January 5, 2019

PART 1 – Will my E46 subframe crack? How will I know?

This article is intended to help those new to the E46 3-series chassis or not overly familiar with the rear subframe mount cracking issue. Be advise, CMP Auto Engineering is not liable for any damage to property or harm to yourself or others that occurs while inspecting your car. How the car is inspected and the safety precautions taken is at the discretion of the individual.

Despite the beliefs of some, all E46 3-series BMW’s are prone to cracking at and around the rear subframe mounts. No matter the shape, model or way it is driven, subframe cracks are inevitable.

The common use term of ‘subframe cracks/failure’ is technically incorrect as it is not the subframe that cracks but rather the section of the chassis the rear subframe mounts to known as the Rear Axle Carrier Panel (RACP) although, the most frequent early sign of failure within the structure occurs at the subframe mounts themselves hence the terminology. During this article we may change back and forth between RACP & subframe so sorry in advance if it causes some confusion.

There are common misconceptions that lead people to believe the issue isn’t universal to all E46’s such as convertibles having thicker floors, some models having additional or differently designed structures or that it only occurs in the earlier manufactured models not the later face-lift models.
Unfortunately, all of the above is technically incorrect. All E46 3-series models built roughly after Feb-2000 share the same Rear Axle Carrier Panel and there is no E46 entirely safe from ‘subframe cracks’.
Although, there is in fact some truth in the above that different models and years will fail differently and may need attention in areas that others do not however, we will come back to that later.

Before getting any further into the where’s and why’s, we feel it would be best to have a fundamental knowledge of fatigue related failure to help you the reader better understand why the E46 subframe mounts crack.

Why do cracks occur?

Cracks do not occur because the applied stresses (force over area) are simply so great that the steel tears but rather that the stresses are great enough repeatedly flex (strain) the structure back and forth in cycles. These cycle stresses are well below the strength limit of the material however, with time the sheet metal is progressively weakened and made more brittle to the point it fractures (cracks). This is known as fatigue failure.

The point at which fatigue failure occurs is known as the fatigue life. Fatigue life is dictated by the magnitude of the stress and the amount of cycles applied over time. To apply this theory to the RACP, this means that a car with a higher power output (higher stress) will theoretically fail sooner than an identical car of lower power output (lower stress).
This is why failure is most common and severe in the M3’s as opposed to lower power non-M variants such as a 318i however, remains inevitable none the less. The greater the stress, the greater the strain (chassis flex) the shorter the fatigue life.

The other variables being the amount of stress cycles applied over time can be perceived by the amount of km/miles travelled however, the reason not all cars fail at the same mileage is because not all cars are driven the same way.
A car that’s racked up consistent highway miles cruising along is experiencing a smaller, constantly applied driving forces against roll/wind resistance as it is only maintaining velocity in a taller gear for a prolonged period of time while another car with much lower km’s would could fail sooner if regularly taking off, changing gears and having to slow back down to a stand still for traffic reasons applying a constantly varying stress on the subframe mounts due to the changing wheel torque from changing throttle inputs & gear ratios. The result is that the stress applied to the rear subframe mounts is greater due to the lower gearing and cycled frequently from the constantly changing speed.

This means that even low mileage pristine E46 examples may have subframe cracks as bad and potentially worse than their high mileage counter parts. This is supported by the fact that we have personally witnessed (and still own) a previously city driven 2002 E46 M3 which suffered fairly severe cracking at the front right and rear lefts subframe mounts at only 96k, km/~60k miles and another M3 of the same year with 245k km/ 150k miles from long highway trip with only very minor, early cracking. There have been many such examples of this however, outliers always remain.  

Now that we know that subframe cracks are inevitable and the exact point in time it will occur is inconsistent and cannot be predicted accurately, the best course of action is to act preventatively before the structure is excessively weakened and fails however, this may not be an option for all and many who do act preventatively often find cracks previously not noticed or hidden beneath components or sealer.
As these cars continue to age, subframe cracks are becoming increasingly frequent on even the lower power variants and unfortunately more severe.

Regardless of when the issue is being addressed, inspecting your vehicle regularly while budgeting to do the repair is highly recommended to ensure your safety and that the state of failure is in fact repairable. Further inspecting the RACP during installation is critical to achieving a comprehensive repair & reinforcement.

How do I inspect my E46 for subframe cracks?

This section of the article will cover simple areas able to be inspected while the car is still assembled to help those unsure about the state of their RACP perform an inspection without having to pull things apart. Due to it being based on a still-assembled back end, it is a simplified list that won’t detail all areas that can fail, but the most common, early signs of failure. The remaining failure prone areas will be detailed in a following article followed by another covering why and how to address it.

The subframe itself is mounted using four points of contact beneath the car between the rear wheels and as mentioned earlier, the most common first sign of failure within the RACP structure is cracks forming at the rear subframe mounts. You may also recall that we said every E46 built after ~Feb 2000 has the same carrier panel however, these cars were produced prior to then (1998) and thus you will need to adjust what you’re looking for based on what year model it is. 

Pre-Feb 2000 build date Vs. Post.

If you have found this article it is likely you’ve already done a bit of research on the issue and scoured the internet finding many images of E46 subframe cracks & failures.
What you may have noticed is that the subframe mounts themselves fail in two distinctively different ways.
the first being that cracks either form at and around the rear subframe mounts and all sorts of other places as shown above or, the much more intimidating type of failure that results in a gaping hole in the floor due to the threaded insert within the RACP tearing out entirely.

This discrepancy is because there was a change made to the design of the threaded insert within RACP that altered the way it was secured within the chassis. This minor revision made a dramatic change in the way failure occurs and by default, what to look for when inspecting the subframe mounts for failure. Be aware that there was a transition period where some could be either or so if you’re unsure, how to inspect the carrier panel to confirm will be covered soon.

The images below illustrate the physical appearance of the revision. The pre-Feb 2000 insert is secured within the RACP using three MIG welds at the top and some spot welds on the bottom. The result was that the top welds tear from the unibody and the threaded insert progressively punches its way out the bottom. The later revision from February 2000 onward included additional flanges contacting the vertical faces with an extra four spot welds and two MIG welds acting in shear.

The addition of these welds eliminated the possibility of the threaded insert ripping out as it did prior. Although the change had positive result, failure was no longer isolated to just the rear subframe mounts but rather caused fatigue failure in stress risers throughout the RACP.

How do I know which insert my E46 has?

For starters, only non-M3 E46’s had the earlier threaded insert however, if your E46 is built within or close to the transition period there is a relatively simple way to check which threaded insert your car has.
When under the vehicle (and safe to do so) look at the vertical face in front of the rear subframe mounts and behind the diff. If you can see two rectangular indents on that face in front of the mount points (shown below) then your car has the later, if not, the earlier.

Where should I be looking?

For a preliminary inspection, there are three key areas that are both the first and most severe points of failure and can be easily inspected with only minor effort (e.g jack up the back of the car & remove the rear wheels). The remaining points of failure that will be covered in a later article are quite a lot more difficult to inspect and may require some mechanical components removed, interior removed and some sheet metal cut out to inspect beneath.

The subframe mounts.

Pre-Feb 2000 models;

For the earlier examples, the best way to check for failure without cutting into the car from above is to look around the front, right and rear, left bushings on the underside face for cracks and bulging. If cracks in this area are visible it means the top welds have likely failed and catastrophic failure is imminent. The risk here is that failure escalates very quickly and it can take just a few short drives to go from a small crack to a gaping hole.

Front right;

For later examples, the front right mount is still prone to failure where the bushing contacts the chassis specifically on the right side of the bushing eventually traveling horizontal on the radius between the underside & vertical face towards the diff.
At this level of failure creaking could be heard while in low gear coming on and off throttle.

Rear left;

The second common point for fatigue failure are cracks just inbound of the rear left subframe mount above the rear sway bar bushing initiating on the radius between where the underside steps down and the vertical face behind the diff.

Rear Wheel arch;

The rear wheel arch often fails at the same time as the subframe mounts however, frequently goes unnoticed and neglected during a repair/reinforcement plate install only to cause trouble later on. In some cases, the secondary failure due to this area can be far worse than the initial failure if left unchecked, the floor will begin to tear away from the rest of the chassis potentially making the car un-repairable.

Even if it had not failed at the time, it often occurs soon after and is consistently the most common point of secondary failure.

The particular area to examine is directly behind the rear left dampener at the bottom of the wheel arch. Viewing from the wheel arch, failure can be seen as a split in the brushed on seam sealer ~5mm above the bottom as shown below.
If viewing from beneath the car, popped spot welds can be seen on the inner side of the wheel arch flange that protrudes downward.  

Alternatively, the rear right wheel arch seam does not pop the spot welds but rather often cracks on the inbound side just above the spot welds. It can often be hard to see however, with time will run from the bottom face then, up the vertical face towards the spring perch.

There are many other areas prone to failure however, they are often subsequent to the above failing first and for now, we’d like to keep the list simple and short to not cause any sleepless nights or overwhelm any readers new to the issue and/or E46 chassis.

What’s the worst thing that can happen?

To conclude on this article, we thought it would be wise to touch on the seriousness of the issue and consequences of what can happen if RACP failure is neglected.
The subframe mounts and thus the remainder of the RACP cracking is not just a negative for noise and chassis rigidity but, is a serious risk to all occupants of the vehicle and other road users/pedestrians.

The rear subframe houses the differential within and also provides mount points to the control arms and sway bars. The upper control arms have the rear springs mounted on them, divorced from the dampener and supports the weight of the vehicle. Therefore, if the subframe moves, these components move to. If the subframe where to separate from the chassis, so would much of the suspension and driveline.

For this to happen, and it can, portions of the RACP or for early models, just the threaded insert within, will have completely separated from the remainder of the unibody structure. This means that the integrity of the chassis is not just compromised in accidents for occupant safety but that the rear end stability and the ability to control the vehicle can be to. When a complete loss of control occurs, an accident often soon follows.

Our recommendation is that if you haven’t already addressed this issue, start planning and budgeting now. It’s something much easier to tackle preventatively than retrospectively. If you have, be sure that it was done correctly so your car doesn’t suffer unnecessary secondary failure like that shown in figure 14 & 15.

Need someone to talk to?

We will be releasing articles in future detailing all the other areas prone to fail and why, as well as what is needed to achieve a permanent peace of mind solution. If your car has suffered from subframe failure, you’re wanting to act preventatively or simply have questions about the above, please do not hesitate to contact us directly.
We are available to discuss this issue or any other aspects of your E46 with you and willing to answer any questions you may have via Email or Facebook.

We are E46 enthusiasts like many of our customers and love the chassis despite its flaws and open to provide whatever guidance we can to help others deal with this issue correctly and permanently in a manner that suits your goals and budget. Whether it’s chassis issues, suspension recommendations or build goals we’re always available to chat.

E: info@CMPAutoEngineering.com

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