1993 BMW E36 Street and Track Build - Rear Subframe
The E36 chassis is an excellent platform for a performance car. However, there are some drawbacks that come with it as well. The E36 was the first chassis that BMW designed with CAD (Computer Aided Design), and though computers are pretty awesome, they are only as smart as the programmer of the software used on it. Now, before the E36, all BMW chassis’ were tested in the field with trial and error, which meant that they were all beefy, and can take a beating. Of course this may mean they came with some more weight. Well, one of the weak spots on the E36 chassis is where the rear subframe attaches to the unibody shell. When the rear subframe bushings wear out, the rear subframe flexes more and flexes the unibody, eventually causing a failure of the sheet metal where it attaches. This is bad.
Luckily, I was able to catch this car before it got to that point, and I welded in the OEM BMW M3 subframe reinforcements. Oh, yeah, an M3 comes with these from the factory!
These four holes are where these reinforcements will be welded in, but first the underbody coating and other street muck must be ground off and cleaned up.
Next was to weld in the reinforcements and paint them up. The cleaning of the area to be welded is NOT a fun process. I recommend a wire wheel attached to an angle grinder, along with a dust mask, hearing protection, goggles, gloves, and perhaps a brew, or four.
Another weakness of the rear subframe is where the rear sway bar attaches to it. With a factory sized sway bar it is most likely sufficient, but when you step up to a Turner Motorsports rear sway bar, these mounts are known to bend under the forces. Therefore, there is a bracket available to weld in and strengthen it up. Well, since we have a fab shop at our disposal, some reinforcement brackets were designed, cut, and welded in as a perfect fit.
The reinforcement was then welded in. A gusset was also welded in to provide some triangulation and eliminate twisting of the bracket. 14ga steel was used in the bracket, which is the same material used in the rest of the subframe. It is surely much stronger now than it ever was before. Now we can track the car without worry of the sway bar breaking the brackets in which hold it to the car.
Along with the rear subframe reinforcements welded to the unibody, AKG poly bushings were installed in place of the weak rubber ones that came with the factory. This not only reduced subframe motion, but also strengthens the rear end much more.