roll center movement question

[GSpeedR]

Originally posted by Alex Kwan:
So... after reading the Olley book some more, he says it is "usual to ignore the slight side shift of the roll center O which occurs when the car rolls". The Milliken guys say, uh, actually "the side shift in the roll center may be large and it may make sense to use a more sophisticated type of analysis".
Is there some type of simplified equation for lateral roll center migration with roll like Olley's for bump? I'd like to know exactly which parameters affect it, and which don't.

I like the procedure given by Mark Ortiz in one of his RCE articles. He uses a method analogous to modeling anti-effects (pitch, dive, etc) in the roll front-view plane. The lateral location of the RC is not dependent upon the orientation of the instant centers but on the distribution of lateral force between the inside and outside tires (specifically he calls it "undefined"). He develops a "resolution line" 75% of the track width away from the outside tire to account for the outside tire generating 75% of the total force between the pair. The roll center is never outside of the track width, which I completely agree with...if the RC is outside the track then it's more a "heave node" or something (remember the pitch and bounce nodes in the x-z plane). In any case, ignoring the effects of the tires is a poor approximation. The RC height is the average height of the force-line intercepts on this resolution line...similar to the SAL method everyone has always used.

75% is an estimation since we don't know how much force a tire is producing between a pair. If you have the tire data you can determine a more realistic lateral force percentage based on the measured inputs to the tire (vertical load, alignment, estimated temp, etc.). I never trusted it when ADAMS tells me my RC is 3.72 miles from the center of the vehicle in roll.

[GSpeedR]

Originally posted by Marshall Grice:
my main concern would be car setup. If you design the RC to stay fixed relative to the ground regardless of ride height and you set the car up with a different ride height(parallel wheel travel) what does that do to your roll stiffnesses? Example, you raise the car 1" front and rear. This should translate to 1" higher CG. So now your CG is 1" higher and your roll centers are at the same spot. This may not have an effect on overall balance if the roll centers do stay in the same spot relative to each other. It would increase roll however.

In many/most cases I think the RC will also raise when you raise ride heights, as the force-line angles will increase (ie raise the IC heights). You'd need to recalculate the roll moment to say whether it's bigger or smaller.

[nopanon]

Discussing the influence of roll center movement on roll stiffness, it shouldn't be forgotten that there is more to roll stiffness than RC height. I think that the progressiveness in your suspension, which is most likely there, will have a much greater influence on how your roll stiffness distribution alters with picth, than what the roll center movement does in the normal case.

[Marshall Grice]

edit: Olley's formula (if my math is correct):
height of RC change =
t/(2h) * (H2/R2-H1/R1)
where:
t = track
h = distance between UBJ and LBJ
H1 = height UBJ
H2 = height LBJ
R1 = length upper A-arm
R2 = length lower A-arm

so exactly what does equation tell you? it's obviously unitless. My numbers give me "1.75" and i'm just curious if that is like an in/in relationship or what

[kwancho]

Yeah, inch of RC movement for inch of bump, I think... Methinks I wrote down that equation wrong. I'll check tomorrow.

[awhittle]

Design the roll center height to be wherever you want it verticaly and such that in any position where the suspension can get to the roll center does not move horizontaly more than an inch or so. If the camber curves are what you are wanting to prove to the judges, you are done and move on to the next subject. The vertical distance from the CG to the RCH will change some with vertical suspension travel. You just have to accept that.

[GSpeedR]

Unless you're thinking with a kinematic RC I don't believe you can constrain it laterally.

[awhittle]

DohertyWins

Try these numbers in your suspension program. You can get real close to static lateral.

AW

[GSpeedR]

Originally posted by awhittle:
DohertyWins

Try these numbers in your suspension program. You can get real close to static lateral.

AW

With a kinematic RC, yeah I agree you can keep it at the center of the car. There are different ways of calculating the RC location (force center like ADAMS, or tire-based like Ortiz) and a method based on control arm angles seems to be the least accurate way.

[Marshall Grice]

AW,
I'm curious why you say to keep the horizontal RC location as stationary as possible. I've never seen/heard that the horizontal position is relavant to handling balance. Care to explain?