Is this a fallacy ?

[Huzo]

I’m beginning to accept that there will be an interdependence of one factor over another.
There are a lot of “Dusty” corners in my knowlege that remind me how old I’m becoming. I just perceived that the stock statement of nimbleness and flickability being a function of low C of G and 3/5th’s of 5/8th’s of SFA else....
Needed some examination. I like Tusayan’s approach to this stuff, he’s ahead of me but I like his approach to working man’s Physics discussions.
I don’t exactly know what a curmudgeon is, but I think I’m becoming one, because I bristle a bit, when I hear illadvised or inaccurate statements trotted out blithely, accompanied by the sage nodding of heads.
I’ve been pulled up here on that very issue by blokes of deeper knowlege occasionally and am the better for it.

[Huzo]

Oh, and response #16.
Duty has done a nice job, but I prefer Dusty’s take on things, although I’ll admit he does have a devotion to Duty...
Also, how does one manage to get the C of G below street level ?

[yogidozer]

I’m beginning to accept that there will be an interdependence of one factor over another.
There are a lot of “Dusty” corners in my knowlege that remind me how old I’m becoming. I just perceived that the stock statement of nimbleness and flickability being a function of low C of G and 3/5th’s of 5/8th’s of SFA else....
Needed some examination. I like Tusayan’s approach to this stuff, he’s ahead of me but I like his approach to working man’s Physics discussions.
I don’t exactly know what a curmudgeon is, but I think I’m becoming one, because I bristle a bit, when I hear illadvised or inaccurate statements trotted out blithely, accompanied by the sage nodding of heads.
I’ve been pulled up here on that very issue by blokes of deeper knowlege occasionally and am the better for it.

You and others don't need to stop asking questions, and not every answer is going to be correct.
But it makes us all stop and think. And that's always a good thing.

[yogidozer]

Oh, and response #16.
Duty has done a nice job, but I prefer Dusty’s take on things, although I’ll admit he does have a devotion to Duty...
Also, how does one manage to get the C of G below street level ?

Dig a tunnel? 

[Huzo]

You and others don't need to stop asking questions, and not every answer is going to be correct.
But it makes us all stop and think. And that's always a good thing.

Now THAT Sir..is my point exactly..!

[oldbike54]

 Hell , even the *experts* have a hard time coming to a consensus on this . The fact is , what works is what works , bending the math to suit comes later .

 Dusty

[Huzo]

Hell , even the *experts* have a hard time coming to a consensus on this . The fact is , what works is what works , bending the math to suit comes later .

 Dusty

[oldbike54]

 Peter , there is a young man in the physics dept at a large state U who worked for me in college , we run these motorbike experiments every so often, he lays all of the math out , then we discuss the real world effects , then we normally laugh about all of it . Carpenter math only gets me so far 

 The concept of raising the forks to decrease rake angle comes up once a year or so , another misunderstood concept , because when you run the math , raising the fork 26 MM reduces rake by like .2 degrees using 28 degrees as a baseline . What most riders are feeling is increased weight on the front tire from tilting the chassis , and the increased feeling of control from having their body tilted forward . Once again , motorbike physics are fun , aren't they ?

 Dusty

[Huzo]

Peter , there is a young man in the physics dept at a large state U who worked for me in college , we run these motorbike experiments every so often, he lays all of the math out , then we discuss the real world effects , then we normally laugh about all of it . Carpenter math only gets me so far 

 The concept of raising the forks to decrease rake angle comes up once a year or so , another misunderstood concept , because when you run the math , raising the fork 26 MM reduces rake by like .2 degrees using 28 degrees as a baseline . What most riders are feeling is increased weight on the front tire from tilting the chassis , and the increased feeling of control from having their body tilted forward . Once again , motorbike physics are fun , aren't they ?

 Dusty

Yes old mate.
Same with people who think that increasing preload on shocks, makes them stiffer...

[kingoffleece]

Ha ha.  I corrected my typo-apologizes to DUSTY!

As for GOG below street level-it was a simple graph that illustrated an intersection of various factors that determine where COG is.  That intersection can be below the bike to the extent that the lines intersect below grade.  It's not that hard a concept.  Something tells me I saw it in Camaron's book.
I'll have to dig it out and check.  Nonetheless, if you understand that moment of inertia, COG, center of mass, and others are determined by vectors (I think that's it IIRC) than the concept of one or more of those measures being located above or below is not unreasonable.

The entire subject sounds like a perfect topic for "Ask Kevin" at CycleWorld.

[Huzo]

Ha ha.  I corrected my typo-apologizes to DUSTY!

As for GOG below street level-it was a simple graph that illustrated an intersection of various factors that determine where COG is.  That intersection can be below the bike to the extent that the lines intersect below grade.  It's not that hard a concept.  Something tells me I saw it in Camaron's book.
I'll have to dig it out and check.  Nonetheless, if you understand that moment of inertia, COG, center of mass, and others are determined by vectors (I think that's it IIRC) than the concept of one or more of those measures being located above or below is not unreasonable.

The entire subject sounds like a perfect topic for "Ask Kevin" at CycleWorld.

I think you may be referring to the concept of taking moments to determine C of G..
It can be in relation to determining weight and balance when calculating loading in light aircraft or similar and C of G position to determine if it is within limits.
When calculating moments any point can be used and if the system is in equilibrium, the resultant will be zero.

[Testarossa]

As for GOG below street level-it was a simple graph that illustrated an intersection of various factors that determine where COG is.  That intersection can be below the bike to the extent that the lines intersect below grade.  It's not that hard a concept.  Something tells me I saw it in Camaron's book.

More likely the point below pavement was the center of rotation when the bike rocks forward and aft with braking and acceleration. Confusion may arise from the fact that a body in flight may rotate around the CoM (or CoG) (aerodynamics permitting) but a machine on the ground certainly will not.

[Huzo]

More likely the point below pavement was the center of rotation when the bike rocks forward and aft with braking and acceleration. Confusion may arise from the fact that a body in flight may rotate around the CoM (or CoG) (aerodynamics permitting) but a machine on the ground certainly will not.

Agreed..
But if you consider a bike coming head on that initiates a turn.
Let us say the bike turns to (our) right, or a left hander for the rider.
The wheels will be displaced to (our) left and the rider’s head will be displaced to (our) right. Somewhere in between these two points is a spot where the lateral displacement vector is zero, and that is the C of M of the system.
In the case of a 130 kg rider on a 10 kg pushbike, the lateral displacement of the wheels will be considerably more than that of the rider’s head, this is because the C of M is quite high.
The same rider on a wide glide with a lightweight flag above his bike to represent the same height as example #1, will exhibit a relatively small lateral movement of the wheels versus the top of the flag.
This is because the C of M is very low.
Guess which one is more umm.... “flickable”.

[Testarossa]

But if you consider a bike coming head on that initiates a turn.
Let us say the bike turns to (our) right, or a left hander for the rider.
The wheels will be displaced to (our) left and the rider’s head will be displaced to (our) right. Somewhere in between these two points is a spot where the lateral displacement vector is zero, and that is the C of M of the system.
In the case of a 130 kg rider on a 10 kg pushbike, the lateral displacement of the wheels will be considerably more than that of the rider’s head, this is because the C of M is quite high.
The same rider on a wide glide with a lightweight flag above his bike to represent the same height as example #1, will exhibit a relatively small lateral movement of the wheels versus the top of the flag.
This is because the C of M is very low.
Guess which one is more umm.... “flickable”.

Can't agree with this. The tires are prevented from moving significantly outward from the arc by engagement with the pavement, and in fact the banking rotation has to be centered on the contact patches of the tires. Any outward movement of the front wheel -- maybe an inch or two -- is induced by the steering geometry and is what in turn induces lean angle when countersteering. I suspect the rear wheel actually tracks INSIDE the arc of the turn unless throttle-induced wheelspin breaks the traction for a power slide.

Comparing steering quickness or flickability with a bicycle or push bike is unrealistic not only because of the weight differences but also because bicycles typically have a 17 degree rake and trail is at the low end of what would be acceptable with a motorcycle -- just a bit more than 2 inches.

[wittangamo]

This is the first thread on this forum I’ve ever read all the way through without understanding a single word. Congrats, guys!
 

[Huzo]

Can't agree with this. The tires are prevented from moving significantly outward from the arc by engagement with the pavement, and in fact the banking rotation has to be centered on the contact patches of the tires. Any outward movement of the front wheel -- maybe an inch or two -- is induced by the steering geometry and is what in turn induces lean angle when countersteering. I suspect the rear wheel actually tracks INSIDE the arc of the turn unless throttle-induced wheelspin breaks the traction for a power slide.

Comparing steering quickness or flickability with a bicycle or push bike is unrealistic not only because of the weight differences but also because bicycles typically have a 17 degree rake and trail is at the low end of what would be acceptable with a motorcycle -- just a bit more than 2 inches.

I think some of that is a bit sketchy, but I don’t have enough raw understanding to be able to engage.
The only thing I’d add, is that the engagement with the pavement is not “preventing” the tracking outside the arc, because the wider arc is initiated by the first countersteer input and that is only possible “because” of that engagement.
Imagine if you were on ice and there was no engagement, the first input on the ‘bar would result if an immediate crash.
The tyres are describing a larger radius than the rider’s head, gravity (downwards) is preventing the rider being thrown outwards (as if in a car), and centrifugal force (sic), is preventing the rider falling inwards due to lean, both are in equal measure.
The reason I introduced the heavy rider on the light bike, was to give an example of high C of G.
However..
The waters are becoming muddied and my original question is becoming blurred at the edges. Also I think I need to be schooled by someone way better than me at Physics, because I may be carrying some false ideas that have manifested over the years.
That great Physicist and Philosopher Albert Einstein put it best when he said..(and this applies to me)

“If you cannot explain something simply..
You probably don’t understand it properly yourself...”
So I guess is off to Princeton with me.   (Love the chat though..)

[Knuckle Dragger]

I think some of that is a bit sketchy, but I don’t have enough raw understanding to be able to engage.
The only thing I’d add, is that the engagement with the pavement is not “preventing” the tracking outside the arc, because the wider arc is initiated by the first countersteer input and that is only possible “because” of that engagement.
Imagine if you were on ice and there was no engagement, the first input on the ‘bar would result if an immediate crash.
The tyres are describing a larger radius than the rider’s head, gravity (downwards) is preventing the rider being thrown outwards (as if in a car), and centrifugal force (sic), is preventing the rider falling inwards due to lean, both are in equal measure.
The reason I introduced the heavy rider on the light bike, was to give an example of high C of G.

Plus that aforementioned gyroscopic force from the wheels (& to a much lesser extent the rotating masses of the engine) which conspire to keep the bike perpendicular to the surface.  The countersteering effort required to initiate the turn is to allow these gyroscopes to tilt sideways in the appropriate direction.

One can feel this effect quite easily with a power planer.  At speed the planer's cutterhead (the rotating mass that forms a gyroscope) will try to stay perpendicular.  If the planer is turned (rotated) around a vertical axis it will naturally try to tilt in the opposite direction, just as a bicycle (motorised or pedalled) will.  Conversely, if the planer is tilted sideways, it will naturally wish to rotate horizontally opposite.

This clearly illustrates the interrelationship between gyroscopes and countersteering.

Any vehicle's wheels will be displaced outwards in a curve.  Irrespective of how many individual wheels or even axles are involved, the steering axle (front in most vehicles, but rear in some machinery like the pea viners I used to drive in my school holidays) will always be displaced outwards in a curve.  The steering axle must always describe an arc of greater radius than a fixed axle.  The only exception that I'm aware of is articulated chassis vehicles, where the arcs tend to equalise.

When I was designing forest roads for large 22 wheel log-lorries in the 70s they were built as narrow as practicable.   Curves were designed to connect the straightaways (obviously), but were carefully 'transitioned' (i.e. halved) for the first & last chord of radii to compensate for the differential radii described by the jinker as it enters & leaves a curve.  The widest arc offset was from the steering axle of the prime mover, the tightest from the third & last trailing axle on the trailer.  There was quite a difference in these radii:  the 'tighter' the curve, the greater the offset required.

There was fairly basic geometry applied to the curve formula, but it was so long ago that it's all but disappeared into the sphagnum bog of trivia that clogs my mind....  Yes, there's probably some slight additional horizontal displacement (rotation) of fulcrum points as the mass of a bicycle in motion leans as it transitions through an arc, but the subtle interplay of Physics & Geometry is now well beyond my ageing comprehension to calculate, or even adequately describe.

[oldbike54]

 Geez Peter , I explained it as simply as possible , what do you want ?

 Dusty

[Huzo]

Geez Peter , I explained it as simply as possible , what do you want ?

 Dusty

I think you’re joking so I’ll chuckle then go on to something else Dusty.
Examples of what Honda did, or how different bikes “feel” and various analogies, do help to form a picture and provide some understanding, but I tend to look for explanations based on sound principles of Physics that are dumbed down to my level.
My University Engineering background has more dust on it than a Cedar Vale picnic table, so I lose track quicker than I used to..
I think I not only don’t remember some stuff that I used to have at my fingertips, but I’m starting to spruke  and apply beliefs that are not soundly based.
Most of the responses here are equally as valid as mine or more so in some cases, but I can see patches of light coming through what is supposed to be a solid wall of factually based knowlege.

[Tusayan]

Can't agree with this. The tires are prevented from moving significantly outward from the arc by engagement with the pavement, and in fact the banking rotation has to be centered on the contact patches of the tires.

Hard countersteering can steer the tires outward from under the CG.  The effect of that model is that the bike rotates around the CG, which simultaneously translates downward.  The suspension extends a little until the bike settles. 

In actuality the bike rotates in a more complex way, somewhere between rotation around the tire patch and the CG...  but I think that the harder the countersteering the closer the bike comes to rotating around the CG.

[Huzo]

Hard countersteering can steer the tires outward from under the CG.  The effect of that model is that the bike rotates around the CG, which simultaneously translates downward.  The suspension extends a little until the bike settles. 

In actuality the bike rotates in a more complex way, somewhere between rotation around the tire patch and the CG...  but I think that the harder the countersteering the closer the bike comes to rotating around the CG.

Ok.