Quote:
Originally Posted by ZDan
The problem I have is with the RC height curve shown here (with context):
The roll center height plot is saying that:
1. At -1.05" ride height, roll center starts at zero (ground level) at zero roll, and rapidly goes CRAZY HIGH (way above ground) with any roll in either direction.
2. At -0.95" ride height, roll center starts at zero (ground level) at zero roll and rapidly goes CRAZY NEGATIVE (way below ground) with any roll in either direction.
Even with perfectly rigid spherical bearings all around, no way that's what happens. Such a car would exhibit zero roll in cornering at -1.05" ride height, but would immediately flop all the way to the bump stops in corners at -0.95" ride height.
I still strongly believe the math model is breaking down with roll applied when static RC height is at ground level and giving unrealistic RC height change with roll.
The roll center height calculation is based on an intersection point of a line that is tending out towards infinity. In that region, you get to a point where you can't calculate RC height with enough precision to give reasonable results.
You can run the numbers and get results, but when basing RC height which is on the order of inches on a line that's going out billions of miles and beyond, the calculated RC height value becomes kinda meaningless...
I would bet that a physics-based vehicle simulation model would give different results, and I'm *certain* that an actual FR-S/BRZ won't behave like it had infinite roll stiffness at -1.05" ride height and zero roll stiffness at -0.95" ride height even with spherical bearings all around.
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I second this motion
. These are limitations to the geometric method. That's why I would like to see a force based simulation, or track data with wheel force transducers (at $50k/wheel I don't think anyone has this...) so that I could calculate the force based RC myself. Or as @
Amaya was hinting at (even though he didn't know it..) was skid pad data. They don't put rigid suspension in for this since they want the car to behave as it's supposed to.. the hardest part about skid pad testing is getting a driver to be able to maintain constant lateral g (and you need lots of sensors). They DO however put solid suspension rods in when doing K&C testing (which can also give you force based roll centers!), which is an entirely other animal used to test the compliance in a vehicle (how much does it move when forced through metal bending and bushings compressing).
The geometric RC has some merit (it's better than nothing and is typically all that is used for FSAE teams to design a little racecar) and is much easier to attain, but it doesn't tell the entire story and it's important to understand where it does and doesn't work (like all simulation analysis).