Here is the Garagistic E30 (and Z3) rear trailing arm reinforcement kit. Every kit includes 2 shock mount gussets and 2 "wing" style gussets that connect between trailing arm mounts.
WHY REINFORCE THE TRAILING ARMS?
Well, the E30 trailing arms have a lot of flex in them. BMW knew this and when they added more power in the Z3 m coupe to that rear suspension they reinforced the arms.
These reinforcement connect the 2 mounts creating an A type shape (similar to the m coupe). Similar gusset concepts were used in DTM cars. Be sure to add the rear sway bar reinforcements at the same time. It is important to go slow and be careful not to warp the arms when welding these.Will require welding. The wings must be bent as they are welded to fit shape on control arm (see instructions).