DingusMcGee said:
No Robo,
inertia works all the time and you pay for each change in movement -- no free lunch here.
Put simply, in a rotational vs. linear response system, the rotational inertia is computed ( l x l x m) when treated as a lumped mass system . So if the center of gravity of your battery is 20" from the ground when mounted low and 60" from the ground when carried in your back pack the resulting ratio of inertial change is 60 x 60 / 20 x 20 = 9. Get that, 9 times the rotational inertia for right and left leaning motion change when carrying your the battery on your back over having it low on the bike.
It is non-linear. Higher get worse very fast.
Here is the thing, your assuming the system works as a complete singular mass, but it doesnt.
I don't know the terminology as well as I should, but it seems you keep referring to center of gravity, but I think you should be considering moment of inertia and center of inertia instead.
Let us consider the requirements to execute a turn.
A bike, unlike a car, uses a concept known as slip angles to turn. The handlebars don't turn the bike, they only lean the bike. Once the bikes wheels are leaned over the sidewall running across the ground causes the wheel to "climb" into the lean, and while this happens the rider simply has to balance themselves and the bike against the centrifugal forces. Now I agree that the sum total of the rider and the bikes CoG is what matters when transitioning to a new direction or vector, once the bike is settled in the lean. But it's the moment of inertia that determines how "flickable" a cycle is. That and gyroscopic forces that we can ignore right now for this discussion, they are equal in either configuration.
So, I submit that the bike doesn't flick with the center of rotation being at the ground when moving. When done with maximum efficiency, a bike rotates at the center of inertia, as the bike sees it. And if your standing on the footpegs/pedals you can decouple the bikes center of inertia from the rider by rotating the bike well above the ground, putting the axis at the center of its mass. Did I make myself clear? Probably not. Let me explain what I mean. Imagine looking at a bike/rider from the front as it travels into a corner we think that the axis point of a bike leaning over is where the tires touch the ground. It's not. Because the bike is moving we can countersteer the handlebars this causes the tires to move to the outside as the top of the bike moves to the inside of the bikes path of travel, the axis that the bike rotates on is not the tires contact patches, it's the center of inertia, inside the frame. If the bike were stationary, then yes flicking the bike would require more input force because it has to rotate at the tires contact patch, which would result in a much higher moment of inertia when flicking side to side.
In fact, let's look at different types of motorcycles. A harley has a low center of gravity, compared to my Yamaha R1. But the R1 has a much smaller moment of inertia. Even if yoy account for weight,which one do you think is more flickable?
Ok, done. I may not have explained that well, sorry. I hope I got my point across there.
So how does any of that apply to a bike on the trail? This is where every one of you have more experience that me on an ebike, but the principals are the same, just at different proportions.
So, as I stated at the beginning of this story, the point is to lean the tires into the corner. The rider can stand and twist the bike on its axis independent from the rider when the rider pivots his mass on the pedals and can actually suspend himself/herself to act independent to the bikes lean. And, the rider can use this ability to actually change when and how their weight takes effect in the total mass calculations, so to speak. What do I mean by when? In a sense you can prepare for a corner as you approach it by positioning your body inside the direction of the corner while the bike is still standing verticle, allowing you to brake deeper into the corner when it is advantageous. Although that is more of a road bike technique.
In this scenario the pack being on the rider allows the bike to be more flickable if best practices are used to execute the turn. If the pack were on the bike then you theoretically would need to accelerate more mass side to side to get the tire in the correct orientation to the ground, which is on the sidewall.
