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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    The above quote was in a discussion about V-Strom 650 vs 1000, but I have heard similar discussions comparing V-Strom's to BMW GS models and other bikes

    Please note that I am not a dirt rider. I have ridden around a bit on dirt bikes when I was young but that was a long time ago, so there is much about riding in dirt that I probably won't ever understand

    I have heard a lot of people talk negatively about high center of gravity (CG) but I have a hard time understanding how that matters when moving on a bike, especially one that isn't being used for racing.

    I can understand it matters when picking up the bike, or when moving it around the garage (especially if you lean it a bit much one way or another). I very much understand how it matters a huge amount on a sports car but the dynamics are very different. On a Sport bike I understand the benefits meaning quicker change in direction, but every dirt bike I have been on is very tall to give a lot of ground clearance and as a result has a pretty high center of gravity. Once moving I think the higher CG can help with stability as it would tend to resist side forces more. When sliding on dirt the higher weight of something like a GS or V-Strom (or any of the large dual Sports) would probably make them a handful compared to a pure dirt bike

    And why does high CG matter when riding slowly, assuming you are moving? Isn't it easier to vertically balance something in your hands that is long and thin (like a ruler) vs. something that is short and stubby? A Harley rider I knew told me that a V-Strom 1000 won a slow-moving contest at a local Harley club meet.. Evidently they blew away the Harleys, which of course have a much lower CG.

    So, for a bike whose main purpose isn't racing on the track or the ultimate high performance street racer, or isn't sliding around on the dirt a lot, why does a low Center of Gravity matter?

    ..Tom
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  2. mousitsas

    mousitsas Long timer

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    Lets say we have two bikes with the same weight but different CG's. Two counter-acting arguments apply:

    1) The higher the CG is, the larger the rotating inertia of your motorcycle will be when getting off balance.
    However you use your own body mass to adjust when on the go, so the higher the CG is, the more difficult it is to control it, since it feels heavier to you.

    2) On the other hand, a high CG (or high rotating inertia) means also a slower response to a destabilizing force (i am talking here about a force that does not cause it to slide, but only to bank). Which means you have more time to react. This makes a high CG bike easier to control in certain cases than a low CG bike.

    So, in which cases we prefer a high CG than a low CG?
    Easy...when we move slow and the bike is not sliding. Why? because when the bike is sliding, then the whole argument of rotating inertia collapses, since the bike is no longer rotating on the contact patches of its wheels to the ground, but free falls.

    So, generally speaking, in slow conditions, sometimes its better to have a high CG, provided there is enough tracion and our body mass is large enough in order to be able to have an effect on the bike when we move around on the seat.
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  3. Round-I-Go

    Round-I-Go I won the Pullet Surprise

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    Interesting discussion, "V-Tom". My thoughts are in your quote below. Respectfully, please understand this does not come from any Degree in Engineering or Kinematics or any of that stuff, just personal experience...
    ... just my $0.02:rayof
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  4. sideshow

    sideshow debaser

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    Grab a hammer. Hold the hammer end first and wave the handle in the air. Now hold the bottom of the handle and wave the hammer end in the air. Flop it from one side to the other. Now think of the extra force required to do this for every corner for every inch that you bike has higher than the next bike.

    When you stop at a stopsign, think of the hammer head being on top of a V-Strom, and at the bottom for a cruiser. What's easier to hold in place at balance with your foot?
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  5. amk

    amk Been here awhile

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    That had to be covered in grade 6 or 7: parallelogram of forces&#8230;<o:p></o:p>
    The lesser the speed of the bike the greater the static forces the rider has to apply to compensate the deviation of CG from its neutral position. If the bike moves with a speed, the rider easily compensates such a deviation with dynamic forces, usually with centrifugal one and with wheal gyroscopic forces. <o:p></o:p>
    <o:p> </o:p>
    Most of dirt bikes have relatively high CG due to their long suspension, however it is not the only factor. The whole picture might be improved or otherwise by allocation of the motor, transmission, and the gas tank. <o:p></o:p>
    <o:p> </o:p>
    Speaking about Stroms, which are not dirt bikes, the allocation of the motor, transmission, and the gas tank moves the CG to its theoretical highest point, add here their monstrous masses and&#8230;<o:p></o:p>
    <o:p> </o:p>
    Just try to do an 8 figure, like when getting the license, on Stroms and on S50 &#8211; S90. Which one is easier? <o:p></o:p>
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  6. SOS

    SOS Ignorance is a gift

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    I think that the CG of a bike has alot less to do with how the bike handles and how easy or hard it is to control at low or high speeds than the steering geometery. I had always assumed that CG made a big difference on how well the bike handled; it certainly makes a big difference when you push the bike around in the garage. But in my ongoing quest to improve my riding skills (and I'm sorry to say that I have a loooooooong way to go) I read a book by David Hough (Proficient Motorcycling). He is the guy that basically laid the groundwork for the motorcycle safety course and how to control a motorcycle while riding. He has a very good and easy to understand section on motorcycle dynamics. In there he describes what is actually happening when a bike is moving, whether in a straight line, in turns, braking, etc. Here is a quote,

    "When you hear someone attributing a motorcycle's good or bad manners to the elevation of its center of gravity (CG), remember that it's mostly steering geometry that makes it feel sluggish or top heavy in turns. ... Next time you hear of a bike that's described as being top heavy or having a high center of gravity, check the rake/trail numbers and consider the sizes and profiles of the tires."

    This quote follows a detailed description of how a bike stays upright in a straight line, how it turns and how the design of the steering and chassis affect the process.

    The book is a very good read and has helped me with my wn riding skills greatly.:norton
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  7. SOS

    SOS Ignorance is a gift

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    Here are the 2002 DL1000 specs from the factory service manual. I am assuming that they are still the same.

    Caster (Rake) 26 deg 30 min
    Trail 111mm (4.3 in)
    Front tire 110/80R19
    Rear tire 150/70R17

    The tire specs obviously only come into play when you are turning or hitting a bump, but the rake and trail contol the bike in straight-line stability and in corning.

    Hope this helps.
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  8. ridewestKTM

    ridewestKTM Long timer

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    I am an engineer and have studied this stuff, and you all are of similar opinion to mine, but I'll try to clairify it a bit. The bike and rider's CG will always (except for a brief period when pro stearing) stay in a position such that the the line between the CG and the tire patch forms an angle to the ground proportional to the accelerations - so straight riding the angle is 90 degrees. Turning- the angle is that which is created by the vector addition of the horizontal acceleration plus the vertical. Draw a picture and it will seem obvious. Now how it got there,- you counterstear to place the tirepatches on the outside -left to turn right right to turn left. The bike and rider cg falls to the planned inside of the radius. This rotation will occur nominally around the cg. So if the cg is high the tirepatch has to go farther to acheive the same angle. This takes longer- so slower response but because it has to go farther it's more stable.
    Consider these points and perhaps play with a weight (like clay) on a stick (weight above you hand -your hand is the tire patch). You might agree with what I am saying.
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  9. SOS

    SOS Ignorance is a gift

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    I certainly wouldn't doubt your point. I am sure that the CG position on a bike/rider does affect bike's handling, but which factor has the overriding impact? I believe it is probably the chassis/steering design. All bikes have a very high CG when there is a rider, especialy my 225lbs sitting on a 33" seat.
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  10. Coach

    Coach Coach

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    Hello All,

    Interesting thread. As I read the posts I'm impressed with the depth of the discussion. May I add a couple of factors for your consideration?

    I suspect that the reason this topic gets confusing is the physics change with speed.

    At about a fast walking speed, say 7 to 10 miles per hour or less, we use a counter-balance turn technique. Why? Because we are only dealing with gravity. Moving your dynamic bitt to the outside of the turn counter-balanced the bikes static center leaning to the inside of the turn. In other words, the combined center of mass is over the tip-over line (drawn from contact patch to contact patch).

    Above 7 to 10 miles per hour centrifugal force and wheel pression come into play. Now "lean angle," rather than body position, is used to maintain balance. In other words, proper lean angle allows gravity in the inside of the turn to counter-act centrifugal force pushing us to the outside of the turn.

    This is where "moment" comes into play. The distance from the tip-over line to the "combined" center of mass (the bike and the rider): the higher the CofG (CofM is more precise), the less lean angle is required for a given turn radius at a given turn speed. This is the BIG pay-off for off-road riders, we don't have to lean over as far is poor traction.

    To explain the physics and mechanics in detail takes hours and lots of visuals in my off-road classes-- so this may not make sense here. But it is worth adding speed to your discussion. Thanks again for a great thread.

    Ride Well,
    Coach
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  11. ridewestKTM

    ridewestKTM Long timer

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    Coach: The angle (combined CG tirepatch vs ground) is independnent of CG height and the angle is tangent of cornering acceleration and vertical. The reason is the higher the cg the higher the moment arm of horizontal and vertcal - proportionally. Because the effective arms are longer it may look bigger. Sorry to dissagree on a ralatively minor point.

    I believe I can agree on your speed sensitivity points though. On light bikes one can let the tirepatches move and maintain the CG vector (what I called pro-stearing).

    All: Some easy experiments include: The clay on a stick as said before, a weight on a string swung around -look at the angle and then change the length, or for the bike wet the ground ride thru the water to dry and look at the tire paths, ride on a line and try to manover away without leaving the line the wrong way. With safety in mind ride with one hand and being very careful try left hand on right grip. If you don't crash right away you can learn much.

    best wishes-happy holidays
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  12. ridewestKTM

    ridewestKTM Long timer

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    Lost what I last wrote.

    Yes steering geometry is important. Ever ride a trials bike fast?? Scary regardless of CG (sitting or standing).
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  13. Pike Bishop

    Pike Bishop Pull Down the Ponzi.

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    I'm totally new to all of this, but a friend of mine who has been riding all his life said that the higher CoG of my bike (DR650) should make it flick into turns far faster than you could do with, say, a cruiser which has a much lower CoG. He said it's much harder, and takes longer, to get a cruiser leaned over than a bike with a higher CoG because basically, the cruiser is more "stable" and wants to continue in a straight line moreso than a bike with a higher CoG. (Then again, now I'm wondering how much of a role rake and trail play in all of that...)

    I really don't know whether it's true, but he has ridden and owned all kinds of bikes throughout his life, and I respect his opinion, so I figured I'd throw it out there for the other inmates to chew on.
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  14. mick

    mick speedtrippy

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    Companies philosophy about CoG placement seems to change every few years. Now, Mass centralization seems to be the major concern with most manufactures now. CoG has been experimented with for a variety of reasons. Too low Cog proves to cause some handling problems. Too high Cog causes a "Tippy" feel. CoG has been moved to center rotating mass in some bikes, and have been moved away from center in others. Basically CoG changes because of design philosophy at the time of engineering.
    Suzuki seems to have high CoG's and still handle well. Yamaha lead the way to stacked transmissions to concentrate mass, and the new Husaberg engine is a design break through that looks promising.
    Try to find a bike that matches your style of riding without the worry of exact physics in design. There is no telling what we will be riding in ten years from now.
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  15. markk53

    markk53 jack of all trades... Super Supporter

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    Narrowness and weight have a lot to do with why CG has less effect on a single cylinder dual sport. If you get one on a trail you find out quickly how important the CG becomes. But to have good travel for off road, the bike has to be relatively high. Just think what it would be like with a wider inline 4.

    The CG can also be too low. Honda found that out when they tried to make a GP bike with the CG as low as possible running the fuel tank under the motor and pipes over the top. It never worked well.

    I don't fully know the physics and science, never really studied it. I know it seems if the CG is in line with the axle line front to rear it works well. That's about where it is on a road racer. Off road is a whole new world.
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  16. motu

    motu Loose Pre Unit

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    After 10 years of adventure bikes (XLV750,XT600,DT230,note I kept going down in size) I have now set my BMW R65 mono up for the same type of riding,namely gravel and tight twisties.I had great problems on the R65 with the standard low narrow bars - it took so much more effort to tip the stable low CG BMW into corners than the adv bikes with quick steering,wide bars and high CG...they just flip flopped through the curves very easily...I was a total mess on the R65.

    Then I fitted flattrack handlebars,giving me the same riding position as a dirt bike (same as the XT600) and then changed to narrow profile Dunlop K70 tyres.The bars give me the leverage to flick the bike over much easier,and the more upright body position allows me to move my body around more.The narrow tyres mean the contact patch of the tyre remains closer to the centerline,requiring less angle of lean....and less input from road irregularities.

    On gravel the lower CG and more central balance is a totally new ball game (well,an old one for me,as I'm going back to what I used to ride).It highlights the problems of a high CG on gravel - a high CG bike is unstable in a slide,and in thick gravel corners at low speed the front wheel tries to slip out or tuck,and the bike wants to fall into corners.With my new bars and tyres the R65 is a dream in gravel,much better than an adv bike - the front end is very stable,I can lean the bike more in turns,and slides are something I can play with - it just slides like a dirttracker,no more lurching,just power out and go.
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  17. Round-I-Go

    Round-I-Go I won the Pullet Surprise

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    I think in the comparison you present here, there are many other forces in play than just CG. Weight, for instance. A DR650 probably weighs not much more than half of your average cruiser. Power-to-Weight ratio has it's effect. Steering head angle is another factor. Cruisers always have much more rake than most any other type of bike (other than a chopper), most notably a dirt oriented bike. This makes the cruiser dislike turning by it's very nature, it would much prefer a straight line. If it didn't have a low CG, it's dislike of cornering would be even more greatly exacerbated. Try "flicking" a Fatboy.

    This is why motorcycle design is all about compromise of purpose IMHO, as I stated in my post above. There are many factors involved, not just weight, not just ergonomics, not just chassis geometry, not just power delivery, not just CG.
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  18. V-Tom

    V-Tom Long timer

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    My gut says no, it isn't the same. When I am moving the bike around the garage I am standing on the ground and the bike does not have natural stability. It would fall over if I wasn't holding it from the side so a low center of gravity makes it easier to hold up (priciple of the lever?) When riding, it stays stable itself by keeping the apparent forces over the tires.


    The stability of a 4 wheeled vehicle depends on how the forces act on the tires. The vehicle does not stay centered over the contact patch in the way a motorcycle does so high center of gravity means that there are more forces acting on the outside tires. If the center of gravity is high enough and cornering forces are great enough the vehicles can tip over (ie: SUV tendencies to roll over). This doesn't happen on a bike, at least it doesn't when moving if the tires retain traction and no other part of the bike contacts the surface. No matter how high the center of gravity is on a bike the cornering forces would not be affected in any steady state manoeuvers. (ie: constant radius turns.)

    If we are in a situation where the bike isn't sliding and the rider does not have his feet on the ground then I would suggest, for steady state manouvers, the CG doesn't matter. The wheels would be as well planted regardless of the CG.


    Wouldn't more weight always be a negative as it would take more control forces to make the bike react? If the bike weighed more then there would be more need for traction, and in a slide then the rider would have to work harder to keep the bike up. (Now the rider's foot has to carry some weight.)

    Here is how I would look at it: When moving on a bike the combination of trail and the gyroscopic forces of the wheel in motion act in a way that keeps the tire track under the CG of the bike. Weight on the handlebar would make this less effective (in the same way as over-tight bearings on the steering assembly make a bike tougher to ride.) As you go faster it is easier to keep the track under the bike so the bike stays more stable.

    This is why I said I understand a lot CG making it easy to handle in the garage, etc. But on a bike the static and dynamic situation is quite different. For example, while sitting still on your bike and not on a stand take your feet off the ground and tell me what happens. (In most cases it will fall over.) Do the same while riding. (It normally doesnt fall over, even if you let go of the handlebars.)


    When sliding on dirt the higher weight of something like a GS or V-Strom (or any of the large dual Sports) would probably make them a handful compared to a pure dirt bike. Yep, and that's part of the compromise in difference of purpose.

    I think very different, as the gyroscope is stationary with respect to the ground.

    Perhaps, but I keep coming back to thinking that a long tall object (yardstick?) is easier to balance in my hand than a short stubby one, and slow speed riding to me seems more like balancing things on my hand. By moving the handlebars I move around the equivalent of my hand as I shift the contact patch to keep it under the bike.

    Extra weight is going to make this harder, but again the dynamic situation is different as we balance on our feet quite differently than on a bike. I suspect that our standing has more in common with a car's balance than a bike's (although we are more dynamic with muscles and all that I am sure we can make a model of a person be able to stand still on two feet, we can't easily do the same with a bike unless the tire bottoms were wide and flat.)

    ..Tom
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  19. V-Tom

    V-Tom Long timer

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    Yes, and that was why I understand low CG being good in moving the bike around the garage. We have to keep it up so in effect the leverage against the weight of a low CG makes it easier for us to handle. This is like holding tight on the hammer and leaning it side to side.. it is easier with the head being at the bottom (low CG).

    But when moving at any speed, isn't this more like trying to balance the same hammer on your hand without holding it? It is far easier if the head is on the top (high CG). It "falls over" more slowly so it is easier for us to recover and keep our hand under it.

    ..Tom
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  20. V-Tom

    V-Tom Long timer

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    That is what I am thinking. Some have said a high CG makes the bike less stable but it seems to me it would be more stable when riding. It would react more slowly to inputs in the steering.

    I think a low CG on a sport bike is desireable for that reason.. it makes the bike less stable, therefore easier to change directions. Small inputs in the steering make proportionally larger movements of the bike's attitude (it falls over faster), and this makes it easier to change directions quickly.

    ..Tom
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