Cracked subframe on an E46? Well its unfortunate that this flaw is very common and we’re getting really good at fixing them. So good that we put our lifetime warranty on all of our subframe repairs!
So what does it mean to have a cracked subframe? Well the rear suspension on a BMW all bolts together and is mounted to the car by 4 large bushings. The cracks that we’re talking about are surrounding where these bushings mount to. The metal in the frame in this area is just a little but too thin so when we reinforce it we weld in much thicker material to ensure no future cracks.
Another contribution to cracked sub frames is worn differential and subframe bushings. When the bushing is worn it will cause a clunk. What that means for the the subframe is a massive amount of load put in those already weak areas. If you’re hearing any clunking from the rear of your car its probably a worn bushing and its probably causing stress to these weak frame areas.
When we repair the subframe cracks we first weld the crack itself, then install a reinforcement kit which is welded in on top of that. When we’re all done we paint the welded areas and install the rear suspension components back into the vehicle.
Since the whole rear subframe needs to be removed to access these cracks its typically a good time to replace the subframe, differential, and rear trailing arm bushings. The labor is heavily discounted since the frame is removed from the car and these bushings typically need to be replaced anyways.
Some cars having this work done are used for track and performance applications in which we choose to upgrade the bushings with urethane or solid aluminum bushings. These upgrades are awesome for track cars but do add some discomfort for most street driven cars. We recommend going with a solid 75D red urethane subframe bushing if you’re going to upgrade the bushings. This bushing will seriously tighten up the rear end of the vehicle and allow zero movement from the rear frame and differential assembly. The down side of these is you now hear ALL of the clunking and whining from the differential – Not a big deal in a race car but can get on your nerves when you’re stuck in traffic. A solid aluminum bushing is the most “race car” bushing we can go with. Installing these bushings mounts everything metal to metal and creates an awesome whine from the differential while driving and makes your car sound kind of like this – https://www.youtube.com/watch?v=eSJ-HzNWJGM . Unfortunately with these bushings we don’t warranty the reinforcement for life but we’ve never had one tear on us. All of the new M3 and M4 cars come with a rear subframe that’s mounted directly to the chassis so BMW ended up doing this design anyways – No idea how they got rid of the differential noise in the new cars.
Here is a car we installed solid aluminum bushings in – https://www.youtube.com/watch?v=xFZwSlkBiXE&t=79s
No matter which bushings you decide to go with please take our advice on the R-tabs or rear trailing arm bushings and in some cases – stiffer is not always better. See our article on RTABS