

How Bikes Steer Themselves - celias
http://www.sciencefriday.com/program/archives/201104154

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aston
<http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics> has all you
need to know on the subject.

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lutorm
I'm not sure I agree with the section on countersteering. It says that "out-
tracking", the fact that you steer the wheel out from under the bike, is more
important than than gyroscopic precession. For motorcycles on highway speeds,
I think this is incorrect. If you've ridden a motorcycle at high speed, you
know that the handlebars are basically like buried in solid concrete. With
normal steering force, I don't think I deflect the handlebars by anything
noticeable. The key is that at high speed, the mere application of _torque_ on
a spinning wheel, will make it precess, ie turn around the orthogonal axis.
I'm convinced this is far more important (but I'm too lazy to do the numbers
now).

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sliverstorm
A motorcycle can be turned at speed two ways. Out-tracking is only one of
those ways. (Leaning is the other)

However, it is the way that lets you take a deep set most quickly, and can
help a lot with tight lines.

You may think that you are not out-tracking when you turn at freeway speeds,
but remember the amount of necessary out-track decreases as speed increases.

~~~
lutorm
_You may think that you are not out-tracking when you turn at freeway speeds,
but remember the amount of necessary out-track decreases as speed increases._

If you think that out-tracking is responsible for entering turns at speed,
there's an experiment to test this: If the bike leaned by moving the wheels
out from under it, the bike should pivot around the center of mass. If you
actually look at a bunch of road racers in the twisties, it's abundantly clear
that the bikes pivot almost completely around the _contact point_ , not around
the center of mass. (Unlike bicycle racers which really do out-track.) Moving
the COM without moving the contact point can _not_ be done through wheel
forces on the pavement, because they act at the contact point. It's impossible
for those forces to lean the bike _and_ move the COM is the same direction.
The only thing that can accomplish such a motion is the precession resulting
from applying the handlebar torque to the front wheel.

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andrewtbham
I'm also not sure how this is going to help someone build a better bike...
just because it's possible doesn't mean it's a good idea. bikes with steep
head tube angles are jerky.

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sliverstorm
Rake is your friend.

This became patently obvious when I rode motorcycles that ran the whole gamut
of rake angles. Even more obvious when I then remounted a bicycle, which has
drastically less rake than even the most aggressive motorcycle.

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stretchwithme
This is a great example of not having to understand a phenomena in order to
exploit. Of course, understanding will let us take it even further.

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tomstuart
See also: <http://www2.eng.cam.ac.uk/~hemh/gyrobike.htm>

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sliverstorm
As best I can tell, it's a mix of forces.

Gyroscopic action helps keep the bike upright by slowing falls. (at 100mph on
a motorcycle, you must apply significant force to lean over _quickly_ )

Rake (trail) makes it less likely the front wheel will be deflected by small
bumps, and I suspect it also helps settle the bike back into neutral position.
In other words, decreases the number of significant self-steering corrections
that must be made.

Then, some third force does the wheel turning. I personally suspect it is tied
to the way the front wheel falls to the right if you suspend the bike and lean
it right.

Note: Honestly, from the diagram at 2:03 it looks like they did not eliminate
gyroscopic action and trail. They merely greatly reduce gyroscopic action,
reduce rake, and invert trail.

~~~
ams6110
To turn/lean a bike at 100MPH you have to push the handlebars opposite of the
direction you want to turn. If you need to swerve right, lean right and push
the handlebars left. Not knowing this is probably related to many loss of
control accidents among novice riders (speculation).

~~~
sliverstorm
Of course. My point is, the gyroscopic forces of a pair of 20lb wheels
spinning at nearly 1,000rpm are not trivial. It takes a noticeable amount of
force to make the bike respond _quickly_.

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andrewtbham
Do people really not know a bike will steer itself? it's been called ghost
riding for a long time... even before that ridiculous song.

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lutorm
know _that_ a bike will steer itself != know _how_ a bike steers itself. It's
a pretty nontrivial coupling of geometry with conservation of momentum and
angular momentum.

~~~
andrewtbham
at 0:31 the guy says "I always have to tell people that bikes do stay up by
themselves." I find that surprising... that people don't know.

