AAF E30 Solid Rear Sub-frame Bushing Set. A pair of light weight aluminum bushings keeps the rear sub-frame from shifting under load. Since the rear sub-frame is not only the mount for the differential but also the suspension pick up points, it is crucial that it remains stiff. Rubber or poly mounts will allow the suspension pickup points to move under load, and affect the vehicles wheel alignment. These solid bushings prevent this, and that equates to more grip. If you have an E30 race car do not waste your time with poly or rubber, you need solid bushings.