When we lower our cars, the baseline suspension geometry changes. For most people, this is an acceptable trade-off for the benefits of a lowered car. We can use Roll Center Correction Kits to fix roll steer in the front, but in the rear, things are more complicated. Further, since I am considering a RWD conversion, the rear axles need to be leveled out so the cv joints will last longer.
Correcting Rear Roll Center
Currently, there are only two off-the-shelf options that I have found to correct rear roll steer on a GD chassis: Wisefab and TSSFab. Both add around 50mm of roll center correction, however, there are some differences between the two designs.
Hub & Bearing
Wisefab uses the STi press-in rear bearing (27 spline), which means you need to run R180 (27 spline) friendly axles. If you want to keep the R160 (25 spline), you will need to buy hybrid axles or make them yourself. GR WRX axles might work, but there could be some differences between press-in and bolt-on 27-spline hubs that prevent the CV joint from fully seating. The TSSFab rear knuckle uses the GR/BRZ bolt-on rear hubs, so you will need GR WRX or STi rear axles depending on your rear diff.
The Wisefab knuckles will accept the GD rear parking brake setup with some modifications to the knuckle for the brakes. You might be able to use sliding calipers since this is determined by the backing plate. I wouldn’t count on this solution without mods, though.
TSSFab uses GR STi or BRZ rear rotor. The GR parking brake should work since the backing plate is sandwiched between the hub and the knuckle. Since the caliper is directly mounted to the knuckle; GR STi Brembos bolt up to GR WRX knuckles, and BRZ rear knuckles are supposedly the same rear knuckle; it may be possible to use floating calipers from a BRZ/LGT. I have now, only with the smaller parking brake (170mm vs. 190mm).
However, the safest assumption is that Brembo calipers will likely be needed on either setup.
As a car is lowered, the angle of the axles increases. The wheel has a fixed height, so the lower the suspension, the lower the diff is from the hub. The only way to level out the axle on a lowered car is to raise the differential. The opposite happens when you lift a car, and the diff needs to be lowered. TSSFab and MSI (Moore Sport) both make rear subframes; however, info on the MSI subframe specs is hard to come by. And I don’t see subframes on their website anymore.
TSSFab has a “Geometry Special” rear subframe/T-bar that lifts the rear diff up, although I am not sure how much correction the relocation adds. Modifying the stock t-bar might be pretty straightforward; the mounting points need to be moved up; however, TSSFab sells a T-bar as well.
Either way, these are two mods that will probably sit on the backburner for a while. They are pretty expensive parts that require other fairly expensive components to work on my WRX. However, I think it is always a good idea to look ahead at mods coming down the pike.