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There's a lot to chew on when you start talking about moving roll centers around. Lowering the car will move roll centers around. Changing spindle location will move them around. So does moving suspension pickup points, like a 935 style suspension kit does.
As big or worse though is that when you move the suspension substantially away from its normal range, extraneous movements become more pronounced, like:
- side to side movement of tires as they travel upward and downward (scrub)
- toe change / bump steer gets worse
- bottoming can be an issue when lowering the car
Roll center changes can also become more extreme, but it becomes secondary to the above. As I believe David said above, the things that change roll center aren't always intuitive. The only way to really know where your roll centers are is to measure everything and map it. But it's not usually necessary to do it for a 911, as the mods that work are pretty well understood.
If you play around with suspension modeling, one thing you'll quickly realize is that everything is a compromise, and getting the "optimal" setup winds up being very specific to a given set of parameters (ride height, tire/wheel sizes, etc.), and as soon as you change a parameter, you have to change other things to re-optimize.
Raising the spindles addresses the issues pretty well in most cases when lowering a 911. 935 suspension kits take it a step further.
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