All, I have a complex question about spring preload and its' effects, hoping someone has some solid info.
The front strut on a BMW is an interesting spot. You want to lower the car to improve the CG, while at the same time keeping the spring perch above the tire to maximize inboard tire space. The effect of this is that my struts have a very short total travel. With the damper part of the strut fully extended, I have around 2.0" of distance between the strut body and the bumpstop. (total travel)
Preloading the spring is the only way to adjust ride height or bump/droop travel. So depending on how high I set the spring perch on the threaded strut body, the spring will compress, and the car will sit higher or lower at rest. (static ride height) Depending on where the car sits, I could have 1" of each bump and droop travel, .5" bump & 1.5" droop, or 1.5" bump and .5" droop, you get the idea. I get one or the other, depending on where the car is sitting, which is determined by spring preload.
My corner weights are 885 & 829 for the front, and I'm using 5.5" 650lb hyperco springs. I have a solid inch or so I can move the spring perch up before I get to the top of the strut body.
So without any preload, (I can turn the springs on the perches by hand, but no up/down play) at full droop (wheel off the ground) I have 13.75" of ride height. (center of wheel to fender) At rest it's at 12.5" ride height, so I have about .75" bump travel and 1.25" droop travel. (Meaning the springs compress about 1.25" with the car sitting static)
So my actual question is, if I preload the spring, and raise the car, giving me more bump travel, does the car actually have farther to roll, and will it roll the same amount, or less? Am I making the 650lb spring act like say a 750lb spring, or am I actually reducing the effective spring rate, and just changing the roll starting point?
So for an example/exercise, what is the difference between a 6" 650lb spring compressed to 5.5", and a 5.5" 650lb spring with no preload?
The goal is to lower the car as much as possible, (advantageous CG) without either getting into the bumpstops or getting coil bind. Control arm geometry, from all reports, does not seem to be a big problem on the front of the E46.
It seems to me that if I want to avoid getting into the bumpstops, I should preload the crap out of the springs, (1"+?) or use a bigger front bar or stiffer springs or all of the above. But if preloading just sets the "starting point" for the roll, and the car will roll to the same overall position, it won't matter where I set the preload.
This is sort of a followup to the troubles I had last season with warm-weather push when I had 550lb springs up front, I was getting into the bumpstops whenever grip was high. at 650lb springs and stiffer bar settings, the cable ties say I'm getting right up to the bumpstops, but not as bad, and the car is definitely behaving better. I want to make sure this won't be a problem when I go to Milwaukee or Lincoln,