the same size (which is, of course, irrelevant to the original question).
Another thought on this subject: Unibiker. He made a machine that was a unicycle with derailleur gears. It was not dissimilar to a bicycle with no front wheel or forks, but he adapted the seat and made other modifications.
I guess (but don’t know) that his wheel was 26 - 28 inches, but he probably had it geared up to twice that.
I recall Unibiker posting reports as he aimed to cover 15 miles distance in an elapsed hour.
Sadly no longer with us.
The gyroscopic effect has little effect on bicycle stability at typical speeds and would have even less effect on a unicycle. The stability of a bike comes from the geometry of the bike rather than the physics of the wheels spinning like a gyroscope. The Wikipedia article on the bicycle gives a good summary of bicycle physics. Google will also find other articles about the gyroscopic effect and balancing a bicycle.
I don’t see the gyroscopic effect having any significant influence on a unicycle whether the wheel is geared up or not.
Reducing weave is more a factor of rider skill and the tire. Skinny high pressure tires are skittish and will cause more weave. Fatter and lower pressure tires track straighter and will have less weave. Tread design, tire compound and the shape of the tire would also have an effect. All of those factors would probably overwhelm any kind of a gyroscopic effect from the wheel at the speeds attainable on a unicycle.
I don’t think the weight of the wheel and tyre has as much to do with wobble/weave as does rider technique and short cranks.
To get maximal efficiency a wheel should have very little *wobble/weave. This occurs at at different speeds for different people, usually as a factor of how relaxed they are when pedalling. However, there comes a point when even the steadiest riders will wobble- at the limits of their maximal cadence for a particular crank length. Once you go past that it’s difficult to keep the wheel tracking straight because you’re fighting to stay on the Unicycle.
Which is also why you would wobble less if you were on a geared Unicycle of same equivalent wheel size/crank length to a bigger wheel- because your cadence is lower, there is greater control.
For example on the AUT- I could descend with much more control on 110mm cranks than the 125’s. At my speed limit for the longer cranks I was wobbling all over the place, limbs flying everywhere despite my best efforts to stay relaxed. The 110’s require a much higher cadence before I felt any wobble. Wobbling is an indication that you are putting a twisting force on the wheel. The aim is to keep it tracking in a straight line as much as possible.
(*by wobbling I don’t mean tilting from side to side- although that might be a factor. I mean riding in a squiggly line)
so MY CONCLUSION IS…i think it will be worth it to build a geared up giraffe, and i will expiriment with the weight proportions:D