What do you think about when stillstanding?

Re: Re: Re: What do you think about when stillstanding?

Ok, after confidently saying that fore and aft ballance is easier, I now have to agree with Klaas that it’s probably harder. This is turning into a fascinating study. Ok, here goes:

There are two methods to still-stand. The first, that everyone here seems to get, is the “tip your upper body hard in the direction of the fall to get the CG behind the contact patch, wait until you tip back a bit, then straghten up again” method that I butchered the description of in an earlier post. This method is the only method that works for things with a small footprint or radius near the ground (pogo sticks, unicycles in lateral mode, wire-walkers, etc.)

The second method is what surfers use - “gently tip the body away from the fall to rock the board into it” - that works when the radius to the ground is very large. Imagine you are standing inside a large hoop. If you tilt away from the direction of the fall you roll the hoop into the direction of the fall, which moves the contact patch and you’re saved.

Both cases have a lower pendulum with a rounded rocker, and an upper pendulum that hinges off the lower one with rider-control over the angle In the case of normal unicycle still-standing, the hinge is at your waist. In the case of the giant hoop, the hinge point is at the rider’s feet. Each is a special case of a general class of ballancing problems.

Idling is also a special case of this type of balancing problem where the lower pendulum is the wheel, the upper pendulum is your body and frame, and the “hinge” at the hub is controlled by your feet on the pedals. The cool thing is that math for all three cases is the same. I solve the equations once and I have all three answers. Cool, eh?

Ok, back to Klaas’s insight. There are two dimensions that are important - the radius of the rocker and the height of the hinge above the ground. The only difference between the front-to-back balancing while still-standing and idling a unicycle is that in one case the hinge point is at your waist, and in the other it’s at the hub. But in the idling case you tip your body back away from the fall by rolling the wheel into the fall, and in the other case you tip your body hard into the fall and rock the wheel away from the fall (as Klaas noticed).

In engineering terms this is called a control reversal. Hinge point low, tip away from the fall; hinge point high, tip into the fall. Whenever there is a control reversal there must be some mid point where there is no control; i.e., there is some height of the hinge point where no matter what you do you fall over.

Since the long rocker of the fore-and-aft motion is closer to this no-control point than the short rocker of the side-to-side motion, it stands to reason that there is more control in the side motion than there is in the fore and aft motion.

Now I’m bloody tired of equations, so I’m going to suit up and go fall over for a while. I’m still riding with wide wobbles side to side because I’m stomping the pedals. I haven’t relaxed into the motion yet. And I still can’t free mount. Grrr… It’s no fun being an old guy!

Re: What do you think about when stillstanding?

On Sun, 16 Nov 2003 20:17:35 -0600, cjd
<cjd@NoEmail.Message.Poster.at.Unicyclist.com> wrote:

>OK, thanks, now I think I get it.
You got it!

>The image I have is of an oreo cookie at the end of a pencil, with half
>of the cookie connected to the eraser and the other half free to
>rotate.
I don’t know what an oreo cookie is. Cyberbellum described the image
of a motorised rotating wheel at the location of the seat which worked
well for me.

>Is it noticably easier to stillstand on a square tire, like a
>gazz vs a halo/duro?
I don’t have experience with the Gazz but it might be. If a tyre is
square and wide enough, there would be no side balance problem at all.
OTOH, as per cyberbellum’ insight, a squarer but not supersquare tyre
might actually /hinder/ stillstanding.

Klaas Bil - Newsgroup Addict

“My butt has a crack in it , but I can still ride. - spyder”

Re: What do you think about when stillstanding?

On Sun, 16 Nov 2003 21:06:33 -0600, cyberbellum
<cyberbellum@NoEmail.Message.Poster.at.Unicyclist.com> wrote:

>Ok, after confidently saying that fore and aft ballance is easier, I now
>have to agree with Klaas that it’s probably harder.
I think your sushi bar reasoning was perfect but not to the point. I
blame that to beer. You wrote you had ‘another’ beer which implies
several of them.

>This is turning
>into a fascinating study. Ok, here goes:
Thanks cyberbellum, that was fascinating indeed. It might seem from
your argument that there is a critical (large) size of wheel on which
stillstanding is impossible. But then stillstanding might yet be
(theoretically) possible if the rider statically moves his CG up or
down e.g. by standing on the pedals, to get away from the ‘control
reversal point’.

You’ve earned yourself a lot of riding time I say!

Klaas Bil - Newsgroup Addict

“My butt has a crack in it , but I can still ride. - spyder”

Re: What do you think about when stillstanding?

On Fri, 14 Nov 2003 22:32:05 -0600,
<Ben.Plotkin-Swing.w0miy@timelimit.unicyclist.com> wrote:

>A unicyclist could use two balance poles (at right angles) to make the
>still stand easier.

Since you only have one pair of hands, rotating will be much easier in
one direction than in the other. I guess that makes that ‘other’
direction useless because it will be too slow. Hmm, maybe you can tie
your underarms somehow to it (as opposed to just holding it with your
hands), to increase leverage. Also, you’ll need to hold the cross
beside your body or above your head.

Oh and this is obviously Ken’s post I’m replying to, but it says
“Ben.Plotking-Swing” in the header. Strange.

Klaas Bil - Newsgroup Addict

“My butt has a crack in it , but I can still ride. - spyder”

Re: Re: What do you think about when stillstanding?

I blame it on ignorance. Could have been the dull stub of a pencil I was using, too…

Yeah, that’s the fascinating bit. I forgot to mention something in my earlier post. Control reversals can take on different characteristics.

There is the classic control reversal where at the neutral point the effects of the control inputs exactly cancel out. Think of a teeter-totter - if you push down on one end it tips one way and if you push down on the other it goes the other way. Push down near the middle and the effect is reduced; push down at the ballance point and nothing happens. This is a nice, simple theoretical model but in the real world it gets more complicated.

Imagine that the teeter pivot point is rusty and tends to stick. In the previous case the control degraded smoothly as you moved in from the ends to the pivot point, but now instead of a no-control point there is a wide no control zone because pushing near the pivot point doesn’t have enough leverage to overcome the stiction.

Then there is the kind with overlaping control at the reversal point. This can happen with complex systems that have multiple coordinated control inputs - like Andrew with his arm-waving, head nodding style, or your suggestion of standing taller if nothing works on a particular wheel.

My little though experiments use very simple models out of necessity. The current model I’m solving has a wheel with a torque at the hub, a rigid lower mass (equivalent to the legs/frame/hips of the rider) and a rigid upper mass (equivalent to torso/arms/head). There are three degrees of freedom in this model (travel along the ground, tipping of the two mass system, and bending between the two masses), so there are three governing equations. Two have control inputs (torque at the hub, bending at the “waist”) and the other (travel along the ground) is determined by side effects from the two control inputs. Each of these governing equations has about a dozen terms, and it’s almost impossible to untangle them. I MIGHT be able to get a clean, closed form solution, but I’m not sure. I’ll probably just have to simulate the whole mess and try to derive answers.

I guess what I’m saying is that these simplified math models are fine for gaining insight, but they can’t give a definative answer on what can really be done with a human body on a unicycle. I wouldn’t say that there is a wheel size where still-standing is impossible. because I think that human bodies have so many degrees of freedom that there will always be a style that works. Some wheel sizes may be more difficult than others, and the still-standing style will certainly change on wheels of different sizes.

I don’t think standing taller will make much difference. I think the solution lies in varying the rate, amplitude and timing of your arm swings and waist bends. The best, most realistic simulator for working these control strategies out is obviously a 16 year old killing time on a unicycle in a parking lot.

For what it’s worth,

Tim