
Moving bikes stay upright but not for the reasons we thought - DanielRibeiro
http://arstechnica.com/science/news/2011/04/moving-bikes-stay-uprightbut-not-for-the-reasons-we-thought.ars
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zyphlar
"Most people have seen a gyroscope in action, so the stability of a rapidly
rotating wheel should be fairly intuitive, making this a focus from the start.
People have built bicycles with counter-rotating wheels and found that they
still remain upright, so that can't be all of the story."

Um, counter-rotating wheels are still gyroscopes. In fact they're extremely
stable gyroscopes in that they won't impart rotational velocity on the frame.
So by having the wheels on a bike rotate oppositely, you're actually making
the gyroscopic effect even stronger.

I think Ars is pulling from this article, which isn't about staying upright
but is about the self-correcting steering of a bike wheel (i.e. the fact that
you can ride hands-free.)
<http://www.sciencemag.org/content/332/6027/339.abstract> \-- in which case
the conclusion is correct and likely due to the geometry of the wheel. For
example tractors have convex pulley systems that allow leather belts to self-
center despite not being perfectly aligned. It's counterintuitive but it
works.

~~~
ScottBurson
> Um, counter-rotating wheels are still gyroscopes.

Um, no. A system consisting of two identical wheels mounted on the same axle
and spinning at the same speed in opposite directions has a total angular
momentum of zero. It will behave like a solid object of the same mass.

~~~
rwmj
I'm still a bit confused about how a bicycle with counter-rotating wheels can
move.

~~~
chad_oliver
The counter-rotating wheels are not the same as the two that the bicycle rolls
on. So there's four wheels, two of which only exist to cancel the rotational
inertia of the 'normal' wheels.

~~~
pbhjpbhj
This. However I wonder what it would be like to ride a bike on two rolling-
roads so that the front and back wheels moved in opposite directions. Or,
indeed, a trike/quad with large coaxial wheel separation and contra-rotating
rolling-roads.

------
slug
There's more info at one of the co-authors website:
<http://bicycle.tudelft.nl/>

I submitted this earlier, with the title of the paper, I guess it was too
technical to catch people's attention :)

------
6ren
> negligible (4mm) trailing

As you tilt a bike, the effect of trailing on steering increases (i.e.
rotational force). This effect is cumulative over time (the velocity of the
handlebars turning changes their position in each successive instant; plus the
force causes acceleration, increasing the velocity). Small effects become big
effects: I would expect that _any_ trailing would induce sufficient steering
effects, and so even 4mm trailing is not "negligible".

Hmmm.... you could test this with a "skate bike" - no wheels, just blades on
ice, but with a curved front-blade, free to steer, to facilitate "trailing".

But this seems obvious, and it looks like they have been very thorough, so I'm
probably missing something (I haven't read their full paper).

* Stabilization occurs because when the front wheel steers that way, it guides the whole bike in a curve, which creates force tilting the bike in the other direction - therefore righting it.

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rmathew
There goes another traditional physics text-book explanation for why things
work the way they do.

Somewhat related, Jef Raskin on how the Bernoulli effect is not really what
helps create the lift on a plane's wings (as taught by most physics text-
books): <http://karmak.org/archive/2003/02/coanda_effect.html>

------
brianpan
Interesting video of a recent WSBK motorcycle staying upright in a crash:
<http://www.youtube.com/watch?v=Yj9c5poj8Ww>

------
jdietrich
A basic principle of bicycle framebuilding is that if you think you understand
bicycle dynamics, you don't. Frame geometries that should ride perfectly
develop terrifying handling problems in the metal. Frame designs that are
unridable in theory turn out to be relatively practical - perhaps the oddest
example of this is the Python recumbent, which steers in the middle, drives at
the front and has no handlebars[1]. Bicycles that are self-stable and balance
on their own aren't necessarily good to ride and vice versa.

Making sense of bicycle dynamics is particularly bewildering because they are
such a natural and direct extension of the rider. Learning to ride a bicycle
is a completely subconscious process and what your body is doing runs counter
to what your brain thinks is happening. Most people who ride bicycles believe
that they steer in the direction of a turn, when in fact the opposite is the
case. A child who learns to ride a bicycle with training wheels actually takes
longer to learn to ride on two wheels because of this - they have to unlearn
steering before they can learn to countersteer.

Bicycles are _weird_.

[1] <http://www.python-lowracer.de/geometry.html>

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kbatten
This article had zero scientific conclusions (or in fact any science at all)
and the title said "we" in the non-royal sense. Why is this here?

And the article even says they don't know whey a bike stays upright:

"What their math can't apparently tell them is why so many different bike
designs tend to stay upright."

To me it sounds like their math is the problem. Its like the bad programmer
who blames the compiler for the segfault.

~~~
dexen
_> This article had zero scientific conclusions (or in fact any science at
all)_

The article summarizes a piece of scientific work. The work itself does 3/4th
of typical scientific work: 1) analyzes the current state of art (previous
publications), 2) provides model and expected values, 3) analyzes
observational data & deviations from the model. It falls short of 4) providing
answer to the final ``why'' -- as the answer is still unknown to the authors.
What it does _not_ lack is scientific honesty and integrity.

 _> To me it sounds like their math is the problem._

Indeed, that's what the article says. ``What their math can't apparently tell
them is why so many different bike designs tend to stay upright.'' -- i.e.,
they haven't found the proper formula(s) yet. They don't blame a compiler (the
math itself), but find the mathematical formulas they selected to be not
sufficient for creating a complete model.

And your point was...?

------
indrax
Clearly the universe is fine tuned by an intelligent designer who likes stable
bicycles.

~~~
brianpan
See, the sarcasm doesn't work here because _intelligent people designed stable
bikes_. You don't look at a bike and think, thank you evolution.

Sorry, pet peeve.

~~~
allwein
I think that part of his point was that in this case, intelligent people tried
to design _unstable_ bikes, and failed. Even their _unstable_ bikes were
stable. He's doing the obvious equating of Intelligent Designer and God. And
since "God created the Universe", he made the universe like stable bikes.

------
georgieporgie
Ever since the previous post, I've been wondering where people are getting
their bicycles. I've never had a bike that would stay upright on its own at
less than ludicrous speed, and the crusty ten speed in the other article had
forks that were very obviously bent back, drastically changing the rake and
trail.

I also recall an old physics video from High School where the man flipped the
fork and handlebars around backwards, specifically so the bicycle would
stabilize itself.

~~~
usaar333
How did you test a bike staying upright on its own?

Being stable only requires the bike to resist some reasonable force that would
knock it down. It takes much more force to tip a moving bike than a stationary
one.

~~~
georgieporgie
A variety of crashes. Also, as a kid, for reasons I don't really remember,
jumping off our bicycles was the pinnacle of fun for awhile.

Oh, and I very regularly ride (or try to ride) without hands. One or two
bicycles were fairly stable, but most were highly unstable.

~~~
khafra
If you have a hill without anything breakable at the bottom of it, and a bike
you don't mind breaking, try pushing the bike down the hill. I don't have any
hills here in Florida, but I recall that working pretty well as a kid living
in more interesting terrain.

