July 13, 2019

PART 2 – It’s Not Just The Subframe Mounts?!

That is correct. As we mentioned in the previous article, fatigue related failure is not limited to just the rear subframe mounts. This issue is so much more complex that it expands not just beyond the underside of the car but into sections of the unibody structure beyond the Rear Axle Carrier Panel (RACP) itself.

In our previous article ‘Part 1 – Will my subframe crack? How will I know?’ we laid the foundations of understanding fatigue and the basic reasoning as to why cracks occur at the within the unibody structure of the E46 chassis. We also aimed to provide guidance on where and what to look for when performing a preliminary inspection to determine the current state of your E46’s subframe mounts and nearby failure points within the RACP.

This article is intended to expand on the understanding gained by the previous by focusing on all the other areas prone to failure in the E46 chassis as a result of this subframe/RACP issue. This article will be a little longer than that last as there are many things to cover and we also included our personal recommendations on how to approach repairing and reinforcing these problem areas.

This article is intended for those taking their subframe reinforcement project from the planning stage to actively repairing & reinforcing the RACP and surrounding structure. Some of the inspection areas and repair methods detailed in this article require cutting and welding which as you can imagine, is not suggested (nor possible in some cases) on a car that is not completely disassembled and knowingly out of commission for some time. Performing these works can take several days and can leave you stranded if needed the vehicle and unforeseen variables caused project delays.
This is a very serious job to undertake and we can assure you, not one you’ll want to do twice. Planning is critical and if you feel uncertain about it, don’t hesitate to contact us for general guidance whether it’s the repair itself, bushings suggestions or you just feel like talking cars. We’re very passionate E46 owners at CMP and always love to speak to likeminded members of the community.

In the hope of preventing this article from becoming a novel, this article will be limited to just the specifics of where failure occurs and just a general method of repair for each one. This information is just an abbreviation of the installation manuals that come with every subframe/RACP reinforcement kit sold by CMP Auto Engineering Pty ltd that provide more detail.
An analysis as to why each of these modes of failure exist and why we encourage the reinforcement kits detailed in this article and how they function will be detailed in a third and final article.

We hope that this technical series will be beneficial to any reader wanting to expand their knowledge of the issues whether performing the reinforcement works in a DIY fashion or professionally with the goal that by sharing this information, every reinforcement job will be fully comprehensive and permanent.

It is worth mentioning now that some of the suggestions we will be making are designed to repair damage however, in some cases not correct the inherent flaw that caused it.
This means that there is a probability and some cases inevitability that it is prone to occur again in the same area or the areas between the repairs & reinforcements added. Due to the inherently flawed design, the structure is simply prone to fatigue in key areas that underside reinforcement plate kits and stitch welding cannot solely correct.
For permanent peace of mind, additional chassis structure installed from above the RACP is required to provide reinforcement, correct load paths, eliminate stress risers and increase structural integrity overall.
If you would like further information on the RACP/subframe reinforcement kits CMP offers for the E46 chassis, there is a brief detail of each product on their respective product page on this website. If you would like further information, you are more than welcome to contact us directly and we will do our best to provide you with answers to your questions and help you take the right path to resolving this issue on your vehicle. We personally enjoy having the opportunity to discuss things such as this with our customers so that the extent of reinforcement we refer best suits the customers long term plans with the vehicle and budget with other factors taken into account such as their vehicle shape, year, model as well as driving style and project build goals.

If you’d like to take us up on this offer please feel free to contact us Facebook, Email or phone anytime and we’ll get back to you as soon as possible.

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Forms of failure

Throughout the RACP and surrounding structure, the failure can be simplified to two key forms;

  1. Cracks forming
  2. Failed spot welds

The reasoning may differ depending on location however, the method of repairing each form of failure is generally consistent regardless. For this reason, we thought it would be wise to detail how we suggest repairing these forms of failure up front to avoid repetition through the article.
If any areas have suffered significant failure to the extent that the RACP structure has begun to shift or deform, provided it is still deemed repairable, the carrier panel must be returned to its correct and original position prior to repair.

This information is intended as a guide only and assumes the car is safely elevated and correctly disassembled, appropriate Personal Protective Equipment (PPE) is being worn and with essentially all components beneath the car rearward of the gearbox removed.
How the vehicle is inspected and the safety precautions taken is at the discretion of the individual(s) doing so. The fabricator performing the works in this guide should sufficiently research the appropriate welding methods and settings prior to undertaking the works. CMP Auto Engineering Pty Ltd is not liable for any damage to property, possessions or harm to readers or other persons present while inspecting, repairing or operating a vehicle.

Please be mindful that we do not commercially offer this service and the imagery available to us is limited to our own development vehicles and those shared by customers we assist remotely. If the image shown does not exactly match the description provided, please follow the wording of this article. If you have any images you feel are better than the ones we have used, please feel free to share them with your permission and we can update things accordingly.

Repair Methods

Crack Repair;

Cracks can sometimes be quite obvious like that shown in Figure 1 or, not as obvious like those shown in Figure 2. Take your time to thoroughly inspect all areas between the wheel arches in line with the front and rear mounts as well as between the four mounts themselves. If a crack goes un-repaired. I very likely will continue to grow.  
If any cracks are found within the RACP or surrounding structure, CMP Auto Engineering suggests apply the following methodology to repair them.

  1. Inspection – Inspect the extent of the crack(s) formed and determine the working area.
  2. Prep the surface – The working area must first be stripped back to bare metal using a wire cup/brush fitted to a grinder or small die grinder.
  3. Clean the surface – The raw metal can then be cleaned with and evaporative cleaner (e.g. acetone) to minimise weld contamination.
    Note: If the cracks are not easily visible, die penetrant testing can be performed to better reveal the full length of the crack.
  4. Stop drill the cracks – To prevent further crack propagation which can occur even after welding, each end and any fork points of the crack must be stop drilled. We suggest using a 3mm or larger drill bit with assistance from a hole centre punch if needed.
    Note: Excessive force with the hole centre punch at the crack end can cause it to grow.
    Note: The drilled hole will need to be filled with weld material. An excessively large hole can make doing so more challenging.
  5. Opening the crack – It is suggested that a small cutting disk fitted to a die grinder is used to lightly open the crack in a ‘V’ configuration to aid weld pooling and ensure weld penetration right the way through the material thickness.
  6. Welding the crack shut – Once prepped correctly, the cracks can be welded shut to restore the structures cross section.
    Note: Some of these welds will require grinding down to not inhibit fitment of reinforcement kits.
    Note: Apply a weld through primer to any bare metal surfaces that will be concealed beneath reinforcement kits.

Spot weld repair;

A spot weld is a form of welding where two or more layers of sheet metal are joined in a localised area by heat obtained from resistance to an electrical current passing between two electrodes. Much like the cracking that’s prone to occur, spot weld failure is the result of fatigue. A spot weld often fails when one of the two layers of sheet metal crack around the perimeter of the weld as shown in Figure 3, eventually tearing out a hole in one layer and a circle of that torn material still adjoined to the opposing layer as shown in Figure 4. A spot weld needs to be repaired from the side that has visible failure as shown below.

Once found;

  1. Prep the surface – the working area must first be stripped back to bare metal using a wire cup/brush fitted to a grinder or small die grinder.
  2. Clean the surface – The raw metal can then be cleaned with and evaporative cleaner to ensure no weld contamination occurs.
  3. Remove the damaged material – The factory spot welds are 6mm in diameter and can be drilled out using an 8mm spot weld drill bit when facing the damaged side shown above.
    Note: be sure to only drill through the damaged layer and not entirely through the opposing layer.
    Note: In places too tight to drill, a carbide bit fitted to a die grinder can be used to remove damaged material.
  4. Plug welding the hole – the newly drilled hole can now be filled with weld material producing a plug weld.
    Note: Some spot welds may initiate small cracks and may require both methods of repair.

Chassis Stitch welds;

In addition to repairing discovered cracks and spot welds, several areas throughout the chassis are suggested to be stitch welded to increase the strength and stiffness of the various joins between panels.
Stitch welding can be done along the edge of a layer of sheet metal where it overlaps another. It is suggested that all stitch welds are applied between existing spot welds. Examples of the stitch welds suggested will be shown in their respective step using the following methodology;

  1. Prep the surface – The working area must first be stripped back to bare metal using a wire cup/brush fitted to a grinder or small die grinder.
    Note: A pick tool may be needed to remove rubberised material stuck between layers.
  2. Clean the surface – The raw metal can then be cleaned with and evaporative cleaner to ensure no weld contamination occurs.
  3. Mark out the stitches – Using either chalk or a marker, mark an area 10mm – 15mm long down the length of the panel edge centred between the factory spot welds.
    Note: The length of the stitch welds should be adjusted depending on the vehicle application and spacing of existing spot welds. Do not seem weld the entire edge. Stitch welds should form the following ratio of the edges full length depending on application; Street = 1/3, Track = 1/2.
  4. Stitch weld the join – Proceed to weld between the marks to achieve a stitch welded join.

Modes of Failure

Subframe Mounts & Rear Wheel Arches

As per the previous article the major points of interest are the rear subframe mounts (especially the rear left & front right), the rear wheel arch area. If fatigue cracking is found at or around the rear subframe mounts or inbound of the wheel arch join, the above crack repair method should be applied.

For the rear wheel arches, the failed spot welds can be observed on the inbound face of the joining panels. Due to the location, it is often not possible to get a drill bit into the area so the carbide bit method detailed above will be required to remove the damaged spot weld material. Be mindful that some of these spot welds may also initiate a crack that expands to the edge of the panel as shown in Figure 7.

Once all the cracks and damaged spot welds in these areas are repaired, it is suggested that the rear wheel arches are stitch welded as per the methodology above. This should be done from the spring perch back to prevent this join failing again as shown in Figure 8.

These areas are the first and primary points of failure within the RACP structure and if you’re lucky may be all the repair work needed prior to installing reinforcement kits with the exception of a few extra stitch welds.
However, if not addressed soon enough or reinforced preventatively, damage can spread to other areas of the RACP and surrounding structure resulting in additional cracks, failed spot welds and in some cases significant structural deformation.

The following areas and their mode of failure are listed in the order in which they are known to generally occur. The failure pattern is often consistent however, can vary and thus all areas should be inspected regardless of the extent of damage at the time of inspection/repair.

Chassis Rail to Boot Floor

The chassis rail is secured to the RACP via a series of spot welds along a flanged edge inside the boot in front of the spare wheel well. To inspect this area, it is best to start beneath the car looking for broken spot welds inbound of the spring perch running from the rear two, rear subframe mounts to the front two, rear subframe mounts as shown in Figure 9. The spot welds closest to the four mounts are the very often the first to fail as per Figure 10.

If enough spot welds are allowed to fail, a gap often forms between the boot floor and chassis rail as per Figure 11. Deformation like this is not visible from above till things have escalated quite far. To inspect this area, you will need to remove the boot trim and potentially some sound deadening mat as shown below.

If any spot welds have failed or look to have an unusually deep recess, repair them using the spot weld repair method detailed above. If a gap is present as shown in the above figure, the deformation must be reversed and this gap closed before repairing and reinforcing the area.  

In addition to repairing the spot welds, it is suggested that the chassis rails flanged edge is stitch welded to the boot floor from the spare wheel well to the rear seat backrest as per the methodology detailed above.

Behind the Rear Subframe Mounts

Looking just behind the rear two, rear subframe mounts, you should find the continuation of the chassis rail highlighted in Figure 14. This area represents the rear most portion of the RACP and joins to the underside of the sheet metal that forms the spare wheel well. To find the four spot welds in this area, the brush on seam sealer will require removal as shown in Figure 15.  Once located, it is suggested that the nearest two spot welds are replaced with rosette welds as per the spot weld repair method detailed above regardless of whether popped or not.

 ‘V’ Shaped Beam

The V shaped beam refers to a pressed metal beam that travels along the underside of the RACP between the front and rear, rear subframe mounts and can be seen just above the highlighted area in Figure 9. When the above modes of failure let go and sufficient movement begins to occur, it is possible for cracks to form where this small beam terminates on the vertical face of the subframe mounts as shown in Figure 16. In the event this failure occurs, this area can be repaired using the crack repair method detailed above.

Topside of Rear Subframe Mounts

This area is not easily found and is located beneath the front most portion of the spare wheel well. If the car is not currently undergoing the installation of a topside reinforcement solution and cutting away a reasonably large section of sheet metal is not an option, this area can be inspected with an inspection camera from either within the boot or below the car if rubber grommets are removed.
As can be seen in Figure 17, the crack is initiated by a failed spot weld outward of the bend in the underside face at the rear left mount.
It not done earlier, repair the spot weld from beneath using the spot weld repair method detailed above. If a crack has initiated, you will need to cut into the boot floor to access the area then apply the crack repair method.
How to access this is detailed in the installation manuals for our stage 1 & stage 2 reinforcement kits and will not be covered in this article.

We feel special mention should be made for the spot welds visible at the top of Figure 17. These spot welds are visible from beneath the car just above the rear two subframe mounts however, do not often appear visible failed from that perspective. In the event these spot welds have failed, they can be repaired using the spot weld repair method.
If a Stage 1 – Reinforcement plate kit is installed prior, it is possible to use a spot weld drill bit to drill to the hidden face of the plate and successfully bond all existing and the new layers simultaneously with a rosette weld.

The above list concludes the areas specific to the rear two of four, rear subframe mounts. The following areas detailed in this article are relevant to the front two, rear subframe mounts. Some of these methods of failure are unique to just coupe chassis and sedans with split folding rear seats due to the varying types of rear bulkhead structure in this area between the range of E46 shapes.

Rear Bench Seat to Chassis Rails

Much like the chassis rail to boot floor, the sheet metal beneath the rear bench seat is also prone to separate from the chassis rails and very often neglected during most reinforcements jobs.
This form of failure is very common on coupe’s and occasionally observed in sedans however, not seen on convertible or touring models due to the additional support from the aforementioned bulk head structure. 

In this area, failure occurs in the form of popped spot welds. Given this panel overlaps the chassis rail rather than lies beneath, failure is observed from above within the cabin. The spot welds in questions can be found beneath a heavy layer of seam sealer at either side as shown below. This seam sealer can be removed with a simple chisel and mallet followed by a wire brush to tidy it up.

If failed spot welds are found, repair them using the same spot weld repair method. Once repaired or if failed spot welds were not found, apply the stitch weld method from the lowest point of the rear bench seat backwards till the panel overlaps with the boot floor as shown in Figure 26.

Chassis Rail to Boot Floor… Again

We thought this area would be worth mentioning again as this portion is relevant to the front two, rear subframe mounts often gets missed as repairers look at those closest to the rear two mounts however, neglect those nearer to the front two.
Unlike the rear bench seat area detailed above, this portion must be inspected from below. As before, the spot welds closest to the front two subframe mounts are known to pop. These particular spot welds represent an area where the bench seat, chassis rail and RACP overlap.

These spot welds often appear like that shown in Figure 21. With time, these spot welds are also known to initiate cracks like that shown in Figure 22 and thus may require both a spot weld and crack repair.

Around Seat Belt/Baby Seat Anchors

Due to the way the various layers of sheet metal are adjoined and stresses are transferred, cracking is also prone to occur around all threaded holes beneath the rear bench seat that the seat belt and baby seat anchors fasten into like that shown in Figure 23. This failure is generally most common at the outer most connection points however, is known to initiate most often above or below any of the bolt holes like that in Figure 24.

This method of failure is far less likely in convertible and touring chassis as opposed to coupe & some sedan variants however, should be inspected regardless.
This failure can be repaired using the crack repair method detailed above.

RACP Front Edge to the Underside of the Rear Seats

Failure in this area is fairly uncommon however, worth mentioning as it does contribute to the RACP structures integrity. Neglecting the RTA pockets, this area represents the most forward end of the entire RACP. This portion of the RACP is spot welded across the width of the rear bench seat.

These spot welds are known to tear away from the cabin floor leaving a small hole visible from above. To inspect these spot welds, it’s necessary to remove some of the deadener material bonded to the rear seat bench. Unfortunately, our development vehicle did not have this form of failure present and thus we don’t have any imagery of our own to share however, if failed spot welds are discovered, apply the spot weld repair method from above.

In addition to that, applying stitch welds using the above method along the red lines drawn over the front edge and overlap edge in Figure 27 is suggested to further reinforce this join.

Front Two, Rear Subframe Mount Top Welds

This area is prone to fail on all shapes and models of E46 unlike some areas around the front two, rear subframe mounts and like many others detailed in this article, is often neglected during a reinforcement job only to cause trouble down the road.
Examining this area is only recommended while the vehicle as it does require a section of the floor to be cut away and the use of power tools within the cabin. For convertible E46 chassis, an additional layer of sheet metal from the rear bulk head also requires removal. 

Cutting into the rear bench seat area exposes the top side of the female threaded inserts the subframe bolts into as per Figure 28. These inserts are secured with several spot welds from beneath as well as a pair of MIG welds on the vertical rear and top faces as per the below figure.

These factory MIG welds on the top face are known to shear away from the remainder of the chassis and on occasion initiate cracks. This issue is far more likely and also an issues for the rear two, rear subframe mounts built before Feb 2000 as per the ‘Part 1’ article.

Models built after November 2004 had an updated welding procedure which improved the bond to the unibody structure. Earlier models had a short weld applied to only one side of the stamped slot as per Figure 29, while the later were entirely filled. The half welds are much weaker and provide stress risers at either end providing a point of initiation for crack propagation while the later are much more evenly distributed and less prone to fail. 

Repairing this area may require two approaches. If cracks are visible, stop drill the cracks as per the repair method detailed above. If the welds are not entirely filled or are damaged in any way, use a carbide bit fitted to a grinder to grind down the original weld material recreating the profile of the stamped slots. Proceed to weld shut the cracks and fill the entirety of the slot much like the later revision.

This point of the article concludes the list of failure prone areas relevant to the rear subframe mounts however, not the entirety of the RACP.

Rear Trailing Arm (RTA) Mount Pocket

Catastrophic rear trailing arm pocket failure is uncommon however, still probable none the less. In a worst case scenario, the spot welds securing the RTA pocket to the RACP have the potential to shear as shown in Figure 31 & 32, resulting in the front of the rear trailing arm dropping from the vehicle. As you could imagine, this would result in a complete loss of control of the vehicle and lead to a potentially fatal accident. 
Given the hazard it presents, CMP AE Pty Ltd strongly suggests that this area is also inspected and addressed whether damage is visible or not.
Inspect the inner most portion of the flanged edge highlighted in Figure 33, for a slight split in the seam sealer around the bolt hole. This split represents that area being pried from the RACP.

To eliminate the potential for failure, apply the stitch weld method to the perimeter of the pocket with a focus on the radius at the most inbound bolt hole. If a small gap has started to develop like that in Figure 34, it’s likely that the nearest spot welds will require repairing as well.

We are aware that there are reinforcement plates available on the market that are intended to prevent this failure. These RTA mount reinforcement plates are 100% necessary on the E36 chassis however, not the case for the E46 3-series. This area had a major design change between the two generations of 3-series eliminating a need for such things.
In our opinion, fitting a plate over the E46 trailing arm pocket contributes nothing in terms of strength and only negatively affects the squat characteristics of the rear suspension. We personally believe stitch welding the trailing arm pocket is more than sufficient to prevent ongoing issues.

The rear trailing arm pocket concludes the section of this article that details all the potential failures within the RACP and surrounding structure.

We do apologise for this article becoming quite a lengthy read however, as we’ve said this issue is so much more complex than just fitting a set of underside reinforcement plates and calling it a day. There is a wide range of areas prone to fail and we believe they were all worth covering. We feel as if any repair that does not address all these areas is not a complete job.
If you’re car has been previously reinforced, we strongly suggest that you try to inspect all these areas to confirm they were not neglected and have been addressed correctly.  

As mentioned earlier, some of the works detailed above paired with underside reinforcement plates repair and reinforce the weakest links present in the RACP & surrounding structure however, in some cases, this results in the areas between to become the new weakest link that may inevitably lead to secondary failure.

For this reason, CMP AE Pty Ltd offers additional stages of reinforcement starting with our stage 1 – underside reinforcement plate kit following by 3 additional stages depending on your E46 shape and future build objectives.

Pairing a stage 1 – Underside Reinforcement plate kit with the stage 2 – Topside beam kit and stage 2.5 – Front mount extension kit is designed to provide a completely comprehensive reinforcement for all four rear subframe mounts and the surrounding structure for the E46 chassis while also providing a significant increase in chassis rigidity and integrity capable of withstanding very abusive driving conditions.

Due to the additional bulkhead structure over the front two, rear subframe mounts on convertible, touring (wagon) and in some cases sedan models the Stage 2.5 – Front Mount Extension kit non-compatible. In exchange, this additional structure prevents fatigue related failure where the rear bench seat sheet metal meets the chassis rails on these models. As a result, a completely comprehensive reinforcement can be achieved by pairing just stages 1 & 2 however, all the areas detailed above still require attention.

For those who thoroughly enjoy driving their E46 to its full potential and want to further expand on the subframe/RACP reinforcement and perfect the vehicles dynamics, CMP has expanded the design to incorporate the Stage 3 – 6-Point Race Brace.
This stage 3 kit is the ultimate E46 chassis stiffening brace and the final stage of our RACP/chassis reinforcement range. The single piece construction brace triangulates all four subframe mounts with the rear shock towers for a substantial increase in torsional chassis rigidity for both improved handing and structural integrity capable of withstanding the most extreme engine outputs and suspension modifications such as true coil over rear suspension.
Best of all, this kit is entirely bolt in should full boot functionality be needed and fits with a complete original cabin interior, back seats and all.

After a Reinforcement Plate Install?

What to do next really comes down to each individual owner’s long term plans and goals for their vehicle. Not all customers can justify a complete top to bottom reinforcement on their cars and we don’t always encourage that.
The reality is that once underside plates are installed and provided all the problem areas in this article and the previous are correctly repaired, it could take several years for secondary failure to initiate. However, much like initial failure, some occurred very early on and some many years later. The down side is that secondary failure depends an even greater range of variables that initially and is even harder to predict when it will happen. Other than the usual power output, driving style etc, a new variable to take into account is condition of the subframe mounts & RACP prior to the stage 1 reinforcement. A car that had suffered more significant failure would be further into its fatigue life even in places where secondary failure can occur compared to a car that was repaired preventatively. This is where we appreciate the opportunity to discuss such things with customers to ensure they feel confident that the repair performed meets their personal needs without being potentially financially compromised.

The reinforcement plate kit offered by CMP has key design features to direct as much stress as possible as directly as possible to the chassis rails to dissipate stress away from problem areas as much as feasibly possible and provide the longest possible fatigue life after reinforcement.

However, secondary failure is still deemed inevitable. Underside plates provide a localised increase in strength to the RACP relative to their coverage however, are only a means of thickening material section and do not alter the major structure to correct the inherent design flaws present within the RACP so please keep that in mind.
The below is an extreme example of secondary failure following a reinforcement plate installation. We do attribute much of the damage to an incompetent installation (done by a professional) of an inferior stage 1 reinforcement kit not developed by CMP AE Pty Ltd and feel as if the extent of damaged could have been far less severe if done correctly.

Naturally, we imagine no one would ever want to be confronted with such a thing especially having already undertaken or paid for such a significant job to be to be done.
This is an issue that should not have existed in the first place let alone be suffered twice. Our hope is that by sharing this information no one will and every reinforcement job will be comprehensive and permanent whether DIY or professional.

We at CMP would like to thank you for taking the time to read this article and special thanks to our customer who choose to support and give us the opportunity expand the range of products and resources we can offer so that we can continue to expand and give back to the E46 community and hopefully one day the wider BMW enthusiast community. 

Feel free to follow us on Facebook and stay tuned for the third and final instalment of our E46 subframe/ RACP article series.

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