Project BMW E46: Chassis and Subframe Reinforcement Time

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All BMW E46 models are susceptible to this wallet-destroying problem.

Welcome back to Project BMW E46, the weekly segment from 6SpeedOnline‘s editor on his latest money pit, a $1,500 BMW E46. If you’re new to the series, then, hello, glad to see you’ve stopped by. You have some catching up to do.

This week’s inane rant regards the biggest epidemic surrounding the BMW E46 chassis: Subframe failure. If you’ve never heard of this epidemic, count yourself lucky. In a previous episode, I found subframe damage on the car. This is not at all surprising, all E46 models are at risk of this issue, and my car, being 19 years old and having over 215,000 miles on it, is a prime candidate.

What is subframe failure?

For those of you who didn’t click that last link, I’m not judging you (too much), I will keep this short and sweet. Basically, the rear subframe on the E46, like all cars, has an important job. It attaches the differential and rear suspension to the chassis by four posts. These large studs protrude from the rear floor pan and allow the rear subframe to be bolted to the chassis. All of this is quite conventional. However, in their infinite wisdom, BMW took an unconventional route when designing all of this. BMW E46 Project Car Build Thread

Normally, manufacturers make the portion of the rear floor pan where the subframe bolts up nice and strong. After all, that subframe is heavy and the suspension has an important job, we don’t want it going anywhere. Unfortunately, BMW engineers were persuaded by the bean counters in accounting, likely after many large steins of German bier, to use several layers of very thin sheet metal in that crucial area. BMW likely saved several fractions of a cent per car in production costs doing this, which made the bean counters very happy.

Less happy were the owners of these cars when, after a few years of driving, a large thunk and bang proceeded the rear subframe falling out of their $40,000 luxury sedan. The soft rubber subframe bushings that locate the subframe to the chassis eventually fatigue, allowing for excessive subframe movement under load. Combine that subframe wiggling around with the general movement of the rear suspension, andt that weak mounting area can’t handle all of the stress. This results in the sheet metal tearing apart as the subframe wiggles around. Eventually, the 150 pound subframe moves so much that it falls out of the car, tearing the chassis apart with it. That’s bad. BMW Project E46 Subframe Cracking Failure Damage Inspection

Holy sh*t

Yeah, no kidding. People unknowingly drove their cars into early graves, as this can result in extensive, and expensive damage to the car. There were also accidents, and lawsuits because when the entire rear suspension falls out of your car as you’re driving along, it tends to result in crashes. BMW eventually acknowledged the issue and offered a half-baked fix involving structural foam. Many people did not even get the fix because they were unaware it existed.

The issue is compounded when a car is driven hard, like a drift or track car. Fortunately, the aftermarket has addressed the problem head-on. A quick Google search shows various chassis reinforcement kits, with the most prominent being Turner Motorsport’s kit. So, I bought that kit, and now it’s time to do some more welding.


Jake Stumph is the Content Editor who runs 6SpeedOnline, and several other Internet Brands Automotive websites. He enjoys track days, drifting, and autocross, at least, when his cars are running right.

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