Originally Posted by lucid
Thanks for the write-up JAJ.
Not sure about your conclusion above though regarding the stiffness of the "lowering" springs. It is possible to use shorter but stiffer springs to achieve the drop (shorter natural length but higher k). That could offset some of the drawbacks of using up the "avaliable" travel built into the stock suspension since the stiffer spring would compress less under the same loading conditions. In that scenario, the main issue is changing the spring stiffness without modifying the damping ratio. In a properly tuned system, they need to be modified in conjunction if the modifications to the stock system are significant. Of course, one can debate what "significant" means.
The perch-perch distance at full droop can't be changed on the stock setup. The shortest spring you can install must be long enough to get from one perch to the other with no gap. Otherwise the spring will come adrift when you jack the car up.
So, the length is what it must be. The only other parameter is the rate, and the spring rate is determined by how far you need it to compress at static position. The softer the spring, the lower the ride.
Because BMW set the suspension up with preload, they created design flexibility to shorten the spring and stiffen it to about 200# per inch while keeping the stock ride height.
At that point the damping will be out of tune. However, if you now reduce the spring rate to lower the car, the damping comes back into tune as the car gets lower. The reduction of travel is a whole other matter - the best damper tuning can't compensate for the rapid climb in spring rate created by the bump stop.
The only "get out of jail free" card in this equation is the prospect of doing a bimodal progressive wind on the spring. It starts with a softer section that allows the car to settle most of the way to static, then that section goes into bind and the stiffer section takes over and controls the motion in the narrow stroke range above and below the static height. The Bilstein PSS10 spring shows this exact winding design.
However, the progressive spring strategy requires new dampers because the stock dampers are tuned for much lower spring rates. The car would be pretty bouncy on stock dampers. Of course, now that you have to replace the dampers, you might as well make them height adjustable - oh, wait! We've just designed a Type Two (street sport) kit!