Hmmm, or you could put them opposite each pedal to counteract the pedal-pushing torque about the frame axis… Better to do that all with technique, I think (agreeing with JF).
Yeah, that would tend to damp out the wobble. Though I find the “drunken sailor” aspect of the wobble quite nice for lateral stability sometimes.
There is a lot of room for experimentation here. I may have to go out and buy some lead.
yeah, well, ok, if u must, i was thinking about weights that would ‘adjust’ themselves ‘automatically’ thanks to the centrifugal forces inherent to the rolling wheel without u having to get off and on
u might as well keep a 24/26 strapped to your back for the technical sections then
Do I assume you mean that if the pedals are at 3:00 and 9:00 positions, the weights would be at 6:00 and 12:00 positions?
For the record tho, I’m not concerned with wheel wobble. I agree that technique will handle that. But speeding along, that heavy Coker wheel/tire has less wobble because its heavy.
To restate what I’m looking for in a 29 inch crosscountry unicycle for this racecourse: Manageable downhills, more effortless flat speed runs, wheel momentum over bumps in the trail so I can stay on the machine. I’ve found that a light, responsive wheel/tire is too affected by trail irregularities. Using the Coker only as an example (our extreme in this one aspect), it plows right over the little stuff and keeps going. Almost as if, once you’ve got it going, IT does the work at those critical instances and the rider just rides through it. Being the extreme example, however, carries with it the extremes of downside effects mentioned earlier by members of this BrainTrust of worse tight manouvering and downhills etc.
Tommy (who, it turns out, has been secretly monitoring this thread) mentioned a video of a Coker blasting through the woods until the wheel tacoed. The average Coker wheel is notoriously weak anyway. The average Niner wheel must then be stronger (circle physics).
Tomorrow is Saturday. Can’t wait. The scientist hits the lab (trail).
(Secondary brainwork for this racecourse: I’ve also paid attention to you (my Uni-BrainTrust) and concluded the my next choice of a ride FOR THIS RACECOURSE would be a 26 or 24 MUni with shorter…145s or 150s…cranks. The 3" tires have the weight and momentum and can cush over the irregular trail stuff. Save the 170s for a course more inundated with severe ups and downs. (Lakeland Park in our area). This course has some, but 80-90% is rolling hills).
Again, thanks for all the help. Tom.
Let me clarify this. I was also thinking that, for this racecourse, a Coker with 170s would be good. I still think so, I just don’t have one.
I’m not convinced it would work like that. To rotate the wheel you’d still have to put enough energy in to accelerate a large mass. Being on the pivot axis won’t help if the mass is still a long way from the pivot.
John
I think that’s “rotate” as in left and right rather than forwards and backwards, such as turning on the spot. In this case the masses stay in the same place.
Phil
A Gazz Jr. on a 26" DX32 rim is 27 3/4" in diameter and the tire weighs the same as the 24" Gazz. Put 140s or 150s on that and you have an almost-29er with a super shock-absorbing tire that doesn’t mind low pressure, and yet still has enough mass for stability, plus plenty of leverage to climb hills and descend without the weight of a brake. Not only that, but it won’t fold under when you’re side-hopping the last little bit of that rise.
The difference in tire diameter (27 3/4 versus 29) is about 5%.
I should have been more specific since there are always a few pivot axes. I was refering to the vertical axis that goes from the tire patch up through the hub and seat, not the horizontal axis that goes through the hub that the wheel spins around.
As U-turn pointed out, it isn’t clear that having the lead at 6 and 12 o’clock when the pedals are at 3 and 9 o’clock is optimal. I was thinking this would add mass to the wheel but still allow quick left and right changes in direction while at a standstill with the cranks horizontal.
It would also make for some very wierd forces while carving a turn at speed, since the wheel will resist turning when the cranks are at 6 and 12 but be easy to turn when the cranks are at 3 and 9. Turning would not be smooth - the path would have straightish segments joined by snap turns. But this is what unicycles do in turns anyway, due to the pedaling action, so I suppose it’s not a big change.
U-turn’s point that putting the masses near the pedals would have an opposite effect makes sense. The natural wobble would be reduced and snap-turning at near zero speed would become harder. So what about putting the lead at 7 and 1 o’clock (looking from the left when the pedals are horizontal)? Or 5 and 11 o’clock? There is nothing special about symmetry on the unicycle since the pedaling action isn’t all that symmetric.
Food for thought.
I’m sorry but I don’t believe that putting the weights opposite the pedals will have much, if any, effect on wobble. Most of the wobble is created by the fact that the pedals are on the left and right, not because the pedals are unbalanced about the axle. In fact, the pedals are balanced about the axle (two pedals on either side of the axle cancel, just as the weights would). They are unbalanced about axis of the frame, not the axle.
If the pedals were unbalanced about the axle, the effect would be that the wheel would speed-up and slow-down. Imagine a weight at the rim of the wheel. The off-balance would have to stop to be in contact with the ground and go twice as fast as the whole unit to go over the top. Throwing the system forward and back (up and down, actually, but it would feel like forward and back), not twisting it left and right. If there were two weights (pedals), they’d cancel.
Now imagine two weights, on opposite side of the wheel, but one is on the rim and the other is a foot wide, on both sides (left and right) of the rim, but they’re the same weight. They’re still balanced, both about the axle and the frame, and there is no wobble.
Now imagine two weights, but while they are on opposite sides of the axle, they are also a foot beyond the edge of the wheel, one on the left and one on the right. Now when the one on the left is accelerating, the other is decelerating, on opposite sides of the frame, producing the dreaded wobble.
In order to balance the pedals, and thereby reduce the wobble, you’d have to have weights that would have the same moment of inertia about the axis of the frame – another set of pedals (or weights that are effectively the same distance from the frame), on the left and right sides and opposite the axle from the original ones. (Hmmm, pedal walking?) This is why when you’re getting your car wheels balanced correctly, two weights are placed - one on the inside and one on the outside of the tire (tyre).
OK, quick, off subject, what part of a train is always going the opposite direction of the rest of the train? Hint: see the second paragraph.
something in the electric motor?
or in the steam loco, the rotating wheel that powers the ‘arms’ powering the wheels?
I disagree, Brian, but only slightly. Adding weights opposite to the pedals on the rim does help reduce wobble. It increases the rotational mass about the frame’s axis. So, for a given pedaling force in that sense, the acceleration, hence distance, is less. F=ma.
The reason you put the weights at that position instead of elsewhere is because, at the position when the wobble-inducing pedal force is highest (at 3 and 9), the rotational mass about the frame’s axis will be highest, since the weights are as far as they will ever get from the frame.
This is thinking statically, and ignoring gyroscopic forces which I never did get a full grasp of.
Of course, dynamics of pedaling may move things a few degrees, but that’s the general idea. Whether or not it would work in practice… who knows.
This is kind of a silly question, isn’t it? If a train is going from point A to point B and has a part that is going from point B to point A, then the part is near the train for only a short interval of time. How can this part be considered a part of the train?
As for the comments about dynamically balancing the wheel, your analysis is all very true, but I’m not sure how it relates to the rest of the thread.
[Dr. Physics mode]
One of the important ideas in physics is that all rigid objects have three “principal axes” that are always at right angles to each other. It doesn’t matter what shape the object is - potato, aircraft, unicycle wheel, or frozen sheep - there are three “special” rotational axes that are fixed with respect to the object body, and these axes are always at right angles to each other.
By symmetry, all unicycle wheels have one of these axes on a vertical line from the contact patch up through the center of the hub while the wheel is upright and the cranks are at 3 and 9 o’clock. If there were no cranks to ruin the left-right symmetry then the second axis would go through the center of each bearing, as it does for bicycle wheels. However, there are cranks on the wheel so this axis is tilted slightly away from the cranks. When the wheel spins it naturally wants to spin about this axis, but it can’t because it has to spin on the bearing axis, hence the wobble.
[/Dr. Physics mode]
Your point about adding masses to dynamically balance the wheel is correct. If you bring the mass distribution into symmetry about the bearing axis then the principal axis is the same as the bearing axis and the wheel spins smoothly.
Memphis’s point was that adding mass increases momentum, which may lead to better behavior in some situations with adverse behavior in others.
My point was that it might be possible to achieve Memphis’s goal of added momentum without a strong penalty in slow-speed maneuvering.
U-Turn’s point was (I hope I’m getting this right) that the asymmetric pedaling action induces wobbling forces that could be damped by adding mass in the right places, and that these places are exactly the opposite of where I suggested the masses should go.
One thing we all aparently agree on is that none of us have any empirical experience of riding a mass-modified unicycle. I think we all agree that the first post with experimental results will be very good reading.
Oh yeah.
Absolutely. In addition, each rider who performs some tests will likely do things a bit different from the others. So all posted results will be greatly appreciated.
Nope, that was me, sorry. I should know better than to post before 8.30, my brain just isn’t up to speed by then. (It takes a while coz it has such a high speed to get to, okay? 
)
It’s the outside edges of the wheels. They stick out past the bit which runs on top of the rail, so for the part of the revolution when they are below the rail they go backwards. Then at the top of the revolution the same bit go’s faster than the train.
John
Gild gave me an idea. . .
If anyone is really passionate about having a self adjusting, equally distributed source of extra weight for their wheel, you could attatch maybe eight seperate weights first, to some sort of track (maybe even use 8 spokes!) that runs from the hub to the rim, and secondly, to a loosely tensioned spring attatched on the other end to the hub. As the wheel spins more quickly, the weights reach the rim as you reach toward top speed for flat sections, adding inertia, and maybe making higher-speed turns a little more graceful and solid feeling. When you’re on the rocks and meed to turn, the weights are retracted in at the hub.
Although i can see problems with the mechanism if there was mud involved, it could prove to be useful, and also avoid the problem of having the weights on the exterior of the circle while accelerating, and while ascending.
-JoshOh jeepers. You asked what part of the train was “always” going the opposite direction. But in the answer we find out that “part” is constantly swapping mass with the part next to it. In other words, not particularly useful.
All the discussion about where to put the counterweights is interesting. What sounded like the most knowledgeable advice, from a physics point of view, was to put the weights in locations that aren’t practicable, because your feet and legs have to pass through those areas when you pedal. The weight has to be more or less confined to the usual places, leaving room for the unicycle to function with you on it.
Another idea that has been proposed in the past as a counterweight thing was to add weight to the crank arms, on the end past the wheel axle. In other words, to add a big blob of mass that will extend above the axle when the pedal is at the bottom of the stroke. This could function, but the mass would be so close in to the axle that you’d have to use even more of it to have a useful effect.
In most cases, I think adding weight to counteract the “wobble” effect is a fruitless cause, because to make it really work you’d have to add more weight that you’d probably be willing to ride with. But if people want to experiment, I’m sure we’d all be interested to hear their results.
Meanwhile, adding weight in the rim area to increase inertia is a more workable plan, and the more level the trail is, the more useful it can potentially be. I will still argue that improved technique will work better for you than extra weight, but I think the original idea was for a more comfortable ride, not necessarily higher performance.
I remember back to hearing the story of Cathy Fox’s (yes, a girl) 100 mile Guinness record from early 1980 (10 hrs. 37 min.). She was accompanied on her ride by racing great Floyd Crandall. Floyd was on a wooden wagon wheel unicycle his father had built, that weighed around 50 lbs. Cathy had a much lighter 40" Tom Miller big wheel. Though training is by far the most important factor, Floyd was unable to complete the ride on his heavy wheel, while Cathy went on to break the previous record (as far as I know, that’s still the girls’ 100 mile record).
I love this website! I have to admit, I didn’t read this entire thread, but I love the fact that we discuss things!
On Wed, 3 Mar 2004 13:44:22 -0600, johnfoss wrote:
>Adding weight to your rim will enhance
>this wobbling effect.
Why is that John? I believe adding weight will decrease the wobbling effect if the weight is evenly distributed along the rim because there is more symmetrical mass, overwhelming the effect of (i) the asymmetrical mass in the pedals and cranks, and (ii) the asymmetrical force on the pedals (that together cause the wobble). The required force on the pedals is virtually the same for weighted and non-weighted rim, provided you ride on the flat and at a constant speed.
Klaas Bil
Posted on the newsgroup two days ago but still not on the forum. 
I should add that any response that I should see, is best sent via email.