The E46 rear driveline and suspension are very similar to that of the E36. An unfortunate side effect of this is that the E46 also shares some of the same problems as the E36. One of these problems is the rear mounting points for the subframe, which can rip out from their spots in the sheet metal. This happens because the rear differential transmits the torsional load from the engine through the subframe, then into the chassis. This constant loading and unloading weakens the sheet metal and causes it to fatigue and separate from the chassis. BMW tried to fix this problem in the E46 by placing a cross member on the front two mounting points of the subframe. This cross member keeps the front mounting points from having problems by placing the bushing and the stud in double-sheer, thus reducing the twisting action on the sheet metal. The unfortunate side effect of this is that the load from the front of the subframe now gets transmitted to the left rear of the subframe. This is where Turner Motorsport has witnessed many failures of the chassis on low mileage street cars and race cars. BMW has also tried to prevent the issue by injecting foam into the open space between the underside of the chassis and the top of the trunk. This seems to work but that does not address the issues of the spot welds failing on the sheetmetal. It can also make any additional work there harder and almost impossible to weld against. The Turner Motorsport reinforcement kit has proven over and over again to be effective and the easiest to work with in the long run.
This product reinforces the chassis sheet metal at the subframe mounting points. Turner accomplishes this by thickening the metal and distributing the load over a larger area. This kit will save time when repairing the chassis, and is also strongly recommended as part of preventative maintenance.
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