My Experiment...

Ben C said:

Quoted message said:

Ben C said:

This
corresponds quite well to balancing on a bike with locked steering
(extremely hard),

Quoted message said:

Have you, or anyone else, actually tried this experiment?

No, but then I am not good at circus tricks.

OK, but the claim (that I am interested in) is Simon's one that
steering is _generally_ performed by moving the centre of gravity
laterally relative to the contact patch, rather than vice versa.

I certainly wouldn't claim that it was technically impossible to do
this sort of thing (on the contrary, it clearly is possible in
principle), but I think it is very clear that such fine control is
well beyond the bounds of what normal people can and do achieve in
everyday riding.

A person who can move their CoG significant distances sideways at will
under control while sitting normally on a bicycle should be able to
balance a stationary bicycle with the steering locked ahead. I'm quite
certain that the vast majority of cyclists cannot manage this, even
though such an achievement is technically possible.

In contrast, steering the contact patch around under the CoG (and out
to one side to initiate a turn) is trivial, any child can learn in a
few days, it requires no circus-level skills and it is clearly
demonstrated by the weaving tracks that you can see when a cyclist
rides though a puddle on an otherwise dry road.

James

Quoted message said:

I would respond by saying that if it were possible then it would also be
possible to move your centre of mass so that you could balance on a
knife edge (as if you were wearing an ice skate while stationary). You
would also not need to fall over when your wheels drop into a tramway.

To lean the bike you'd just have to unbalance it to the side of your
choice, effectively falling off. This would be stopped by the turn
(hopefully 😉.

To *balance* the bike after making a lean (or it otherwise becoming
unbalanced) would require a *much greater* degree of control and a much
greater shift of mass to bring the CofG back on track.

Quoted message said:

OK, but the claim (that I am interested in) is Simon's one that
steering is _generally_ performed by moving the centre of gravity
laterally relative to the contact patch, rather than vice versa.

Hang on a mo. Moving the centre of gravity won't steer the bike, but
simply enables the rider to steer it without falling off.

IIRC Simon thought that riders didn't use countersteer to move the CofG,
but could do it by shifting their body.

Quoted message said:

I certainly wouldn't claim that it was technically impossible to do
this sort of thing (on the contrary, it clearly is possible in
principle), but I think it is very clear that such fine control is
well beyond the bounds of what normal people can and do achieve in
everyday riding.

The fine control is needed to balance. Not much control is needed to
simply move the centre of gravity.

Quoted message said:

A person who can move their CoG significant distances sideways at will
under control while sitting normally on a bicycle should be able to
balance a stationary bicycle with the steering locked ahead.

No. It would require much greater effort to recover and control the CofG
than it would to simply move it one way or the other.

Quoted message said:

I'm quite
certain that the vast majority of cyclists cannot manage this, even
though such an achievement is technically possible.

Quoted message said:

In contrast, steering the contact patch around under the CoG (and out
to one side to initiate a turn) is trivial, any child can learn in a
few days, it requires no circus-level skills and it is clearly
demonstrated by the weaving tracks that you can see when a cyclist
rides though a puddle on an otherwise dry road.

Yep, this is both very easy to do and very hard to prevent. I can't
prevent the small turns of the bar that help keep us upright, tho I can
keep them very small.

In article <[email hidden]>, Ben C
[email hidden] says...

Quoted message said:

Here is a video I found of a man trying to balance on a ladder. He falls
off a few times but I think it demonstrates the principle well. The feet
of the ladder stay in the same place at all times, and he corrects
himself from falling over (not always successfully, but enough to prove
the concept) by moving his centre of mass relative to them.

I think he's moving the contact point.

. . / .
| | / /
|___| `-./
^ ^
| |

Rob Morley said:

In article <[email hidden]>, Ben C
[email hidden] says...

Quoted message said:

Here is a video I found of a man trying to balance on a ladder. He falls
off a few times but I think it demonstrates the principle well. The feet
of the ladder stay in the same place at all times, and he corrects
himself from falling over (not always successfully, but enough to prove
the concept) by moving his centre of mass relative to them.

I think he's moving the contact point.

. . / .
| | / /
|___| `-./
^ ^
| |

Relative to the centre of mass of (ladder + himself). But it's not
sliding around on the ground I don't think.

Ben C said:

Here is a video I found of a man trying to balance on a ladder. He falls
off a few times but I think it demonstrates the principle well. The feet
of the ladder stay in the same place at all times, and he corrects
himself from falling over (not always successfully, but enough to prove
the concept) by moving his centre of mass relative to them.

I'm not sure I agree. As Rob has already mentioned, it appears to me that
much of the correction comes from tipping the ladder, shifting the point of
contact of the feet of the ladder with the ground.

I don't dispute it is possible to generate a lateral force on the contact
point of a bicycle/ladder. As a simple example, imagine standing near the
base of a tall ladder (i.e. below the COM of the ladder) and pushing away
your their hands and feet. If there was no lateral force at the base, the
ladder would just slide along the ground, rotating in the opposite
direction. The fact that there is an external force means that the combined
COM will move. I think this is the effect that you have been suggesting?

However, to generate significant force at the contact patch requires large
movements. My intuition is that the movements required will be necessarily
larger than the resultant shift in the COM*, but I haven't tried to do any
calculations. Compare this with how much countersteer you need to shift the
contact patch by a certain distance. My calculations give that at 20mph,
0.1 degrees of countersteer for 1 second will move the contact ~10cm
laterally**. Consider how much flailing would be required to achieve the
same effect. Which seems more plausible as a real world mechanism for
initiating the lean?

Anthony

* - certainly this seems likely for the ladder example above, and this would
be close to 'best case' for moving combined COM.

** - This is also why Simon's experiments have failed to prove countersteer:
the required rotation of the bars is minuscule.

Quoted message said:

Ben C said:

Quoted message said:

On Feb 26, 8:26 am, Ben C <[email hidden]> wrote:
> This
> corresponds quite well to balancing on a bike with locked steering
> (extremely hard),

Quoted message said:

Have you, or anyone else, actually tried this experiment?

No, but then I am not good at circus tricks.

OK, but the claim (that I am interested in) is Simon's one that
steering is _generally_ performed by moving the centre of gravity
laterally relative to the contact patch, rather than vice versa.

I see, I didn't realize that was Simon's claim. But riding the bike this
morning I tried weaving around and turning corners while trying to use
introspection to determine what I was doing. I reckon I was just leaning
into the turns to initiate them, not countersteering. But introspection
is not reliable.

Quoted message said:

I certainly wouldn't claim that it was technically impossible to do
this sort of thing (on the contrary, it clearly is possible in
principle), but I think it is very clear that such fine control is
well beyond the bounds of what normal people can and do achieve in
everyday riding.

Agree.

Quoted message said:

A person who can move their CoG significant distances sideways at will
under control while sitting normally on a bicycle should be able to
balance a stationary bicycle with the steering locked ahead. I'm quite
certain that the vast majority of cyclists cannot manage this, even
though such an achievement is technically possible.

Exactly.

Ben C said:

Quoted message said:

I think he's moving the contact point.

. . / .
| | / /
|___| `-./
^ ^
| |

Relative to the centre of mass of (ladder + himself). But it's not
sliding around on the ground I don't think.

It doesn't need to slide on the ground -- since the ladder feet aren't sharp
points, tipping the ladder 'rolls' them. Look at Rob's ascii diagram.

Anthony

Ben C said:

I see, I didn't realize that was Simon's claim. But riding the bike this
morning I tried weaving around and turning corners while trying to use
introspection to determine what I was doing. I reckon I was just leaning
into the turns to initiate them, not countersteering. But introspection
is not reliable.

Indeed. I'd go as far as to say that you'd *expect* not to be able to detect
the countersteer considering the magnitude of handlebar rotation required.

Anthony

Anthony Jones said:

Ben C said:

Quoted message said:

I think he's moving the contact point.

Quoted message said:

Quoted message said:

. . / .
| | / /
|___| `-./
^ ^
| |

Quoted message said:

Relative to the centre of mass of (ladder + himself). But it's not
sliding around on the ground I don't think.

It doesn't need to slide on the ground -- since the ladder feet aren't sharp
points, tipping the ladder 'rolls' them. Look at Rob's ascii diagram.

I think Ben's right. The connection with the ground is the key. With
the ladder it's possible to get some reaction force from the ground
and use this to shift the C of G. I can do the same trick while
leaning backwards on a chair so that it's on two legs. I can keep it
balanced for some time, and it's the reaction force from the ground
that enables me to move my C of G backwards and forwards slightly.

As I mentioned in another recent thread I'm sure it's also possible to
do this on a bike but it requires strong positive movements. This does
not happen in normal riding where the bike and the rider's body remain
in the same plane. To see for yourself how hard it is to influence the
C of G on a bike try locking the steering straight and getting someone
to support you at the side. Then have them tip you a tiny amount past
the point of balance. No matter how gently they do it, it's impossible
to recover.

--
Dave...

On Feb 26, 8:52 am, Mark Thompson

pleasegivegenerously@warmmail*_turn_up_the_heat_to_reply*.com said:

To lean the bike you'd just have to unbalance it to the side
of your choice, effectively falling off.

Just! :-)

--
Dave...

dkahn400 said:

Quoted message said:

It doesn't need to slide on the ground -- since the ladder feet aren't
sharp points, tipping the ladder 'rolls' them. Look at Rob's ascii
diagram.

I think Ben's right. The connection with the ground is the key. With
the ladder it's possible to get some reaction force from the ground
and use this to shift the C of G.

It's difficult to tell whether it's this or moving the contact point that
has more effect in the case of the ladder video. I think that's important
to note because it demonstrates how difficult it is to separate the
different effects.

Quoted message said:

I can do the same trick while
leaning backwards on a chair so that it's on two legs. I can keep it
balanced for some time, and it's the reaction force from the ground
that enables me to move my C of G backwards and forwards slightly.

That's an excellent example -- I couldn't think of any situation where it
was possible to actually *balance* on an edge without shifting the contact
point by some amount, but this covers it nicely.

Quoted message said:

As I mentioned in another recent thread I'm sure it's also possible to
do this on a bike but it requires strong positive movements. This does
not happen in normal riding where the bike and the rider's body remain
in the same plane.

My sentiments exactly.

Anthony

Anthony Jones said:

Ben C said:

Here is a video I found of a man trying to balance on a ladder. He falls
off a few times but I think it demonstrates the principle well. The feet
of the ladder stay in the same place at all times, and he corrects
himself from falling over (not always successfully, but enough to prove
the concept) by moving his centre of mass relative to them.

I'm not sure I agree. As Rob has already mentioned, it appears to me that
much of the correction comes from tipping the ladder, shifting the point of
contact of the feet of the ladder with the ground.

I don't dispute it is possible to generate a lateral force on the contact
point of a bicycle/ladder. As a simple example, imagine standing near the
base of a tall ladder (i.e. below the COM of the ladder) and pushing away
your their hands and feet. If there was no lateral force at the base, the
ladder would just slide along the ground, rotating in the opposite
direction.

The way it works is like this: the dumbass rearranges the position of
his COM relative to the ladder feet. Once the vertical line through the
COM to the ground is off the line between the feet, there's a torque on
the dumbass+ladder. Lateral frictional forces at the feet oppose that
torque, causing the ladder to pivot around the feet rather than rotating
around the dumbass+ladder COM.

Quoted message said:

The fact that there is an external force means that the combined
COM will move. I think this is the effect that you have been suggesting?

Kind of, without any friction at the feet the only way the COM is moving
is straight downwards.

Quoted message said:

However, to generate significant force at the contact patch requires large
movements. My intuition is that the movements required will be necessarily
larger than the resultant shift in the COM*, but I haven't tried to do any
calculations.

The calculations are quite hard as you'd have to do a sort of volume
integral, also taking into account that the dumbass is not of uniform
density. But your intuition sounds reasonable.

Quoted message said:

Compare this with how much countersteer you need to shift the contact
patch by a certain distance. My calculations give that at 20mph, 0.1
degrees of countersteer for 1 second will move the contact ~10cm
laterally**. Consider how much flailing would be required to achieve
the same effect. Which seems more plausible as a real world mechanism
for initiating the lean?

I have no idea. We're now in the territory of puddle tests, these
calculations you're talking about, and other things. Whether leaning is
impossible is one question (I'm satisfied that it's not); whether it's
what riders actually do, or whether they countersteer, is quite another,
and much harder to answer.

Ben C said:

Quoted message said:

I don't dispute it is possible to generate a lateral force on the contact
point of a bicycle/ladder. As a simple example, imagine standing near the
base of a tall ladder (i.e. below the COM of the ladder) and pushing away
your their hands and feet. If there was no lateral force at the base, the
ladder would just slide along the ground, rotating in the opposite
direction.

The way it works is like this: the dumbass rearranges the position of
his COM relative to the ladder feet.

What I was getting at is establishing *how* the COM can move if the contact
point remains fixed. It has to be via an external force (Newton's first
law). This force has to come from the contact point, since nothing else is
in contact. This force is then most easily explained in terms of
conservation of momentum and angular momentum.

Quoted message said:

Once the vertical line through the
COM to the ground is off the line between the feet, there's a torque on
the dumbass+ladder. Lateral frictional forces at the feet oppose that
torque, causing the ladder to pivot around the feet rather than rotating
around the dumbass+ladder COM.

All you're saying is that something that isn't balanced falls over, I don't
think this needs much further explanation. 🙂

Quoted message said:

Quoted message said:

However, to generate significant force at the contact patch requires
large movements. My intuition is that the movements required will be
necessarily larger than the resultant shift in the COM*, but I haven't
tried to do any calculations.

The calculations are quite hard as you'd have to do a sort of volume
integral, also taking into account that the dumbass is not of uniform
density. But your intuition sounds reasonable.

I don't think it requires that level of calculation if you're just
establishing a lower bound. I might try sketching something out if I get a
chance...

Anthony

Anthony Jones said:

Ben C said:

Quoted message said:

I don't dispute it is possible to generate a lateral force on the contact
point of a bicycle/ladder. As a simple example, imagine standing near the
base of a tall ladder (i.e. below the COM of the ladder) and pushing away
your their hands and feet. If there was no lateral force at the base, the
ladder would just slide along the ground, rotating in the opposite
direction.

The way it works is like this: the dumbass rearranges the position of
his COM relative to the ladder feet.

What I was getting at is establishing *how* the COM can move if the contact
point remains fixed. It has to be via an external force (Newton's first
law). This force has to come from the contact point, since nothing else is
in contact.

Yes, good point, and that is where it comes from, if the COM is to have
any sideways component of movement. To move straight down requires only
gravity, which doesn't act through the feet.

Quoted message said:

Quoted message said:

Once the vertical line through the
COM to the ground is off the line between the feet, there's a torque on
the dumbass+ladder. Lateral frictional forces at the feet oppose that
torque, causing the ladder to pivot around the feet rather than rotating
around the dumbass+ladder COM.

All you're saying is that something that isn't balanced falls over, I don't
think this needs much further explanation. 🙂

Well, this is the mechanism by which the man on the ladder can maintain
balance though.

Quoted message said:

Quoted message said:

Quoted message said:

However, to generate significant force at the contact patch requires
large movements. My intuition is that the movements required will be
necessarily larger than the resultant shift in the COM*, but I haven't
tried to do any calculations.

The calculations are quite hard as you'd have to do a sort of volume
integral, also taking into account that the dumbass is not of uniform
density. But your intuition sounds reasonable.

I don't think it requires that level of calculation if you're just
establishing a lower bound. I might try sketching something out if I get a
chance...

Good stuff.

On Feb 26, 6:25 pm, Mark Thompson

pleasegivegenerously@warmmail*_turn_up_the_heat_to_reply*.com said:

Quoted message said:

OK, but the claim (that I am interested in) is Simon's one that
steering is _generally_ performed by moving the centre of gravity
laterally relative to the contact patch, rather than vice versa.

Hang on a mo. Moving the centre of gravity won't steer the bike, but
simply enables the rider to steer it without falling off.

IIRC Simon thought that riders didn't use countersteer to move the CofG,
but could do it by shifting their body.

Quoted message said:

I certainly wouldn't claim that it was technically impossible to do
this sort of thing (on the contrary, it clearly is possible in
principle), but I think it is very clear that such fine control is
well beyond the bounds of what normal people can and do achieve in
everyday riding.

The fine control is needed to balance. Not much control is needed to
simply move the centre of gravity.

Quoted message said:

A person who can move their CoG significant distances sideways at will
under control while sitting normally on a bicycle should be able to
balance a stationary bicycle with the steering locked ahead.

No. It would require much greater effort to recover and control the CofG
than it would to simply move it one way or the other.

Just think about it a bit.

The claim is that the CoG can be moved laterally a substantial amount
(consider starting from a position in which you happen to be
instantaneously out of balance to the right, and diving into a sharp
left turn - the CoG would have to move some tens of centimetres
sideways in a fraction of a second and then abruptly stop moving). No
credible explanation of the motions and calculations of the forces
required to make this happen have ever been presented. The claim is
further that this is _not_ something that can be done in a smooth and
controllable manner, or else balancing on a stationary or unsteerable
bicycle would be straightforward. It's known (Newton's laws) that
lateral movement can only be generated by substantial rotational
accelerations to generate a lateral force at the ground contact patch.
Straightforward examination of cyclists reveals no hint of such
movement, and indeed a non-ridden bicycle will generally balance
itself for as long as it has sufficient speed. Straightforward
examination of tyre tracks after riding through a puddle and making a
sharp turn will inevitably reveal that the front wheel steered away
from the turn.

Simon, after starting this particular thread, has run away from the
consequences of it, as is typical for a troll.

James

Ben C said:

Anthony Jones said:

Ben C said:

>I don't dispute it is possible to generate a lateral force on the contact
>point of a bicycle/ladder. As a simple example, imagine standing near the
>base of a tall ladder (i.e. below the COM of the ladder) and pushing away
>your their hands and feet. If there was no lateral force at the base, the
>ladder would just slide along the ground, rotating in the opposite
>direction.

The way it works is like this: the dumbass rearranges the position of
his COM relative to the ladder feet.

What I was getting at is establishing *how* the COM can move if the contact
point remains fixed. It has to be via an external force (Newton's first
law). This force has to come from the contact point, since nothing else is
in contact.

Yes, good point, and that is where it comes from, if the COM is to have
any sideways component of movement. To move straight down requires only
gravity, which doesn't act through the feet.

Didn't anyone notice that the ground is soft in this demonstration? He
digs the legs of the ladder into the ground and now has some rotational
friction which allow him to move his centre of mass. It's the friction
and/or the springiness of that pivot that allows him to generate a
turning moment. Without that friction I don't think it will work.
--
Roger Thorpe

My email address is spamtrapped. You can work it out!

On Feb 27, 7:57 pm, Roger Thorpe <[email hidden]>

Quoted message said:

Ben C said:

On 2007-02-26, Anthony Jones <[email hidden]> wrote:

Quoted message said:

Quoted message said:

Ben C wrote:

Quoted message said:

Quoted message said:

>>I don't dispute it is possible to generate a lateral force on the contact
>>point of a bicycle/ladder. As a simple example, imagine standing near the
>>base of a tall ladder (i.e. below the COM of the ladder) and pushing away
>>your their hands and feet. If there was no lateral force at the base, the
>>ladder would just slide along the ground, rotating in the opposite
>>direction.

Quoted message said:

Quoted message said:

>The way it works is like this: the dumbass rearranges the position of
>his COM relative to the ladder feet.

Quoted message said:

Quoted message said:

What I was getting at is establishing *how* the COM can move if the contact
point remains fixed. It has to be via an external force (Newton's first
law). This force has to come from the contact point, since nothing else is
in contact.

Quoted message said:

Yes, good point, and that is where it comes from, if the COM is to have
any sideways component of movement. To move straight down requires only
gravity, which doesn't act through the feet.

Didn't anyone notice that the ground is soft in this demonstration? He
digs the legs of the ladder into the ground and now has some rotational
friction which allow him to move his centre of mass. It's the friction
and/or the springiness of that pivot that allows him to generate a
turning moment. Without that friction I don't think it will work.

I wouldn't describe it as "friction", so much as the ability to change
the centre of pressure of the reaction force. Yes I had certainly
noticed that the ladder was fairly firmly planted in gravel. Burying a
bicycle up to its hubs in sand would help to balance that too :-) OTOH
such balancing tricks are possible in theory even without such tricks,
and may even be reasonably straightforward given possession of a large
gyroscope, or long pole like tightrope artists use (note however that
standing on a rope enables the contact patch to be shifted sideways
under the person which may play a role in balance in that case).

James

Quoted message said:

On Feb 27, 7:57 pm, Roger Thorpe <[email hidden]>

Quoted message said:

Ben C said:

On 2007-02-26, Anthony Jones <[email hidden]> wrote:

Quoted message said:

>Ben C wrote:

Quoted message said:

>>>I don't dispute it is possible to generate a lateral force on the contact
>>>point of a bicycle/ladder. As a simple example, imagine standing near the
>>>base of a tall ladder (i.e. below the COM of the ladder) and pushing away
>>>your their hands and feet. If there was no lateral force at the base, the
>>>ladder would just slide along the ground, rotating in the opposite
>>>direction.

Quoted message said:

>>The way it works is like this: the dumbass rearranges the position of
>>his COM relative to the ladder feet.

Quoted message said:

>What I was getting at is establishing *how* the COM can move if the contact
>point remains fixed. It has to be via an external force (Newton's first
>law). This force has to come from the contact point, since nothing else is
>in contact.

Quoted message said:

Yes, good point, and that is where it comes from, if the COM is to have
any sideways component of movement. To move straight down requires only
gravity, which doesn't act through the feet.

Didn't anyone notice that the ground is soft in this demonstration? He
digs the legs of the ladder into the ground and now has some rotational
friction which allow him to move his centre of mass. It's the friction
and/or the springiness of that pivot that allows him to generate a
turning moment. Without that friction I don't think it will work.

I wouldn't describe it as "friction", so much as the ability to change
the centre of pressure of the reaction force. Yes I had certainly
noticed that the ladder was fairly firmly planted in gravel. Burying a
bicycle up to its hubs in sand would help to balance that too :-) OTOH
such balancing tricks are possible in theory even without such tricks,
and may even be reasonably straightforward given possession of a large
gyroscope, or long pole like tightrope artists use (note however that
standing on a rope enables the contact patch to be shifted sideways
under the person which may play a role in balance in that case).

James

Yes, the tightrope thing is something that hadn't occurred to me until
recently, there must be some very interesting stuff going on. The
tightrope will have a natural tendency to oscillate and the problem of
what you do at the ends poses a problem. I seem to remember that the
rope is usually anchored behind the platform that the walker starts and
finishes on. (this could be wrong). I imagine that, like balancing on a
bike it's only possible if you do without thinking.
Roger

--
Roger Thorpe

My email address is spamtrapped. You can work it out!

Roger Thorpe said:

Fortunately nobody has mentioned the way dropped cats manage to turn
over in mid air.

Ooops (puts hand to mouth)

Imagine you are stood[1] in a space suit in space not rotating and not
attached to anything else, and you want to turn "upside down"[2]. If you
swing both arms in circles in the same direction parallel to your body,
you will rotate the other way as long as your arms are moving. Once you
stop swinging your arms, you body will stop rotating. [3]

I think cats must do something similar with their legs, tail and
possibly head.

Of course the real question is how does toast always manage to land
butter side down, as it does not have limbs or intelligence :-)

[1] or whatever the word is for zero gravity.
[2] ditto
[3] of course if this is your only method of turning, you are probably
dead, even if you try and pull a David Bowman[4]
[4] His h*lm*t saved his life because he took it off and threw it away :-)

Martin.