High COG...good or bad - BMW Luxury Touring Community
 
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post #1 of 40 Old May 18th, 2007, 8:44 am Thread Starter
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Question High COG...good or bad

High COG = Bad

I thought I'd post this because there was some discussion about a high COG actually being a good thing.

I remember reading an article in Cycle World that explained this. It was a full page long with lots of formulas and technical information, but on the next page was a very simple and easy to understand explanation. Here it is in a nut shell. Stand in front of your bike and mentally draw a line vertically starting at a point where the tires contact the ground. Are you drawing? Good. Now stop the line at the same height as the handlebars. This is your lever arm. Now imagine this lever arm weighs 800 lbs. Now imagine that 795 of those 800 lbs. are 1/2” off the ground. This is a very low COG. Now reach out and grab the handlebars, I mean the top of the lever arm, and jerk it as fast as you can right to left. Are you jerking? I knew that you where. Fast, easy and requires very little effort. You can STOP jerking now.
Now imagine that 795 lbs. is ˝” from the top of the lever arm. A very high COG. Now jerk that! A good doctor might be able to pop that back into place.

The point of the article was, all other things being equal; a bike with a lower COG will handle better than a bike with a higher COG.

Now to really stir the pot; when do you think your COG is lower: standing on the pegs or sitting on the seat?

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post #2 of 40 Old May 18th, 2007, 8:53 am
 
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Wink

Now to really stir the pot; when do you think your COG is lower: standing on the pegs or sitting on the seat?[/QUOTE]

Standing on the pegs...
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post #3 of 40 Old May 18th, 2007, 9:07 am
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In the pegs!

Not to burst your bubble but by the same argument on easy to move it. To maneuver you don't move the TOP of the bike (easy in low CG) rather you move the BOTTOM of the bike (easy in high CG). Try it the next time you are out. Hit the bars and see what the bike really does in the corner. You will find the wheels move out from under you to establish the lean angle.

That is why a high CG is more maneuverable. It makes the bike more unstable and and the less stability you have to over come the easier it is to maneuver.

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post #4 of 40 Old May 18th, 2007, 9:25 am Thread Starter
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Do three rights equal a left?

At some point in this turn you have to right the beast and sometimes (very quickly) go the other way. A low COG is what makes a bike “flickable”. Try it the next time your out. Go right and left.

Standing on the pegs is correct. I am always impressed by the difference in my KTM when riding on the pegs though a really nasty section.

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post #5 of 40 Old May 18th, 2007, 9:33 am
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Quote:
Now to really stir the pot; when do you think your COG is lower: standing on the pegs or sitting on the seat?
Quote:
Originally Posted by steamboatjohn
Standing on the pegs...
Is that because your weight bearing onto the bike has been transferred to the pegs, which are lower?

Following up on that, while the bike's CG may be lower at that point, would it not be fair to say that the rider's CG is higher, making the rider less stable on the bike?

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post #6 of 40 Old May 18th, 2007, 10:09 am
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+1. great thread. More or less what I've thought to be the case.

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post #7 of 40 Old May 18th, 2007, 10:12 am
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With all of your weight bearing on the pegs, you still do not displace the mass of your tourso....it's a movement and "moment" issue. The theory hold true in aviation geometry regarding weight and balance. It is where the mass is located that determines the COG.

The lower your butt is on the seat, the lower the COG...remember that Gravity is constantly pulling perpendicular to the ground...so if you are standing on the pegs through a corner, gravity is pulling just as hard (albeit harder) on your upper body, and upsetting the intended balance of the motorcycle.

You don't see superbike riders standing on the pegs going through corners in a race, do you ?...they get as low as possible, and sometimes hang their bodies off the side of the bike to "lower" the COG,and apply more traction to the tires of their machine.

...and then there is centrifugal force to deal with, but that's another thread...

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post #8 of 40 Old May 18th, 2007, 10:18 am Thread Starter
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Very Good

Quote:
Originally Posted by deputy5211
Is that because your weight bearing onto the bike has been transferred to the pegs, which are lower?

Following up on that, while the bike's CG may be lower at that point, would it not be fair to say that the rider's CG is higher, making the rider less stable on the bike?
Yes, while standing does lower the COG it might feel less stable, but while standing you are taught to squeeze the tank/seat with your legs. This also works while sitting on a street bike. You should feel more “planted” if you squeeze the bike, especially while cornering. Try it.

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post #9 of 40 Old May 18th, 2007, 10:22 am Thread Starter
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They do put weight on the pegs

Quote:
Originally Posted by K1200LTryder
With all of your weight bearing on the pegs, you still do not displace the mass of your tourso....it's a movement and "moment" issue. The theory hold true in aviation geometry regarding weight and balance. It is where the mass is located that determines the COG.

The lower your butt is on the seat, the lower the COG...remember that Gravity is constantly pulling perpendicular to the ground...so if you are standing on the pegs through a corner, gravity is pulling just as hard (albeit harder) on your upper body, and upsetting the intended balance of the motorcycle.

You don't see superbike riders standing on the pegs going through corners in a race, do you ?...they get as low as possible, and sometimes hang their bodies off the side of the bike to "lower" the COG,and apply more traction to the tires of their machine.

...and then there is centrifugal force to deal with, but that's another thread...
Superbike riders do put weight on the pegs to lower the COG. They weight the outside peg as much as possible.

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post #10 of 40 Old May 18th, 2007, 10:45 am
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Quote:
Originally Posted by ez_rdr55
They weight the outside peg as much as possible.
But that is for counter balance, not to change the CG.
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Oy! With all due respect, this thread is about as useless as a pig in a prom dress. (At least for me that would be "useless". Some of you guys might find a pig quite attractive in a purty pink prom dress. ) But I digress...

The fact that the term "flickable" was used in a thread within the LT forum backs me up, IMO. The only thing "flickable" about the LT is the high-beam switch. We are still talking about an 850-pound motorcycle here, right? The LT does amazingly well in the twisties for it's size...but let's not get carried away. Next we'll be hearing all those stories about the guys blowing away all the sportbikes. As if! All that means is that the schmuck on the sportbike couldn't ride. Those stories mean nothing to me in terms of how well the LT can handle the twisties.

Put the same rider on an LT, my GT, and a new Yamaha R1, let him/her do some laps at Laguna Seca and the proof will be in the lap times. The LT will be embarrassing compared to my GT, and my GT will be embarrassing next to the R1.

The bottom line: The LT has an extremely high COG which is "bad" when going slow. Fortunately, this can be overcome with patience and practice. Furthermore, the LT's high COG is also the reason why it allows the rider so much lean angle. If BMW lowered it's COG, it would scrap far too early. I think BMW did a pretty good job at finding a happy medium.



And now for the quiz-of-the-day: If a higher COG is BAD, why did Kawasaki RAISE the COG on their newest ZX-6R?
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post #12 of 40 Old May 18th, 2007, 12:17 pm
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Funny, I don't have any problem going from right to left or left to right in the twisties at speed on the LT.

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post #13 of 40 Old May 18th, 2007, 12:26 pm
 
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Originally Posted by Steve_R
Funny, I don't have any problem going from right to left or left to right in the twisties at speed on the LT.
Is that why you were a disappearing speck in my mirrors on the Blue Ridge Parkway? Ya see Steve . . . it's all relative. Your "at speed" and my "at speed" are two different terms. Just as "a high COG" is a relative term.
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post #14 of 40 Old May 18th, 2007, 12:48 pm
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Absolutely true. And I would never push my LT like you did yours or the GT for that matter. It IS MY RIDE.

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post #15 of 40 Old May 18th, 2007, 1:54 pm
 
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Quote:
Originally Posted by Steve_R
Absolutely true. And I would never push my LT like you did yours or the GT for that matter. It IS MY RIDE.
Atta Boy Brother! And AMEN!!!
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post #16 of 40 Old May 18th, 2007, 1:56 pm Thread Starter
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The fact that the term "flickable" was used in a thread within the LT forum backs me up, IMO. The only thing "flickable" about the LT is the high-beam switch.

I believe that was my point.

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post #17 of 40 Old May 18th, 2007, 1:57 pm
 
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so does shorter rake raise the COG or lower the COG, what about forward weight distribution, or forward controls, big tire and little tire,...what about this grasshopper; if a 1200lb horse can carry 33.333percent or 400lbs...does he handle differently when a 225lb man changes from a 30lb saddle to a 45lb saddle ? ( i might should not have posted this).
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post #18 of 40 Old May 18th, 2007, 2:03 pm
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...and just how long would it take a dog screwing a pail full of snowballs to melt it...

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post #19 of 40 Old May 18th, 2007, 2:05 pm
 
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...and just how long would it take a dog screwing a pail full of snowballs to melt it...
That's my favorite reply yet!
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post #20 of 40 Old May 18th, 2007, 2:10 pm
 
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im glad we're all on the same page....
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post #21 of 40 Old May 18th, 2007, 2:16 pm
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Pi=3.1415

My knowledge of physics is shown above. That's it, shot the load.

All I know is weight transfer to the pegs helps immensely in the twisties. I don't 'stand' on them, just transfer a little weight off my butt to the pegs. This doesn't increase torso weight, as some have argued, but transfers maybe 50 lbs (guessing) about 2 feet lower.

Doesn't make her any more "Flickable" though....

I know it works - I'll let the engineers figure out the "why-for"...

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post #22 of 40 Old May 18th, 2007, 2:24 pm
 
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you mean geometry...(to pie that is). I find i push everything a little too far and dont have time to compute what i am doing, like i said on a previous thread, when i scrape thats my alarm ...go in slow come out fast ("come out" being the operative words).
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post #23 of 40 Old May 18th, 2007, 8:15 pm
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Quote:
Originally Posted by ez_rdr55
At some point in this turn you have to right the beast and sometimes (very quickly) go the other way. A low COG is what makes a bike “flickable”. Try it the next time your out. Go right and left.
I still contend I am right. Go out and find a straight stretch of road with NO traffic. Ride down the centerline and (oh my gosh here it comes...) flick the bike from side to side. On the LT your body will remain ON centerline while the wheels will go either side. My LT is more "flickable" than my old Suzuki Water Buffalo (low CG).

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post #24 of 40 Old May 19th, 2007, 10:35 pm
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Lightbulb Time for some physics

Wow, interesting stuff. My opinion and explanation follows. Remember you get what you paid for...

BMW is trying to find a balance between comfort, ease of riding and traction.

Horizontal location locaton of the CG vs Vertical location of the CG. Althought the two are obviously related I believe the issue is better explained thinking more about the horizontal issues realted to CG rather than the vertical. Ok let the confusion begin.

The real issue for turning is the rotational inertia, or the tendancy of the bike to remain vertical when you are moving. This tendancy has the bike becoming more stable as you speed up. You LT guys know this from experience; It sucks at low speeds but improves over 5 mph. It actually gets better as you go faster. This is the affect I am talking about; As you move faster, it gets harder to 'disturb' this 'state' of vertical position, (but never too hard). The whole push right to turn right is based on the physics of this phenomina.

Back to the horizontal issue... Think of the CG being high, say at the tank. Now picture a vertical line through you, the bike, the wheels and striking the road. As the bike leans, the high CG (or the tank) gets further away horizontally from this vertical line. As the CG gets further away horizontally from the vertical line the less stable we get. Also the heavier the weight away from this line the less stable we get. And when combined, a heavier weight further away, well, you get the idea. And yes a high CG means a heaveier weight getting further away.

Without going into it, a larger weight further away (high CG) from this vertical line will require additional horizontal load, or TRACTION, at the wheel to keep the bike stable (read not falling over). Now remember that you only have so much TRACTION available at the wheel. If some of it is taken up by stableizing the higher CG, less is available for cornering. In simple terms, a lower CG will allow more traction for 'ripping it up'. That is why loading the foot pegs is a good idea, and why it works.

Now consider a very low CG, say for discussion, at the level of the pegs. You can, I hope, picture that as the bike leans the pegs are still very close to this vertical line relative to how far the tank is away. Thus a very stable position without using up TRACTION. More is left to play with.

A higher CG will allow easier initiation of the turn. Why? Because it is less stable, and it is easier to move something that is less stable. By definition, things that are very stable are hard to move. A lower CG is better, much better, for conering as there is more traction available. However a very low CG bike will have the tendancy to want to right itself quicker if you remove the turning force on the bars.

That is the game I think that BMW is playing with the LT. The lower CG GT can corner better than the higher CG LT. But the LT will initiate the corner easier and will take less effort to hold it there. I think this is the trade off that BMW is playing with in the design of a tourning bike and a sport touring bike. The results are never ending trade offs. I think they have found a good balance (punn intneded) and will continue to improve.

Just my 2 pennies. And oh by the way the rotational inertia, or the tendancy to remain vertical discussed above, is GREATLY changed by the size and weight of the wheel. None of which is lost by the Engineers in their designs.

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post #25 of 40 Old May 20th, 2007, 6:03 am
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Quote:
Originally Posted by K1200LTryder
With all of your weight bearing on the pegs, you still do not displace the mass of your tourso....it's a movement and "moment" issue. The theory hold true in aviation geometry regarding weight and balance. It is where the mass is located that determines the COG.

The lower your butt is on the seat, the lower the COG...remember that Gravity is constantly pulling perpendicular to the ground...so if you are standing on the pegs through a corner, gravity is pulling just as hard (albeit harder) on your upper body, and upsetting the intended balance of the motorcycle.

You don't see superbike riders standing on the pegs going through corners in a race, do you ?...they get as low as possible, and sometimes hang their bodies off the side of the bike to "lower" the COG,and apply more traction to the tires of their machine.

...and then there is centrifugal force to deal with, but that's another thread...
On a dirt bike, or any bike for that matter, standing on the pegs allows the bike to be flicked around much easier, because the rider is not tightly coupled to the bike and can let his body mass stay relatively in place while the bike can pivot around it's COG. If the rider was absolutely rigid in place on the bike and stood up, then it would be more difficult, but that is not the case. Watch a dirt rider flick the bike around under him while his torso moves very little and you can see that there are two center of masses here, the bikes, and the rider's, but pretty much uncoupled from each other. When the rider sits back on the seat, there is a "soft" coupling then, which raises the total center of mass, but on a light dirt bike the rider may still flick the bike around while counter moving his torso.

On a street bike, where the rider stays pretty much at the same relation to the bike all the time, then yes, the combined Center of Mass stays relatively the same when leaning the bike.

Also, turning a street bike works the same when righting it as it does turning it. The wheels are steered out from under the COG to turn it in, and the reverse to right it, so the wheels DO move far more relative to the COG than the rider does.

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post #26 of 40 Old May 20th, 2007, 8:45 am
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Quote:
Originally Posted by hdfan
Without going into it, a larger weight further away (high CG) from this vertical line will require additional horizontal load, or TRACTION, at the wheel to keep the bike stable (read not falling over). Now remember that you only have so much TRACTION available at the wheel. If some of it is taken up by stableizing the higher CG, less is available for cornering. In simple terms, a lower CG will allow more traction for 'ripping it up'. That is why loading the foot pegs is a good idea, and why it works.
"Loading the foot pegs" does not change the center of gravity, as David also noted above. In fact it will raise it if your butt is lifted off the seat in the process. The only way a rider can lower the COG is to get physically lower on the bike.

I'm not convinced of the "weight further away from the vertical line requiring more traction" argument either. Regardless of COG location, it seems like it would be inline with your vertical line, thus needing the same amount of traction regardless no matter how high it is.

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post #27 of 40 Old May 20th, 2007, 9:28 am
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Quote:
Originally Posted by kdog
"Loading the foot pegs" does not change the center of gravity, as David also noted above. In fact it will raise it if your butt is lifted off the seat in the process. The only way a rider can lower the COG is to get physically lower on the bike.

I'm not convinced of the "weight further away from the vertical line requiring more traction" argument either. Regardless of COG location, it seems like it would be inline with your vertical line, thus needing the same amount of traction regardless no matter how high it is.

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So Joel where did you get your engineering degree
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post #28 of 40 Old May 20th, 2007, 9:49 am
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So Joel where did you get your engineering degree
Evidently a better place than your got yours, Ken.
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post #29 of 40 Old May 20th, 2007, 11:22 am
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Ok I'm cool with that; maybe I can learn something today. By the way at the risk of embarassing the school I got mine at the University of WA. You?

My assumptions were based on the fact that the that a higher CG would have a greater moment arm and would thus require a greater amount of centrifual force to keep the bike in a balanced condition. This increase in centrifugal force would then need to be balanced by an increased centripital force in the form (we hope) of static friction at the wheel. What am I missing? Thanks for any info.

Ps you don't have to lift your butt to weight the pegs Ken
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post #30 of 40 Old May 20th, 2007, 2:34 pm Thread Starter
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Talking Standing on the tank should help

Maybe the new gen LT will have an even higher COG for superior cornering and even better handling. Until then I'll try standing on the tank.

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post #31 of 40 Old May 20th, 2007, 5:26 pm
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Ok I'm cool with that; maybe I can learn something today. By the way at the risk of embarassing the school I got mine at the University of WA. You?
So what did you major in, dick jousting? I fail to see why this is relevant to the discussion.
Quote:
My assumptions were based on the fact that the that a higher CG would have a greater moment arm and would thus require a greater amount of centrifual force to keep the bike in a balanced condition. This increase in centrifugal force would then need to be balanced by an increased centripital force in the form (we hope) of static friction at the wheel. What am I missing? Thanks for any info.
In my opinion, countersteering sets up a moment arm by moving the COG off the center axis of the bike. The higher the COG, the greater the moment arm, the faster you enter the turn. I think you agree with that. However, once initiated, the bike becomes balanced throughout the turn. The forces are directed straight down to the contact patch, and the affects of the moment arm are nullified. So with higher COG, you initiate the turn more quickly, and progress through the turn without negative affects. The argument is silly anyways because it's not like LT's are skidding off the road. In fact, they stick like glue. A bike with a lower COG will not stick to the road any better. Sure, there's some conjecture here on my part. I may be wrong, but you haven't given me any information that convinces me otherwise.

Quote:
Ps you don't have to lift your butt to weight the pegs Ken
Once again, weighting the pegs does NOT lower the COG one iota. Of this, I am sure. In order to lower the COG you'd have to physically move some mass lower. The fact that you don't understand this doesn't help your other argument.

Regards,
-joel
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post #32 of 40 Old May 20th, 2007, 6:58 pm
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Quote:
Originally Posted by kdog
So what did you major in, dick jousting?
Sorry I was just curious.

LOL, this is why there will never be a reality show about engineers. The commercials will be the highlight of the show...

Now dick jousting... I know for a absolute fact that an erect one has a higher COG.

[/QUOTE]However, once initiated, the bike becomes balanced throughout the turn. The forces are directed straight down to the contact patch, and the affects of the moment arm are nullified. So with higher COG, you initiate the turn more quickly, and progress through the turn without negative affects. [/QUOTE]

I agree. If there is any change in velocity or changes in the pavement causing the bike to shift etc,(like that never happens) the bike will come out of balance and I believe, within reason, that the reduced stability of a higher COG is less desirable.

[/QUOTE]Once again, weighting the pegs does NOT lower the COG one iota. Of this, I am sure. In order to lower the COG you'd have to physically move some mass lower.[/QUOTE]

You are correct that the COG/COM will not change by 'weighting' the pegs. Applying the force of the body's weight to a lower portion of the bike will not change that. I do believe there is an increase in stability from applying the force of the body's weight to the bike at the pegs and not the seat. Just riding around a few corners with ones feet off the pegs is an easy way to check that out.

[/QUOTE]The argument is silly anyways because it's not like LT's are skidding off the road.[/QUOTE]

Ahmen!

[/QUOTE]A bike with a lower COG will not stick to the road any better. [/QUOTE]

I would have to agree, but really do not know that for sure. The point I tried to make in the first post is that it is the balance between ability to initiate the turn and the stability within the turn that defines the ride in my opinion. And many bikes have different purposes and as such have different characteristics.

Being heavy into the mountain biking world I am keenly aware of how 1 degree in frame geometry makes a world of difference. There is much much more to handling than the issue of CG.

thanks for the thought provoking thoughts.
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post #33 of 40 Old May 20th, 2007, 8:11 pm
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Quote:
Originally Posted by hdfan
Being heavy into the mountain biking world I am keenly aware of how 1 degree in frame geometry makes a world of difference. There is much much more to handling than the issue of CG.

thanks for the thought provoking thoughts.
I'm just glad someone finally used the most correct acronym - CG not COG.

Ok, continue.

John

2004 - LT - Anthracite
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post #34 of 40 Old May 20th, 2007, 8:31 pm
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Quote:
Originally Posted by hdfan
If there is any change in velocity or changes in the pavement causing the bike to shift etc,(like that never happens) the bike will come out of balance and I believe, within reason, that the reduced stability of a higher COG is less desirable.
May be, but I would prefer the increased maneuverability.
Quote:
thanks for the thought provoking thoughts.
Thank you, as well. BTW, for the record, my own academic history is a bit checkered, but suffice to say, doesn't include much in the way of physics.

Cheers,
-joel
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post #35 of 40 Old May 20th, 2007, 9:11 pm
 
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Quote:
Originally Posted by kdog
So what did you major in, dick jousting? I fail to see why this is relevant to the discussion.

In my opinion, countersteering sets up a moment arm by moving the COG off the center axis of the bike. The higher the COG, the greater the moment arm, the faster you enter the turn. I think you agree with that. However, once initiated, the bike becomes balanced throughout the turn. The forces are directed straight down to the contact patch, and the affects of the moment arm are nullified. So with higher COG, you initiate the turn more quickly, and progress through the turn without negative affects. The argument is silly anyways because it's not like LT's are skidding off the road. In fact, they stick like glue. A bike with a lower COG will not stick to the road any better. Sure, there's some conjecture here on my part. I may be wrong, but you haven't given me any information that convinces me otherwise.


Once again, weighting the pegs does NOT lower the COG one iota. Of this, I am sure. In order to lower the COG you'd have to physically move some mass lower. The fact that you don't understand this doesn't help your other argument.

Regards,
-joel
One MC rider pees from a bridge and says the water is cold. The other bike rider says it sure is deep.
Applying one,s weight to the pegs, rather than the seat lowers the CG. However, if one has a fat ass and rides slowly, does it really matter?
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post #36 of 40 Old May 20th, 2007, 9:12 pm
 
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Quote:
Originally Posted by kdog
So what did you major in, dick jousting? I fail to see why this is relevant to the discussion.

In my opinion, countersteering sets up a moment arm by moving the COG off the center axis of the bike. The higher the COG, the greater the moment arm, the faster you enter the turn. I think you agree with that. However, once initiated, the bike becomes balanced throughout the turn. The forces are directed straight down to the contact patch, and the affects of the moment arm are nullified. So with higher COG, you initiate the turn more quickly, and progress through the turn without negative affects. The argument is silly anyways because it's not like LT's are skidding off the road. In fact, they stick like glue. A bike with a lower COG will not stick to the road any better. Sure, there's some conjecture here on my part. I may be wrong, but you haven't given me any information that convinces me otherwise.


Once again, weighting the pegs does NOT lower the COG one iota. Of this, I am sure. In order to lower the COG you'd have to physically move some mass lower. The fact that you don't understand this doesn't help your other argument.

Regards,
-joel
One MC rider pees from a bridge and says the water is cold. The other bike rider says it sure is deep.

Applying one's weight to the pegs, rather than the seat does, for a scientific fact, lower the CG. However, if one has a fat behind and rides slowly, does it really matter?
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post #37 of 40 Old May 20th, 2007, 9:32 pm
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It's one thing for doctors to have different opinions (different diagnoses while looking at the same test results, for example), but I'm thinking physics should be cut-and-dried -- no room for interpretation, that is. I don't understand why people can have different interpretations of the same physics-related issue.

Don't get me wrong: I'm not a physicist, and physics was definitely not my strong suit in college. (I'll even admit that the only course I ever failed in college was a physics class. I enjoyed the class and I understood the concepts (or so I thought ) but I couldn't pass an exam to save my life). What I'm getting at is: what is the one real answer to these questions?

I think it would be terrifically educational and interesting to have a BMW Motorrad engineer discuss these points -- keeping things in relative lay terms, of course.

Howard Schisler
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post #39 of 40 Old May 20th, 2007, 10:24 pm
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Generally the issue is not with the physics but in the questions and the answers and how they relate to the terms (mass vs weight vs force for example). These terms are used, incorrectly by many, myself too at times, but most everyone usually understands what folks are asking. But there are folks who will get hung up on the terms and that is where the confusion can begin. What gets lost is usually the concept that people are trying to get a handle on, as they get burried in technicalities. For example, Center of Gravity is technically about mass (center of mass)and not about weight or force and thus it is easy to get wrapped around the axle on terms etc. So, does the CG or more accurately, center of mass, change when you apply a force, or put your weight on the pegs. No. But can it make a difference to how the bike handles, yes. As your average person does not draw the distinction between these terms when they ask the questions, there are times when the answers don't jibe and the confusion begins.

There is a lot at play on a moving bike that will make a big difference in how the bike handles. All terms aside...the weight of the bike, the weight of the rider, where that weight is, the wheel base, the location of the handgrips, the location of the rider, the location of the pegs, the geometry of just about every part of the frame, the size of the wheels, the weight of the wheels, the weight of the tires, just to name a few.

So maybe questions that relate around results rather than terms can help. For example instead of asking about CG one could ask, what can I do as a rider to make it easier or quicker to transition from a right hander to a left hander? Just an idea.

Have I said that I love my LT lately. ken
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post #40 of 40 Old May 21st, 2007, 6:44 am
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Quote:
Originally Posted by kdog
----------------
Once again, weighting the pegs does NOT lower the COG one iota. Of this, I am sure. In order to lower the COG you'd have to physically move some mass lower. The fact that you don't understand this doesn't help your other argument.

Regards,
-joel
When the rider is sitting on the bike, the center of mass of the combined bike/rider is higher than the bike alone. When the rider stands up off the seat, the STATIC center of mass rises, but the DYNAMIC center of mass depends on the rider's movements. If the rider keeps his main body in ridgid relationship to the bike, then that center of mass is relatively the same as the bike turns. However, as in a dirt bike, if the rider lets the bike move side to side, but moves his main body opposite (or basically keeps it from moving side to side with the bke), then the DYNAMIC center of mass in the side to side direction is little different than that of the bike alone. The bike's movements will be resisted by mostly just the bike's weight and affected by mostly just the bike's center of mass. The center of mass relative to fore and aft will be mostly that of the rider and bike combined since the rider does not move as much front to back as side to side.

Remember, we are not dealing with rigid coupling here, unless you are made of metal and welded to the bike.

Dirt bike and motocross riders try to keep their body's center of mass as much divorced from the bike as possible to let the bike move around under them. MotoGP riders want the coupling to be kept as solid as possible so that hanging off moves the combined center of mass to the inside of the turn, reducing the required lean angle, and as low as they can get it. Low does not mean more maneuverable, but reduces the affect of centrepital force trying to throw the bike and rider back up on a high speed turn.

Big street bike riders basically fit the "welded and stiff" mode, so the combined bike/rider center of mass changes little. I do agree that "weighting the pegs" does nothing if the rider does not get his butt off the seat allowing him to move his body relative to the bike. You have to seperate the center of masses of the bike and rider to have any affect.

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