The physics behind rail riding and stillstanding?



A

andrew_carter

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Some of us Australian trials and muni riders are putting together a
combined movie project that we plan to sell and that we hope will raise
awareness, etc of unicycling in Australia. As well as my sectin of the
movie, I'm making a rail riding tutorial to fit into an extras menu and
I want it to be useful and to make sence...

...so I'd really like to know a little about the physics of rail riding.
When I'm riding a rail, if I'm falling to the right then I'll throw my
arms and upper body to the right to somehow regain my balance. Could
somebody please tell me what's going on here? Being higher above the
ground, and often heavier, I would have thought that the upper body
would pull the rider even further off balance if it was moved in the
direction he/she was falling. Or is it that when the rider moves their
upper body to the right, they are pushing their lower body to the left,
and keeping their weight over the rail?

Also, I've sometimes found myself swinging one arm in circles in a final
attempt to regain balance, and sometimes it's worked. Actually, I just
looked at a video of a rail I rode once and it's sometimes more of a
push of the arm from the back to the front. In the case I just observed
I was falling to the right and I did this action with my right arm to
regain balance. Can someone please explain this?

And one last thing, is the forward momentum doing anything to keep the
rider upright? Should narrow rail riding (with no lateral wheel
movement) be any easier than stillstanding?

I've attached some frames fro ma video to try to explain the actions I'm
referring to.

Thanks a lot. I'm sure I could attempt to analyse these goings-on
myself but I'd rather hear even the basic stuff from somebody who knows
what they're talking about.

Thanks,
Andrew


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I'm not particularly good at riding rails, but from the physics point of
view the idea of swinging the arms is that the reaction from your arms
swinging moves your weight in the opposite direction, in the same way
that you can do sharp turns y swinging your amrs around you, just in a
different axis.
I think its also important that the arms are aove your head, which means
they are further away from the pivot (the bottom of the tyre) so the
force from swinging your arms produces enough of a moment (turning
force) to regain your balance without moving your cnetre of gravity even
further over to the point that you fall anyway.
I suppose that technically with no lateral movement a rail should be the
same as a still stand, but with most rails the thickness of the tyre
might mean you can move a little to regain balance. From the pictures
you can probably answer that better than me.:)

Hope that helps,

John


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Yes, that was helpful, thanks John. I think that with rails that size
and smaller any lateral wheel movement is unintentional for me.

Keep the explanations coming!

Andrew


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Hi Andrew,

Sounds like a neat project. I don't have much time now but I'll give it
some thought and post later. One of the stillstanding threads had a
good discussion of what is going on. You're on the right track with
your hunches. (Oh - sorry about the double pun... :) )

Tim


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Conservation of momentum: Your total momentmum must be the same at all
times in the left right direction, so if you give your arms momentum to
the left, then the rest of you must go to the right to 'cancel' the
momentum to the left.

Same concept as throwing something heavy when you're standing on really
slippery ice: the thing you threw goes one way (Like your arms) and you
go the other way (Like your torso and uni).


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Okay thanks, but how about these ones...

"Also, I've sometimes found myself swinging one arm in circles in a
final attempt to regain balance, and sometimes it's worked. Actually, I
just looked at a video of a rail I rode once and it's sometimes more of
a push of the arm from the back to the front. In the case I just
observed I was falling to the right and I did this action with my right
arm to regain balance. Can someone please explain this?"

"And one last thing, is the forward momentum doing anything to keep the
rider upright?"

And I noticed that the guy (in the Moab section) riding that brown rail
seems to prefer to keep his upper body very low. This seems strange to
me.

I found a good clip yesterday to add to the tutorial. It's just me
stillstanding on a wooden rail but you can see clearly how I'm trying to
push my hips to the side by moving my upper body to the other side.

Andrew


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andrew_carter wrote:
> *Should narrow rail riding (with no lateral wheel movement) be any
> easier than stillstanding?*
Of course. In a stillstand you are not allowed the freedom to roll
forward and back. The rolling also allows you to make some left-right
corrections with slight changes in your line.

There is the whole Newton's law and conservation of energy/momentum
issue, but this doesn't explain how a Kris Holm can spend 3 minutes
stillstanding on a piece of railroad track. Clearly he was doing
something the rest of us were not.

Add *time* to the physics info from above. I believe that is a factor.
Also if you make your body movements in a wave, that kind of gets wider
as it goes up. Those are two brief descriptions of how I think you get
around the normal laws of physics to balance where common sense says you
can't.


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Did i read that right? THREE MINUTES? is there a special technique for
stillstands?


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This would be easier with pictures, but I haven't figured out how to do
that easily in a regular post so here goes:

Imagine two vertically elongated masses with one stacked on top of the
other and joined together with a powered hinge. The lower mass
represents your legs, hips and the unicycle. The upper mass represents
your torso, head and arms. The hinge and power source are your waist
and abdominal/back muscles.

Since you can roll on a thin rail you can control your fore and aft
balance with normal pedaling. This frees your abdominal/back muscles
from the need to balance a still stand in that direction so there is
more "power" for still-standing in the lateral direction.

So, imagine that you are looking down the rail. Mentally "float" the
two mass system an inch over the rail. ("Matrix" physics rules are in
effect - gravity is suspended.) Imagine the system is perfectly still
and aligned perfectly straight and vertical. What happens if the hinge
"motors" are actuated to bend the top to the right? Conservation of
momentum (Newton's first law) says that the center of mass stays put,
since there are no externally applied forces (the "tire" isn't touching
the rail). Further, there is no reason for the system to rotate as a
whole since there are no externally applied torques either. So the top
and bottom of the system move to the right, and the waist moves to the
left. The center of mass of the upper mass stays on the vertical line
but moves down, and the center of mass of the lower mass stays on the
line and moves up. The overall center of mass stays put.

Now, mentally straighten the masses out again and turn gravity on. The
mass system drops onto the rail and balances. If everthing is PERFECTLY
alligned it will just stay there, however this perfection is only
possible in cartoons and computer simulations. In the real world it
will never happen. Eventually it will begin to rotate around the
contact point on the rail and fall. Why?

With gravity turned on the mass system pushes down on the rail, and the
rail in turn pushes up on the mass system. When the system is perfectly
vertical these two pushes are exactly in alignment and cancel out. When
the mass system tips, even a tiny amount, the two pushes become slightly
offset. This creates something called a "couple" in physics that
creates a pure twisting force or "torque". Since these forces are
externally applied Newton's second law comes into effect - the masses
accelerate. As the masses tip they also start to accelerate sideways
because the rail "up" push is no longer exactly vertical.

Now imagine that the masses are still aligned straight with each other
but have tipped a tiny bit to the right. What happens if the "motor" in
the waist is actuated to rotate the top mass strongly to the right?

In the zero gravity thought experiment the top part rotated to the right
and the lower part rotated to the left. But in the gravity-on case the
"tire" is in contact with the "rail" so the bottop part isn't free to
rotate anymore. In the free-air this floating tire tread would have
moved to the right too, but since it is in contact it can't, so the
whole system is pulled to the left.

Now, mentally stop time: What happens if you bend the system at the
waist and then straighten it in the same instant? The answer is
nothing. It will go back to exactly how it was. So managing time must
be part of the solution for still standing. Ok, start time again:

When the tire patch causes the system to move to the left it causes the
center of mass to move to the LEFT of the rail. Now the tipping couple
is reversed and the masses start to fall to the left. After a suitable
period of time imagine the motor is actuated again to straighten the
system. This causes the center of mass to move to the right, but since
the masses spent some time falling to the left it doesn't go back to
where it started from. If the timing and movements are done perfectly
the center of mass will wind up right over the rail and the system is
back in balance.

So what about the arm windmilling? Well, it's the same thing but with
three masses (and a new powered hinge at the shoulder.) When you start
your arm rotation you cause the tire to pull your center of gravity one
way, and when you stop the rotation it pulls you the other way. Get the
timing right and you stay over the rail.

This form of balance isn't limited to unicycleing. Watch any good
soccer player in slow motion and you will see the arm-swings and waist
bends helping him (or her) maneuver. Or just go down to the bar and
watch the really drunk folks - since their balance and timing systems
are impared you can see in an exagerated sense the waist bends and arm
swings that we all do to just stand and walk on our feet.

Hope it helps. For the video I'd have an illustrator make some
animations. Long leather coats optional for the "Matrix" physics
section.

Tim


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In my opinion, I think that one of the most important points to mention
in any video would be to explain that the primary focus of balance when
doing stillstands and riding railings (or for that matter any other
unicycling move) is not your arms- it's your core. Developing a good
sense of core balance will do way more for still stands than
concentrating on balancing with your arms.

This means thinking in terms of shifting your hips around to maintain
balance, rather than just flapping your arms.

Kris.


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Thankyou very much guys, that combined was exactly what I was looking
for. Of course in a short tutorial on rail riding which will rely only
of video, maybe a diagram, and text (because I don't have a microphone)
I can't go into too much detail at all but all of that info has really
helped me to understand what's going on. I just need to practise now.

I find stillstands and rail riding easier when sitting down on my trials
uni (rather than either sitting or standing on my freestyle uni with a
high seat). It's clear to me now why that is.

Thanks a lot,
Andrew


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Jack Wiley has a line in "The Unicycle Book" that goes:
Trying to maintain balance by wild arm and body motions is something
like trying to balance a wet noodle.

Unfortunately, that's exactly what you have to do when still standing or
riding a rail. In normal riding the object is to keep the wheel
underneath you. When still standing or riding a rail the object is to
keep yourself over the wheel. It's opposite of the way that you
normally ride. Learning to ride on a rail requires learning a new way
of doing balance corrections. It's not like normal riding at all.

That's the gist of what you want to get across to the lay person.
That's why riding on a rail is difficult and a feat of balance while
normal riding is not so difficult and doesn't require extraordinary
balance.


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Yes, that's what I've been telling any of my unicycling friends who have
wanted to learn to ride rails in the past. I think it's a very
important point that instead of keeping the wheel beneath you you're
keeping yourself over the wheel.

George,
I have many many many clips on my computer from the movie we've been
filming. I'll put a downloadable version of the movie up when it's done
(very soon...just sorting out music now). Thanks for the kind words but
I'm really not fan-worthy. :)

Andrew


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andrew_carter wrote:
> *I'm really not fan-worthy.
>
> Andrew *


That's a lie and you know it!


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On Fri, 23 Apr 2004 21:35:43 -0500, "andrew_carter" wrote:

>I'm really not fan-worthy. :)

Andrew, I'm a fan of you!

Klaas Bil - Newsgroup Addict
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