Inspecting Project E46 for Subframe Damage
No BMW E46 can escape the wrath and ravages of age and abuse when it comes to chassis damage.
Oh hello, and welcome to another electrifying episode of Project E46, the build thread based on the financial burden parked behind my house! A lot has been happening with the E46 as of late, so, for the time being, expect a regular segment about this budget Bimmer to roll out every Monday morning, or, at least until I am dethroned as Editor of the site (place your bets in the comments below).
In the previous episode, I tackled the rather large project of dropping the whole rear subframe and suspension out of the E46. Of course, I did this for multiple reasons. First off, replacing all of the 19-year-old suspension bushings in the rear end of the car will be imperative for track work. Then the more pressing issue at hand: Addressing the chassis and possible chassis damage on this $1,500 BMW.
When I bought the E46, I knew right away that I would be working my way towards this point. All BMW E46 models, with the exception of the convertibles, are prone to excessive chassis fatigue and failure. While there are many areas prone to being damaged with age, or hard driving, there is one area in particular that must be addressed, no matter what. E46 BMWs are notorious for so-called “subframe cracking,” or tearing, and if that sounds terrifying it should.
What is subframe cracking?
The rear subframe, which is the mounting point of the rear suspension, and differential is under a lot of stress. The suspension moves around, the driveline moves around and the differential distributes engine torque through the axles, driving the wheels, there’s a lot going on here.
That subframe is also affixed to the chassis of the car by four mounts, which is quite normal. The issue is that those mounts use soft, rubber bushings, to minimize noise, vibration and harshness (NVH) that the passenger may feel in the car from all of those things moving around. Again, not that uncommon. However, as those bushings fatigue from old age, hard driving, they allow that rear subframe to move around. All of that energy has to go somewhere, so it goes into chassis. Now, this would all be fine and dandy if BMW made the rear chassis structure nice and strong. But they didn’t.
Instead, the chassis is composed of multiple layers of thin metal where the subframe bolts up. All of that subframe movement and force will eventually cause the subframe to tear away at that metal. This results in tears and cracks forming in the chassis sheet metal. If left unattended, the subframe will eventually tear away completely from the fatigued chassis. This results in a big hole in the floor, and the rear axle scattering its contents on the pavement. It’s not a good look.
That sounds really dangerous!
Too right you are, it’s very dangerous! A quick Google search will show cars that have been structurally totaled, and, in a few cases, caused people to have serious traffic accidents.
“Jake, I think you’re annoying sometimes, but I don’t want you to die!” Is what you may be saying as you read this, and I appreciate the sentiment! Fortunately, if one is proactive about it, the chassis can be sufficiently reinforced before it becomes an issue. So, with the rear subframe off the car, now is the perfect time to fully assess the condition of the E46 chassis and make a game plan.
Let’s take a look at the four subframe mounting points:
The E46 rear subframe is secured via two studs in the chassis, and two bolts that go up through the subframe and into holes in the chassis. Generally speaking, the areas surrounding those two holes are the more trouble-prone of the bunch, due to the design. When the subframe moves, it allows the bolts to put more leverage against the chassis, increasing fatigue.
That said, initially, everything looked solid. Although I didn’t spot any obvious damage under the car when I bought it, it’s not really able to be diagnosed properly until the subframe comes out of the car. I was worried that my shot differential bushing could have been chassis damage in the making, but it seems like this car has been driven rather gently over its 19-years of existence.
While it’s a relief that there is no obvious massive damage on Project E46, that’s not the end of it yet. Since I will be drifting this car and thrashing it on the track, chassis reinforcement is still a must-do.