# Center Of Gravity High Or Low

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Racer, I agree with you that datadan could have chosen a better word than "highside" for what he was saying. I interpreted his words to mean that the lateral force tends to stand the bike up, while gravity tends to make it fall in. The two vectors should zero each other out.

Have a good evening out and I'll look forward to what more you have to say.

On the idea that we're overthinking this, I'm with you, in that the discussion may not help your riding. Being a little nerdy, I like the subject anyway.

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wow. thanks tz. i regret i don't have the time i wish to devote to this so please forgive me. maybe later today. istarted writing but way too muddy. need sleep now and in pain from broken toe. very distracting. ow.

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Racer, I agree with you that datadan could have chosen a better word than "highside" for what he was saying. I interpreted his words to mean that the lateral force tends to stand the bike up, while gravity tends to make it fall in. The two vectors should zero each other out.

Have a good evening out and I'll look forward to what more you have to say.

On the idea that we're overthinking this, I'm with you, in that the discussion may not help your riding. Being a little nerdy, I like the subject anyway.

yeaaaah, now we r talking. yes, higher COG will make the bike stand up(wat i called outward force in my eariler post), but u want to keep it down, thats why u wld like to keep as low a COG as possible. however, at some point, traction, surface conditions, and other factors wld come in, and therefore, u want to look for that "optimum" COG, and not necessarily the lowest COG, as somebody posted above.

now next few lines, only those inclined towards theory shld read.:-)

as i posted once earlier, a simplified equation for balance in corner is

m*g*x=m*v^2*d/r

now its simple, lower COG is lower 'd'. now other factors remaining constant, to balance out m*g*x, it gives me a chance to have a higher 'v'(v=velocity). SIMPLE baby!!!!

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as i posted once earlier, a simplified equation for balance in corner is

m*g*x=m*v^2*d/r

now its simple, lower COG is lower 'd'. now other factors remaining constant, to balance out m*g*x, it gives me a chance to have a higher 'v'(v=velocity). SIMPLE baby!!!!

Not as simple as that. Your equation doesn't address the height of CoM, does it? I couldn't find your ealier post with this formula, so I don't know for sure what all your variables are above, though most of them are intuitive. I'm not sure what x and d are. Can you clarify?

Even so, your formula doesn't address lean angle at all; I suspect it addresses equillibrium of traction vs. lateral acceleration. This formula does deal with lean angle:

m * g * cmh * sin(theta) = m * a * cmh * cos(theta)

Variables:

m is mass

g is gravity

cmh is height of CoM

theta is lean angle from vertical

a is lateral acceleration (force tending to push the bike to the outside of the turn)

Remember that whatever formula you use for this must take "cmh" into account with respect to both gravity and lateral acceleration. Your earlier post on this thread used F1 cars as an example, but the example doesn't apply, as F1 cars don't lean into the corner.

To summarize my point, the height of CoM affects some things, but it does not affect lean angle required in a given turn at a given speed. Hanging off does, because you are moving the CoM of the bike/rider towards the inside of the turn. This certainly allows you to stand the bike up a little.

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Howdy all.

Kalkat's equation was first stated and explained by him in "rule #1" started by scarabrae. if you click on a member's name and click on profile options you will be able to choose "find posts" or "find topics" and see all posts and topics started by that memeber. it took me awhile to find that option again after the site software was upgraded last month.

anyway, quickly, here. the merry go round example is a demonstration of centrifugal effect. not gyroscopic effects. and a better experiment would be to hold the bloody yardstick in your hand at the edge and let go. what happens to the yardstick? does it move outward perpendicular to the merry goround? i thinknot. I think it will move like a baseball thrown from a pitcher tangentially to the angular moment.

a merry go round is a turntable with a permanantly fixed (we hope) axle.

hold a mc wheel on an axle and spin the wheel. even at the weak speed of 5-10 mph i can generate with my bare hand, i can feel the wheel resisting my effort to move it about. THAT is gyroscopic effect. and it increases greatly at speed. hence why it is more difficult to turn in a mc the faster you go. and hence why a motorcycle does not need a rider to balance at speed.

i must apologize for previously interchanging certain words like momentum and inertia. also talking of turning speed, i mean the speed at which you can make the bike turn in. or lean in. or roll in. or flick. NOT cornerspeed.

now, there are two issues at question posed by bozdijar.

1. lean angle vs c/g .

2. speed at which bike turns in vs c/g.

my concern or interest is about the idea that gravity is responsible for the speed at which a bike turns in. i believe when you yank on the handlebars or lightly push on them, that action determines the momentum at which the bike leans over. hence something other than gravity is responsible for the speed at which a bike turns in. and if moving the engine up in the frame helps that, it is not because gravity has a bigger lever. yes, once the bike starts to tip and then the more it tips, the more gravity will have an effect. and it would seem that gravity would direct the action downward, as lateral g's direct it upward, once you unbalance the equation by countersteering, but i think that component of acceleration due to gravity/lateral g is very much secondary to the momentum generated by the act of countersteering. again, this about the SPEED at which a bike turns in. that is what i'm on about.

think of a car rolling over when you yank the steering wheel. yes, it will lean over, just takes more force because it is wide and has four wheels and more stable. a car that rolls when it fails to negotiate a turn is experiencing the same force that rolls a bike. momentum/inertia. not gravity. in fact it is gravity that fights the car flipping as it has a long lever from the width of the car. but the force from momentum and inertia will overcome that. yes, a lower car is more stable. lower c/g = less lever for inertia.

anyway, that's all for now. hope i do more to clarify than to confuse.

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Also...

I think it takes time for gravity to accelerate a falling object. Far more time than can be accounted for in the, what, half a second it takes a fast racer to flick a bike in? I know there's a s/s*2 formula for acceleration due to gravity. I don't have it handy but...again, my gut says there ain't enuf time to accelerate the bike's "falling" momentum with gravity. I also think there is literally tonnes of energy bound up in the foward momentum of the bike and its inertia. So, that feels like the likely force to be tapped for a quick flick.

Like I said before, a ghost bike has to practically stop before gravity overcomes the gyroscopic effects of the wheels and makes it fall down, and when it does tip it seems pretty slow to me, so how much does gravity do to overcome the gyroscopic effects of the wheels of a bike traveling say 40 mph or 90 mph???

One reason I want to read up on gyros is the idea of gyroscopic precession having something to do with this process. But I can't say now.

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It's not gyroscopic precession that keeps the ghost bike up. When the bike starts to fall over the rake/trail cause the wheel to turn to the inside of the turn and will right the bike.

It's inertia that causes the initial "snap" in of lean angle. A high CG will facilitate this. Stand a slegde hammer on either end and kick the bottom... One way will have the hammer falling just like a bike (at initial counter steer), the other will produce a broken toe.

Take that same hammer and balance it on your hand... Now let it fall 5 degrees to one side... With a long handle (high CG height) you'll need to move the base farther to regain balance. With a 2" handle, you'll hardly have to move the base at all...

Now my question is... What are the changes in handling characteristics when already leaned in? I would think that the lower CG'd bike would be easier to make line changes with and would be more stable.

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OK. I'm not sure where to start with all of this. So, I'll go in order of M1's comments...

1. I agree that precession is not what keeps a ghost bike up. Not to be defensive, but, I never meant to imply that it was.

2. The action you describe by the rake/trail catching the bike sounds like a tank slapper to me. Or the action that initiates a bike countersteering back up frm a lean. But not what keeps a ghost bike up.

3. The stabilizing effect of the gyroscopic action of the wheels combined with the trail of the front axle IS what keeps a ghost bike or any bike balanced and traveling in a straight line. Until the bike slows and the gyro effect diminishes enuf for gravity to overcome it and pull the bike off balance...

As for the sledgehammer...

Um...yeah. And if I stand next to a stationary bike and kick the wheels out from under it, breaking my toes, the bike will fall over like a sledgehammer.

I guess. And...?

The mass distribution of a motorcycle and rider is nothing like a sledgehammer. And I'm not sure what you are trying to imply with this analogy. Though, funny you should mention hammers as I'm learning to juggle...hammers...spinning hammers that is.

I'm wondering how far that handle will go before the sledge hits the ground. Will it get horizontal? Vertical? Will it SPIN around the center of mass past vertical and horizontal again before it hits the ground if you kick it hard enuf?

You brought up center of roll in a previous post and I asked you where that was. You never answered.

So I repeat the question. And...

What if the hammer had two handles? Wouldn't this be more like the mass distribution of a bike and rider?

(Or an ice skater turned horizontal?)

Why will a high c/g facilitate "the initial 'snap in' of lean angle"? And what are you implying with the word "initial"? A secondary action of some sort? I mean, once the bike is leaned, what else is there?

Where is the fulcrum for the lever in your analogy if the contact patch is the handle of the sledge?

I've postulated that gravity assists or directs or has some effect...but...

What falls faster...a short handle or a long handle? What falls faster...a 5 lb sledge or a 10 lb sledge with the same length handle? Hint: Nothing falls faster.

Acceleration due to gravity is a constant. Hence, if the short handle hits the ground sooner, it's because it has less distance to travel. And vice versa on the balancing trick. Not sure of the relevance of that analogy either. Are you implying that the distance your hand travels is somehow related to inertia? I don't get it.

Or is it about the lean angle thing vs c/g? Well, what if the hammer has two handles? What if you have a slap hammer and you secure the weight at different points along the slide? I still don't get it. Five degres is five degrees is five degrees. The distance your hand travels is related to the physical length of the handle. Nothing else. Mass is irrelevant.

I have to stop here now. Lunch is over. As for the c/g vs lean angle thing, tzrider has done some research on that...I'm not there yet.

Cheers.

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"Like I said before, a ghost bike has to practically stop before gravity overcomes the gyroscopic effects of the wheels and makes it fall down"

My bad... I thought you wereimplying that gyroscopic precession was the dominant force keeping the motorcycle up until it slows down.

2 - They're all the same. The mechanics of a tank slapper are exactly what keep a ghost bike upright. It just doesn't turn into a tank slapper (hopefully anyway).

3 - Same as above I think.

WRT the hammer analogy, I was trying to make the point that with a high CG, the bottom will move a lot easier. With a low CG, the bottom will require more force to be moved. To answer your question, yeah, the handle would most likely rotate all the way around until it hits the ground again and then the hammer would begin falling to one side. That goes beyond the scope of the discussion though. My only point was how much force it would take to get the bottom moving (which is where our contact pactch is, and is the only location we have to affect change in the direction of our bike).

"The mass distribution of a motorcycle and rider is nothing like a sledgehammer."

I disagree, but that's ok. It's not "exactly" like a sledge hammer balanced on one end, but it's close enough to make the point I was aimed at, which was that the further away from the CM that our tire patches are, the easier it is to initiate lean angle.

"You brought up center of roll in a previous post and I asked you where that was. You never answered."

Sorry about that . I'm not entirely certain, but it's somewhere between the combined CM of rider/bike and the tire patch.

"And vice versa on the balancing trick. Not sure of the relevance of that analogy either. Are you implying that the distance your hand travels is somehow related to inertia? I don't get it. "

Certainly not . I'm implying that with a long handle (higher CG) the CG itself moves a greater distance at 5 degrees of lean angle, and that to bring the tire patches (or bottom of the handle) back under the CM to balance the hammer, it will need to be moved farther than one with a short handle (or lower CG relative to our tire patches). I wasn't talking about inertia there, I was talking about a system being either in or out of equilibrium. The way to change direction on a bike (or, to take a bike somewhere other than where it's current state of equilibrium would have taken it), whether it be mid-corner, corner entry, or on corner exit is to remove it from equilibrium. This analogy shows ME that a bike with a low CG will require less movement of the bars to affect change to it's state to either remove it from or return it to equilibrium. It may be described as some to be "twitchy", but only if they are being ham fisted with the bars.

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OK. As usual, I have only a moment...

Re: #3 previous...

I kinda combined the two things. So, I'll try to separate...

Trail helps to keep the wheel in line with the rear while the bike is upright and rolling forward. Hence, going straight, but, the key is upright. Why does a ghost bike start to lean over while it is still in motion?

Countersteering is accomplished by the application of an "unbalanced" force In physics language, meaning, there is nothing to balance that force. So...what force pushes the bike over? And Why when it slows? But long before it stops? If it was just forward momnetum, the bike could lean over at any time. Or another way to look at it...why doesnt the bike come to a halt and just fall over? What unbalances the equilibrium? And if it is gravity, we know gravity hasn't changed, so what has changed? What force has been removed to unbalance gravity?

If it was the 'trail effect' that was keeping the bike upright, then the thing would be flopping constantly, as the wheel has to deflect to then 'steer it back up' as you say. But doesn't the trail effect keep the wheel in equilibrium unless you push on it? isn't trail what stops a tank slapper? if i let go of the bars during a tank slapper, the slapping stops. it's my weight on the bars that amplifies it, it's gyroscopic effect and trail that dampen it or smooth it out.

i've watched mc's ghost for like a quarter mile perfectly straight...not a twitch. in fact one incident at loudon with an rz350 whose rider got sucked off by the wall onthe front straight and continued all the way down to the starter and ran him over and then continued on. here's to louie. he lived.

by the way, i dont think ive ever seen a ghost bike lever itself back up. or go tank slapping. all the ones ive seen sort of gently curve out, no 'trail effect' pushing them back up. dont you have to countersteer a bike back up by pushing on the bars?

and frankly the more i think about it...it doesn't even make sense. as the bike falls over the trail effect would put the wheel into equilibrium and continue to do so as the bike leaned over more. and unless something disrupted that equilibrium, the bike would continue to curve gently down like all the ones ive seen. steering damper? hmmm...

how many ghost bikes have you seen steer themselves back up?

I'm gonna leave it there. By no means do i have all the answers but i'm pretty sure about this one. Try the experiment i mentioned earlier with a wheel on an axle in your hands...even a bicycle wheel will work. The faster it goes the more it will resist any attempt to move it about. Which also leads to the precession effect, but...i'm getting ahead of myself.

For now, I'll say this about examples, analogies and experiments. I prefer to keep it real. I try to use examples of things that closely mimic the force vectors of a bike in motion. In reality. I try to keep the abstraction to a minimum because frankly, it aint real. I cant reproduce it. Forgive me tzrider, i think your self esteem canhandle being an example..."imagine being able to balance a yardstick on your finger on a merry go round and then imagine theres no atmosphere on earth and then imagine what might happen..."

Riiight.

Hence, I use the car flipping thing, or the ice skater,or the gyro in your hand...things I or someone can demonstrate in real life. I mean, we're already abstracting an abstraction of an abstraction to have this conversation...

So, i get where you're going with the hammer thing, but, it's precisely for the reason I say above that i think it's not accurate and I'm going to continue to pick it apart any way I can. And frankly, i'd really appreciate it if you'd do the same for me. I don't have this clearly sorted and to go to some book and read the answer takes the fun out of it for me. As does quoting someone else because they are published. Thats even worse to me.

So in that spirit, i've asked a slew of direct questions that you haven't answered. Because i say so twice doesn't cut it any more than repeating yourself louder...

cheers

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Newton said...

"Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it."

We counter steer to the right, the bike flops over to the left because all of it's mass would like to continue on in a straight line. We can then proscribe a left hand turn. The height of the CM is effectively defining the length of the lever arm that we use to make this happen. The lever arm would be defined as the distance between the tire patch (where we are exerting force) and the CM. Simple mechanics will tell us that the length of a lever arm will directly affect the efficiency of our system. I think we both understand that.

On to the ghost rider...

I agree... The bike does fall over slowly but you'll find that it never proscribes a constant arc, and if it does a slow wobble, it never goes "straight". It's an ever tightening arc until it slows down enough that the inertia isn't great enough to support the system. It's newtons first law that causes this too. The inertia built into the system as a mass at a given (but slowing) velocity is what holds it "mostly" upright. As the system loses velocity, it has less inertia to hold itself up, but as it leans more the rake/trail turn the wheel inwards as it's designed to and tightens the turn all while bleeding the least amount of energy as possible.

I propose that if you were to lock the steering head straight ahead then a bike would fall over just as quickly at 100MPH as if it was just standing still. Obviously it's gravity that causes this, but I believe that gravity is a very small part of why a bike drops into a turn quickly. Inertia is, IMO, the VASTLY overwhelming force. Once IN a turn and at equilibrium, then yes, it's inertia (or angular momentum?) that is directly countering the force of gravity. Now, again, we get to the low vs. high CG thing... Which attains a higher cornering speed given the same lean angle?

Here's my take... a low CG requires less lean angle with a given speed and velocity. The angle at which the gravitational and centrepital forces are acting through the tire patch is defined by the lean angle. You essentially waste less force pushing outwards when you use less lean angle. Of course, each tyre type will have an optimal lean angle, so the real answer is that you want the CG height to be at the height that takes advantage of the tires characteristics.

"If it was just forward momnetum, the bike could lean over at any time."

It does... constantly... back and forth. It's the self righting geometry combined with inertia and gravity that cause this. Bikes never go straight until after something like 250MPH where they cease to be a counterstering system. That said, they return to being a countersteering system again somewhere above 300MPH.

I've done the wheel on an axle in my hands experiment. I understand gyroscopes. As a side note, it's newton's first law that makes gyroscopes work the way they do as well.

As far as picking apart the hammer thing ... have at it .

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I'm really getting confused...

"It's inertia that causes the initial "snap" in of lean angle. A high CG will facilitate this."

"a bike with a low CG will require less movement of the bars to affect change to it's state to either remove it from or return it to equilibrium."

These would seem to be contradictory statements. But maybe it's apples and oranges...

"The height of the CM is effectively defining the length of the lever arm that we use to make this happen. The lever arm would be defined as the distance between the tire patch (where we are exerting force) and the CM."

If we are exerting force at the tire patch and that is like the end of your sledge handle, and the sledge handle or tire patch is moving out form under the sledge or cm... then where is the fulcrum of that lever?

How much of a bike's mass is in the motor? how much of the motor's mass is in the crankshaft and flywheel? With a human low and long as recommended by the experts for turn in/cornering....just where do you figure the cm to be? Up high on top like a sledgehammer balanced on the end of its handle? Really?

"the further away from the CM that our tire patches are, the easier it is to initiate lean angle."

I agree with tzrider who said something about easier not being the same as faster.

Hint: If the ice skater pulls in their arms they spin faster. Think of a hammer with two handles.

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OK...

What if... there's so bloody much energy bound up in the mass/momentum/inertia of a mc at speed that the length of the arm it has to act on to initiate turn in is practically irrelevant. Like smashing a bug with a sledgehammer, it really doesn't matter how long the handle is. There's more than enough to get the job done no matter how long the moment arm is.

Unlike a car that has a wide base, the bike is relatively inherently unstable and compared to the resevoir of energy available requires very little to knock it over. The only issue is how fast.

And...at the end of the day, what if the centralization of mass might be the defining parameter for turn in speed.

Being that the cm is basically in the middle of the bike and everyone seems to think the wheels deflect yet seems to think of the top leaning over...might it be that like a spinning ice skater turned horizontally that there are TWO moment arms to work with and the center of roll is the fulcrum?

Initiating the turn is one thing. Once the energy is fed in, the speed then generated is another. Like the skater approaching the jumping spin, the arms are extended and wound up to help initiate the spin and lift off then brought in for speed?

So, the longer the arms, the less effort needed to initiate spin. But the slower the roll/spin will be.

The shorter the arm(s), the more effort needed to initiate spin. But the faster the mass will roll/spin.

The closer the cm is to cr, effectively speaking, the shorter the arm(s) will be.

Then again, maybe there's just one arm and the shorter it is, the faster the spin, and, the longer it is, the easier to turn. And the cr is the fulcrum and the closer cm is to cr...it still works faster. Like spinning a propeller by grabbing one end and pulling? As long as the prop is well balanced and mounted at the cm...it will be the most efficient. Hmmm....sounds ok.

So...if a human in racing gear weighs between 150-200 lbs and an engine weighs in the same neighborhood..where is the center of mass?

Where is the bulk of engine weight? Pretty low on the machine if you ask me.

And if the rider is above the engine, then the cm is very much in between them. Especially when the human is low and long.

Hmmm...still sounds ok.

So...where is the force applied? Is any applied at the top? Or the center? Or just at the patch? Hmmm...

Time for bed.

as for precession...ive done some reading and it seems to confirm what ive been thinking. but until i study up on it all, i'm gonna let it lie.

hey, why do stock cars and sprint cars run speedways counterclockwise?????

huh! sprint cars, now why didn't i think of that before. talk about getting airborne and flipping/spinning...ive seen them do a complete 360 in mid air and come back down on the wheels.

everything i say might be completely wrong.....

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I think I get what M1 is saying about the short handle deal...

With a short handle, M1 says it takes less movement of the bars to accomplish the same lean angle.

I've been thinking in terms of time so, from that standpoint...

I believe it would take less time of holding pressure on the bars to move a shorter arm to the same angle.

hence, the hand balancing the handle analogy.

whereby the hand balancing the shorter handle moves less distance to unbalance or rebalance the handle based on how long it is.

The distance is a function of the length of the handle as I said before, and dueo the less distance traveled would take less time with the equivilant "movement of the bars" as M1 puts it.

So, a shorter 125 GP bike flicks faster than ZX10.

But, does that mean if you lower the cm of a ZX10 it will flick faster?

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WRT the higher CM making it "easier" to initiate lean... That's all I was saying . Not that it would also be inherently faster.

The fulcrum would of course be the roll center. Look at a teeter-totter... the fulcrom is the roll center. Of course, ours isn't attached to anything in particular, but it's there.

"With a human low and long as recommended by the experts for turn in/cornering....just where do you figure the cm to be? Up high on top like a sledgehammer balanced on the end of its handle? Really?"

No. My guess would be just a few inches above a line drawn from the top of one tire to the top of the other in side view.

I certainly see your point with the two handled hammer thing but it didn't serve the purpose of the point I was trying to make. I wasn't assuming that the RC of the hammer was at the hammer head.

"I agree with tzrider who said something about easier not being the same as faster."

Me too .

"And...at the end of the day, what if the centralization of mass might be the defining parameter for turn in speed."

Centralization is obviously important but of course you'll run into the law of diminishing returns with that and the gyroscopic properties of the crank shaft will need to be looked at as well. That said, it's all a trade off. Larger flywheels add stability. It's all a package... Which is why I think that "at the end of the day"... The CG height needs to be placed where it needs to be placed to maximize traction when leaned greater then 45 degrees. I could care less about "lever effort". I care about quick. That's why I ride an XB12R. It is a little harder to snap into lean at higher speeds, but it does it very quickly if you use your muscles. It's still a whole package though... radical geometry for sharpness, large flywheels to stabilize, VERY centralized mass etc...

"So, the longer the arms, the less effort needed to initiate spin. But the slower the roll/spin will be.

The shorter the arm(s), the more effort needed to initiate spin. But the faster the mass will roll/spin.

The closer the cm is to cr, effectively speaking, the shorter the arm(s) will be. "

I would agree with those three statements. There's still a fine line though... You don't want a bike that takes so much lever effort that it becomes difficult to manage the strength that you exert, or makes you prone to using both hands to initiate lean.

"So...where is the force applied? Is any applied at the top? Or the center? Or just at the patch? Hmmm..."

At the tire patch obviously, but also from the top (your other arm ). We "should" hang off well before we initiate turn in. This gives gravity a longer lever to pull the bike over using our mass. Forces applied in the center... Twisting forces I would imagine. Torque applied to the frame by the steering head and running through to the swingarm.

"hey, why do stock cars and sprint cars run speedways counterclockwise????? "

Because most people find it easier to turn left.

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I guess my main hitch is that it still feels like a running man tripping and "falling flat on his face". Like there's a fulcrum at the feet and his momentum is what slams him down. Even if he was running on the ceiling with gravity boots he would still "fall" up. N'est-ce pas?

As long as forward momentum/mass/inertia was enough to overcome gravity. So, some math might be handy there. But to turn that around for the bike that isn't on the ceiling...

Actually, it might be more like the running back or soccer player who plants his foot to initiate direction change...

And, frankly, I don't think the wheels deflect nearly as much as the top of the bike.

But that's a perception as a rider. Not an observation from head on...so...???

In any case, yeah, I'm thinking about the steering head thing and how rake plays in with that hinge and what lever effect might be happening there,and, still thinking about gyro precession effect and the gravity component...but still trying to gut it out so to speak intuitively in my head...but at the end...I'm afraid I might have to draw diagrams and calculations.

But like circuit analysis, i keep thinking i can get a grip on one system of vectors at a time in my head...

And then integrate, if i could just nail down the most important system or anchor to work from, but the whole bloody thing is in motion!!! Which still feels like the key to me. inertia of momentum. so, i'm working from the assumption that everything else will be less than that. ass u me...

Are you SURE about the turn left thing about speedways? I know Johnny Stock Car is used to turning left but as a professional, i think he could handle turning right...although, most of those boys have some difficulty at road courses, i think it might have as much to do with setting up the suspension ...I mean, seriously...I can see the left turn issue for bike riders but...in a four wheel vehicle, you don't have to freak out about lean angle vs traction or anything...no, i cant buy that. i remember the feeling of being uncomfortable in right turns on a bike...and i never felt that in a car, even on track.

What does precession do to a car with an engine mounted front to back with a crankshaft spinning one way ...or another? I realize we're about bikes, and the crank in a car is a lot heavier but is a BMW bike any different to ride tha a bike with a horizontally mounted engine? I know you don't believe in gyroscopes like i don't believe in gravity, M1, but....humor me...

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Racer, I don't know if you've gotten an answer to your question of where the fulcrum is. M1 said something about it a few posts back, but your subsequent posts make it seem you're not certain where the fulcrum would be. Getting agreement on this is going to be a key to understanding the rest of the discussion, I think.

M1 stated that the roll axis is somewhere between the CoM and the contact patch. For practical purposes, I think the roll axis is almost exactly at the CoM. Wherever it actually is, this point is the fulcrum you've been asking about. When you apply force to a lever connected to a free body (such as the handle of a hammer balanced on your hand), the fulcrum will be the roll axis.

BTW, as you do, I like to keep it real too. Analogies are good though, because they can help isolate various pieces of a more complex system to understand the dynamics of each aspect independently. You've used a few analogies yourself along the way and seem to find them useful within limits too.

In your trip-the-running-man example, I would suggest that the fulcrum is his CoM. The lever end is the point at his ankle where you trip him.

This continues to be interesting. Any comments on where the fulcrum is?

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Interesting... I was looking at the running man from a completely different direction... I would ass u me that that fulcrum is where you trip him and the end of the lever arm would be his CM. The fulcrum is also the RC (ass u ming a rigid skeleton...).

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Yeah, my intuition keeps telling me the front axle is the deflection point and that the rest flows from that somehow, but im not quite ready to rectify that with the mass issue vs roll center unless of course moving the rider off the bike lowers the roll center....hmmmm...

but im trying to get there with some kind of reasoning instead of what just feels right.

its kinda like working backwards for me and filling in the gaps that a real scientist or engineer gets to step by step which has the advantage of being backed up by proof. but, i once said that you can do it without math...so, im trying not to eat my words...or choke on them...

and thanks for your input. i agree that the fulcrum is the key.

nothing personal about the analogy thing, my point was things i can reproduce in reality. I really can juggle the hammer. but i have a hard time with pretending there's no atmosphere , or actually performing an imaginary experiment to prove a theory. and no, i cant kick the wheels out from under a bike, but that was my whole point. (albeit a bit cheeky) and i think i can perform the kick the sledge thing with a stiff pair of boots and a light sledge. i was getting confused with the sledge on top of the hammer and i felt that the handle alone would be better to illustrate the length of the lever...separating it from the mass issue for a minute. and i tried to put that analogy in M1's voice and then use my own example of real bikes...125 vs zx10. but...ill allow that i may need to use an imaginary one if youll accept that only a govt scientist with access to avaccuum chamber can perform the merry go round and even then, to balance the stick while the thing is moving would be a superhuman feat to start with...or we can move on as if...

and i think you can tell that im thinking a bit beyond what im on about...not because i dont want to be wrong, tho i dont wanna be any wronger than i have to be, haha...i dont want to confuse the issue any further than necessary or have to backtrack and undo a whole line of thot, which im already doing. but better to sort one thing for certain in my mind then move on.

btw being wrong is good thing in my book. beginning of all learning. Why would i want to step in the ring with someone whocouldnt beat me? what would be the point? winning? what does that teach you? what does one learn from the easy path? or a soft comfortable life? be a friend and fight me. give me your best. (or play chess) show me my weak spots where i need to improve...but i digress...

anyway, nice hint with the 'ankle'.

cheers

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I think the problem that we may be running into is that IMO there isn't a "answer" to whether the CG should be high or low. I think it depends on what you want out of the bike. I think a bike with a high CG will feel like a "light steering bike" and a low CG will feel heavier. The "answer", as I've said before, is that you define what you want the bike to DO first, then you go about getting there by tuning rake/trail, CG height, wheelbase, angular relation between the swingarm and rake, bar leverage, frame stiffness, seating position, suspension setup, wheel size, etc, etc, etc...

The ONLY other thing we can do IMO is decide what the effect of raising or lowering the CG is when leaving everything else alone.

That said... I ride an XB12R. It's widely regarded as one of the best handling bikes you can buy (like top three, and most places put it AT the top, but really , I'm not here to bench race ) and it's got one of the lowest centers of gravity of any production bike and it's mass is EXTREMELY centralized. It also has a very steep rake angle and very little trail and heavy flywheels when compared to most other bikes so I really can't tell you which of those is "most" responsible for the good handling. As I said above though, I suspect it's the entire package...

In any case... The roll center should be between the combined CM and the bikes CM... Probably very close to the bikes CM. It would make sense to do it that way because that should allow the bike to be as light feeling as possible because it's rolling around a spot very close to it's own CM.

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Yeah, what he said.

I'm not after what makes a bike "handle well" as that can be subjective.

I have a specific goal of working toward answering bozdijar's question of relating turn in speed to c/g.

And, at bozdijar's unfortunate expense, choosing to sort it all out myself without any books, and doing that in a public forum. And enlisting the discourse of others along the way.

Without going further into why I choose to do this when I can easily find the information without a library card is another matter of a personal nature.

I imagine, in addition to my personal goals, this entire process might be very interesting from an educator's point of view, or, how shall I say, someone interested in how a particular mind works or how the process of communication might be analyzed and/or improved. Or how to go about finding the source of a misconception and then custom tailoring the effort to bridge the gap, so to speak...

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nothing personal about the analogy thing, my point was things i can reproduce in reality.

I don't offend and you're being nice anyway. No worries, my friend.

ab

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