BMW E46 Rear Subframe Failures
As advanced and solid as the E46 feels it has a major weakness in the rear suspension: the subframe rips away from the chassis. The subframe is bolted to the chassis at four points under the trunk floor. The attachment points where it gets bolted are spot welded to the sheet metal of the floor. As the rear differential transmits the torsional load from the engine through the subframe, then into the chassis, the sheet metal flexes and twists and will break the welds with additional cracks forming through the sheet metal. Worn rubber subframe mounts add fuel to the fire and promote further twisting. This constant loading and unloading weakens the sheet metal and causes it to fatigue and separate from the chassis (see images below). The issue is widespread on all 1999-2006 E46 sedan and coupe models (we have not seen any E46 convertibles with this issue). A very similar issue preexisted on the E36 models and BMW fixed it by welding reinforcment plates on the floor.
BMW tried to fix the issue during the design of 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 we have seen many failures of the chassis on low mileage street cars and race cars.
Our Turner E46 rear subframe reinforcement kit thickens the sheetmetal over a greater area. This spreads the load over a wider and stronger area than before. We also include instructions and new sheetmetal for reinforcing the top of the subframe mounts in the trunk floor, an often neglected location that also fails. This kit will save time when repairing the chassis, and is also strongly recommended as a preventative maintenance item. Click here for more information on this kit.
BMW's official fix for the E46 was to inject structural foam into the frame rails and the void between the trunk floors. This increased the rigidity but did not address the weak spot welds holding the subframe mounts to the body. It also makes chassis repairs more difficult and affects the rear crash structure of the chassis. It also doesn't help if you already have cracks in the rear floor. In our opinion, from having repaired street and track cars, the best fixes for the subframe problem are:
a) making the attachment points stronger by completeng the spot welds
b) reinforcing the sheet metal floor by adding more metal and spreading the load over a greater area (the tried and true solution done on the E36)
c) using solid subframe mounts that will not twist under load (BMW went to solid mounts on the F10 M5 and F8X M3/M4)
Below are photos of subframe damage we have repaired in house.