Great thread.. thx for sharing. I am also very worried about availability of parts going forward.

I have two extra sets of 996GT3/Cup uprights with hubs if anyone is in a bind and needs a set. One pair is virtually brand new with about 6k miles

Hey Pete,
I have a pair of used 996GT3/Cup rear wheel carriers (side members) if you need a set. Again, I would be very cautious of blindly using Cup/RSR carriers or uprights as they are based on a car running 27.8” rear wheels at super low ride heights.

Tire size plays a HUGE part in altering the roll center. An RSR runs a 27.8" tall rear tire in conjunction with a 100mm rear ride height which is allowed to go as low as 88mm. That is a 2.6" taller tire than the the stock tire on a 6GT3 which means that you would need to lower the rear ride height a full 1.3" to get the chassis to the same starting point. By doing so, you are drastically lowering the roll centers as a result (due to the change of the inclination of the rear control arms) and creating a bigger spread between the CG and the roll center, which is not good. To correct for this, the pick up points on the RSR wheel carriers are lowered another 1/2" or so (compared to the 6GT3RS) and the RSR side member pick up points are raised by a similar amount. This is all done to raise the roll centers back up to get the geometry to an optimal point. Now, take into account that the RSR runs yet another 1" - 1.5" lower ride height than the 6GT3 which further necessitates the raising of the roll centers. When you use the RSR components on a street car with it's small tires and high ride height, you far from optimum geometry wise. To correct, you would need to increase the rear tire diameter as much as practical and lower the car as much as possible. Due to the constraints of the street ABS, wheel well limitation and other factors, you are very limited with what you can do on a street GT3.

One thing I would do as a starting point if trying to optimize a suspension is to take a look at an RSR or a Cup car and carefully measure the degrees of inclination/declination of both the front and rear LCAs, thrust arms and upper control arms at static ride height with weight on wheels. Best way to do this is on an elevated alignment rack. If one is not looking to do a highly sprung race set up, then looking at these items on the late generation 997GT3s is also beneficial if you are looking to get a starting point of a dual purpose track/street car that runs similar ride heights to our cars. I would then shoot for those numbers as Porsche spent a lot of R&D getting the suspensions dialed in on the late gen GT cars like the 3.8RS or the 4.0.