horizenjob wrote:
Explain why you consider lateral movement of the roll center an issue?
Who says I have a responsibility to educate? I have a long memory, and people who treat me as an a$$hole get treated as assholes themselves.
If mjalaly's friends are the same ones who told him that his simple, curved-tube spaceframe has a torsional stiffness in excess of 38,000lb.ft/degree, then their butts
ought to be well calibrated, admittedly, 'cos that's where they keep their brains. But I rather think that it's more likely to be a case of Colin Chapman's assertion that 'any suspension will work if you don't let it' (ie. make it stiff enough, and it won't move enough to get you into trouble).
Short answer to your question, though, is that cars have two ends, but only one sprung mass.You therefore need to think in three dimensions, and if you need to ask the question, I'm probably going to struggle to explain it to you without resorting to diagrams, but I'll have a go:
Easiest way to visualise it is to imagine the car static, with a magical piece of string attached to the CoG of the sprung mass, which can pass unimpeded through the structure. Pull the string directly to one side, and you're simulating roll force; directly fore or aft and you'd be simulating braking or acceleration. With me so far?
When you pull on this piece of string to simulate roll, the car does NOT try to roll around the front roll centre or the rear roll centre. It
tries to roll around the geometric roll
AXIS - the line that connects the two. I say 'tries', because it won't actually succeed... the differing F:R roll resistances from the springs will resist it to differing degrees, to force it into an attitude of 'skewed roll', with a component of pitch, but let's not muddy the water with that just yet.
If you give it a bit of thought, you'll appreciate that
any deviation of the roll axis from a straight, horizontal fore-and-aft alignment will geometrically try to force the car to try to 'lean' on one corner as it rolls. If you give it a bit more thought in terms of a very large lateral movement of the roll centre at one end, you'll hopefully be able to appreciate that it will introduce a large fore-aft component in the weight transfer.
Not necessarily a problem if the relationship of the roll axis to the sprung mass is fixed (ie. roll centres both ends fixed relative to sprung mass), and indeed you can use it as a tool to tune understeer:oversteer balance. But hopefully you'll be able to appreciate that if the roll axis is swinging around wildly
in whatever orientation, it will be playing merry hell with the diagonal weight transfer as it does so.
Anyone who tells you that lateral roll centre location is not important is thinking in only two dimensions.