We recently had a customer who was running the Intraxx true real coil over on the E46 M3 and wanted to have the rear shock towers reinforced along with a rear strut brace welded in. Here’s what we thought was best.
The design is a compromise as the owner doesn’t have a roll cage (which would be ideal). The vertical loads are distributed to the vertical tubes coming up directly attached to the rear subframe mounts. Those were reinforced at the same time. 1/8″ cold rolled steel was used to reinforce the shock towers and the rear subframe mounts. We designed this all with the expectation that the customer may eventually want to have a full roll cage installed so we made sure there would be good nodes/attachment points to tie the rear downtubes into. The tubing is 4130 alloy 1.5″ .083 wall thickness. The customer wanted this as light weight as possible while still adding rigidity so we chose to use the chromoly with the thinner wall. We feel this is about the best solution you can have without having a roll cage installed.
Because of the S54’s torque the rear differential causes constant pulling and twisting on the rear subframe mounts in this area which can eventually pull them out of the sheet metal entirely. It’s important to reinforce the top and bottom mounting areas of the rear subframe. The nice thing about this setup is it also distributes some of that load from one of the rear subframe points to the other.