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Center Of Gravity High Or Low


bozjidar

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I hear a lot of people talking about lowering your center of gravity when going around corners. Yes, lower center of gravity helps you turn faster, but higher center of gravity gets you around the corner with less lean angle. I see a lot of people confusing getting down and low with leaning off. Many people would be better off with higher bars, so that they can sit up more when going around corners, keep their center of gravity higher, and reduce their lean angle.

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Guest waynedunham

I hear a lot of people talking about lowering your center of gravity when going around corners. Yes, lower center of gravity helps you turn faster, but higher center of gravity gets you around the corner with less lean angle. I see a lot of people confusing getting down and low with leaning off. Many people would be better off with higher bars, so that they can sit up more when going around corners, keep their center of gravity higher, and reduce their lean angle.

 

I agree! This is why I prefer my naked bikes and the higher bar! During my last session with CSS in Class 4 at VIR, Cobie helped me so much. He told me that instead of my flopping all over the bike(which was scaring the hell out of him!) before I got into the turn, to stay more on top of the bike. He said it would decrease my lap times and it did! Cobie got me to work at stopping my flopping all over the side of the bike and going into the turns too fast.

His words of wisdom were to set up before the turn, by staying planted more on the seat and go "In slow and out fast!". By Golly if my lap times didn't improve and I started having more fun! I finally saw Da Light!

Thanks Cobie!

I still want to know if you would take this 66yr. old body on to train as a "track coach"! I told you that I

will work for nothing! What the hell, I'm retired, want to have some fun, and learn from the Master.

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I've been wondering about this too. It appears to me that riders are perched up real high on Superbikes and such. The seat looks to be level with the handlebars and the pegs are, lokks to me to be, about 4 inches higher than production bikes. I think the gas tanks are shorter but is the seat higher or are the bars shorter. I think overall the bikes are lower meaning the whole bike but, in conjunction with the wheels is the seat higher. I'd like to compare the geometry specs with stock specs. Vedy Vedy intedesting!

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My first observation here: Different bikes require different riding styles and my second: Different riders prefer ever varying riding styles.

 

Superdave88: Superbikes have more or less regular seat and bar heights, as they have to be homologations of the road bikes they are based on. that said, GP bikes are very much the same heights, therefore proving the manufacturers have it right for supersports bikes.

 

Footpegs are raised from standard road position for track racing on all but the strictest one make racing series. This is to increase available ground clearance when leaning the bike over on the track. Regular supersports road machines are built slightly lower for comfort on long runs and for Joe Public. If you have ever sat on a real race bike or a road bike modified as such, you'll get leg ache after an hour. Footpegs do assist in using the whole body as a steering lever, but I think that's too technical for this question and a bit off subject.

 

The idea of a lower centre of gravity providing quicker turning is true, you can't beat it. It is a basic lever effect. (Lesson in physics there!) If you are really on the pace on a track day and the bike is being riding to within a millimeter of loosing traction completely, then you need as low a centre of gravity in corners as possible, to reduce stresses on your tyres. Hanging from the inside of the bike helps to get you body weight lower and (more importantly) closer to the centre of radius of the turn. This really does help, trust me! For example, go to an Ice Rink somewhere and watch some pro Speed Skaters. Watch their body stance in the corners. They lean so far over that they drag their inside hand on the ice. (to gauge their lean angle, much the same as sticking your kneeout on the track) In comparrison to bikes, they could try leaning the blades but no leaning with their bodies. I can assure you they would have to go alot slower to stay upright. They lean their bodies right over to get a lower centre of gravity and to get the majority of their weight closer to the inside of the turn, to allow gravity to slightly assist in getting them round the corner quicker. The same rules apply for bikes. Why do you think a MotoGP star has never tried sitting up more or using upright bars during a race in modern history?

 

Having higher bars and staying more upright (keeping a higher centre of gravity) in the corners will give you much more confidence in the twisties, I won't deny that. I used to ride a Honda Hornet CB600 and it was very reasuring. I found it to be the same on a Yamaha FZ600 Fazer. This is because you have a wider bar to lever the front wheel with, giving you easier control over steering. The high position keeps you further from the floor, assisting with nerves and the natural desire to stay alive.

 

Waynedunham: I believe, from what you have said below, that you were slightly un-nerving your instructor with your hanging off style, possibly not something you do naturally on the street? Basically, by telling you to stay more upright and focus on the corners and set up (in slow out fast), you lost several seconds per lap. You learnt something very important there - Keep it simple! By cutting out all the effort and work of hanging off and trying to sort everything else out as you approach a corner, you were able to focus all your thoughts and feelings on the deceleration, down shifting and cornering itself, therefore becoming much more relaxed on the bike. This helped you get round quicker.

 

Now, enter Mr Valentino Rossi on your bike at the CSS. He hangs off on every corner. Assuming he has set a few control laps at speed, you tell him to stop hanging off and keep himself mostly centred around the seat and try to keep his lap times the same. Imagine what would happen. He would probably be seen braking very harsh into corners, back wheel sliding and bobbing off the floor every where. He would be seen carrying huge amounts of lean angle into and through corners and getting so hard on the gas that the bike would be spinning up everywhere. His lap times would also be slower, "for sure"! Racing has proven that lower, shorter bars provide better front end loading, better aerodynamics and better riding position. (for racing at least). The relatively high seats (in comparrision to the bars) assist in getting weight over the front and allow the rider better feel for what the rear tyre is doing. The high footpegs are for ground clearance more than anything. Racers hang off for a lower centre of gravity. Proven fact I'm afriad!

 

Just as a quick sum up: Everyone rides differently. High bars and higher centre of gravity makes cornering better and quicker for some (less to think about) but not for others. I personally get a feeling of running wide and imminent death if I don't hang off my bikes. Racing has proved that when really pushing a bike and tyres to the limits, hanging off and lowering the centre of gravity, as well as low bars and high footpegs is definitely the way to go for faster lap times. One last fly to throw into the ointment - Supermoto bikes! I recently watched them compete on the track against Aprilia RSV1000's, CBR600RR's and a ZX10R Kawasaki. The Supermoto's were right at the front and came first and third. The riders of the supermotos stayed upright in the seat and leant off the wrong way whilst the supersports riders hung off. Does this mean they have it right or does it mean they have different machines which require different riding styles? You see, this is an endless debate if you are not considering different styles of bikes. Not neccessarily saying anyone in here was, I just decided to go for it on this one and write down all my feelings, with some of the more complex Physics left out. lol

 

Hope I didn't bore any of you too much?!

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I hear a lot of people talking about lowering your center of gravity when going around corners. Yes, lower center of gravity helps you turn faster, but higher center of gravity gets you around the corner with less lean angle. I see a lot of people confusing getting down and low with leaning off. Many people would be better off with higher bars, so that they can sit up more when going around corners, keep their center of gravity higher, and reduce their lean angle.

 

 

Really? Where did this info come from. I was under the impression that a high CofG allows a bike to turn quicker (fall in faster) and a low CogG allows for less lean angle.

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Guest waynedunham

Andy, I found your comments of interest! However, I have been under the same impression as Stuman.

My observation while at CSS was that two of the fastest instructors there don't hang off much.

I may stand corrected, but regarding the above two guys, I don't think that James or Will hang off that much!

Kevin Kane, old buddy, if you are out there reading this thread, "What say you"

Wayne

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Wayne,

I have observed that some instructors do not hang off as much but I believe it is because they are not going at a pace where they "need" to hang off more. Where you or I may believe we are going fast and just barely keeping it on the track, they are typically not working up a sweat to keep up with us (well at least me). If you have the opportunity to see them light it up like I did at CodeRACE two years ago - you will see that they all can and DO hang off at those speeds.

...my 2 cents.

 

Kevin

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Really? Where did this info come from. I was under the impression that a high CofG allows a bike to turn quicker (fall in faster) and a low CogG allows for less lean angle.

 

The height of CofG makes no difference at all to the lean angle required to get through the turn, assuming the speed and line remain the same. Lowering the CofG on a motorcycle makes it easier to turn quickly by reducing the moment arm from the CofG to the contact patches. Kind of the same thing manufacturers talk about with regard to mass centralization; the closer all the mass is to the roll axis, the more easily you can turn the bike.

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The roll center (or roll "axis" if you prefer) is not at the contact patch, and the LONGER the moment arm is the easier/quicker it'll roll in.

 

The cg height certainly DOES affect the lean angle required to get around a turn... More importantly the COMBINED cg height matters as well.

 

Draw a free body diagram and you'll understand.

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Being that we're so close to the winter olympics...

 

I find myself thinking of a figure skater spinning in place with their arms extended. What happens when the arms are brought closer to the body? The skater spins faster. Hmmm. However, in this picture, the skater is ALREADY spinning when the arms are brought in. (inertia/angular momentum, etc) Where are the arms when the skater INITIATES the spin? Are they extended? Are they extended more before a spinning jump that requires more effort?

 

How much effort needs to be expended to turn a motorcycle? Can a rider expend a bit more effort to capitalize (small investment/big return) on a lower c/g (or c/mass being closer to c/roll)?

 

If the c/r is not at the contact patch, is it advantageous to think of a motorcycle 'falling' into a corner? And, if the c/r is not at the contact patch, where is it?

 

If one is travelling perpendicular to a gravity field, what happens to 'weight'? When you run, do you feel lighter? In addition, considering gyroscopic action, how much does a bike at speed really 'fall'?

 

 

 

I agree that the COMBINED 'c/g height' (of bike and rider) is paramount...

 

With regards to getting around a turn at a particular lean angle and speed (for a given bike/rider combination), something about a sidecar monkey tickles the back of my brain...

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The roll center (or roll "axis" if you prefer) is not at the contact patch, and the LONGER the moment arm is the easier/quicker it'll roll in.

 

The cg height certainly DOES affect the lean angle required to get around a turn... More importantly the COMBINED cg height matters as well.

 

Draw a free body diagram and you'll understand.

 

 

Yeah, I was a little off base with my comment above, I was half right. I did a little research last night and found something pretty interesting.

 

1) High CofG will help a bike turn quicker as explained in the quote above. A good example of this is the changes made to the '06 ZX10R. Kawasaki wanted to make the bike turn in faster, so the raised up the engine in the frame to raise the CofG. When you turn a bike you basically steer the wheels out from under it and it falls over, the higher the CofG, the longer the lever gravity has, the quicker the bike falls. So I think I had this part right before.

 

2) The interesting thing I found last night reading through Tony Foale?s book is that a lower CofG requires more lean angle then a higher CofG. My misunderstanding was that I thought when you hang off a bike you were trying to lower the CofG so that the bike could remain more upright. But that is not how it works, when you hang off you move the CofG to the inside of the corner and that is why you don?t have to lean the bike so much, not because the CofG is lower but because it was moved left or right.

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OK. If one basically steers the wheels out from under a motorcycle, is the bike falling toward the ground or rolling along the z-axis? And is it doing this because of gravity????? I think not. As I have stated my opinion before, I believe gravity may assist, but, I believe the overiding force responsible for a bike leaning into a turn is the force applied at the handlebars. As I can overcome gravity and gyroscopic force by standing the bike back up.

 

If the bike "falls" into a corner, why does it not hit the ground? Must you "catch it"? Or does it go over as far as you push it, a controlled roll?

 

Did Kawasaki raise the motor to raise c/g ...or to move c/m closer to c/r?

 

I mean how much did they raise it? A couple inches? And THAT is going to make a BIG difference in how fast it flicks because of gravity having a bigger lever???

 

Hey M1! Where is c/r?

 

 

 

Which brings my next question...

 

Why does a lower c/g require more lean angle?

 

 

 

I don't know. My brain has been wrapped around sailboats and airplanes and I only have a few minutes to think about this, so, please set me straight at your leisure or I will track down this Tony Foale's book and edumacate myself. And maybe I'll grab Twist II and Soft Science while I'm at the library. About time I give them a read as well.

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Hmmmm....

 

"The interesting thing I found last night reading through Tony Foale?s book is that a lower CofG requires more lean angle then a higher CofG."

 

A lower CG wil have a shorter moment arm. Therefore greater force will be needed to overcome the force of gravity (or a longer arm). The force being the centripidal variety being exerted towards the outside of the turn (speed causes this). You can either use LESS lean angle to increase the length of the arm, or you can use more speed to exert enough force on the arm you have. Keep in mind though, that your limit is traction. There's no point in designing a bike that's a great deal more capable than it's tires. My thoughts seem to contradict what you are saying Tony says... There must be some confusion somewhere...

 

It is my opinion that "within reason" there are no benefits to moving the CG up or down. The CG needs to be at the "correct" height. That "correct" height will be defined by the available amount of traction and lean angle. It needs to be in a place that is complimented by the other factors. Of course moving it will have an effect on the way the bike feels (higher CG should make a bike more flickable, lower CG should make a bike feel a little more stable and should find equilibrium at less angle of lean with a given speed).

 

You can't isolate one area. Other things inherent in the geometry of a motorcycle can have the same or similar effect. Less rake makes a bike more flickable as well. Larger flywheels can make a bike more stable.

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The roll center (or roll "axis" if you prefer) is not at the contact patch, and the LONGER the moment arm is the easier/quicker it'll roll in.

 

I understand your point above, but would point out that "easier" might not always equal "quicker." I assume we agree that the roll axis is at or near the center of mass when we're changing lean angle and that the lean angle change comes about as a result of deflecting the tires one way or the other under the CoM. With a longer moment from the contact patch to the CoM, there is greater leverage on the mass of the bike/rider to make it roll, but the tires must also deflect farther to cause a given amount of lean angle change. By contrast, lowering the CoM will give you less leverage on the mass of the bike, thus making it feel harder to turn, but will also mean that the tires don't have to deflect as far to cause the same lean angle change. You could probably reach a point of diminishing return either way by raining or lowering the CoM too much. When you lower the CoM, the bike should turn more quickly, assuming the rider is stong enough to work with the reduced leverage. When you get to the point that the rider cannot press the bars hard enough to turn the bike faster, you are completely right that raising the CoM would allow the rider to turn the bike faster.

 

 

The cg height certainly DOES affect the lean angle required to get around a turn... More importantly the COMBINED cg height matters as well.

 

The height of the CoM really doesn't matter at all. Assuming you don't shift weight from side to side, as in hanging off, the only variables left that determine lean angle are speed and turn radius. The speed of the motorcycle squared divided by the radius of the turn equals the lateral acceleration of the vehicle.

 

When a motorcycle is leaned into a turn, lateral acceleration is acting on the CoM to stand the bike up, while gravity is acting on the CoM to make the bike fall into the turn. The bike is in equillibrium when the lateral acceleration and gravity are equal. See the following diagram:

 

Height_of_CoM.jpg

 

On the left part of the diagram, the leaned object has a high CoM, which lengthens both the moments of gravity and lateral acceleration. The diagram on the right shows a lowered CoM at the same lean angle, and you see that the moments of both gravity and lateral acceleration are shorter. If you'd prefer to look at this mathematically this thread on the Cycle World forum has a good proof from "DataDan:" Link

 

Here is an excerpt from the relevant post:

 

One reason a motorcycle must lean into a turn is to counterbalance centrifugal force tending to highside it. With the motorcycle leaned over and going around a constant radius turn at a constant speed, the moment produced by centrifugal force and which tends to highside the bike is:

 

mh = m * a * cmh * cos(theta)

 

where m is mass (rider + bike), a is the lateral acceleration due to centrifugal force, cmh is the height of the center of mass, and theta is the lean angle from vertical.

 

The moment produced by leaning, which tends to make it lowside is:

 

ml = m * g * cmh * sin(theta)

 

where g is acceleration due to gravity.

 

The motorcycle is in equilibrium?neither highsiding nor lowsiding?when the highside and lowside moments balance out. The equation quickly simplifies:

 

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

 

a / g = sin(theta) / cos (theta) = tan(theta)

 

In words, the lateral acceleration expressed in units of g, acceleration due to gravity, must equal the tangent of the lean angle.

 

Lateral acceleration depends on speed v and turn radius r, so the lean angle can be expressed as a function of speed and turn radius:

 

a = v^2 / r

 

tan(theta) = v^2 / r*g

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Hi, one and all,

I have read the replies to this topic and must admit to being more confused now than at the start! As a 61 year old returning to bikes after my Norton Dominator back in 1961, age and old bones prevents me from leaping about on my GSXR1000 approaching corners. My observations are of a more practical nature. Following my 25 year old son on his SP2, dodging the trail of sparks from his knees, but still remaining in touch with him, leads me to believe that "getting one's knee down" is highly overated. I wonder if Rossi would have been faster than Mike Hailwood or Agostini had they been around together! I firmly believe that a myth has evolved regarding hanging off and probably suited a particular type of rider. Whats the betting everyone will just as quickly stop doing it when I become world champion!!! I don't understand anyone having a problem in getting a modern superbike to "turn in", I do the normal, natural, countersteer and the bike throws itself into the lean angle required. Maybe the bikes too slow and I should look around for something more challenging!!! Just joking, I really enjoy all of the opinions expressed. Gixergeezer!!

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There's no such thing as centrifugal force. Nor does it cause highsides. :P

 

When describing an arc, due to rolling around the inside of a drum, the inside of a gravity well, or the inside of a mc tire...inertia will cause a centrifugal effect.

 

Highsides are another effect of inertia. Usually the inertia from sliding sideways being suddenly released by the rear wheel hooking up and the compressed rear spring adding its energy. I can highside a bicycle from a crossed up nearly straight up and down position traveling in a straight line using rear brake. No 'centripedal' anything.

 

Why does a car roll and flip when you yank on the steering wheel hard enuf? Gravity?

 

Will it roll more/flip sooner if it's taller or top heavy like a van? Yes. Is that because of gravity?

 

Why does an ice skater extend their (moment(um)) arm before a spinning jump? Where is that momentum coming from? Inertia.

 

What is countersteering? Why does it not work at slow speeds? Perhaps because there isn't enuf lateral g "centrifugal force" (INERTIA) to knock it off balance?

 

Why doesn't it work at hyperspeed? Perhaps because the gyroscopic forces of the wheels are so high they restabilize the machine enuf to overcome inertia?

 

It seems to me that the driving force of knocking a bike off balance into a turn is inertia. Countersteering up again would also be driven by inertia.

 

I'm thinkin that lever is being acted upon by inertia. Maybe once it's over, gravity helps or things balance out....but...

 

Sorry, my gut just can't get beyond the mass of a mc moving at speed being less force than 1g at 30in. What's that deal...hitting a brick wall at 40mph is like jumping off a ten story building...or something like that?

 

What takes longer? A standing still bike tipping over from gravity, or Rossi flicking it in?

 

My imagination says the static bike will take longer...

 

But what do I know?

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And...Rossi is overcoming gyroscopic forces to boot. Gravity? Riiiight.

 

How long does it take a ghost bike to fall over? Seems like a slow motion replay to me. Going, going, going...going...still going a little...and...boom, gone.

 

Once the bike is tipped it seems gravity would come into play, perhaps with diminishing effect relative to speed (inertia) or the force with which you countersteer...until you release the bars, when the front wheel finds equilibrium and starts to track, driving the forward momentum to the outside of the turn to create lateral g's and balance gravity...

 

So, what happened to the momentum (inertia) of the bike's downward 'fall' into the turn? Is that energy translated or balanced by "lateral g's" but doesn't that leave something left over? If those g's balance a certain speed and lean angle won't the downward inertia require more to catch? Hmmm...

 

Time to read up on gyroscopic force and the mysterious world of angular momentum...oo aaa.

 

Any physicists or engineers out there? I'm just going by my gut here. Please correct my thinking before I confuse everybody. Everybody else that is... :blink:

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Well, I'm going to have to look at this over the weekend, do a little studying I think.

 

Some of the points I will comment on: regarding riding and hanging off---it's a skill and has uses, but I would not rank it above the other skills taught say in level 1. I have seen riders go very, very fast w/out hanging off. Ultimately one is going to be able to go faster (as the bike would be leaned over less) by a well-executed hang off.

 

Sometimes hard to convince guys, but get the basics into play first, then add the fancy stuff. On that note, when done, many also hang off too far. Rossi doesn't.

 

Cobie

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I dont know why we always end up going tangentially into theory all the time. probably too much knowledge and too little application.

i dont know much, and i dont care for much. All I care for is speed. watever gives me higher speed thru the corner is the RIGHT answer. and answer in this case is LOW.

i dont know how to explain it well, but they dont make a formula1 car 100m high, do they?point here is that u want to MINIMIZE the force that pulls u outwards, bcos other things constant, u will get a chance to have higher speed then!

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Time to read up on gyroscopic force and the mysterious world of angular momentum...oo aaa.

 

I'm a little confused by your last couple of posts -- not sure what issue you're addressing. If you happen to be talking about lean angle while in a turn, I don't know that reading up on gyroscopic force will help.

 

Here's an example that may help distinguish the notion of equillibrium while leaning in a turn from anything to do with gyroscopes. Imagine that you are standing on the outer edge of one of those big turntable things you sometimes find in a child's playground. You know, they're about 12 feet across, have handrails on them and you can run around to get them spinning and then ride them until you vomit. Let's say you're riding one of these while you're spinning and decide to balance a yardstick on your finger. If you could manage to balance the yardstick (let's pretend that the breeze wouldn't make this next to impossible), would the yardstick be dead vertical or would it be leaning in some direction? Why?

Edited by tzrider
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The tires have a sweet spot. In lean angle, slip angle temperature etc. That's exactly where you need to be for best speed. Of course... That's obvious which is why the conversation is about CG height. The CG height needs to be at a height that allows you to take best advantage of the tires sweetspot. That's the answer. There's no real advantage to moving the cg up or down other than that. It needs to be in one place for flickability, it needs to be in another place for maximum traction. You move it to that other place by hanging off.

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I was addressing the idea that a higher c/g gives 'gravity' a longer lever so a bike will turn faster. Stated by someone regarding kawasaki's changes to the ZX10. And playing with the idea that gravity might not be the significant factor in turning faster. Znd that perhaps that moment arm has more to do with the momentum of forward inertia...as opposed to gravity.

 

AS it relates to lean angle and the Bozdijar's questions and the answers provided by you?

 

Quoting someone named datadan from cycleworld post?

 

I was too tired to play with his math last night but the bottom line seemed to be that the sideways force balances the down force...hence the bike don't fall over and c/g height is irrelevant. I have more to say about that but no time now.

 

In any case, he Dman, made some statements that I disagree with. Like Centrifugal force causing highsides...I was being a bit of a wiseacher, but I think it's important to be accurate about all that. And why I don't think gravity is the force behind countersteering a bike which may or may not have much to do with c/g affecting turn speed...but whatever the answer is ...I think it matters to know what the deal is.

 

Was it really so much nonsense? In any case, I just logged on to see if anyone had any direct answers to any of my questions or refutations to my ideas. I don't have time to go back and read thru it all just now as I'm out the door for my one night out a week...yipee.

 

Cheers.

 

 

 

 

Not that my "theory" was well developed or anything. Just more like thinking out loud.

 

Hi Kal kat. Yeah, you right. I can't help myself. pushing my limits. :D

 

b cool

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last word walking out the door til later...

 

by no means do i have a proof for any of this...it just sounds good to my ear or feels on track to my gut. but truthfully, i might as well being trying to sell you some real estate in florida. hence why i ask for the input of an expert to sort my meandering late night stream of semi consciousness. just rather try to think it thru than quote a quote of a quote of ...

 

i'd be grate ful if someone said "you're spam dude." the forward momentum is all conserved and sorted to corner speed and here's the math to prove it. :)

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