balance

Quoted message said:

Simply by rotating the handlebars one way or the other you can move
the bike from side to side even though the wheels aren't moving
forward, obviously it's because of the rake and trail of the
steering mechanism. Let's see someone try to trackstand on a bike
built with no rake or trail. Carl?

Don't be so narrow minded. Have you ever been to a circus? They have
guys there who climb to the top of a plain ladder, sit there and
juggle dissimilar objects. The ladder has neither wheels, nor rake.

You may be surprised, but "simply by rotating the handlebars one way
or the other you can move the bike" forward. This is an obscure way
of generating forward motion but it works, albeit slowly.

Jobst Brandt

Ben C said:

David L. Johnson said:

Ben C said:

I will attempt to explain.

Momentum is the product of force and velocity.

Try again. Momentum is the product of mass and velocity.

Sorry! Yes of course it is.

Quoted message said:

Quoted message said:

Impulse is the product (integral strictly) of force and time. If you
give something a knock with a hammer,

Your example is closer than your description, which is not correct, and
calling it an integral doesn't help.

So what is an impulse then?

According to a dictionary:

3. (Mech.) The action of a force during a very small interval
of time; the effect of such action; as, the impulse of a
sudden blow upon a hard elastic body.
[1913 Webster]

In the only technical sense I see the word used, it would be a limit of a
particular input of energy (work, not force, representing the change of
kinetic energy) that occurs essentially instantaneously, modeled by a
Dirac delta function, which would take some explanation.

Quoted message said:

F = ma.

Integrate both sides against time, you get Impulse = Change of momentum.

No. Integrate the right hand side with respect to time and you get
momentum, not change of momentum. Change of momentum is the right hand
side.

--

David L. Johnson

And though I have the gift of prophecy, and understand all mysteries,
and all knowledge; and though I have all faith, so that I could remove
mountains, and have not charity, I am nothing. [1 Corinth. 13:2]

David L. Johnson said:

Ben C said:

David L. Johnson said:

On Sat, 20 Jan 2007 10:31:00 -0600, Ben C wrote:

> I will attempt to explain.
>
> Momentum is the product of force and velocity.

Try again. Momentum is the product of mass and velocity.

Sorry! Yes of course it is.

Quoted message said:

> Impulse is the product (integral strictly) of force and time. If you
> give something a knock with a hammer,

Your example is closer than your description, which is not correct, and
calling it an integral doesn't help.

So what is an impulse then?

According to a dictionary:

3. (Mech.) The action of a force during a very small interval
of time; the effect of such action; as, the impulse of a
sudden blow upon a hard elastic body.
[1913 Webster]

In the only technical sense I see the word used, it would be a limit of a
particular input of energy (work, not force, representing the change of
kinetic energy) that occurs essentially instantaneously, modeled by a
Dirac delta function, which would take some explanation.

I just poked around a bit in wikipedia and found another definition of
"impulse" as the integral of a force over time, so I stand corrected on
that score, and that would then be momentum according to Newton's second
law. But the "knocked with a hammer" notion of impulse is the way I'm
used to using the term, which is also in wikipedia. Note there the
definition is "the term "impulse" is also used to refer to a change in an
object's momentum caused by a fast-acting force. This type of impulse is
often idealized so that the change in momentum happens with no change in
time." -- which is where you got the "change in momentum" part.

Quoted message said:

Quoted message said:

F = ma.

Integrate both sides against time, you get Impulse = Change of
momentum.

No. Integrate the right hand side with respect to time and you get
momentum, not change of momentum. Change of momentum is the right hand
side.

--

David L. Johnson

And though I have the gift of prophecy, and understand all mysteries,
and all knowledge; and though I have all faith, so that I could remove
mountains, and have not charity, I am nothing. [1 Corinth. 13:2]

david said:

I can assure you that a troll was not intended!
I would just like a clear scientific answer.

Obviously a bicycle, moving or not, cannot balance itself anymore than
it can, by itself, defy gravity.

The deciding factor can then in no way be the bicycle itself and,
instead, must be the mounted rider in the sense of the rider being the
mechanism which corrects (as per Newton's: for every action there is an
equal reaction) the natural tendancy of a bicycle to yield to gravity.

The rider makes the corrections by keeping him or herself in balance
with the bicylce as a mere extention of his or herself.

Obviously this is possible when the bicycle is in motion just as, for
example, a spinning top can be kept upright by the corrective force of
a light finger taps.

If the top were not spinning, however, the only way the top could then
be kept upright would be by applying constant correction which would be
the equivlent, however, of someone keeping a bicyle upright while not
mounted on it.

david said:

I can assure you that a troll was not intended!
I would just like a clear scientific answer.

Quoted message said:

david who? said:

Can somebody please explain why a moving bicycle is easier to
balance than a stationary one.

Is this a troll or what! How about trying this with an in-line skate
or ice skate. I'm sure you can glide on one skate effortlessly until
you come to a stop. As speed approaches zero balancing becomes much
more difficult. You may discover what keeps a single track vehicle
upright if you give this some thought.

Jobst Brandt

I'm beginning to get a sense of why trolls might congregate on these
newsgroups. Any question seems to be a basis for a flame war with
unending one-ups-manship posts.

I did a quick google search at the time when this was first posted and
my initial impression of the gyroscopic effects of the wheels spinning
being the significant factor seemed to be wrong, though I didn't quite
get the counter-rotating wheel proof. The question of why turning the
front wheel should be easy if the same force is supposedly keeping the
bike upright seems a strong argument against any significance to
gyroscopic tendencies. But I'm now open to the flames too. I did find a
fairly good explanation on a Wiki page.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

It seems to have more to do with the position of the steering wheel,
being in front of the center of gravity mainly, as a self correcting aid
to balance. Anyway the explanation isn't that technical though there are
technical parts. It's still fairly understandable.

Apparently a rear steering bicycle is almost impossible to balance.
Ironic in that following a recent link to a video of an unusual bicycle
I saw another link to a "side ways" bike that had both front and rear
steering wheels. Most people could ride it on their first try.

On 2007-01-21, David L. Johnson <[email hidden]> wrote:
[snip]

Quoted message said:

I just poked around a bit in wikipedia and found another definition of
"impulse" as the integral of a force over time, so I stand corrected on
that score, and that would then be momentum according to Newton's second
law. But the "knocked with a hammer" notion of impulse is the way I'm
used to using the term, which is also in wikipedia. Note there the
definition is "the term "impulse" is also used to refer to a change in an
object's momentum caused by a fast-acting force. This type of impulse is
often idealized so that the change in momentum happens with no change in
time." -- which is where you got the "change in momentum" part.

The impulse doesn't have to occur instantaneously in order to produce a
change of momentum.

A given force produces a given rate of change of momentum, and so a
given force applied for a given time produces a given change of
momentum.

You're right that the word is usually used when the time period is
short, or idealized down to nothing, but the concept is the same.

I was trying to explain to the OP the physics behind the idea that
things with more momentum seem to resist being knocked off course.
Impulse and change of momentum are useful in visualizing that problem I
think, rather than going back to talking about mass, force and
acceleration.

david said:

Can somebody please explain why a moving bicycle is easier to balance
than a stationary one.

Ride a bike through a puddle and look at the tracks the wheels make.

don Gabacho said:

david said:

I can assure you that a troll was not intended!
I would just like a clear scientific answer.

Obviously a bicycle, moving or not, cannot balance itself anymore than
it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord slingshot.

If the riderless bike starts to lean left, the geometry of the steering
is such that it will turn left and right itself. It will be headed in a
new direction but it will be stable.

When it is going too slow, it cannot right itself and will keep turning
ever sharper one way and spiral in until it drops.

MkTm said:

david wrote:

Snip
Apparently a rear steering bicycle is almost impossible to balance.
Ironic in that following a recent link to a video of an unusual bicycle
I saw another link to a "side ways" bike that had both front and rear
steering wheels. Most people could ride it on their first try.

I have tried riding my fixie backwards and cannot manage it. I suspect
that my efforts provide great amusement for the neighbors though and are
therefore worth the effort. I have seen videos of trick riders doing it
so it must be possible.

Dan said:

MkTm said:

david wrote:

Snip
Apparently a rear steering bicycle is almost impossible to balance.
Ironic in that following a recent link to a video of an unusual bicycle
I saw another link to a "side ways" bike that had both front and rear
steering wheels. Most people could ride it on their first try.

I have tried riding my fixie backwards and cannot manage it. I suspect
that my efforts provide great amusement for the neighbors though and are
therefore worth the effort. I have seen videos of trick riders doing it
so it must be possible.

I have found it easier to do it by sitting on the bars facing backward.
Something about the direction I am facing seems to affect things quite
a bit. When I try to ride backward while facing forward, it is all I
can do to manage a few pedal strokes and I have no control over where I
go as steering is just to keep me from falling down.

Joseph

On Sun, 21 Jan 2007 08:01:13 GMT, MkTm <[email hidden]> wrote:

[snip]

Quoted message said:

Apparently a rear steering bicycle is almost impossible to balance.

[snip]

Dear MT,

Riders like this woman haven't heard that rear-steering bicycles are
almost impossible to balance:

http://www.glumbert.com/media/bikerobatics

She seems quite happy to ride backwards in circles while standing with
one foot on the seat and the other foot on the handlebar.

Cheers,

Carl Fogel

Quoted message said:

have found it easier to do it by sitting on the bars facing backward.
Something about the direction I am facing seems to affect things quite
a bit. When I try to ride backward while facing forward, it is all I
can do to manage a few pedal strokes and I have no control over where I
go as steering is just to keep me from falling down.

If you can ride backward more than a pedal revolution,
you are not far from mastering the trick. Keep trying,
you'll get it, and before too long it will seem easy.
Almost all the messengers in my town can do this;
I'd like to see one of em do a Dubai backward
cyclone burnout though. Shouldn't be too long now.

Robert

MkTm wrote:
-snip-

Quoted message said:

Apparently a rear steering bicycle is almost impossible to balance.

-snip-

I once rented a room to a champion cyclist (Bob Mionske) who, riding to
work in the morning, would climb up on the handlebars and continue a
conversation while riding ass-first with arms crossed facing backwards,
then climb back on the seat, all smoothly and gracefully.

I'll reserve judgement of 'almost impossible' for now.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Dan said:

don Gabacho said:

david said:

I can assure you that a troll was not intended!
I would just like a clear scientific answer.

Obviously a bicycle, moving or not, cannot balance itself anymore than
it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

don Gabacho said:

Dan said:

don Gabacho said:

david wrote:

>I can assure you that a troll was not intended!
>I would just like a clear scientific answer.

Obviously a bicycle, moving or not, cannot balance itself anymore than
it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

The Wiki article I linked to earlier had a link to a video of a
riderless bike coasting along seemingly without a care and even able to
take some abuse along the way.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

--------------------------------------------------
Gyroscopic effects

The role of the gyroscopic effect in most bike designs is to help steer
the front wheel into the direction of a lean. This phenomenon is called
precession and the rate at which an object precesses is inversely
proportional to its rate of spin. The slower a front wheel spins, the
faster it will precess when the bike leans, and visa-versa. The rear
wheel is prevented from precessing as the front wheel does by friction
of the tires on the ground, and so continues to lean as though it were
not spinning at all. Hence gyroscopic forces do not provide any
resistance to tipping.

At low forward speeds, the precession of the front wheel is too quick,
contributing to an uncontrolled bike’s tendency to oversteer, start to
lean the other way and eventually oscillate and fall over. At high
forward speeds, the precession is usually too slow, contributing to an
uncontrolled bike’s tendency to understeer and eventually fall over
without ever having reached the upright position. This instability is
very slow, on the order of seconds, and is trivial to counteract for
most riders. Thus a fast bike may feel stable even though it is actually
not self-stable and would fall over if it were uncontrolled.

Self stability

Between these two extremes, there may be a range of forward speeds for a
given bike design at which the effects described above steer an
uncontrolled bike upright. However, even without self-stability a bike
may be ridden by steering it to keep it over its wheels.

See a video of a riderless bicycle exhibiting this self-stability.
http://www.tam.cornell.edu/~ad29/JBike6/JBike6_self_stable_files/bicycle_stability.mpeg
--------------------------------------------------

don Gabacho said:

Dan said:

don Gabacho said:

david wrote:
> I can assure you that a troll was not intended!
> I would just like a clear scientific answer.
Obviously a bicycle, moving or not, cannot balance itself anymore than
it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

I don't follow your logic. How can the slingshot be a corrective force
after the bike has left the slingshot?

Try it. Take a bike out to a large open area, get it going and let go.
Run and give it a big push.

MkTm said:

don Gabacho said:

Dan said:

don Gabacho wrote:

> david wrote:
>
>> I can assure you that a troll was not intended!
>> I would just like a clear scientific answer.
>
> Obviously a bicycle, moving or not, cannot balance itself anymore than
> it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord
slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

The Wiki article I linked to earlier had a link to a video of a
riderless bike coasting along seemingly without a care and even able to
take some abuse along the way.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

--------------------------------------------------
Gyroscopic effects

The role of the gyroscopic effect in most bike designs is to help steer
the front wheel into the direction of a lean. This phenomenon is called
precession and the rate at which an object precesses is inversely
proportional to its rate of spin. The slower a front wheel spins, the
faster it will precess when the bike leans, and visa-versa. The rear
wheel is prevented from precessing as the front wheel does by friction
of the tires on the ground, and so continues to lean as though it were
not spinning at all. Hence gyroscopic forces do not provide any
resistance to tipping.

At low forward speeds, the precession of the front wheel is too quick,
contributing to an uncontrolled bike’s tendency to oversteer, start to
lean the other way and eventually oscillate and fall over. At high
forward speeds, the precession is usually too slow, contributing to an
uncontrolled bike’s tendency to understeer and eventually fall over
without ever having reached the upright position. This instability is
very slow, on the order of seconds, and is trivial to counteract for
most riders. Thus a fast bike may feel stable even though it is actually
not self-stable and would fall over if it were uncontrolled.

Self stability

Between these two extremes, there may be a range of forward speeds for a
given bike design at which the effects described above steer an
uncontrolled bike upright. However, even without self-stability a bike
may be ridden by steering it to keep it over its wheels.

See a video of a riderless bicycle exhibiting this self-stability.
http://www.tam.cornell.edu/~ad29/JBike6/JBike6_self_stable_files/bicycle_stability.mpeg

--------------------------------------------------

Excellent video but I am not convinced that gyroscopic effect is the
main source of stability at normal speeds. A bike will turn into a lean
even if it isn't moving. Hold a stationary bike by the seat and lean it
one way and the front wheel will turn into the lean. This is because of
the geometry of rake and offset which creates a small moment about the
steering axis. When walking a bike it is easy to steer it this way while
holding the seat. Gyroscopic effect may help but it is not the primary
factor at normal bicycle speeds.

Dan said:

don Gabacho said:

Dan said:

don Gabacho wrote:

> david wrote:
>
>> I can assure you that a troll was not intended!
>> I would just like a clear scientific answer.
>
> Obviously a bicycle, moving or not, cannot balance itself anymore than
> it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord
slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

I don't follow your logic. How can the slingshot be a corrective force
after the bike has left the slingshot?

Try it. Take a bike out to a large open area, get it going and let go.
Run and give it a big push.

Already replied to another post but maybe cross posted somewhat.

http://www.tam.cornell.edu/~ad29/JBike6/JBike6_self_stable_files/bicycle_stability.mpeg

MkTm said:

don Gabacho said:

Dan said:

don Gabacho wrote:

>david wrote:
>
>>I can assure you that a troll was not intended!
>>I would just like a clear scientific answer.
>
>Obviously a bicycle, moving or not, cannot balance itself anymore than
>it can, by itself, defy gravity.

Actually, if it is going fast enough, a properly designed bicycle is
stable and will stay in balance until it slows down. I've done this by
firing a bicycle across a parking lot with a large bungee cord slingshot.

The propulsion provided by the "slingshot" was then the, though waning,
'corrective' force.

By itself, a bicycle can still not balance itself.

The Wiki article I linked to earlier had a link to a video of a
riderless bike coasting along seemingly without a care and even able to
take some abuse along the way.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

--------------------------------------------------
Gyroscopic effects

The role of the gyroscopic effect in most bike designs is to help steer
the front wheel into the direction of a lean. This phenomenon is called
precession and the rate at which an object precesses is inversely
proportional to its rate of spin. The slower a front wheel spins, the
faster it will precess when the bike leans, and visa-versa. The rear
wheel is prevented from precessing as the front wheel does by friction
of the tires on the ground, and so continues to lean as though it were
not spinning at all. Hence gyroscopic forces do not provide any
resistance to tipping.

At low forward speeds, the precession of the front wheel is too quick,
contributing to an uncontrolled bike’s tendency to oversteer, start to
lean the other way and eventually oscillate and fall over. At high
forward speeds, the precession is usually too slow, contributing to an
uncontrolled bike’s tendency to understeer and eventually fall over
without ever having reached the upright position. This instability is
very slow, on the order of seconds, and is trivial to counteract for
most riders. Thus a fast bike may feel stable even though it is actually
not self-stable and would fall over if it were uncontrolled.

Self stability

Between these two extremes, there may be a range of forward speeds for a
given bike design at which the effects described above steer an
uncontrolled bike upright. However, even without self-stability a bike
may be ridden by steering it to keep it over its wheels.

See a video of a riderless bicycle exhibiting this self-stability.
http://www.tam.cornell.edu/~ad29/JBike6/JBike6_self_stable_files/bicycle_stability.mpeg
--------------------------------------------------

Dear Dan,

A bare hoop rolls nicely.

A riderless bicycle is fairly close to a bare hoop.

But consider what would happen if the riderless bicycle had a 175-lb
crash dummy strapped to it.

You'd have about ten times as much mass, the center of mass would be
up higher, and a much smaller percentage of the total mass would be
spinning.

Cheers,

Carl Fogel

Quoted message said:

Dear Dan,

A bare hoop rolls nicely.

A riderless bicycle is fairly close to a bare hoop.

But consider what would happen if the riderless bicycle had a 175-lb
crash dummy strapped to it.

You'd have about ten times as much mass, the center of mass would be
up higher, and a much smaller percentage of the total mass would be
spinning.

Cheers,

Carl Fogel

Carl-

I am the only dummy that rides my bicycle.

The higher mass may just create a larger righting moment due to the
centrifugal cornering force. The mass would need to be rigidly attached
to the bike. I can already see myself duct taping a load of bricks to my
wife's bike to see if it works.

I first thought about this when a friend had a radio controlled
motorcycle. It was self stabilizing and very stable so long as it was
going fast enough. The wheels were small and the overall mass was
relatively large. The thing was ingeniously steered by shifting the mass
of the battery from side to side. Two controls, speed and lateral mass.
This is how no hands bike riding works, you simply shift your mass from
side to side. The struggle in ho hands riding seems to be getting the
bike to go where you want it to go, not in keeping upright - so long as
you go fast enough, the bike geometry is right, etc.