[blitzyo]

Right now I have my outer ball joint (front/rear lca) slightly higher than the inner ball joint.
Should I have them exactly level?
Is the outer allowed to be slightly higher or vice versa?
Do people add driver weight in the car when checking this?

I'm not technical enough to calculate the roll center or instant center etc.

Is there a guideline for eye balling it, so I don't have badly setup car at the very least?

Or am I better off not messing with roll center correction and let springs and dampers manage it?

[SCMTX]

Did you ever get any answers on this topic?

[blitzyo]

Originally Posted by SCMTX

Did you ever get any answers on this topic?

Nope, I've just got the all arms fairly parallel to each other and to the ground now.

[rbahr]

Roll centers move dynamically

I would suggest that you measure Bump - front and rear, adjust if necessary, corner balance and leave it at that, and yes put your equivalent weight in the drivers seat. I asked some folks and was unable to get information on roll centers and bump correction.

Ray

[D.Yooras]

Originally Posted by blitzyo

Right now I have my outer ball joint (front/rear lca) slightly higher than the inner ball joint.
Should I have them exactly level?
Is the outer allowed to be slightly higher or vice versa?
Do people add driver weight in the car when checking this?

I'm not technical enough to calculate the roll center or instant center etc.

Is there a guideline for eye balling it, so I don't have badly setup car at the very least?

Or am I better off not messing with roll center correction and let springs and dampers manage it?

General rule of thumb on cars with front struts: stiff front springs, stiff dampers, stiff anti-roll bar. Though you can modify roll center there are many aspects that would need to be addressed, and optimized, before messing with pick up point geometry. This is especially true with modern BMWs, they have the front strut chassis very well setup since they've been developing it for so long.

When it comes to setting up a car it should always be done with the intended ballast loaded in the vehicle. The angle of the lower control arm is a good reference but the real point you want to know the angle of is the center of the in-board pivot and the pivot of the lower ball joint. Generally, if you don't want odd suspension behavior, the lower control actual angle at 1G should be parallel to the ground or below/above parallel by a good margin. What you don't really want is a lower control actual angle that is near parallel to the ground and crosses over that parallel frequently, especially during bump/compression.

The most important thing that changes when the lower control arm actual angle goes below parallel (ball joint higher than inboard pivot) the camber curve flips sign and beings to go more positive which obviously isn't ideal. Additionally, when the lower control arm actual angle crosses that line (parallel to the ground) it's highly likely the bump curve will reverse sign as well. Nearly all modern cars toe out in bump/compression but when the actual angle of the control arm crosses that line, that corner could start to toe in which is generally not confidence inspiring.

If you don't already have one get a big front anti-roll bar (30mm seems to be the sweet spot). That will function as a significant tuning aid and is much easier to adjust at the track than roll center correction or bump steer kits. When tuning these cars, you're generally looking for an ideal pairing of camber and roll stiffness. A well sorted chassis (front struts) will usually be at, or near, 100% lateral load transfer to the outside front wheel during heavy cornering and the big front anti-roll is critical in facilitating that.

That said, it is a balancing act because if you make the front bar too stiff, you'll have the outside front tire become overloaded BEFORE that load transfer happens and the car will understeer. If it's not stiff enough, you'll have excessive body roll which will lead to excessive positive camber gain and an underutilized front outside tire which will also lead to understeer.

Lastly, remember that cars with strut front suspension play by a different set of rules than double wishbone. It's a compromised setup to begin with so the trick of making it work well also involves compromises.