My Experiment...

[Mark Thompson]

>> You steer by moving your body, whether you have your hands on the
>> bars or not.
>
> Correct, you lean the way you want the bike to go.

And you get the lean by countersteering, or by not correcting a previous
coutersteer.

 

[Ben C]

On 2007-02-24, Rob Morley <[email protected]> wrote:
> In article <[email protected]>, Ben C
> [email protected] says...
[...]
>> If the stationary bike is in outer space, whatever flailing around you
>> may do (short of e.g. removing your h****t and hurling it into the void)
>> will not move your COM. But on Planet Earth you do have an external
>> force-- gravity, acting through the tyre contacts.
>>
> But gravity only acts vertically, and we're talking about a horizontal
> force.

There's no horizontal force, only a torque. As the bike starts to tip,
the ground opposes the torque which does actually result in a horizontal
force. If the tyre contacts were frictionless, the bike would still fall
down, but as it tipped left the tyres would slide to the right from
underneath it, and it would be a pure rotation about the COM with an
acceleration vertically downwards.

You can represent gravity as a vertical line from the center of mass to
the ground. You can represent the reaction forces as two vertical lines
upwards from the contact points. Vertical components add up to zero (3rd
law). You can take moments by working out for each of these three forces
F x r where r is the perpendicular distance from the line of action of
the force to the centre of mass. Add up the moments and you will get a
net torque, which tips the bike over.

Put another way, if an object is pulled straight down from its centre of
mass, and pushed straight upwards by an opposing force not acting
through the centre of mass, but offset, it will start to rotate. Net
torque but no net force.

It is possible to make a bike (ladder, or any other precariously
balanced object) tip by moving your centre of mass relative to where you
are contacting the ground. No external forces are required for that--
you can move your COM relative to the rest of you without an external
force.

Once you've done that, if you've ended up in a configuration in which
your COM is not directly above the contact point(s), which is not hard
to do, you're going down. For small errors, it is possible to oppose
external destabilizing forces and regain balance with skill and flailing
around.

Yes, you could trackstand on a bike with locked steering if you were
good enough and the deflections were small enough. Easier to do on one
with some steering, trail and that you can move backwards and forwards
small amounts, because then you can manipulate the positions of contact
points as well. Easier still to do if you're riding along because then
you have the feedback mechanisms already described. The physics is
relatively easy to explain, the precise actions of the rider much
harder.

 

[Anthony Jones]

Ian Smith wrote:
> Which means, presumably, that it's completely impossible for me to
> steer while riding no-handed.
>
> OK, as long as no-one tells my bike, I'll just carry on doing it and
> we'll be fine.
>
> How do the 'you can only turn by shoving the bars the wrong way' and
> 'leaning a bike won't make it turn' lot explain the ability to ride
> no-hands in other than straight lines?

Riding no hands does not preclude turning the bars. You just do it by
tilting the bike underneath you by shifting your hips, gyroscopic
precession and steering geometry takes care of the rest.

Anthony

 

[Anthony Jones]

Ben C wrote:
> It is possible to make a bike (ladder, or any other precariously
> balanced object) tip by moving your centre of mass relative to where you
> are contacting the ground. No external forces are required for that--
> you can move your COM relative to the rest of you without an external
> force.

Huh? Could you further explain what you mean by this?

I think you're missing that if you flail around while on a stationary bike
or ladder, the ground will end up applying a force via friction to the
tyres/base of the ladder. This is an external force.

Anthony

 

[Roger Thorpe]

Ben C wrote:

> You can balance though, with all the waving about, and enough skill.
> People can also do trackstands and all sorts of other things. But riding
> a moving bike is easier, because there's more inherent stability there,
> given I think by the interaction of trail and centripetal force-- bike
> starts to lean, bike turns, that makes it lean back. You still need
> rider input but you don't have to be an acrobat.

yes, I've thought about this a bit more, and to me it seems the secret
to dynamic balancing is being able to move your point (or line) of
contact with the ground.
I can trackstand for ages, but only where there's a camber in the road
and my front wheel is at an angle to the rest of the bike. What you do
is move the front wheel up or down the camber by modulating the pressure
that you put on the pedals (actually, now I think of it I suppose that
you might be able to do it wit a fixed, but any play in the chain will
make it very difficult). I've got a feeling that this is how a tightrope
walker works too. Whilst you can't move your centre of mass, you can
move the rope from side to side. Especially if you're holding a pole
that gives you a big rotational inertia to work against. Do they always
sprint the first and last bits where it won't move so much?
Thanks for the word 'trail' by the way. I knew there was something, but
I couldn't think of it.
Roger Thorpe

 

[Tosspot]

Pete Biggs wrote:

<snip>

> Theory goes that you can steer only by counter-steering first, and that you
> do it subconciously. See http://sheldonbrown.com/brandt/gyro.html

I not sure about this. Anyone who has tried to ride a motorbike (which
you *do* have to counter steer) no handed will realise to turn right you
*must* lean left. I've not noticed this on a bicycle.


> It certainly happens at low speed. Ride through a puddle, turn left
> normally, go back and have a look at your tyre tracks.

That's just staying upright and would be present counter or not steer.

 

[Tony B]

Tony B wrote:

> That's what was happening in the school yard, when I pushed the bike
> away from me. The geometry of the front end of the bike forces the wheel
> to whip left as I push right... immediately after this, the front end
> flops over to the left and the bike goes into a left hand arc.

er, no it's not - I'm getting left and right confused here

What meant to say was, I walk along pushing the bike by the saddle, push
the saddle away from my (to the bike's right) and observe the front
wheel nip left a bit, before returning back to the right and continuing
right, in a flopping manner...

So it would seem that a lean to the right makes the handlebars turn left
a bit, before steering geometry compensates (in fact, over compensates)
and brings the bars back to the right. So, maybe it also works the other
way about and turning the bars to the left causes a lean to the right.

But we all agrees on this did we not? The issue was, is it possible to
steer any other way...

Also, I saw a lad on a BMX stunt bike on TV this am. Dunno what he
thinks about counter steering but he couldn't half ride/balance/hoon
about...

Tony B

 

[Pete Biggs]

Tosspot wrote:

>> It certainly happens at low speed. Ride through a puddle, turn left
>> normally, go back and have a look at your tyre tracks.
>
> That's just staying upright and would be present counter or not steer.

Look at the tracks just before the turn. You'll consistently see the
opposite pattern when you turn the opposite way.

Whether you call it countersteer or not, the wheel turns right just before
it turns left. If it was just the normal balancing weaves that occur when
trying to cycle straight ahead, it would be random, so on about half the
left turns you'd see a move to the left before the main turn to the left.

~PB

 

[Tim Woodall]

On 23 Feb 2007 19:58:55 GMT,
Ian Smith <[email protected]> wrote:
> On Fri, 23 Feb, Pete Biggs <> wrote:
>> Another link:
>>
>> http://www.sheldonbrown.com/brandt/descending.html :
>> "What is not apparent, is that two wheeled vehicles can be
>> controlled ONLY by countersteer, there is no other way.
>
> Which means, presumably, that it's completely impossible for me to
> steer while riding no-handed.
>
> OK, as long as no-one tells my bike, I'll just carry on doing it and
> we'll be fine.
>
> How do the 'you can only turn by shoving the bars the wrong way' and
> 'leaning a bike won't make it turn' lot explain the ability to ride
> no-hands in other than straight lines?
>
Ian, is this a windup? I can't believe you don't know how this works.

If you really don't know then stand with your hand on the saddle of the
bike holding it upright with the wheel pointing straight ahead. Now lean
the bike either way and watch what the steering does.

Nothing at all prevents you from leaning the bike while riding it. What
is prohibited is moving the centre of mass from a straight line without
moving the centre of mass of something else the other way.


I'm pretty sure I'm being wound up here but, in all it's glorious
detail:


To corner no handed from riding in a straight line.

For simplicity we'll assume the rider starts going in a perfectly
straight line straight ahead with no left/right lean or steer and the
CoM directly above the contact patch.

The rider now leans the bike to the right (and moves their body to the
left) so that the centre of mass stays in the same place (actually it
probably goes down slightly and the Earth moves up to compensate)

The steering geometry[1] now causes the wheel to turn to the right. The
centre of mass continues in a straight line so the centre of mass moves
to the left of the contact point. I'm not sure exactly what the rider
does with their body - they may sit up slightly taller in which case the
CoM stays at the same height as they start to fall to the left or the
CoM may start moving towards the ground.

[1] and possibly gyroscopic effects

The rider now leans the bike to the left, and their body goes to the
right to compensate. The steering geometry now causes the wheel to turn
to the left.

The rider limits the leftward lean of the bike so that the leftward
steering is not enough to move the contact patch back under the centre
of mass. Instead the bike goes round in a circle, centripital force
generated by the reaction of the leaning bike on the ground resolved
towards the centre of the turn accelerating the CoM in a circle, the
normal component of the reaction force against the ground balancing the
force of gravity on the CoM (and to keep momentum conserved, the spin of
the Earth changes slightly)

To finish the turn the rider leans the bike a bit more to the left,
steering turns a bit more and the CoM moves under the contact patch. The
rider allows the bike to come upright so that the steering straightens
at exactly the right rate to put the CoM over the contact patch when the
bike is exactly upright. While the bike is moving upright, the CoM of
the riders body is moving the other way.


A rider can also come out of the lean by accelerating slightly.


In practice of course, going in a straight line is a constant left/right
lean so that the contact patch oscillates from left to right under the
CoM and the left/right oscillations continue while cornering although
the contact patch stays to the right of the CoM to maintain the lean.


But I'm sure you knew all that already.


Tim.


--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/

 

[Ben C]

On 2007-02-24, Anthony Jones <[email protected]> wrote:
> Ben C wrote:
>> It is possible to make a bike (ladder, or any other precariously
>> balanced object) tip by moving your centre of mass relative to where you
>> are contacting the ground. No external forces are required for that--
>> you can move your COM relative to the rest of you without an external
>> force.
>
> Huh? Could you further explain what you mean by this?
>
> I think you're missing that if you flail around while on a stationary bike
> or ladder, the ground will end up applying a force via friction to the
> tyres/base of the ladder. This is an external force.

Yes, I didn't mean to say it wasn't.

Sorry if I was unclear. The suggestion was: you need an external force
to get tipped over. You do, the question is how do you get hold of one
stranded at the top of a ladder. You do it by moving your COM around
such that the ground applies an external force in a place that results
in a net torque.

You don't require friction at the base of the ladder though, you can
still fall down with a frictionless contact, but you will rotate about
your centre of mass as you fall instead of being flipped to one side.

 

[Simon Brooke]

in message <[email protected]>, Ian Smith
('[email protected]') wrote:

> On Fri, 23 Feb, Pete Biggs <> wrote:
>> Another link:
>>
>> http://www.sheldonbrown.com/brandt/descending.html :
>> "What is not apparent, is that two wheeled vehicles can be
>> controlled ONLY by countersteer, there is no other way.
>
> Which means, presumably, that it's completely impossible for me to
> steer while riding no-handed.
>
> OK, as long as no-one tells my bike, I'll just carry on doing it and
> we'll be fine.
>
> How do the 'you can only turn by shoving the bars the wrong way' and
> 'leaning a bike won't make it turn' lot explain the ability to ride
> no-hands in other than straight lines?

There really is no point. It's a religion - an unquestionable belief. The
rational arguments have been made, and the experiments are there for
anyone to try. The true believers will carry on truly believing,
regardless of the evidence of their own senses - so what possible good
will argument do?

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Madness takes its toll. Please have exact change.

 

[Alan Holmes]

"Mark Thompson"
<pleasegivegenerously@warmmail*_turn_up_the_heat_to_reply*.com> wrote in
message news:[email protected]...
>>> You steer by moving your body, whether you have your hands on the
>>> bars or not.
>>
>> Correct, you lean the way you want the bike to go.
>
> And you get the lean by countersteering, or by not correcting a previous
> coutersteer.

I do not understand the term 'countersteer' if you lean to the left you turn
the handlebars to the left and vis versa.

Simple really!

 

[Rob Morley]

In article <[email protected]>, Simon
Brooke
[email protected] says...
> in message <[email protected]>, Ian Smith
> ('[email protected]') wrote:
>
> > On Fri, 23 Feb, Pete Biggs <> wrote:
> >> Another link:
> >>
> >> http://www.sheldonbrown.com/brandt/descending.html :
> >> "What is not apparent, is that two wheeled vehicles can be
> >> controlled ONLY by countersteer, there is no other way.
> >
> > Which means, presumably, that it's completely impossible for me to
> > steer while riding no-handed.
> >
> > OK, as long as no-one tells my bike, I'll just carry on doing it and
> > we'll be fine.
> >
> > How do the 'you can only turn by shoving the bars the wrong way' and
> > 'leaning a bike won't make it turn' lot explain the ability to ride
> > no-hands in other than straight lines?
>
> There really is no point. It's a religion - an unquestionable belief. The
> rational arguments have been made, and the experiments are there for
> anyone to try. The true believers will carry on truly believing,
> regardless of the evidence of their own senses - so what possible good
> will argument do?
>
>
On the one hand we have those who spout Newtonian physics but perhaps
haven't gone into it in sufficient depth, on the other we have those who
claim common sense and experiment are on their side but haven't produced
robust empirical data. I'm tending toward the countersteering camp, but
I have yet to muster sufficient brain power to work through Ben C's
hypothesis of torque reaction - high school physics was a long time ago.
Are you sure that /you/ are keeping an open mind?

 

[[email protected]]

On Feb 25, 6:16 am, Simon Brooke <[email protected]> wrote:

> There really is no point. It's a religion - an unquestionable belief. The
> rational arguments have been made, and the experiments are there for
> anyone to try. The true believers will carry on truly believing,
> regardless of the evidence of their own senses - so what possible good
> will argument do?

That seems undeniable. There is a simple explanation invoking nothing
more than school-level applied maths, supported by basically every
scholarly investigation and experiment on the subject, and then there
is Simon Brooke who claims that none of this applies to him, although
he cannot actually explain what near-mystical powers enable him to
balance a bicycle via some as-yet undiscovered phenomenon.

James

 

[Ben C]

On 2007-02-25, [email protected] <[email protected]> wrote:
> On Feb 25, 6:16 am, Simon Brooke <[email protected]> wrote:
>
>> There really is no point. It's a religion - an unquestionable belief. The
>> rational arguments have been made, and the experiments are there for
>> anyone to try. The true believers will carry on truly believing,
>> regardless of the evidence of their own senses - so what possible good
>> will argument do?
>
> That seems undeniable. There is a simple explanation invoking nothing
> more than school-level applied maths, supported by basically every
> scholarly investigation and experiment on the subject, and then there
> is Simon Brooke who claims that none of this applies to him, although
> he cannot actually explain what near-mystical powers enable him to
> balance a bicycle via some as-yet undiscovered phenomenon.

If you're saying there is a school-level applied maths explanation for
why it's not possible for a rider to make his bike lean to one side
without countersteering, I would be very interested to see it.

 

[Phil Cook]

Roger Thorpe wrote:

>Paul Boyd wrote:
>
>> On 23/02/2007 19:01, Roger Thorpe said,
>>
>>> Ok, the thing that I'm not exactly clear about steering is this:
>>> I find that sticking my knee out when going round a tight corner
>>> enables me to adjust the radius of the turn by bringing it in . Is it
>>> just me?
>>
>>
>> Nope - assuming you mean the knee on the inside of the corner. Sticking
>> it out means you can corner sharper/faster, bringing it in will relax
>> the corner. The only problem I find is that my SPDs unclip :-(
>>
>yes, that's exactly what I mean. I've got a sort of feeling about how it
>works, I think that it's about moving your centre of gravity so that you
>need to turn more sharply, but I'm not quite sure abut how the reaction
>forces on the rest of your body work out yet.

It's angular momentum I think. Similar to a skater doing a spin, the
closer you get the mass to the centre of rotation the quicker it will
turn.
--
Phil Cook looking north over the park to the "Westminster Gasworks"

 

[Tosspot]

Pete Biggs wrote:
> Tosspot wrote:
>
>
>>>It certainly happens at low speed. Ride through a puddle, turn left
>>>normally, go back and have a look at your tyre tracks.
>>
>>That's just staying upright and would be present counter or not steer.
>
>
> Look at the tracks just before the turn. You'll consistently see the
> opposite pattern when you turn the opposite way.
>
> Whether you call it countersteer or not, the wheel turns right just before
> it turns left. If it was just the normal balancing weaves that occur when
> trying to cycle straight ahead, it would be random, so on about half the
> left turns you'd see a move to the left before the main turn to the left.

Well I didn't notice it last time I looked but I'll have another look.
The other thing I remember its again obvious on a motorbike riding one
handed when you have to *pull* the bar towards you turn in the desired
direction.

 

[Ben C]

On 2007-02-25, Roger Thorpe <[email protected]> wrote:
> Ben C wrote:
[...]
>> I think it's quite possible that riders do countersteer. I just don't
>> believe that it's necessary or that we know for sure a priori that
>> that's what happens. It's just not true that there's no other way to
>> make the bike lean over, if that's really what was meant by Jobst Brandt
>> in the article on Sheldon's site.
>
> The problem about using Newtonian mechanics to justify this is partly
> that usenet doesn't allow us to use diagrams,

This particular diagram would be nothing to the ASCII-artists of RBT,
who regularly draw wheels, spokes, hub internals and brinneled headset
races with just what their keyboards provide.

> and even if it did then writing the explanation could be pretty
> tedious. It's worth having a go, but in the end some principles, like
> conservation of momentum make it sound like a statement of faith.

It's one thing to question the principle of conservation of momentum,
another some theory about how bicycles might balance!

Of course we could always resort to general skepticism-- how do we know
anything at all?-- but I think most people agree that physics as we know
it is a pretty sound theory for this kind of thing. The difficulty is
getting the application of it right.

[...]
> In order to answer your statement that "It's just not true that
> there's no other way to make the bike lean over" 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).

I maintain that you can, although it's much harder. It's easier if you
can move the skate around, and easier still if you're moving. This
corresponds quite well to balancing on a bike with locked steering
(extremely hard), trackstanding where you can move the steering
(ordinarily hard), and actually riding (as easy as riding a bike). In
each case the rider has more to play with to stay upright, and in the
final case an inherently stabilizing feedback mechanism that he just has
to keep in trim.

> You would also not need to fall over when your wheels drop into a
> tramway.

True, and I maintain that it would be possible to ride along a tramway
without falling over with sufficient practice. Physics does not exclude
it. Although I'm not going to try it myself.

 

[[email protected]]

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

Have you, or anyone else, actually tried this experiment? Assuming you
haven't, please do so, and get back to us with your results.

(I'm assuming you mean the steering is locked as close to straight as
you can judge, not at an angle - the latter would enable a normal
trackstand on a cambered road.)

James

 

[Ben C]

On 2007-02-26, [email protected] <[email protected]> wrote:
> On Feb 26, 8:26 am, Ben C <[email protected]> wrote:
>> This
>> corresponds quite well to balancing on a bike with locked steering
>> (extremely hard),
>
> Have you, or anyone else, actually tried this experiment?

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

> Assuming you haven't, please do so, and get back to us with your
> results.

I don't claim I can do it, only that there is no physical reason why it
is impossible.

> (I'm assuming you mean the steering is locked as close to straight as
> you can judge,

Yes, that would present no problem.

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.

http://www.youtube.com/watch?v=QPdxMG1s_18