wobble/cranks/coker

Unicycles and Equipment
1

Riding with 150’s on my muni seemed so natural till I got into the 29-er with 125’s and now 110’s.

When I took the muni out I was very aware of the amount my legs were having to shift and also of a ‘knees in my armpits’ feel.

So here’s a question for Coker riders; the standard cranks on a coker are 150’s, I’d always assumed that cos the Cokers wheel is so big the feel would be more like a 29-er with 110’s or 102’s, however the turning cirlce of the pedals is independant of the wheel size so would they feel as clumsy as the muni does to me now?

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.

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

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

I’m aware of how crank/wheel ratio affects speed and controllability, but am unsure of how relevant this is to ‘wheel wobble’.

2

I think the most important factor affecting wobble is technique. I usually use 170s on the Coker to provide spinning practice for the MUni. I don’t think I wobble any more than I would with shorter cranks, but I do ride slower than I would with shorter cranks.

At NAUCC the fastest riders on any size were riding straighter than the others as far as I could see.

On the Norway tour, all Cokers, the standard crank length under all conditions was 125mm AFAIK. This comes with skill and a brake is good for steep downhills. The standard shipping length of 150mm seems like a good starting point, though.

Another dimension that people tend to think affects wobble is Q; but judging from David Stone’s, Nathan Hoover’s, and other riders’ experiences with The Strongest Coker I don’t think that’s true, at least to the degree popularly held. Again, my opinion is that the major factor is technique.

One-foot riding is a major helper in spinning smoothly and reducing wasted pedalling motion.

That’s my 2 cents, anyway. :sunglasses:

3

In Norway I used 5.5 inch cranks. The most important part to not wobbling is you have to have lots of weight on your cranks. Since I am pretty light, I wobble a lot. On my 20 inch uni, I use 4.5 inch cranks and don’t wobble at all, unless I am going really fast. Control and weight on the cranks and wheel are the two factors.

-Ryan Woessner :slight_smile:

4

if you want physics, then it’s inertial mass, or I. I increases with radius/diameter and also is greater when more mass is further from the center; therefore fatter wheels have more I. when you spin a wheel it’s got various forces acting on it. some of these are perpendicular to the plane of rotation and therefore resist when the wheel changes plane, or wobbles. since those are forces, then velocity is also a factor. therefore, tall tires, fat tires, and fast tires (or riders) make less wobble. but that’s only the textbook answer

5

Wheel wobble isn’t a machine thing, it’s a rider thing.

Obviously, there has to be a certain amount of side to side movement of the wheel to maintain balance, but people do ride 20s and Cokers on very narrow strips: kerbstones, log bridges, bridge parapets…

I think wobble happens when your pedal action isn’t smooth. If you pedal in a series of hard downwards pumps on the pedals, the uni will tend to turn towards the pedal which is going downwards. If you pedal smoothly and fast then you will travel in a straighter, smoother line.

I think crank:wheel ratio matters a bit. I put 170s on the 26 and found it was quite difficult to avoid the “push push” style of pedalling, rather than the “spin spin” style. The uni was slow, the tyre introduced an element of weave, and I couldn’t keep the pealling smooth enough. On the other hand, I never notice weave or zig zag on the 28 with 110s.

It also seems to be the case that a larger diameter wheel weaves less. I think that’s an absolute, because (unless you scrub the tyre) the uni’s turning circle is dictated by the wheel diameter.

And the Coker seems to be almost a ‘special case’ because the wheel is so big and heavy that you don’t pedal it, you let it spin, and just pump a bit more energy into it. It works like a flywheel or gyroscope.

6

Re: wobble/cranks/coker

onewheeldave <onewheeldave.t0108@timelimit.unicyclist.com> 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 <kfuchs@winternet.com>

7

crankless pedals

While we are at fundamentals physics, I have a question here:
IF the pedals of a uni where not fixed to the axle and transmitting force thru
the cranks
but were fixed in a way more like those of an “impossible wheel”
(by a trick yet to be invented … alongside a bar that goes from axle to rim for instance -though I don’t believe it would be a good idea- )
what would be the effect on riding?

just curious

bear

8

Re: crankless pedals

Do you mean an ‘ultimate wheel’? i.e. pedals fixed directly to the wheel, rather than an ‘impossible wheel’ which has no cranks at all.

If so, I’ve heard that of the two types of ultimate wheel, the one with pedals fitted to a wooden wheel are easier to ride and more stable than ones which are simply unicycles with the seat/frame removed.

I think this is because the first type have a narrower pedal to pedal distance.

9

Re: wobble/cranks/coker

“wobbling bear” <wobbling.bear.t2xvd@timelimit.unicyclist.com> wrote
>
> While we are at fundamentals physics, I have a question here:
> IF the pedals of a uni where not fixed to the axle and transmitting
> force thru
> the cranks
> but were fixed in a way more like those of an “impossible wheel”
> (by a trick yet to be invented … alongside a bar that goes from axle
> to rim for instance -though I don’t believe it would be a good idea- )
> what would be the effect on riding?

Assuming your foot pedal position relative to the wheel is the same and
everything is sufficiently stiff then the effect on riding would be
negligible. The idea is clearly doable although creates a whole lot of
problems with almost no advantages. One could easily have crank arms welded
directly along the centerline of a hub in stead of bolting them to the
outside of the hub. They would just extending outward within the spokes.
But you need a long extension from the end of the crank to the pedal. This
is not where you want an extension. The leverage there would create a lot
of twisting on the crankarm. Nearly everything related to building or
modifying configuration would be more difficult.
-Cubby

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10

Re: wobble/cranks/coker

>Ken Fuchs wrote:

>> 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. …

>From: wobbling bear <wobbling.bear.t2xvd@timelimit.unicyclist.com>

>While we are at fundamentals physics, I have a question here:
>IF the pedals of a uni where not fixed to the axle and transmitting
>force thru the cranks but were fixed in a way more like those of an
>“impossible wheel” (by a trick yet to be invented … alongside a bar
>that goes from axle to rim for instance -though I don’t believe it
>would be a good idea- ) what would be the effect on riding?

To be more specific about the cause of wheel wobble, it should be noted
that it is maximized by two out of wheel plane masses that are 180
degrees out of phase with each other with respect to wheel rotation. In
other words the two masses are attached to a normal set of pedals.

If we place the cranks in phase with each other such as on a kangaroo
unicycle where both cranks point the same direction, the (side to side)
wheel wobble is eliminated. What is new in this case, is a lack of
power during one half of the wheel rotation, but there is no wobble!

I’m sure Wobbling Bear is suggesting something more advanced than simple
kangaroo style pedals, allowing smooth power application throughout
wheel rotation. This also does eliminate wheel wobble and would be far
more practical to ride than a simple kangaroo unicycle!

Sincerely,

Ken Fuchs <kfuchs@winternet.com>