Norm Peterson
Corner Barstool Sitter
To Grant - sorry, but I never got any notice that the other thread had gone anywhere, and I'm now finding it locked. Hence this new one.
But first, a note about "roll center". It is not a point about which the chassis actually rolls, and certainly not in any SAE definition. What a roll center is, is a point at which a lateral force can be applied to the sprung mass without producing roll.
What that means is if a lateral force - meaning horizontal - is applied at any other height than that of the roll center, you will get some roll. The amount of which depends on (a) the magnitude of the lateral force, (b) the distance from the vertical point of application of that force, and (c) the total amount of elastic suspension roll stiffness present.
(a) is sprung mass times your cornering g's, (b) is the difference in elevation between the sprung mass CG and that of the geometric roll centers (both of them), and (c) is your springs, sta-bars, and during transients, dampers.
Since (b) depends on both geo-RCs, vertically relocating one of them will affect the amount of roll, assuming that (a) and (c) are held constant. And greater vertical separation between the CG and the geo-RCs means more roll. Has to.
In the other thread, I think that roll was getting confused with total lateral load transfer. I'm now talking about just the total load transfer here, not its front:rear distribution. Total lateral load transfer is what stays constant when you vertically relocate a geometric roll center, not the amount of roll.
I think it's best to keep in mind that vehicle dynamic effects are force-based (inertial forces due to acceleration, braking, and cornering maneuvers), and that displacement effects (roll, squat, nose-dive, etc.) are only visible indications that load transfer is happening. IOW, roll does not cause "lateral weight transfer", it's the other way around.
Total lateral load transfer (TLLT) has to be equal to (d) total car mass (sprung + unsprung) times (e) the height of the car's total-mass CG height (not the same as (b) above). There are three components that comprise TLLT, those being geometric lateral load transfer (load from the sprung mass transferred through the roll centers, no roll happening from this effect), elastic load transfer (load from the sprung mass transferred through the springs, bars, and dampers, where the roll comes from), and load transfer coming from the unsprung masses (no roll from this, perhaps with an asterisk).
At the point where lowering the rear roll center is on your mind, you're faced with a few choices. Whether to hold the amount of roll constant (outside front wheel camber), whether to hold the current total lateral load transfer distribution constant TLLTD, or allow those to vary for some other purpose.
And just when you think you've got this whole geometry-based roll center thing straightened out, try mentally letting the car roll a bit and watch what happens to your geometric constructions. You may find that this geometrically-defined roll center approach that most of us use isn't adequate to explain what's really going on (convergence points no longer converge, and those geometrically-defined RCs at rest are no longer definable geometrically).
Norm
But first, a note about "roll center". It is not a point about which the chassis actually rolls, and certainly not in any SAE definition. What a roll center is, is a point at which a lateral force can be applied to the sprung mass without producing roll.
What that means is if a lateral force - meaning horizontal - is applied at any other height than that of the roll center, you will get some roll. The amount of which depends on (a) the magnitude of the lateral force, (b) the distance from the vertical point of application of that force, and (c) the total amount of elastic suspension roll stiffness present.
(a) is sprung mass times your cornering g's, (b) is the difference in elevation between the sprung mass CG and that of the geometric roll centers (both of them), and (c) is your springs, sta-bars, and during transients, dampers.
Since (b) depends on both geo-RCs, vertically relocating one of them will affect the amount of roll, assuming that (a) and (c) are held constant. And greater vertical separation between the CG and the geo-RCs means more roll. Has to.
In the other thread, I think that roll was getting confused with total lateral load transfer. I'm now talking about just the total load transfer here, not its front:rear distribution. Total lateral load transfer is what stays constant when you vertically relocate a geometric roll center, not the amount of roll.
I think it's best to keep in mind that vehicle dynamic effects are force-based (inertial forces due to acceleration, braking, and cornering maneuvers), and that displacement effects (roll, squat, nose-dive, etc.) are only visible indications that load transfer is happening. IOW, roll does not cause "lateral weight transfer", it's the other way around.
Total lateral load transfer (TLLT) has to be equal to (d) total car mass (sprung + unsprung) times (e) the height of the car's total-mass CG height (not the same as (b) above). There are three components that comprise TLLT, those being geometric lateral load transfer (load from the sprung mass transferred through the roll centers, no roll happening from this effect), elastic load transfer (load from the sprung mass transferred through the springs, bars, and dampers, where the roll comes from), and load transfer coming from the unsprung masses (no roll from this, perhaps with an asterisk).
At the point where lowering the rear roll center is on your mind, you're faced with a few choices. Whether to hold the amount of roll constant (outside front wheel camber), whether to hold the current total lateral load transfer distribution constant TLLTD, or allow those to vary for some other purpose.
And just when you think you've got this whole geometry-based roll center thing straightened out, try mentally letting the car roll a bit and watch what happens to your geometric constructions. You may find that this geometrically-defined roll center approach that most of us use isn't adequate to explain what's really going on (convergence points no longer converge, and those geometrically-defined RCs at rest are no longer definable geometrically).
Norm