December 30, 2016

Why We Developed Our Kit

Our subframe kit was invented due to a necessity. Like many, we had repaired the rear axle carrier panel using the repair plates widely available to the market and sometime later found failure occurring elsewhere. Not being satisfied with what was available on the existing market and being mechanical engineers as well as E46 enthusiasts, we performed significant analysis and exploration of the concealed chassis components and began development of the preliminary designs that evolved into the kit we now offer.

The post repair failure was discovered in the left wheel arch join and boot floor. As it was not recommended by the manufacturer of the repair plates, nor presented any signs of failure at the time, these areas did not receive additional welding.

As shown in figure 1, the wheel arch spot welds had popped and the join at the wheel arch had begun to separate.

        

This wheel arch join is directly outward from the rear two subframe mounts points (rear left visible in figure 1). For this wheel arch join to fail, it would have to be under load. Given the spring perch is further up, is linked directly to the chassis rail (Shown in figure 3) and had no signs of local failure, it was deemed that the loads in question were those applied by the driveline as discussed in our technical discussion “Why E46 subframes fail”.

Some sheet metal was removed from the boot cavity to explore what the driveline loads were being supported by and why the wheel arch had failed as shown in figure 4.

It was found that the section of the rear axle carrier panel that supported the rear two subframe mounts resembled a hollow rectangle member formed by spot welded, pressed ~1.6 gauge sheet metal. This rectangle member did not directly connect to the chassis rails but was approximately 1 inch below. This member then extended outwards past the chassis rails, necked down into a single sheet, then linked to the wheel arch via the joins shown in figure 1 & 2. The centre point of where the subframe mounts to the carrier panel was measured to be slightly greater than 210mm. A downward force with a perpendicular distance from where it is supported would result in an internal bending moment.

        

Furthermore, a crack was discovered in an area that was not a recommended inspection point by the plate manufacturer as shown below. The origin of this crack is clearly from a failed spot weld. This is the same spot weld highlighted in figure 8.

     

Based on the information gathered, two not to scale technical drawings of the rear left subframe member and how it is ties into the chassis were developed to aid in analysing the chassis as shown below. Red numbered arrows represent the common areas the panel fails as well as the order they generally occur. Blue spots represent known spot weld joins and the green highlighted area show the coverage of two common repair plate systems.

When referring to figures 9 & 10, it is simple to see why the wheel arch join had split. It appears that due to the layout of the design, a significant portion of the experienced loads would be transferred outward to the wheel arch join and up into the chassis rails. Should the wheel arch join fail, the loads would be distributed elsewhere resulting in subsequent failures occurring.

With the added rigidity of the repair plates on the formed rectangle member, less flex would occur within the localised area preventing localised metal fatigue however, with less deflection, the peak force applied to the wheel arch join due to the experienced impulsive loads in theory would increase. As a result, the total of three spot welds supporting the split area in figure 1, are under even greater stress and more likely to fail.

Once the wheel arch splits, the loads are dependent on the left and right most spot welds joining the rear axle carrier panel to the chassis legs as detailed in figure 10. These spot welds inevitably fail as they are not intended to act independently resulting in the boot floor separating from the chassis legs.

Should these fail prone joins be reinforced with additional welding it is probable that the increased localised rigidity and thermal fatigue could reproduce the same effect of subsequent failures elsewhere.

At this point it was determined that a system needed to be adopted to relieve the stresses on the rear axle carrier panel and its joins and work in tandem with the factory designed system in order to ensure a permanent fix to this global issue.

Several existing methods all adopting different design considerations were analysed and although potentially effective, were not ideal or not fully optimised for the most efficient result.

Some designs pursued the intended result by linking the known to fail subframe to the also known to mushroom shock towers. Others simply filled the gap between the chassis leg and the sheet formed rectangle member. Of those analysed, some utilised a series of hinges or a bolt in rigid body, others using adhesives or welding. In short, upon comparative analysis, some were deemed as being potentially unreliable, too costly or compromised the purpose of the boot. Others were deemed effective however, not yet fully optimised.

Rather than copy the existing methods and develop a bolt in brace, CMP Auto’s engineers pursued a permanent weld in solution that both eliminated the possibility of subframe failure and improved chassis rigidity for increased driver feel and potentially handling.

For further information on this kit as to how it functions, please follow the link below otherwise additional information is available in our technical section.