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High COG...good or bad

3K views 39 replies 16 participants last post by  dshealey 
#1 ·
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 ½” 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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#3 ·
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.
 
#4 ·
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.
 
#5 ·
Now to really stir the pot; when do you think your COG is lower: standing on the pegs or sitting on the seat?
steamboatjohn said:
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? :stir: :stir: :stir: :v:
 
#7 ·
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... :bmw:
 
#8 ·
Very Good

deputy5211 said:
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? :stir: :stir: :stir: :v:
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.
 
#9 ·
They do put weight on the pegs

K1200LTryder said:
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... :bmw:
Superbike riders do put weight on the pegs to lower the COG. They weight the outside peg as much as possible.
 
#11 ·
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. :D ) 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?
 
#12 ·
Funny, I don't have any problem going from right to left or left to right in the twisties at speed on the LT.
 
#14 ·
Absolutely true. And I would never push my LT like you did yours or the GT for that matter. It IS MY RIDE. :)
 
#16 ·
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.
 
#17 ·
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 ? :rolleyes: ( i might should not have posted this). :( :eek:
 
#21 ·
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.... :rotf:

I know it works - I'll let the engineers figure out the "why-for"...
 
#22 ·
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 :eek: ...go in slow come out fast :D ("come out" being the operative words). :cool:
 
#23 ·
ez_rdr55 said:
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).
 
#24 · (Edited)
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.
 
#25 ·
K1200LTryder said:
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... :bmw:
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.
 
#26 ·
hdfan said:
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.

Regards,
-joel
 
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