eemsreno said:
Apply this to your motorcycle. When you stand up, your mass is attached at the pegs.
This is a Quote from Digitalmoto. This is the only thing he would have had to say on this subject to be 100% correct. With 40 years of racing motocross I can say that standing up feels like it doubles your bikes tire traction.
Well, to the extent that you want to neglect force/weight on the bars, I would agree with that. Seems obvious enough. But that has nothing to with COG, just saying. The ladder model actually goes more to his point about levers and fulcrums and points of attachment.
I agree with most of his numbered points, but would point out one significant issue:
"1. A motorcycle has a generally fixed CG..."
"6. Putting together #1 & #4. A rider can move about a motorcycle. You can lean forward, to the right, stand-up, etc. As a result, the CG for the entire system (rider + motorcycle) is dynamic and depends upon the position of the rider."
This neglects the orientation of the motorcycle. When the bike leans, the COG of the bike moves to the inside of the turn, and may sit outside of the volume occupied by the bike.
But I think this part gets more confusing:
"Standing on the first step, the center or gravity is fairly low. Where is your center of gravity? Still behind your sternum. How have you attached your weight to the ladder? On the bottom step with your feet and with your hands. The overall CG for you and the ladder is near the step you are standing on. "
Agreed with respect to the point of attachment. Assuming you are considerably heavier than the ladder, the overall system COG is pretty close to the point behind your sternum, not near the bottom step.
"Consider how easy it is for someone to move the top of the ladder just an inch or two to the left or right? It is fairly easy to move the ladder and you stay stable on the ladder because you can shift your weight to compensate very quickly. Thinking about #7, your center of gravity is farther away from the top of the ladder, so it takes less effort for someone to tip the ladder. Right?"
Actually, the point here is that the ladder can move and you don't have to. Decoupled. The COG is relatively undisturbed in this case.
"Now imagine sitting on a step roughly at the same height as your butt as when you were standing on the first step. Point your legs inward, between the legs of the ladder. How are you attached to the ladder? Your butt and your hands. How stable is the ladder when you try to do the same hop? It's not! "
Actually, I believe the ladder is much more stable, e.g. harder to move, per his next point.
"How hard is it for someone to tip the ladder? It is much harder to move the mass and is much harder to shift your weight fast enough to compensate. This is an example of #8. Your center of gravity has moved farther away from the ladder's feet and closer to the top of the ladder. More force means you have a shorter lever. "
Your COG has not moved much at all in this example. In fact, the scenario appears designed to demonstrate levers, fulcrums and points of attachment and to do so ensures that the COG is kept constant. So, it's just gotten a bit confused and conflated with this COG idea. In this example what has changed is the ability of the cog to move independently from the ladder. They have become coupled. Additionally, by attaching the load further from the fulcrum (ladder's feet), and higher up the lever (ladder) you increase the force needed to move the load, and need to move it farther to achieve the same effect. The concepts of levers and fulcrums may have some merit and relevance, but I am not seeing COG here.
Now, he says "This is also the same idea as learning to relax when you have a tank slapper. Clamping down on the bars, moves the CG higher on the bike. This makes the tank slapper worse." Clamping down on the bars does not change the COG.
He also says, "The lower you attach your mass to the bike, the lower the overall CG." In fact the point of attachment has nothing to do with the COG.
The COG is exactly as he defined it,
"Center of gravity: A point from which the weight of a body or system may be considered to act. In uniform gravity it is the same as the center of mass."
On the surface of the earth then the COG is the center of mass. The center of mass is independent of any attachments. For instance, if one is interested in the Earth's orbit about the sun one might choose to base a simplified calculation on the combined/system center of mass of the Earth and Moon together, a point somewhere between their two individual centers of mass. But the Moon and the Earth are not attached at all.
The bottom line here is that we can talk about the COG of the bike, the COG of the rider, and the COG of the bike/rider system. When you stand, assuming nothing else happens at the moment, and once the bike/rider system has restabilized: The COG of the bike is unchanged, the COG of the rider is higher, and the COG of the system is higher.
This statement, "Yup. Simply put, by staying on your feet, you can keep the center for gravity lower than you can when you sit down." is not correct.
What may be correct is that you have extended the length of the lever, and attached the mass closer to the fulcrum (where the wheels meet the ground).... i.e. increased the ability of your mass to control the bike. But I am not sure this is right either. First it assumes the rider is stiff and rigid. In fact I think most riders try to do what I said in my earlier post, use their arms and legs as shock absorbers to maintain smoothness and keep the body's central mass moving as nearly in a straight line as possible, e.g. to decouple the rider's COG from the bike's COG and to reduce the overall change in COG that the suspension must deal with... but clearly the rider also uses his muscles to direct inputs to the bike, and when he does that the lever model may be more relevant.
But I think all of these povs are very simplified. In fact what happens is very complex and dynamic.
I don't mean to be rude or disagreeable, and I appreciate digitalmoto's contribution, but it is simply not "100% correct." Ok, g'night.
