Part 3 - Stage 1 Underside Reinforcement

For the final stage of our E46 RACP tech series we were wanting to break down the reasons why we believe why certain areas are prone to fail and thought it would be best to pair that information with how to prevent it which inevitably led to pairing the information with how each of our respective reinforcement kits function to prevent it. 

We are quite proud of our subframe reinforcement kits and if you've read our last two technical articles should be well aware that we do know that it's not the subframe that's the issue but, the Rear Axle Carrier Panel (RACP). 

Given we couldn't fit all the information we wanted to on the product pages, we thought it would be a great idea to go into further detail as to why we believe our reinforcement kits are the best and what unique design features there are and how they perform to give you the best possible reinforcement. 
To avoid an overload of information resulting in articles no one will ever want to read we've separated this information into two respective stages of reinforcement following being underside and topside. 

Before we start, we wanted to point out the obvious well known fact that cracking is common at the subframe mounts. The focus of these last few articles will be on areas beyond the immediate mounting face that also fail and crack and are not addressed by other plate systems or are overlooked entirely. 
Before getting onto that we will touch on the front two of four rear subframe mounts which in their case, cracking occurs due to a small air gap between the sheet metal surface visible from beneath and the female threaded insert within. The tabs protruding from the factory rubber bushings have a metal sleeve within that when in contact with the underside of the mount (as a result of excessive rubber deflection) causes localised strain (flex) due to the air gap leaving room to do so on the other side resulting in fatigue and thus cracking. This can be seen in the images below where the bushing has caused a ~6mm/1/4" dent in the underside. 

 

Figure 1 - Black mark from bushing contact

Cracked E46 subframe mount

Figure 2 - Bushing Indent into mount underside

E46 subframe cracks

This issue exists regardless of which bushing you use hence why underside plates are needed on the front two rear subframe mounts regardless of the additional structure from above. Once you put plates on the front two, you need to do the same for the rear two to not tilt the subframe when installed. 

Cracks are also common to form around the stud hole which is due the fact that a section of the sheet metal is sandwiched between the stud and the threaded insert within causing a stress riser about the edge resulting in a perfectly concentric crack.
In the case of the rear two of four, rear subframe mounts, that will be covered after we touch on some theory as it's just one of several issues that occur due to a flawed design feature.

The major points we want to discuss in this process is load paths and stress distribution, the relation between rigidity, stress and strain and conclude on something that we're surprised keeps popping up and that's the 'epoxy method'. 

For this analysis we'll be focusing on the structural design of the back half of the RACP structure that includes just the rear two of four rear subframe mounts as we feel this is the most critical area.
If you've ever taken a grinder to the E46 chassis and exposed the topside of the RACP which is partially hidden beneath a section of the spare wheel well, you should find this.

E46 RACP
Figure 3 - RACP rectangular profile

What you can see (potentially other than some previously hidden cracks) is that the shape of the RACP structure is roughly a rectangle that is wider than the distance between the chassis rails however, sits about 1" below.

Seeing this you'd imagine that this rectangular shape must continue outward beneath the chassis rail till it gets to the wheel arch creating a solid beam between the two ends. 
Unfortunately this is not the case as the rectangle section actually shrinks down in height till it eventually turns into a single layer of sheet metal with no structural height before it reaches the next face.

E46 RACP profile
Figure 4 - RACP beam end profile left
E46 RACP shape
Figure 5 - RACP beam end profile right

The result is that this major beam supporting both rear subframe mounts is not rigidly terminated but, suspended by a single layer of sheet metal in all directions.
To put it into perspective, imagine standing in the middle of a trampoline. You're going to have a lot of flex in that single layer of sheet metal as it simply has no cross section and thus negligible rigidity. 

Now moving onto stress distribution, you'd begin to wonder if this single layer of sheet metal flexes equally and distributes stress evenly in all directions much like the trampoline analogy?
If you look from beneath the car you'll see that the edge between the subframe mounts underside face and vertical face travels the full width of the rear wheel arches and thus creates a section much like an angle iron although incredibly thin (~1.2mm). As a result, this section of the RACP is the most rigid path and by default experiences the greatest stress compared to other sections of sheet metal when experiencing chassis flex. 

As a result a majority of the of the force acting on the subframe mounts travels along this hard edge to the wheel arch resulting in cracks along several stress risers as well as causing the spot welds joining the wheel arch to the RACP together to pop. This is why cracks can be found at every point the underside face steps higher up as they represent a sudden change in structural rigidity creating a localised spike in stress hence why cracks at the rear left subframe mount are the initiation point for cracking as it is the most significant of the lot. 

E46 subframe mount crack
Figure 6 - RL subframe mount stress riser crack
E46 RACP cracks
Figure 7 - Wider RACP stress riser cracks
Once the spot welds between the wheel arch and RACP at the end of the hard edge pop (behind the dampener), stress can no longer be supported in that direction as there's simply nothing holding it anymore. This triggers a domino effect where the next most rigid (but far weaker than the original) is next to go and so on. 


This is why in our Part 1 Tech Article we suggest inspecting the most significant stress riser above the sway bar bush and the rear wheel seam for a split due to flex between the panels as these are the initial points of RACP/subframe mount failure. This is then often followed by the spot welds behind the rear subframe mounts and then in front where it's joined to the chassis rail inbound of the spring perch. 

E46 RACP spot welds failure
Figure 8 - rear of sufbrame mount spot welds
E46 chassis spot weld failure
Figure 9 - in front of subframe mount spot weld failure

Now moving onto rigidity, Stress (force over area) and strain (elongation/flex) are directly proportionate and dependent on one another. You simply cannot have one without the other. 
How this relates to a structure is that in order for stress to be exerted through a body it must experience a proportional amount of flex. By manipulating the structure to be more rigid, you can either reduce stress and thus flex under the same applied force. (try bending a ruler over the edge of a desk with the flat face up vs with it on it's edge like a knife). 

Much like how the hard angle is more rigid and carries the most stress, we can add rigidity to other areas of the RACP to increase the stress they transfer and thus direct stress away from areas we cannot reinforce. This is why the CMP RACP reinforcement kit appears so different to the market norm. 

The major difference being that our mounts don't just go up the front vertical face of the rear mounts but backward as well despite that area not being prone to cracking. 
What the plate is actually doing is making the arch between where the RACP separates from then re-attaches to the chassis rails stiffer. By being stiffer, less strain occurs in the thinner single later of sheet metal toward the wheel arch resulting in less stress and thus reduced fatigue. 

E46 RACP
Figure 10 - RACP arch from chassis rail

The next challenge then is to ensure that where we're sending this stress will not cause the joining spot welds to pop. This by comparison is a relatively simple and resolved by adding stitch welds along the edge of the chassis rail joining it to the RACP to strengthen the join and support the spot welds to prevent separation. This along with several other areas needing the same treatment is why our installation manual is a whopping 18 pages long. It ensures that you're installing the plate correctly and doing all the additional works possible to ensure your reinforcement is as good as can be. 

There were a few other less obvious design consideration applied when conceptualising our reinforcement plates such as cantilevering the chassis rail to further dissipate stress from the sheet metal toward the wheel arch as well as adding clever weld locations to provide the most direct transfer of stress between the various layers that make up the RACP rather than depending mostly on the perimeter seam weld like most plates. 

Now to wrap up we've been needing to put this one to bed. The epoxy method. As many people in support of the epoxy method like to point out, epoxy is used to bond new cars together, epoxy is more rigid, welding makes metal brittle, using epoxy gives better stress distribution due to the plates surface area. 

We are not by any means against using epoxy for automotive use however, it is not the best solution in all given scenarios. Epoxy bonding a roof panel or non structural elements of a vehicle such as a spare wheel well or door skins together is a very effective method of using epoxy however, the subframe mounts simply are not. 

This is because the stresses acting at the subframe mounts are far greater as they are not distributed like the roof panel. They are localised point loads. What occurs is that the pressure acting on the subframe mount creates stress. Now this is where the surface area and rigidity is pointed out. 
Analysing a materials rigidity is a product of comparing the amount of flex that occurs relative to the stress exerted on it and is known as an elastic modulus. Imagine the stiff vs soft spring analogy. In this situation epoxy and composite material have an incredibly high elastic modulus however, often have a far lower strain failure limit compared to most metals. A strain failure limit is the amount of stretch/flex occurs before it snaps/fails (like how stiffer springs allow less travel). This means that when stretched evenly, the epoxy will fracture far sooner than steel although, would have been experiencing greater stress at the time. 

Relating this information back to the subframe mounts & RACP is that the point load at the subframe mount is going to cause the RACP to flex. If you've read our bushing articles you would know that in order to transmit a full force a sufficient amount of flex must occur (like your cars suspension settling as you drop it back on its wheels). However, the epoxy bonding the two steel plates will resist this flex due to its far higher elastic modulus. Due to it being paper thin it simply does not have the strength to prevent this flex and will be forced to flex with the steel and exceed its strain failure limit. 

Assuming the epoxy was perfectly applied and didn't fail due to other reasons such as inconsistent thickness or air pockets, it will have suffer an adhesive failure where it has cracked down it's centre separating the reinforcement plate from the subframe mount resulting in what we'd consider an over glorified washer. 

The final point in the epoxy analysis is that welding makes metal brittle. This is entirely true and known as thermal embrittlement however, the extent of the embrittlement seems to be blown out of proportion in recent years and people simply seem to glaze over the fact that these cars (like many steel structure) were welded together in the first place. There are controls that can be implemented to minimise this effect however, the reality is that thermal embrittlement is not going to be the cause of failure down the track. 

On top of that, the epoxy method technically does not eliminate the need for welding when reinforcing the e46 chassis. As detailed in our previous article, cracks need to be welded shut, spot welds need to be repaired and certain joins need to be stitch welded together.  

If you're looking to reinforce your e46's subframe mounts we strongly suggest you read our tech articles before giving it a go.

if you're on the market for a reinforcement solution we would like to think that it's become a bit clearer why spending a little more and getting a superior, engineered product over a cheaper band aid fix and attempting to go about the installation using the cheapest method possible is potentially going to cost you more in the long run. 

The labour component to do this job correctly is many time greater than the cost of the components needed and is simply not the place to be cutting corners. If you only want to do it once, do it right.