Part 4 - Stage 2 Topside Reinforcement

By now, many E46 3-series BMW's have already been reinforced with some form of underside reinforcement plate kit especially within the M3 scene. 

As detailed in the previous article, we had engineered several unique design features into our underside reinforcement plate kit in an effort to reduce fatigue in problem areas often to blame for secondary failure.
We know our plates are far more comprehensive than any other on the market 
however, are limited to just thickening the underside sheet metal and from our research and analysis, do believe that even our plates alone are not going to provide a true permanent solution.

While we have never heard of a car reinforced with our kit suffering secondary failure, this conclusion has become a reality for many people who had fitted smaller and simpler plates that cover no more than the face the bushing contacts. Much like how the issue occurred sooner on M3's due to their higher output, these examples have occurred sooner due to their far more basic construction.
We have witnessed several M3's showing broken spot welds and cracks after having these simpler plates installed and can conclude that this is likely the inevitability for all other E46 models as well. 

This was where CMP originated and the justification to developing our first product being the Topside Beam Kit. 

By introducing a new chassis member bridging between the existing in a 'H' configuration we were able to link the female threaded inserts the subframe bolts into directly to the chassis rails providing the shortest possible load path and increasing rigidity more than adequately to eliminate flex from occurring in the sheet metal that previously suspended the rear two of four subframe mounts thus, eliminating the possibility of fatigue and failure permanently in this area. 

E46 RACP Reinforcment
Figure 1 - Topside beam kit exposed

This kit is more than substantial than others on the market as it is designed to not just be adequate to stop a factory original car from cracking but be sufficient to continue doing so when producing power outputs beyond three figures even with aggressive suspension modifications. We'd rather be too strong and ready for anything than just strong enough. 

While that concludes the problem areas prone to occur at the rear two of four rear subframe mounts, the front two as dicussed in Part 2 has many that often go undetected. 

First and foremost, many of these modes of failure are only present on coupe chassis as they lack the additional bulkhead structure their convertible, touring and some sedans do. The kit we developed for this region is only compatible with coupe chassis and the occasional sedan that is fitted with split folding rear seats which was not an option in the Australian market. 

Spot weld failure along the chassis rail under the rear bench seat and cracks forming around the various threaded anchors for seat belts, baby seats etc is much like everything with the RACP, a product of fatigue. 

While the sheet metal beneath the rear bench seat is not considered nor designed to be structural based on its different material and thickness to that of the RACP, it is unfortunately experiencing stresses exerted on it by the front two, rear subframe mounts. 
this is because the front half of the RACP is structurally bonded to the bench seat as the thread for those anchor points is actually within the RACP. By being bonded at these locations the stress acting through the RACP is also acting on the bench seat sheet metal. Given the transfer of stress is localised to small point loads, the localised stress is very focused and dissipates as it spreads however, is sufficient to cause cracking in close vicinity as shown below. 

E46 floor cracks
Figure 2 - Bench seat anchor points

Due to the weaker material of this panels and the few amount of spot welds bonding it to the chassis rail they to are prone to pop as shown below. 

E46 failed floor spot welds
Figure 3 - bench seat to chassis rail cracks

Much like how the topside beam kit functions, our Front Mount Extension kit bridges the distance between the chassis rails and welds directly to the female threaded insert the front studs screw into providing a more direct transfer of stress between the subframe mount and the chassis rail relieving stress from being transferred through the anchor points and onto the spot welds along the chassis rail. 
Given the greater span and limited space in this area, it was further braced by the square section travelling forward from the topside beam kit to add further rigidity to the front two mounts and the original channels travelling between the front and rear, rear subframe mounts to maximise and more evenly add rigidity to the entire RACP structure. 

E46 Topside reinforcement
Figure 4 - Front Mount Extension kit

The final mode of failure not discussed is the top welds on the front mounts. Some believe that his is a result of the front stud wanting to be pried (rotated) however, as we've said before sufficient rotational flex must occur in the rubber bushing in order to transfer a bending force to the mount which simply cannot occur with the other 3 mounts bolted down as well as the fact that the front studs are secured in double shear meaning even if a prying moment was exerted on the stud it act as opposing shear forces at either end. 

Given the factory rubber bushes depending on a single front mount to transfer all the force resulting from cornering G's and that only the top welds on that one side generally fail believe cracks forming at the front mount top welds is the result of failure on the underside of the front mounts where the bushing makes contact.
As cracks form the cornering G forces create lateral movements as the sheet metal the mount is bonded to is no longer secured causing the base of the threaded insert to rock left and right under cornering G's which then much like a simply supported beam would (the stud and threaded insert), exert a moment on the top welds resulting in the fatigue that initiates cracking. 

Therefore, the solution to prevent cracks at the top welds we feel is to both increase the rigidity of the underside using a sturdy reinforcement plate kit and to tie the top welds into a brace structure to transfer the induced stresses into a stronger body than the original thin sheet metal.

In conclusion we believe that all the problem areas that the E46 subframe mounts/RACP can suffer detailed in part 1 & 2 can be resolved permanently by installing a set of solid subframe mount reinforcement plates in addition to sturdy reinforcements directly to the top of all four subframe mounts. 

If you've made it this far through our technical articles we applaud you and appreciate you taking the time to read the information we have to offer to better understand this terrible issue.