This is interesting

[M61A1MECH]

Check out this video.

 

http://www.wimp.com/demonstrationmomentum/

[tz89]

Which is why, in the Ride Like a Pro video, Motorman has you keep your RPM up and let the clutch slip, for slow speed tight turnings.

[Marcarl]

None of this is intuitive. I think I got that right. Hey I'm Dutch as well EH?

[LilBeaver]

Which is why, in the Ride Like a Pro video, Motorman has you keep your RPM up and let the clutch slip, for slow speed tight turnings.

This is absolutely 100% false. [for low speed maneuvering] -- I have heard a number of people say this and claiming a physical explanation to back up this reasoning is complete non-sense.

Keeping the engine speed up, using the friction zone and modifying your speed with the rear brake keeps you from stalling your bike and gives you a very easy 'out' if you start to go over since all you need to do is lift your foot and/or pop the clutch to stand upright under the proper application of principles of linear and angular momentum in addition to opposing forces, etc etc -- but that will be for another discussion.

 

The rotating parts in the motor, while they do contribute to the overall dynamic of the bike the effects are nominal, at best, not to mention in the wrong direction of rotation to have any effect on keeping you upright.

Lets begin by looking at the simple mechanics of what is moving inside the engine. First one ought to note that there is actually quite a bit of symmetry in an engine -- think about it -- the V design is not an accident, nor is the flat design or the in-line designs. So motion of the pistons over a given time period oppose each other and have a net zero effect on the overall dynamic of the bike.

Now for the part that rotates and is a function of engine speed (actually how the engine speed is defined) -- the cam shaft rotates but is laid along the long axis of the bike, rotates in a direction that is perpendicular to the direction of linear motion, parallel to the angular motion (roll) and puts the direction of angular momentum along the axis of the bike itself (that is draw a line connecting the two wheels and that is the line for which the direction of the angular momenta is aligned). Any variation in the rotation rate (assuming constant mass of the cam shaft) makes NO contribution to the 'roll' (if you let me borrow an aeronautical term here) of the bike what so ever.

-- Think about it: if the engine speed itself was correlated affected your ability to stay upright, in order to never have to put your feet down, all you would need to do is rev your engine really high when you stop! How many of us know that the bike drop just as easily when you are in neutral with the side-stand up regardless of whether the bike is running or not?

 

 

Now, if there were active gyroscopes located in your bike and you adjusted the rate in which those were spinning, that would absolutely have an effect on low speed maneuvering -- how do you think that satellites are realigned in space, how large ships are stabilized or how a segway stays upright? Or, more directly, when you are traveling at a higher rate of speed you can lean over further [hint: different and much more massive objects are rotating in a direction that aligns the angular momenta properly to have a direct effect on the roll of the bike -- not to mention the linear momentum and net forces between the tires and the road....].

 

Okay so now let us take a peek at some of the simplified math.

Linear motion and forces are related to the rotational motion and forces by the simple cross product of the position vector and the linear force [or momenta] for which you are describing. Recall that newton's second law relate momentum, p, and force, F, as:

F=dp/dt -- that is force is equal to the way momentum varies as a function of time. Also recall that momentum can be loosely defined as a measure of an objects inertia or more simply put, a way to quantify how difficult it may be to change an object's motion.

In simple math, momentum, p, is equal to the objects mass, m, times its velocity, v.

That is: p=mv

Rotation is accounted for by including a quantity to define the position of a particular particle [or group of particles] for which you are actually describing the motion. In this case, we will call that position vector, r.

What you do not realize is that rotation is already built into the 'standard' simple statement but there is an implicit assumption that there is a constant direction.

Skipping some of the details, one follows through by using what is known as a cross product -- which is one way of combining two quantities [vectors] that include information of both size and direction. In this case the position vector, locates any particular piece of mass and its location from the part of the system that we define as the origin [often times, the place that remains constant or has high symmetry -- such as the center of rotation, ie the center of an axle].

So the form for angular momentum, L, comes out to be:

L=r x mv

which, with some additional fancy footwork to put this into terms that are little more user friendly one can introduce what is known as the 'moment of inertia', I, which is a quantity that basically provides the appropriate scaling for the 'mass' quantity that is rotating. Strictly speaking, the moment of inertia accounts for the varying mass as a function of distance that contributes to an object's rotational properties. That is to say that the moment of inertia is a quantity that scales the resistance to an object's rotation acceleration. Just like a more massive object resists linear motion (it is a lot easier to push a 1 gram paper clip across your desk than it is to shove an elephant -- right?).

 

Skipping boring details, one can make an assumption of a constant position vector and write the angular velocity as the simple expression w=r x v.

Now the direction of the angular momentum is parallel to that of the angular velocity.

The important aspect to remember here is that when there is no net external torque (force) momentum is conserved which includes the size of the momentum vector as well as the direction of the momentum vector. Despite the fact that the direction is based on an arbitrary mathematical definition, since the 'roll' of the motorcycle occurs in a direction that is parallel to the roll of the rotating cam shaft, the external force/torques are present, the angular momentum is not conserved and hence there is no additional stabilization that is a result of the increased engine speed.

 

Disclaimer: I have not proof read this, and maybe I should before posting but I am going to post it now so I don't lose everything I have written [again].

 

I hope this makes sense, if not, please ask away, I am happy to explain whatever requires additional explanation as long as someone is interested in learning.

 

 

----

 

Sorry for the hijack...

Edited September 21, 2013 by LilBeaver

[SilvrT]

so I don't lose everything I have written [again].

 

 

A little "lesson" I learned long ago when typing long "narratives" in places such as this (where you have no "Save" button) is to occasionally press Ctrl+A (select all), then Ctrl+C (copy). This stores what I've type into the clipboard (aka memory). If the app locks up or the post fails, you can easily get the info back by starting over and pressing Ctrl+V (paste).

 

Sometimes I'll also jump into a Word document and past my copied info there.

 

(sorry for the hijack! )

[Sylvester]

My brain aches. Think I will go for a ride and let it heal.

 

Didn't mean to hijack the thread.

 

[bongobobny]

Wish I could have seen the rest of the lecture...

[Trader]

And here I thought I didn't fall down so much because of my riding skills!

 

That formula on the board explains it all in basic terms.

[Black Owl]

This is absolutely 100% false. [for low speed maneuvering] -- I have heard a number of people say this and claiming a physical explanation to back up this reasoning is complete non-sense.

Keeping the engine speed up, using the friction zone and modifying your speed with the rear brake keeps you from stalling your bike and gives you a very easy 'out' if you start to go over since all you need to do is lift your foot and/or pop the clutch to stand upright under the proper application of principles of linear and angular momentum in addition to opposing forces, etc etc -- but that will be for another discussion.

 

The rotating parts in the motor, while they do contribute to the overall dynamic of the bike the effects are nominal, at best, not to mention in the wrong direction of rotation to have any effect on keeping you upright.

Lets begin by looking at the simple mechanics of what is moving inside the engine. First one ought to note that there is actually quite a bit of symmetry in an engine -- think about it -- the V design is not an accident, nor is the flat design or the in-line designs. So motion of the pistons over a given time period oppose each other and have a net zero effect on the overall dynamic of the bike.

Now for the part that rotates and is a function of engine speed (actually how the engine speed is defined) -- the cam shaft rotates but is laid along the long axis of the bike, rotates in a direction that is perpendicular to the direction of linear motion, parallel to the angular motion (roll) and puts the direction of angular momentum along the axis of the bike itself (that is draw a line connecting the two wheels and that is the line for which the direction of the angular momenta is aligned). Any variation in the rotation rate (assuming constant mass of the cam shaft) makes NO contribution to the 'roll' (if you let me borrow an aeronautical term here) of the bike what so ever.

-- Think about it: if the engine speed itself was correlated affected your ability to stay upright, in order to never have to put your feet down, all you would need to do is rev your engine really high when you stop! How many of us know that the bike drop just as easily when you are in neutral with the side-stand up regardless of whether the bike is running or not?

 

 

Now, if there were active gyroscopes located in your bike and you adjusted the rate in which those were spinning, that would absolutely have an effect on low speed maneuvering -- how do you think that satellites are realigned in space, how large ships are stabilized or how a segway stays upright? Or, more directly, when you are traveling at a higher rate of speed you can lean over further [hint: different and much more massive objects are rotating in a direction that aligns the angular momenta properly to have a direct effect on the roll of the bike -- not to mention the linear momentum and net forces between the tires and the road....].

 

Okay so now let us take a peek at some of the simplified math.

Linear motion and forces are related to the rotational motion and forces by the simple cross product of the position vector and the linear force [or momenta] for which you are describing. Recall that newton's second law relate momentum, p, and force, F, as:

F=dp/dt -- that is force is equal to the way momentum varies as a function of time. Also recall that momentum can be loosely defined as a measure of an objects inertia or more simply put, a way to quantify how difficult it may be to change an object's motion.

In simple math, momentum, p, is equal to the objects mass, m, times its velocity, v.

That is: p=mv

Rotation is accounted for by including a quantity to define the position of a particular particle [or group of particles] for which you are actually describing the motion. In this case, we will call that position vector, r.

What you do not realize is that rotation is already built into the 'standard' simple statement but there is an implicit assumption that there is a constant direction.

Skipping some of the details, one follows through by using what is known as a cross product -- which is one way of combining two quantities [vectors] that include information of both size and direction. In this case the position vector, locates any particular piece of mass and its location from the part of the system that we define as the origin [often times, the place that remains constant or has high symmetry -- such as the center of rotation, ie the center of an axle].

So the form for angular momentum, L, comes out to be:

L=r x mv

which, with some additional fancy footwork to put this into terms that are little more user friendly one can introduce what is known as the 'moment of inertia', I, which is a quantity that basically provides the appropriate scaling for the 'mass' quantity that is rotating. Strictly speaking, the moment of inertia accounts for the varying mass as a function of distance that contributes to an object's rotational properties. That is to say that the moment of inertia is a quantity that scales the resistance to an object's rotation acceleration. Just like a more massive object resists linear motion (it is a lot easier to push a 1 gram paper clip across your desk than it is to shove an elephant -- right?).

 

Skipping boring details, one can make an assumption of a constant position vector and write the angular velocity as the simple expression w=r x v.

Now the direction of the angular momentum is parallel to that of the angular velocity.

The important aspect to remember here is that when there is no net external torque (force) momentum is conserved which includes the size of the momentum vector as well as the direction of the momentum vector. Despite the fact that the direction is based on an arbitrary mathematical definition, since the 'roll' of the motorcycle occurs in a direction that is parallel to the roll of the rotating cam shaft, the external force/torques are present, the angular momentum is not conserved and hence there is no additional stabilization that is a result of the increased engine speed.

 

Disclaimer: I have not proof read this, and maybe I should before posting but I am going to post it now so I don't lose everything I have written [again].

 

I hope this makes sense, if not, please ask away, I am happy to explain whatever requires additional explanation as long as someone is interested in learning.

 

 

----

 

Sorry for the hijack...

 

WOW. Thanks for the lecture.

 

Now, can you give us the Readers Digest Condensed version?

 

[bongobobny]

WOW. Thanks for the lecture.

 

Now, can you give us the Readers Digest Condensed version?

 

The wheels on the bus go round and round...

[cowpuc]

Confusion says = On the other hand, if you have a tendency to fall over on your scoot when not revving the motor a little than by all means - blip that throttle....

[tz89]

Interesting.

 

I see it said lots of places - just did a google search...

 

http://www.xs11.com/xs11-info/tech-tips/riding/216-how-to-make-a-slow-tight-u-turn.html

 

[LilBeaver]

Interesting.

 

I see it said lots of places - just did a google search...

 

http://www.xs11.com/xs11-info/tech-tips/riding/216-how-to-make-a-slow-tight-u-turn.html

 

 

As I commented initially, I too have seen and heard the claim time and time again but that does not make it any less wrong. There are plenty of misconceptions in the world, sadly, this concept is one that is often misconstrued or just flat out not understood and still passed around regardless of its accuracy.

This particular link that you posted here claims that centripetal force somehow has something to do with keeping one's bike up; for the record, that explanation may seem good at first because they throw around some fancy words, but it too is nonsense.

By definition, centripetal force is a force that causes a body to move along a circular trajectory. The link you provided would lead you to believe that the 'rotating engine parts' (their words, not mine) exert a centripetal force on the bike which helps keep you upright and makes your turns easier. Again, complete and udder nonsense.

Lets start with a simple example to show what centripetal force REALLY is. Go get a golf ball and put it in a long tube sock. Hold the sock by the end with the hole in it and then begin to swing the sock and ball in a circular motion holding your hand relatively still but allowing the sock to be twirled about. If you were to be a 3rd person, observing yourself twirling this sock and golf ball around you can easily identify the forces at play here. First of all, the outside observer can easily see the golf ball is traveling in a circular motion. In order for the ball to not just go off straight, there must be a force acting on it to keep it moving in the circular path. One can easily identify the fact that the sock is exerting a force on the ball, which is keeping the ball from flying off in a straight line. The sock is being held into place by your hand. Your hand exerts a force on the sock as the sock exerts a force on your hand. The tension in the sock mediates the force between your hand and the golf ball. The CENTRIPETAL force, in this case, is identified as the force required to keep the golf ball moving in its circular path. Furthermore, the centripetal force is equal in magnitude to what you feel in the sock as you feel the apparent weight of the golf ball as it twirls around.

So, looking back at the rotating engine parts, ie the cam shaft, and note that while the centripetal forces involved there have to do with keeping the cam shaft (as a rigid body) from falling apart and has nothing to do with the outside body [ie the stability of the motorcycle itself]

 

The only plausible argument for increased stability due to rotating parts MUST include an explanation that includes conservation of momentum as well as account for the remaining forces in this dynamic system since the rotating wheels and effects of conservation of angular momentum (ie gyroscopic stability) are actually a small part in what keeps you upright.

In the video clip that the original poster showed, you can see that when the wheel is spun and then suspended by the axle, it rotates around the suspension point. Now, if you were to hold the axle in your hands and tilt it either clockwise or counter-clockwise, the system would resist that change and you would feel it in your arms. If you were on a stool that was free to rotate, you would find that the entire system's angular momentum is conserved and the more you tilted the wheel the faster you rotate on the stool in the opposite direction. THIS effect, clearly demonstrates part of why you are able to be much more stable at higher speeds when leaning over. The other contribution has to do with balancing the forces and torques between the rubber and the road, etc etc.

Note that the axle of the wheels is perpendicular to the direction in which the cam rotates. More importantly, the direction of the axle for which the wheels rotate around is perpendicular to both the direction of linear motion of the bike and the direction of rotation of the bike and hence, it actually makes a difference.

But I digress...

 

 

%%%

Video clip of the fairly typical bicycle wheel/stool demo (no, it is not me as I am not quite cool enough to have a fancy white coat...): [ame=http://www.youtube.com/watch?v=7ZDF3oDs-JI]1Q40 30 Rotating Stool and Bicycle Wheel - YouTube[/ame]

FYI: Not all scientists wear lab coats, in fact, I usually take off the loose articles of clothing for demos that involve rotating things because I nearly always get something stuck in the object that is rotating.... All about safety here .

Edited September 22, 2013 by LilBeaver

[LilBeaver]

WOW. Thanks for the lecture.

 

Now, can you give us the Readers Digest Condensed version?

 

 

That was the very very condensed version

 

The usual lecture takes at least 45 minutes -- sometimes it is several 45 minute periods worth...

 

 

----

I'll give it another try.

 

Take a look here: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/bicycle.html

 

This has some good diagrams to help describe what effect the rotating bodies have on a system. The real important bit to take from all of this is within this next statement. Now, from what you learned there, note that on our bikes, the 'rotating engine parts' are rotating in a direction that is perpendicular to the wheel rotation (parallel to the rotation of the cam shaft) and hence any effect from the rotating cam would be felt in the frame of the bike and in pitch and have nothing at all to do with the roll of the bike.

 

I hope that helps some. If desired, I will take some time to write up a much more complete explanation for a general audience -- but it might be a little while before I end up with a sufficient block of time to do such a write-up...

[playboy]

I' would like to hear the version of why the RLP technique for slow turns does work which it obviously does. If what I'm reading and understanding from your statement Lil Beaver it can not work. But I'm confused already so I will stick with the old adage.

 

 

If it aint broke don't fix it.

[dacheedah]

I know the friction zone thing works, don't know why

 

ultimately in terms of driving you can only control two things; direction and speed. I know the energy applied to the drive train and an amount of friction applied at the same time has a positive effect on keeping the bike upright.

[BoomerCPO]

I'm no rocket scientist but I did sleep at a Holiday Inn Express once....

I have been using the RLP technique during slow speed practice sessions which I do 1-2 times per week....whatever the explanation it all works for me.

Boomer....who sez when yer poor dropping de scoot is not an option...

[noahzark]

I agree, the RLP method works. It appears to me Lilbeaver addresses this in his second paragraph, "Keeping the engine speed up . . .". But then I'm not a scientist, but I worked with many rocket scientists in the 80s.

[M61A1MECH]

Man!! Did this post take a weird turn or what, I just thought the video was interesting never expected it to create a controvesy, but the discussions are interesting also.

[tz89]

Best $12 ever!

[JohnT]

I am not a rocket scientist.

My company budget won't allow Holiday Inn Express. (not cheap in Mass.)

I am also to poor to drop my scoot.

 

Um, I forgot where I was going with this.

 

But here is what is left of my thought.

My crankshaft and my wheels spin on the same plane. (probably the wrong term) The faster my wheels spin the more stable my bike is. (or seems)

So, it follows (in my mind) that the faster my crank spins it has to help. When I try to turn a U tightly, I keep my revs up. I don't slip the clutch, but if needed I do engage and disengage it to maintain movement.

I haven't fallen down in awhile.

[XV1100SE]

The wheels on the bus go round and round...

 

Now that I understood.

[Kirby]

It all about finesse and MOMENTUM.

[LilBeaver]

I' would like to hear the version of why the RLP technique for slow turns does work which it obviously does. If what I'm reading and understanding from your statement Lil Beaver it can not work. But I'm confused already so I will stick with the old adage.

[...]

 

Playboy, I do NOT mean to imply at all, that the RLAP method does not work because it most certainly DOES work. I was trained to ride that way many years ago and I will continue to attest to its success. The 'WHY' that gets applied to the explanation is what I was addressing as to what is blatantly incorrect.

 

I agree, the RLP method works. It appears to me Lilbeaver addresses this in his second paragraph, "Keeping the engine speed up . . .". But then I'm not a scientist, but I worked with many rocket scientists in the 80s.

 

Noahzark -- correct, my longer statement was discredit the incorrect rationale specifically related to RPM as being related to stability via the incorrect conservation of angular momentum statements (that routinely get incorrectly applied to the apparent correlation of RPM to stability).

 

My suggestion with respect to the real reason as to why the method works has to do with the appropriate balance of power and the ability to continually 'fine tune' the control one has over the motorcycle and hence provide much better control during those slow speed maneuvers.

My original statement is as follows:

 

[...]

Keeping the engine speed up, using the friction zone and modifying your speed with the rear brake keeps you from stalling your bike and gives you a very easy 'out' if you start to go over since all you need to do is lift your foot and/or pop the clutch to stand upright under the proper application of principles of linear and angular momentum in addition to opposing forces, etc etc -- but that will be for another discussion. [...]

 

I renew my thought challenge:

Think about it: if the engine speed itself was correlated affected your ability to stay upright, in order to never have to put your feet down, all you would need to do is rev your engine really high when you stop! How many of us know that the bike drop just as easily when you are in neutral with the side-stand up regardless of whether the bike is running or not?

 

 

I hope this clears up the confusion.

[LilBeaver]

[...]

But here is what is left of my thought.

My crankshaft and my wheels spin on the same plane. (probably the wrong term) The faster my wheels spin the more stable my bike is. (or seems)

 

There is no argument that the faster the wheels spin (assuming no slippage of the tires on the road) the more stable the bike is.

The PROBLEM here is that the direction of rotation for the wheels not to mention the negligible mass and radius (related to the moment of inertia) as compared to the overall mass of the bike -- independnt of the fact that the direction of rotations for the wheels and crank are perpendicular to each other.

The increased speed of the bike affecting stability has more to do with the increase in forces (and hence torque) between the tires and the road. Rotational equilibrium is achieved by balancing the forces. Yes, of course, momentum plays a BIG role here.

 

So, it follows (in my mind) that the faster my crank spins it has to help. When I try to turn a U tightly, I keep my revs up. I don't slip the clutch, but if needed I do engage and disengage it to maintain movement.

[...]

 

As previously mentioned the direction of rotation of the crank and drive shaft simply have no effect on the roll of the bike independent of the fact that the rotating parts are a small fraction of the overall weight of the bike and hence would have a very small contribution even if the directions were such that the angular momentum of the spinning parts would have any effect at all on the roll of the bike.

 

The technique produces the result, which is excellent. I do not argue that at all. But again, the claim that the crank has an overall effect related to the roll and stability of the bike is bogus. The laws of physics do not support the explanation.

 

Hope this helps some