In the never ending pursuit of creating a strong and light unibody chassis BMW occasionally makes mistakes. The E36 chassis has a few structural weak points, the front strut towers, the rear subframe mounts are some of the most common failures. In order to keep the frame as light as possible BMW used some thin sheet metal in some fairly critical areas. When these chassis are asked to bear some heavier loads than originally intended, they can fail. In the case of the front strut tower this can happen due to higher spring rates and camber plates. The rear subframe can often fail due to the increased torque of a modified engine or higher suspension loads and track abuse.
On the rear subframe of the E36 BMW created a fix for the chassis and made weld on plates that could be welded on retroactively to the early M3 and the rest of the E36 models if necessary. Anytime we have a car that will see track time in our shop we recommend they consider adding rear subframe reinforcements when changing suspension bushings or doing a coil over installation.
Reinforcing the subframe mounts requires that the entire rear subframe be dropped. The plates are then temporarily bolted in place with short bolts. The plates include holes for rosette welds to be made. These attach the plate to the frame and leave a nice flat surface when TIG welded. We typically also add a couple of stitch welds along the exterior being careful not to overheat and warp the sheet metal. We then paint and re-install the rear subframe along with whatever bushings/suspension components we decided to change out at the same time. Since we also have a complete alignment and corner balancing system we can also re-align our customers cars after the reinforcement has been done.
For more information on reinforcing any part of the E36 or E46 chassis please contact us for a quotation.