
We still don't really know how bicycles work - ColinWright
http://www.newstatesman.com/2013/07/mysteries-bicycle
======
asynchronous13
The gyroscopic theory has been proven wrong, yet many people continue to
believe it. But just because many people are wrong, does not mean that no one
knows.

A bicycle in motion adjusts its center of gravity to remain upright. It's very
similar to the inverted pendulum problem.

Look at a bicycle directly from behind with the wheels exactly lined up. Now
imagine that you could frictionlessly slide the two tire patches left and
right. The similarities to the inverted pendulum become more clear.

Of course, it is more complicated than the classic inverted pendulum. Instead
of one point of contact under the mass, there are two. And the two points of
contact (i.e. the wheels) have their own complex dynamics.

Having a rake angle on the front wheel makes a bicycle self correcting (if the
c.g. is on the right side of where the wheels contact the ground, then a right
turn is induced in the front wheel by the rank angle)

There are two major forces that must be in balance to turn a bicycle - the
side force from being off center with respect to c.g., and the centripital
force in the turn.

Ever watch a cyclist train on rollers? That's much closer to an inverted
pendulum. And since there is no forward momentum, there is no centripital
force, which makes it more difficult to remain upright on rollers than on
pavement.

~~~
lambda
> Having a rake angle on the front wheel makes a bicycle self correcting (if
> the c.g. is on the right side of where the wheels contact the ground, then a
> right turn is induced in the front wheel by the rank angle)

No. If you actually read the paper linked (and the Supplementary Online
Materials, which contains a lot of the actual information), it is shown that
rake angle is not necessary for the bike to be self correcting. You can build
a bike with a negative or zero rake angle that is still self-stable (at least,
according to the bicycle dynamics modelling software they were using, JBike6;
they didn't actually build this particular bike, but did build one that had
small negative trail and no gyroscopic effects that was still stable).

As they demonstrate in the paper, none of rake angle, trail, or gyroscopic
forces are either necessary or sufficient for self-stability. All of them can
influence stability, so saying the gyroscopic theory has been proven wrong is
not entirely right either; it is a part of the dynamics that adds stability,
it is simply not necessary or sufficient on its own. In fact, the paper shows
that on the "benchmark bicycle", removing the gyroscopic force makes it
unstable, so on that particular design, the gyroscopic force is necessary for
its stability (thus explaining why it was believed for so long that gyroscopic
force is what provided stability).

What we know is that gyroscopic forces, trail, rake angle, and distribution of
the center of mass of the fork and body of the bicycle all influence stability
(in particular, the center of mass of the body of the bike being substantially
higher than that of the fork); none of them alone are sufficient to provide
stability, and likewise none of them alone are necessary as we can build bikes
without them that are still self-stable.

~~~
emmelaich

      > > Having a rake angle on the front wheel makes a bicycle self correcting ..
      > No. If you actually read the paper linked ...
    

Please don't start replies with a "no", especially when you don't disagree!
You reply that it "is not necessary" which does not negate your interlocutor's
point!

(probably going to regret going meta, but the initial 'no' in forums and irc
really bothers me)

------
barrkel
Well, we know enough to know how to increase stability - primarily by
increasing trail and rake. Decreasing rake angle is the primary way of making
a motorcycle turn faster, with the known risk that it makes it more likely to
head shake or even tank-slap.

It's fairly clear that common two-wheelers are stabilized by the rake and
trail inducing a counter-steering effect when pushed horizontally - try to
shove over a bicycle and the front of the front wheel will turn in the
direction of the push because the point of contact of the tyre with the ground
is behind its axis of rotation (the amount of which is the trail). And if the
bicycle is moving forwards, this turn of the wheel will cause a torquing
effect to roll the bike in the opposite direction of the shove. So long as the
correction is somewhat less than the overall shove, the system should be self-
damping.

But if the dynamics were fully understood, we'd be less likely to get
motorcycles with issues like the well-known Pan Weave[1] and other high speed
stability issues. At the limit, things like aerodynamics, chassis rigidity
etc. start coming into the equation.

[1]
[http://en.wikipedia.org/wiki/Honda_ST1300#Pan_weave](http://en.wikipedia.org/wiki/Honda_ST1300#Pan_weave)

~~~
Terretta
Thank you. We do know how they work, and that it's nothing to do with
gyroscopes and everything to do with (what parent said, but simplifying a bit)
front fork geometry.

Pretty much anyone can see this for themselves. Walk a bike with your hand
holding the back of the seat, not the handlebars. Steer the front wheel by
leaning the bike. Lean father to correct faster. If it's a cheap bike, let go
and watch this happen on its own till it wobbles too far to counteract.

This is one of those idiotic tropes like bumblebees not being able to fly.

~~~
lambda
If you read the actual paper referenced, we know some of how it works, but not
all.

Two of the proposed theories (that it has to do with gyroscopic effects, and
that it has to do with trail), have been disproven by creating a self-stable
bike with no gyroscopic effects and (slightly) negative trail. The paper
introduces one additional factor, the difference in center of mass between the
steering assembly and the rigid body of the bike; the steering assembly having
a lower center of mass causes it to fall faster, providing the necessary
corrective steering to achieve self-stability.

So, there are several factors we know about, which can increase stability. We
know how to locally optimize stability for certain designs. But we don't yet
have a full set of necessary and sufficient conditions for a bike to be self-
stable. We haven't even proven, analytically, the intuitive notion that a
bicycle must lean toward a fall, though given our intuition it is believed to
be true.

Here are the two necessary conditions that the paper provides:

> To hold a self-stable bicycle in a right steady turn requires a left torque
> on the handlebars. Equivalently, if the hands are suddenly released from
> holding a self-stable bicycle in a steady turn to the right, the immediate
> first motion of the handlebars will be a turn further to the right. This is
> a rigorous version of the more general, as-yet-unproved claim that a stable
> bicycle must turn toward a fall.

> Another simple necessary condition for self-stability is that at least one
> factor coupling lean to steer must be present [at least one of Mδϕ, Cδϕ, or
> Kδϕ must be nonzero (SOM text S3)]. These coupling terms arise from
> combinations of trail, spin momentum, steer axis tilt, and center of mass
> locations and products of inertia of the front and rear assemblies.

That's what is meant when people say "we don't know how bicycles work." We do
know some of how they work; we know that the designs that we create steer into
a fall, and do so in such a way that damps the wobbles and eventually goes
straight again. And we do know some necessary conditions for a self-stable
bicycle, like a requirement that something that couples lean and steering must
be present; but we don't know if the steering into the fall is absolutely
necessary, or if you could build a bike that managed to achieve self-stability
without it.

So, I would say that a more accurate summary is "we know how current bicycle
designs work, but we don't know exactly what aspects of them are necessary, or
how to completely characterize the sets of designs that work or don't work."
But that's a bit more of a mouthful than "we don't know how bicycles work", so
that's what gets repeated.

~~~
apalmer
So we do know how the bikes that actually are 'bikes' work at least to a very
large degree (engineering vs math), but we don't know all the possible physics
which can enable an arbitrary two wheeled construct to be selfstablize when
perturbed while in forward motion.

I completely understand the point your making. However, at when making a
technical argument and then generalizing the end results, we can end up in a
situation where the truth of the technical argument no longer strictly implies
the truth of the generalized/summarized result. I feel the statement 'we do
not know how bikes work' has crossed that line.

~~~
lambda
I suppose it depends on how you look at it, or perhaps on whether you're
interested in _how_ a bike works versus _why_ a bike works. How is relatively
easily answered; as the bike tilts it steers into the tilt, moving its base
back under its center of gravity, in a way which damps itself thus getting
back to upright without oscillating repeatedly or falling over.

Why it works is the open question. We know that it's some combination of
gyroscopic effects, rake, trail, and the different centers of gravity of the
frame and fork, but we don't know the precise relationship between them that
allows it to work.

------
schoper
We do know, here is the paper that the journalist refers to:

[http://www.sciencemag.org/content/332/6027/339](http://www.sciencemag.org/content/332/6027/339)

And here is a great video by the author:

[http://www.youtube.com/watch?v=YdtE3aIUhbU](http://www.youtube.com/watch?v=YdtE3aIUhbU)

Short answer: "a bicycle should turn into a fall."

------
JoeAltmaier
Urban legend. Bicycle physics is well-understood. My old job, we even made a
fuzzy logic model of motorcycles/riders for Harley Davidson. No mystery here,
move on.

~~~
crusso
Isn't it amazing how confident people are that their own ignorance is
everyone's ignorance?

It's so much worse when the ignorance belongs to a "journalist" who really
just needed to do a little research before promoting such a poor theme.

Yes, we know how bicycles works. No, it wouldn't be that big of a deal if
nobody was working on solving the mysteries of the bicycle in favor of really
hard problems like "dark matter".

------
netrus
I highly recommend the following TED talk on bike physics by Arend Schwab from
Delft University of Technology:

[http://www.youtube.com/watch?v=2Y4mbT3ozcA](http://www.youtube.com/watch?v=2Y4mbT3ozcA)

It focuses on the question why bikes do not fall, even without a rider on them
(spoiler: coriolis is insignificant).

------
hownottowrite
[http://ruina.tam.cornell.edu/research/topics/bicycle_mechani...](http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/stablebicycle/index.htm)

~~~
dljsjr
Andy Ruina is a great guy to listen to about stuff like this. There's an
annual conference for scientists in all fields (robotics, biology, cognitive
science, etc.) that study walking locomotion called Dynamic Walking; our lab
hosted it in Pensacola last summer and Andy gave a "Greybeard" talk about the
mechanics of sailboats and sailing down wind faster than the wind that was
really entertaining.

I'm not sure where his fascination with esoteric mechanics comes from, but he
has a pretty clever and engaging manner of discussing the stuff.

Edit: Didn't realize that Ruina was a co-author on the paper referenced in the
article. Even better. It all goes full-circle.

------
arh68
If Feynman were still around, he'd set things straight. I don't think a
bicycle staying upright is much different from a train keeping course [1], but
I can't describe why. Instead of the rail veering off and the wheel adapting,
it's like the bike does what it wants and the ground shifts beneath it.

If you've read about Einstein's pail-of-milk-on-a-lazy-susan, it's a similarly
unintuitive frame of reference. Also, if the experts haven't figured it out: I
don't know what I'm talking about.

I find it fascinating that we as a species can observe, define, and exploit
Maxwell's law, neutrino physics, etc, but we can't clearly explain bicycles.
Or why wings provide lift.

EDIT: forgot the Feynman video

[1]
[http://www.youtube.com/watch?v=y7h4OtFDnYE](http://www.youtube.com/watch?v=y7h4OtFDnYE)

------
mhb
How does he not even have a picture of the "Anaconda"? OK, here's a paper
about it:

[http://bicycle.tudelft.nl/bmd2010/CDProceedingsBMD2010/paper...](http://bicycle.tudelft.nl/bmd2010/CDProceedingsBMD2010/papers/kabeya2010simulation.pdf)

------
PhasmaFelis
Facepalm.

I look forward to New Statesman's thrilling coverage of how bumblebees can't
actually fly, and nobody knows why duck quacks don't echo.

------
mkoryak
I had an idea to build a tall bike which contained a large and heavy wheel in
the center of the frame, above the 2 wheels but below the rider. This wheel
would be made to spin somehow and act as a gyroscope to help the bike stay
upright when stopped.

This seems to suggest that my plan would never work. If gyroscopes don't
stabilize, then why are they used on monorail trains etc?

~~~
joshrice
> If gyroscopes don't stabilize, then why are they used on monorail trains
> etc?

They do, it's just the bicycle's wheels aren't heavy enough to provide enough
force to really make that much of a difference.

------
e3pi
...and, like a horse, I still can't get on these newfangled things from the
starboard side.

------
jokoon
isn't it because the front wheel change the contact point with the ground when
you turn ?

when you both turn and inclinate the bike, the contact point is more on the
side which you turn, balancing the bike.

------
tudorconstantin
"Check mate, scientists" :))

------
jccalhoun
magnets

------
whiddershins
Nor do we know how airplanes work. Or whips.

~~~
speeder
What parts we don't know about these?

~~~
rayiner
I don't know about "don't understand" but did you know that there is a lot
more to why a wing creates lift than the "pressure differential" explanation
commonly put forward:
[http://en.wikipedia.org/wiki/Lift_(force)](http://en.wikipedia.org/wiki/Lift_\(force\)).
In particular, the typical assertion that the air at the top has to speed up
so it arrives at the trailing edge at the same time as the air from the bottom
is false.

~~~
speeder
I see...

The speed explanation is not incorrect, except the REASON for it is.

I learned on school that the air on top of the wing move faster to catch up
with the one in the bottom (this never made sense to me...), the air on top
indeed move faster, we only don't know why.

Damnit, I should be a physicist, not a programmer :P

~~~
claudius
If you were a physicist, you’d likely also have to be a programmer – the
inverse, unfortunately, does not hold.

~~~
michael_h
Public service announcement for physicists fancying themselves as programmers:
Learn the following topics before going any futher:

    
    
      * BLAS (please)
      * LAPACK
      * memset
      * memcpy
      * 'Segmentation Fault' is not a synonym for 'exited successfully'.
    

Those who maintain your code will thank you for it. Sometimes it might get you
chocolates.

~~~
CamperBob2
_' Segmentation Fault' is not a synonym for 'exited successfully'._

Next, you'll tell me that 'Thank you for playing Wing Commander' is not a
synonym for 'EMM386 exception 006A at 00B8:3128'.

