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. NJ-Brett

    NJ-Brett Brett

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    Math makes my head hurt.
    I have had street bikes with a low center of gravity (old Triumphs), and dirt bikes with a low center of gravity (old short suspension dual sports, old Triumphs) and always found a low cog to be much better.
    Tall heavy bikes are a handfull in sand, low light bikes are much better.

    Tall bikes with lots of suspension can fly over some very rough stuff but they tend to fall over easy and be hard to control in sand and mud. Its a trade off I guess.
    #61
  2. Tripped1

    Tripped1 Smoove, Smoove like velvet.

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    Quite the opposite I believe.

    When Honda tried to get the CoM on the ground in MotoGP no one could ride it. For comparisons with more modern machinery, comparing my 675, to my buddies GSXR, the GSXR carries its weight and the rider's weight lower.

    The result is that the GSXR is certainly more stable that my Daytona, but it doesn't move side to side nearly as quickly.

    That is a difficult comparison, the geometry is so my different

    OK now that I think about it.

    As I noted above, it has been tried. Likewise, if you remember original CBR600RR centralization of mass and reduction of Moment of Inertia were supposed to be the breakthrough engineering over the F4i.

    As you noted it's a trade off, get the whole thing too high and you become unstable under braking, too low and you have ground clearance and front end issues.This is why its so interesting, if you can ride at even a moderate track pace, a 5mm change makes the bike respond completely different (admittedly that is more a front end geometry thing).

    If I get a change I want to play with that equation a bit, I know what its derived from but I'm trying to prove it out from its components, and its just not happening in my head.
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  3. markjenn

    markjenn Long timer

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    The Dakar guys seem to have no issues with tall/heavy bikes in sand/mud.

    I think its mostly a matter of skill and training. If you're good, you're Okay with a tall bike because it gives you great suspension travel and ground clearance and you have the skills to be using throttle, brakes, steering and body english to keep all that high-up weight in exactly the right spot to make the bike do what you want. But if you're a novice and are paddling around like a kid on an XR75, it all becomes totally unwieldy. Truth be told, most of us are closer to the novice level than the Dakar racer level. I certainly am.

    - Mark
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  4. PeterW

    PeterW Long timer

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    A lot has to do with riding style. You can adapt.

    I found my DL 650 really scary the first few times I took it on dirt then it all clicked. Sit down (the damn thing carries it's weight high already, no need to move more weight up) relax, and just steer with the throttle and small weight shifts.

    Still slower than a good 250 on dirt roads, but man does it eat the miles effortlessly.

    Pete
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  5. markk53

    markk53 jack of all trades... Super Supporter

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    Who cares? I'm not doing physics while riding. I'm riding and doing what works. It's like countersteering. Experience riding will make you do what works without actually knowing the science. If you needed a degree in physics to ride, there'd be a few thousand bikes on the road! :huh


    Now if you are simply curious, that's another thing...:lol3
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  6. markk53

    markk53 jack of all trades... Super Supporter

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    Did you watch any of Dakar? The leader (former after the fact) seemed to have problems with a serious mud hole. Then there were all the crashes they were showing. Heck the lead American entry bailed and I think broke a collar bone. They hose up too. :D

    And they sure didn't have enough time to "science out" the CG! :lol3
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  7. Coach

    Coach Coach

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    Hi Mark, You are so right. When on the bike there is not much time to "science out." But in training there is. For example, knowing something about center of mass helps you make good decisions about where to sit or stand for turns in different terrain or surface conditions.

    Look at the image I posted earlier-- the one with the lines through the seated off-road rider. It is from the last Laughlin race (Best in the Desert). This rider is sitting straight up and down because that's where most are comfortable. But to sit on the fall line of gravity because you think it's more stable means the bike has to lean more under you. This results in less traction because you are on the edge of your knobbies. That's why this bike is sliding out in this turn. The solution? Switch Centers of Mass. If this seated rider were to lean to the inside of the turn then the bike could be pushed away (more vertical) on to a better part of the tire. The effective lean angle through the combined center of mass stays pretty much the same. This is learned in training by doing 100's of seated and standing turns. By the time he or she gets to the Dakar in Argentina it is automatic. The science part is hidden.

    The bottom line for training is KSA. The three stages of learning to ride well. K stands for KNOWLEDGE. Learning the science (physics and mechanics) in a classroom setting. S is for SKILLS. Learning in a protected training environment to acquire the body mechanics and controls function to pull off the fast turn, to bust big, etc. And finally the A. Practicing so much much you now have the ABILITY to apply your new knowledge and skills in a racing environment or under the pressures of everyday road traffic-- without thinking too much about center of mass.

    Coach
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  8. DAKEZ

    DAKEZ Long timer

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    <iframe width="560" height="315" src="http://www.youtube.com/embed/CqT0PjKg0k0" frameborder="0" allowfullscreen></iframe>
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  9. markk53

    markk53 jack of all trades... Super Supporter

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    It's the same as moving. The difference is there is movement in another axis, and if you stick your foot down it's gonna catch pavement, kick back, and possibly hurt like hell. When you talk about leaning it is in a totally different axis than the forward motion. Want proof? If you have the balance of a trials rider and could do so, you'd find it is far easier to balance the bike while standing up than it is sitting down with your feet on the pegs. It's about being able to work the balancing of the bike and when standing, the rider weight on the bike is at the pegs versus on the seat when sitting. Same is true when moving.

    When I'm on a bike standing, the weight load is at the pegs, when sitting a majority of it is at the seat. So, relative to the motorcycle and it's load, the fixed CG is lower with the rider standing. The CG relative to the combined motorcycle and rider will be actually higher when the rider is standing than seated. The thing is the CG of the rider is a dynamic situation when standing or sitting, it changes as they lean in any direction. When standing it has more emphsis because the rider can lean further and with the weight on the pegs, the bike can be moved about easier.

    One other thing that happens too - the rider also weights the pegs different depending on direction of lean, where sitting on the seat that doesn't happen without sliding off one side or the other. And of course when sliding off, the rider will weight the inside peg more, supporting their weight when they slide off, lowering where the weight is distributed again.

    But I know this is a futile discussion for the most part. It's just entertainment for a cold winter day.
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  10. markk53

    markk53 jack of all trades... Super Supporter

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    And you know most riders never do know the science either, even some of the greatest. Heck Keith Code is in a constant learning mode. When he started he knew there had to be reasons for doing what they did that worked, but no one really pushed the issue until he did, for the most part. Most riders learned by experimentation with some minimal knowledge.

    Then in many off road situations there can be multiple correct ways to do whatever. I remember seeing pictures of riders with the bar end in the dirt, the bar virtually resting on their leg, while sitting nearly vertical on the bike. I also saw shots where the rider was leaning in with the chassis near upright. Seems different conditions call for different things to be done. Available traction usually dictating what should be done. Trying to make one cut and dried method stick always gets fouled up by what varying circumstances call for in any of the infinite situations may call for. Do it wrong and you may fall down or, in the case of racing, going slower.

    That rider in the picture you use for example, could keep the tire on line, but odds are that "loss of traction" is done because it is the fastest way to make that little change of direction needed and get forward motion. Supermoto riders and road racers still use "loss of traction" to go faster. That rider's position looks pretty much right for what I'm betting is happening. He goes into the corner at whatever rate, then as the bike starts to slide a bit on the hard pack he's putting a foot out and compensation a bit by shifting his upper body more upright. You see it all the time in off roading. Unless there's a berm or rut to rail around the rider will usually have the bike leaned more than the body.

    One of the more interesting things I remember about some of the flat track bikes was the cornering characteristics of the frame geometries. The Bultaco Astro chassis was extremely suited to leaning in and sliding with the rider more or less sitting in line with the bike or even leaning the bike in a bit more than the rider was leaning. On the other extreme was the Kenny Roberts frames like the Pentons and CanAms used tended to want to sit up and slide with the rider leaning their upper body in. The two chassis didn't work the same, so the rider had to do what was necessary. What worked on one did not work on the other.
    #70
  11. Lion BR

    Lion BR I'd rather be riding

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    I know there are many "experts" here to provide the "right" answer to this question.

    The Moto GP guys at Ducati were puzzled by why the bike and Valentino were not performing well this last season. They were trying a new chassis. Too stiff some were saying, providing little input from the front wheel. Others were saying it was due to the tires not reaching the right temperature. Those guys work on minute changes to a chassis configuration, materials, as being the difference between succeeding or not. And rightly so, as they are pushing the limits. And they have to do that or else they should give up racing. They are not talking about center of gravity. They are talking about fluid dynamics, engine management systems, electronic aids, chassis performance, tires.

    Those Dakar bikes of the 80's, some of them were taller than the riders. The Dakar bikes of today, are also tall. ever hear any of them complaining about a high CoG. "Yes, the bike performed well, but man, that high CoG was a bitch to handle" I haven't heard that from a racer. Yet.

    And here we are, amateurs, making a point about center of gravity, discussing the minutia of things we don't even understand well enough. But we do have opinions, right?

    So here is my opinion. Last year I took a Ducati Monster for a spin, loved it. But then a week later I took a Hypermotard. Tall beast. High CoG. But man, that was a lot more fun to ride than the Monster. The way it "fell" on the corners, but still with a stable front, was just great, absolutely a blast. The people who designed the Husqvarna Nuda mentioned that they wanted it tall, with a higher CoG.

    Now, I have to say CoG really did matter. It mattered it making it more fun to ride the Hypermotard. And some bikes are designed on purpose to get that feel. But that is a question of taste. Not a prove that bikes have to be one way or the other.

    In other words, CoG really matters about what the bikes gives back to you. Test it, find the one that works for you.

    Oh, yes, like the OP mentioned on his original question: CoG really matters when you have to pick up a dropped bike. And yes, when you are trying to get that foot down, because you are just learning how to ride on dirt, and the bike is too tall... Well, being tall and a higher CoG will bring it down faster and more often. But once you learn to ride on dirt you will never know that there is CoG anywhere on the bike. Unless that is the only argument you have to prove your bike is better than someone else's. But even so, the argument could go either way. Depending on what you like.
    #71
  12. markk53

    markk53 jack of all trades... Super Supporter

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    No, Dakar riders just mentin that the bikes carry the weight very high with all that fuel and electronics up there.

    As for when you're riding. you may not have even known of the term Center of Gravity or Center of Mass, but you darn well are aware of the actions they cause. You learn fast if you have your weight too far forward when you hit whoops or too far back going up a steep hill. You don't have to know the terms and all about balance to realize the importance of it when starting to ride a two wheeler. You likely didn't know anything about countersteering when you did exactly that as you rode down a fast hill on your bicycle, but you did it - or crashed your brains out.

    You see, it's darn hard to ignore the laws of gravity and physics. They work whether you know it or not.
    #72