Center of Gravity, why does it matter on a bike?

Discussion in 'The Perfect Line and Other Riding Myths' started by V-Tom, Dec 29, 2008.

  1. V-Tom

    V-Tom Long timer

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    Yes, really when talking about Center of Gravity we probably should more properly be talking about Center of Mass.

    Another issue would be polar moments of inertia. The more spread out the mass is the more it resists twisting motion and vice versa. Large polar moments of inertia would feel more stable to a rider but slower to react to inputs. A taller bike would tend to have a large polar moment of inertia than a shorter bike, and the more spread out the masses of the engine, etc are the more dramtic this effect would be.

    ..Tom
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  2. V-Tom

    V-Tom Long timer

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    I have never ridden a trials bike.. although it is somethign I would love to do in the future. What happens when they are ridden at high speeds?

    How does the geometry of a trials bike compare to an offroad bike? For that matter, how does the geometry vary with Trials bikes, dirt bikes, Adventure Bikes, Sport bikes and Cruisers?

    ..Tom
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  3. V-Tom

    V-Tom Long timer

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    Concentrating the mass reduces the polar moments of inertia and makes the bikes react more easily to inputs. I can see it being a real good thing on a sport bike, or any bike that needs to change direction quickly, but not necessarily as desirable on a cruiser or touring bike.

    ..Tom
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  4. Round-I-Go

    Round-I-Go I won the Pullet Surprise

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    I just scrapped about an hours worth of reply ... I sense you really wanted to answer all your own questions.

    Now, I think you should just pack as much weight as you can on your bike of choice, as high up as you possibly can and go prove your theories true ... I mean, since it doesn't matter. Enjoy the ride! ... at any speed :lol2
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  5. Ardyjay

    Ardyjay Been here awhile

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    After nearly 40 years of riding- that's the clearest explanation of what's actually going on with countersteering I've ever read. Thanks!

    One thing I see as potentially significant that hasn't been mentioned is the gyroscopic forces created as the engine spins. When you spin a bicycle wheel in your hands as you hold its axle, it is difficult to change it's position.
    It seems to me heavy flywheels / engine and drivetrain masses (like Harleys and 4cyl in lines) spinning in the same direction as the bikes wheels turn will affect the rotational manouvering in a measurable way. Lighter flywheels like a KLR should affect it less.
    A BMW and Moto Guzzi twins, for example, with all their spinning masses in line with the travel of the bike must be affected less, and react differently again.
    Any thoughts on this?
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  6. Pike Bishop

    Pike Bishop Pull Down the Ponzi.

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    Here's an idea I haven't heard mentioned yet. Again, I'm not an engineer and only a new rider, so bear with me.

    But it seems to me that when you have a higher CoG, when you move the front contact patch a given distance from the centerline (or original track - forgive my terminology), the length of the lever arm that wants to lean the bike will be longer than it would be on a bike with a lower CoG.

    Since the length of that lever arm is longer, the resulting twisting torque force that wants to lean the bike will be stronger than it would be on a bike with a lower CoG. Thus, the bike with the higher CoG would flick into turns faster than one with a lower CoG...which would agree with my friend's assessment that I mentioned in my earlier post.

    Whatever the case, it's an interesting discussion. Now I want to try riding a cruiser to see how it turns in comparison with my tall dual-sport!
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  7. coppertop

    coppertop occasional meanderthal

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    Pike, I think the problem with your high center of mass bike/longer lever arm (to the contact patches) idea is that the road surface does not remain perpendicular... Initially, the input at the bars changes the orientation of the moving tire contact patch, and the tire deviates from a vertical line through the center of mass...(looking at the bike from the front or rear, mind you). But as the tire moves out, and the bike falls, the amount of leverage you can apply (at the contact patch) is limited by the available traction, which is directly related to the angle between the road and the tire.

    There are a great number of variables involved, and I wouldn't say that it is impossible to understand, but it takes a bit more space than I would care to fill, even if I did understand it.

    In the "balance a broomstick" examples, the key difference is that we control the balance of the bike by inputs at the handlebars, so you've got leverage between your input at the grips and the pivot at the steering head. There is then the second "lever" between the contact patch and the center of mass of the bike. The second lever length can change (due to suspension travel) and the amount of force that can be applied changes (are the tires rolling or sliding?)

    It is an interesting discussion, but it is more fun to test the theories...
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  8. markk53

    markk53 jack of all trades... Super Supporter

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    The key thing to remember it is entirely about leverage, gravity, moving masses, and gyroscopics... in infinitely varying relationships. :huh

    I think we're talking a graduate thesis here, an incomplete coverage of it at that level. :deal

    Some simple generalities that may help can probably be described, but good luck to all who try to give the definitive answer... :eek1

    it just ain't that simple. But makes for good shop talk... :freaky
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  9. Pike Bishop

    Pike Bishop Pull Down the Ponzi.

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    No doubt! [​IMG] The more I learn about riding, the less I understand!
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  10. V-Tom

    V-Tom Long timer

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    I'd love to hear your thoughts, particularilly where I might be misunderstanding things.

    One of the comments pointed to a CG being too low as being a negative in handling on one race bike... why is that?

    On my bike I always have a 52l Givi top case. On medium trips if not camping I'll have an additional bag of stuff on the back seat, for longer trips if I really have to I'll add Givi 41 liter side cases. I don't like carrying a lot of weight if i can avoid it.

    I normally won't go any closer to offroad than an occasional gravel road but love going through windy roads if possible.

    ..Tom
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  11. markk53

    markk53 jack of all trades... Super Supporter

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    I mentioned the racing thing. In the mid 1980s Honda tried to get all the weight low, with being on the ground being the unattainable ideal. They did this with one of their two stroke 500 GP bikes by putting the gas tank in the underbelly and the pipes routed up overtop of the motor. This put the weight pretty much as low as possible. It didn't work well at all. Why, I don't know. I think it was possibly that where it takes extra effort to fight the weight when CG higher than ideal, it also takes extra effort to fight the lack of weight when CG is below the ideal. By the way, none of what I am talking about here has anything to do with parking lot speed maneuvering, only "at speed" where the characteristics of single track steering/countersteering come into play. Refer back to the comment about a graduate thesis...

    What I do think is it had to do with how the bike rolls about the various axes of the bike, relating to leverage, mechanical advantage. It seems from what I can gather, on a total amateur basis, that it seems the ideal is to have the center of gravity located vertically as close to the line drawn from front to rear axle and centered left to right on the wheel axes. The weight distribution (CG) front to rear is another thing entirely. I'm talking only the locating of the CG left-to-right to be centered on the axle/wheel line, and the locating of the CG vertically on the centerline of the axle/wheel line. This will deal with rolling into a turn the location front to rear will deal with wheel traction relative to the turning forces. Again refer back to the comment about a graduate thesis...

    I have no scientific explanation with all the physics formulas, just what I see happening. From what I observe, the two ways to deal with CG positioning are to get it closer to the axle line as road bikes and racers usually do or create a mechanical leverage advantage - aka wide bars - as off road, dual sport, supermoto, and adventure bikes do. Just imagine trying to deal with a fully loaded GS or Gold Wing using a set of 26" wide clip ons and realize how easy it is to toss a tall dual sport or adventure bike around with 33" wide MX style bars. Again refer back to the comment about a graduate thesis...

    By the way, I did have a unique experience with CG. Some friends were going to an overnight race/party at a local track. They were taking bikes, but none of them had a way to carry their huge Coleman cooler full of ice and beer. My Guzzi 850T had a rack on the back, so you can imagine what happens here. I have like 70lb of cooler/ice/beer on a rack slightly behind and about 2 feet above the rear axle. Talk about a light front wheel. Fortunately it was only about 4 miles of good road to get there, although the uphill dirt road to the track was a bit enlightening. I had the worst of two worlds related to CG there. Too high and too far back... by a proverbial mile. Again, refer back to the comment about a graduate thesis...

    Like I said before, it is an extremely complex issue to really understand. Just having a general understanding is adequate. Like I also said, it is all about mechanical advantage and leverage. Using weight and its positioning to make the motorcycle perform as desired. Yes, CG can be too low, but it takes a real unique design other than the common motorcycle layout.

    After all this... refer to the last few beers in the refridgerator and piss on the graduate thesis... :1drink :freaky :1drink
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  12. camfarm

    camfarm Been here awhile

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    This is certainly no graduate thesis discussion. High school physics, maybe. Lots of thought, some good insights, and sharing of concepts. But short on facts and understanding.
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  13. Round-I-Go

    Round-I-Go I won the Pullet Surprise

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    Then you must have some facts and understanding you'd like to share with the class ... ?
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  14. camfarm

    camfarm Been here awhile

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    Try self-improvement instead.
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  15. V-Tom

    V-Tom Long timer

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    One thing about the left to right (longtudinal?) centering of weight is that a bike would pretty much naturally balance longitudinally while in motion if goign in a straight line. If it didn't wouldn't it tend to fall over?

    As far as front to rear CG, I suspect somewhere toward the middle is a good thing. But this really has got to get messed up pretty badly with the effects of weight shift (bikes are quite tall compared to their wheelbase) and the high weight of the rider (which moves around somewhat) compared to the weight of the bike.

    ..Tom
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  16. markk53

    markk53 jack of all trades... Super Supporter

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    At this point, it's pretty much proof of what I said. To try to actually define the whole thing is far from H.S. physics and pretty much is far more complicated than the first two courses of physics in college, per my experience. There are just too many variables to deal with it at those levels. Just overly simple explanations... thus my comment. :deal
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  17. markk53

    markk53 jack of all trades... Super Supporter

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    If you let go of the bars it will definitely lean in the direction with the most weight or force, eventually falling to the side - unless you correct for it by leaning opposite or by grabbing the bars again. Otherwise it wouldn't be necessary to locate the engine as close to centered as possible. If there is more weight to one side there will be a constant need for correction (steering force) to keep from leading off to that side. The old XR1000R actually had the engine significantly off set. I don't remember if there were comments about it or not.

    Also the rotation of the MotoGuzzi crank and such creates a force that is very evident when you take your hands off the bars. You have to lean a fair amount to the one side (I think it was to the left side) to counteract the rotational forces. That doesn't happen on anything with the crank shaft rotation perpendicular to the front to rear centerline of the bike.

    Your figuring on centering front to rear isn't quite true, the current sportbikes have more to the front than rear, even trying to get the engine closer to the front to get better steering. That has always been a bit of an issue for Ducati with the lower cylinder sticking fairly far forward and right in the path of the front wheel, limiting just how far forward the engine can be moved.
    #37
  18. V-Tom

    V-Tom Long timer

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    I understand what you are saying. I guess what I meant is that the bike and rider tend to pretty much form a balanced system or else, as you say, we would be constantly fighting the bars. My V-Strom has only one pipe on the side so I suspect is not perfectly balanced. I am sure that when I am riding it my seating position automatically compensates for it.

    I've heard of the effect on Guzzi's, BMW's, etc, but have never experienced it. I haven't heard a lot of comments from riders of it being a big issue.. is that something you automatically learn to compensate for?

    Everything is a compromise! I can see this making sense on a race bike. It would also tend to help keep the front down when accelerating. Does that imply that braking less important on a race bike than accelleration or that the weight shift of the rider can compensate? In ultra high performance sports cars (like F1) it seems to be that about 60% or so of the weight on the rear wheels is the best setup (vs. the theoretical perfect 50:50 weight distribution.). This gives great traction on acceleration and allows the brakes to be more effective due to apparant weight shift towards the front on braking. If course, the CG is so low on these cars that there is no risk of wheelieing or doing stoppies, and the driver's weight is pretty much static compared to a bike.

    ..Tom
    #38
  19. markk53

    markk53 jack of all trades... Super Supporter

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    Yeah, the Guzzi forces are automatically equalized by the rider, no learning curve needed. Besides hands off riding, the one place where you most notice the rocking motion is when sitting still and blipping the throttle. The wild thing is when you hammer a powershift into second, you have the front end lifting motion and the side to side twist - entertaining for sure.

    You are right about the rider balancing the bike for any weight bias left to right. I doubt any bike is perfectly balanced left to right.

    As for the F1 thing, they put BA wings to create down force at speed, hard to do on a bike, but I think you know that. It has been pretty true that there need be a front bias for a number of reasons on sport and performance bikes. Cruisers not quite so true.

    Good points and nice chat. Thanks.

    Mark
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  20. scotteroni

    scotteroni Been here awhile

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    All physics explanations aside, When I get off my 600 pound low CG cruiser and on my 250 lb high CG dual sport ( 87 XL250R) I feel like I'm riding a motorized mountain bike on the road. I certainly can relate to feeling like the bottom of the tires are moving side to side to maintain the stability of the higher CG ( the bike weighs 40 lbs more than me ) on the bike like the palm of your hand balancing a broom handle upside down. I find riding this 24 hp bike at median speeds on the road is more exhilarating with a greater sense of freedom than anything else I;ve ridden. In fact I neglected my other bike the larger part of last season because of it.
    #40