Why bicycle don't fall?

[kk]

Why a bicycle can keep balanced when it's going but it inevitably falls
when it stops?
I'm keen on physics but I couldn't find an answer for that.
Please send me an e-mail.
Thank you!

Eugenio
e.c...@agonet.it

[Doug Craigen]

kk wrote:
>
> Why a bicycle can keep balanced when it's going but it inevitably falls
> when it stops?
> I'm keen on physics but I couldn't find an answer for that.

First of all a bicycle does not keep balanced when in motion, the rider
keeps it balanced. If you put a sack of potatoes on the seat and put it
into motion, it falls over.

The angular momentum of the wheels does help slow the rate at which a
bicycle would fall (ever done the demo where you hold a spinning bicycle
wheel and see how much effort is reguired to change its orientation?).
However, this slows the rate of fall, it doesn't provide a force to put
it back upright again.

The secret is in the steering. The best way I could think of to explain
it is this: Imagine a rigid perfectly balanced bicycle moving down the
road with no wind or other forces to upset it. It moves in a straight
line. Now put the bike at a small tilt in one direction. It will not
fall but perform a large circular rotation. So there is your
fundamental difference between a moving and stationary bike. The
stationary bike would fall over given a small tilt, but a moving bike
would stay up and go in a circle. It is you manipulation of the
steering which keeps the bike upright and going in a straight line in
spite of effects that create the tilts that would have toppled a
stationary bike.

I understand that it is possible to construct a steering mechanism that
makes a bike unstable.

|++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
| Doug Craigen |
| |
| Eureka Stories -- Stories of discovery for young children |
| http://www.cyberspc.mb.ca/~dcc/eureka/ |
|++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|

[Bill]

Doug Craigen <d...@cyberspc.mb.ca> wrote:

>kk wrote:
>>
>> Why a bicycle can keep balanced when it's going but it inevitably falls
>> when it stops?
>> I'm keen on physics but I couldn't find an answer for that.
>
>First of all a bicycle does not keep balanced when in motion, the rider
>keeps it balanced. If you put a sack of potatoes on the seat and put it
>into motion, it falls over.
>
>The angular momentum of the wheels does help slow the rate at which a
>bicycle would fall (ever done the demo where you hold a spinning bicycle
>wheel and see how much effort is reguired to change its orientation?).
>However, this slows the rate of fall, it doesn't provide a force to put
>it back upright again.
>

Someone years ago modified a bicycle to use skate wheels to illustrate that
the gyroscopic effects of the wheels were not important aspects of a
bicycle's stability. People were reportedly amazed at how well it rode.

>The secret is in the steering. The best way I could think of to explain
>it is this: Imagine a rigid perfectly balanced bicycle moving down the
>road with no wind or other forces to upset it. It moves in a straight
>line. Now put the bike at a small tilt in one direction. It will not
>fall but perform a large circular rotation. So there is your
>fundamental difference between a moving and stationary bike. The
>stationary bike would fall over given a small tilt, but a moving bike
>would stay up and go in a circle. It is you manipulation of the
>steering which keeps the bike upright and going in a straight line in
>spite of effects that create the tilts that would have toppled a
>stationary bike.
>
>I understand that it is possible to construct a steering mechanism that
>makes a bike unstable.
>

The stability and steering aspects are kind of surprisingly complex. I
think there was a Scientific American article some years ago on this
subject. The primary thing that determines the stability is the distance
between the intersection of the projection of the steering tube/fork axis
with the ground and the ground contact point of the front wheel. The wheel
contact point needs to be aft of the intersection for stability, and is
more stable for larger separations.

Bill

[Gregory Loren Hansen]

In article <01bcfb75$a9720aa0$cc7d...@e.capra.agonet.it>,

kk <e.c...@agonet.it> wrote:
>Why a bicycle can keep balanced when it's going but it inevitably falls
>when it stops?
>I'm keen on physics but I couldn't find an answer for that.

>Please send me an e-mail.
>Thank you!

The rider has to adjust the steering constantly, steering one way or
another depending on which direction the bike is tipping. Try securing
your handlebars sometime so you can't move them, then take a ride-- you'll
fall. The bicycle is inherently stable because of the castor on the front
wheel. That is, the fork is tilted. When the bike falls to the left, the
wheel will point to the left and the bike will put itself back upright.
That helps the human, but it's not enough to stabilize the bike on a ghost
ride. If your bike will let you do this, try turning the wheel 180
degrees and ride it. You can still ride it, but it's much more difficult.

--
"And don't skimp on the mayonnaise!"

[Donnie Jones]

IN>Why a bicycle can keep balanced when it's going but it inevitably falls
IN>when it stops?
IN>I'm keen on physics but I couldn't find an answer for that.
IN>Please send me an e-mail.
IN>Thank you!

IN>Eugenio
IN>e.c...@agonet.it

Well I havent taken physics but I will try to answer this using logic.
What I think is that maybe because of the momentum from the force that
has been applied by your legs to the bike to make it move that when you
go and have it balanced it stands up and when you let it roll it stands
up but once you come to a stop it doesnt. Well this is because the
force that was once there to make the bike move is gone; therefore,
without the momentum it will fall. Also I think that the momentum is
what helps to keep the bike balanced. Maybe I havent explained much to
help you and I really dont know much about this but I have tried and I
hope that I have helped you in some way (probably not though).

---
ş SLMR 2.1a ş

[John Popelish]

kk wrote:
>
> Why a bicycle can keep balanced when it's going but it inevitably falls

> when it stops?

> I'm keen on physics but I couldn't find an answer for that.

> Please send me an e-mail.

> Thank you!
>
> Eugenio
> e.c...@agonet.it

The main effect that allows a moving bicycle to remain upright is a
rider reacting to tilt, and steering. If he keeps steering so that the
wheels move under the center of gravity , the bicycle has no reason to
fall. You do the same thing as you run.

If you start to lean to one side, you step a little to that side, and
get your feet back under yourself. If you want to run around a turn,
you first step to the outside of the turn, to cause yourself to start to
fall over in the turn direction, then you step into the turn enough to
keep from falling over any more, but not enough to regain an upright
stance. When you are through turning, you step a little extra into the
turn, and force yourself into a fully upright stance. Then you step out
of the turn, and resume running forward. Try it.

Balancing a bicycle is very similar except that discrete steps are not
involved. Some bicycles have just enough camber (the axle is forward of
the point on the ground that would connect with an extension of the
steering axis) so that as the bicycle leans, the front wheel is forced
to turn just about the correct amount to turn the bike enough to prevent
falling over, or even to bring it back to a fully upright position.
Such bikes will roll a long distance with a dummy riding on them. I had
one of these, and could ride it all afternoon without touching the
handle bars, and just steered with my ass. (by shifting my weight
slightly)

John Popelish

[Eric Stevens]

On Fri, 28 Nov 1997 09:05:43 -0600, Doug Craigen <d...@cyberspc.mb.ca>
wrote:

>kk wrote:
>>
>> Why a bicycle can keep balanced when it's going but it inevitably falls
>> when it stops?
>> I'm keen on physics but I couldn't find an answer for that.
>

>First of all a bicycle does not keep balanced when in motion, the rider
>keeps it balanced. If you put a sack of potatoes on the seat and put it
>into motion, it falls over.
>
>The angular momentum of the wheels does help slow the rate at which a
>bicycle would fall (ever done the demo where you hold a spinning bicycle
>wheel and see how much effort is reguired to change its orientation?).
>However, this slows the rate of fall, it doesn't provide a force to put
>it back upright again.
>

>The secret is in the steering. The best way I could think of to explain
>it is this: Imagine a rigid perfectly balanced bicycle moving down the
>road with no wind or other forces to upset it. It moves in a straight
>line. Now put the bike at a small tilt in one direction. It will not
>fall but perform a large circular rotation. So there is your
>fundamental difference between a moving and stationary bike. The
>stationary bike would fall over given a small tilt, but a moving bike
>would stay up and go in a circle. It is you manipulation of the
>steering which keeps the bike upright and going in a straight line in
>spite of effects that create the tilts that would have toppled a
>stationary bike.
>
>I understand that it is possible to construct a steering mechanism that
>makes a bike unstable.

A bicycle front wheel steers where it is going as a result castering.
The bicycle stays more or less up right because of the effect of
gyroscopic precession on the front wheel. If the cyclist (or sack of
potatoes) starts to fall to the right, precession also makes the front
wheel steer to the right. This steers the bicycle back under the
cyclist (or the sack of potatoes).

The trick is to get the right balance between the self-aligning
castering forces and the steering precession forces. Get the balance
wrong and you have an unstable bicycle.

Eric Stevens

Chaos is found in the greatest abundance wherever order is being
sought. It always defeats order, because it is better organised.
-: Ly Tin Wheedle

[Stephen D. McDonald]

kk wrote:

> Why a bicycle can keep balanced when it's going but it inevitably falls
> when it stops?
> I'm keen on physics but I couldn't find an answer for that.

> Please send me an e-mail.
> Thank you!
>
> Eugenio
> e.c...@agonet.it

Actually I must admit, I have done extensive observations in this area
some time ago. Really, when I was between the ages of 8 ~ 14.
No joke. We used to ride our bikes and jump off of them to see just
how far a bike could go without a rider. Often times hilarious results
occured that would certainly entertain anyone of that age.
(Note: And at the same time drive parents crazy)

Anyway watching the path of the bikes, they tended to curve off
to the left or the right as they began to slow down, unless they collided
with something first.

Two things are acting here. 1) the tilt of the bike, 2) releasing the
handle
bars simultaneously. If there were no initial tilt and the bars are
released
at the same instance, then the bike could continue in a straight line and
"plop" over when it finally stops.

In hind-sight, two other factors keep the bike up while its moving. Both
of them deal with inertia. There is a dynamic interaction between forward
movement, the center of mass and the steering of the bike. (Think about
this one) Also the wheels act as gyroscopes!

After a certain point, when the bike slows down, gravity becomes the
predominant force and that is when chaos takes over.

--
Steve McDonald
Research Associate
University of Oklahoma
School of Meteorology
Norman, Ok