E46 M3 Rear Subframe Reinforcement: Part 1

It is well known that the E46 chassis, especially the M3, suffers from rear trunk floor issues. Also known as the rear axle carrier panel. While BMW produced arguably their best vehicles in the 90s and early 00s, many of the models suffered from rear subframe and trunk floor issues. Namely the E36, E36/7 and E36/8, and of course, the E46 chassis.

Many people say that the condition of the rear axle carrier panel aka the RACP can be observed by looking underneath the vehicle at the mounts. My brother and I are firm believers that this is not true. We believe every single E46 M3 is in some stage of deterioration and this was confirmed with our 2005 Imola E46 M3 with ~100k miles. A very small crack was seen with the subframe bolted in the vehicle, but after removal it was clearly evident the damage was far worse than what many would have thought as can be seen below.

Cracks were even found after thoroughly cleaning the inner and outer wheel arches. The left side of the rear axle carrier panel was starting to separate from the chassis as could be seen by a crack in the seam sealer on the rear driver wheel arch. A crack and popped spot weld was found on the inside of the driver’s rear wheel arch as well. This is also an area that should be looked at and repaired along with reinforcing the other parts of the rear axle carrier panel.

While the damage found was no where near as catastrophic as many have unfortunately experienced it was worse than what most people would have assumed. Extensive research was completed to decide on the best plan which was to reinforce the top and bottom of the RACP to provide a permanent solution. It was decided to use Redish V2 RACP reinforcement plates plus a Vincebar for the rear mounting points plus his reinforcement kit for the front mounts.

Reinforcement plates will address the cracking of the RACP, but it does not address the panel pulling away from the chassis. With VinceSE2’s Vincebar and front mount reinforcement it ties the RACP to the upper part of the chassis creating a complete structural unit.

Parts and tools used:

Hobart 140 welder
electric cut off tool with 1mm cutting discs
1/2 inch belt grinder with 40, 60 and 100 grit belts
spot weld drill bit
various clamps
Ingersoll Rand 90 degree angle grinder with Roloc 50 grit discs
Redish Motorsport E46 M3 RACP V2 Reinforcement Plate Kit
Vincebar (VinceSE2 from m3forum) stealth weld in bar for the top of the RACP and front mount reinforcements and tool kit.
SprayMax 2k Epoxy Primer
Fusor 806D undercoating
3M Cavity Wax Plus

The documentation of the RACP reinforcement begins below. The rear drivetrain and fuel tank along with the trunk interior and rear seat area were removed prior to beginning.

Step 1: Remove seam sealer and cut access panel for top of RACP.

Seam sealer was easily removed using an old thin putty knife. All rubber body plugs were also removed. After that was removed a Dremel tool and cutting disc was used to cut around the panel covering the rear of the RACP.

Panel removed.

Step 2: Remove under seat brackets and drill through top of RACP.

The next step was to remove the two brackets underneath the rear seat. They are located right above the front subframe mounts in the RACP and need to be removed. A spot weld bit was used to drill around the spot welds and remove the brackets. After this was completed the studs for the rear subframe were removed and a drill bit slightly smaller than the threaded holes in the front and rear of the RACP was used to drill up through the top of the RACP to prepare for the top reinforcements.

Bracket removed.

Studs removed.

Drilling through mounting holes and through the top of the RACP.

View from above.

After drilling through for the front mounts the rear mounts were completed next.

Step 3: Modify top of mounting areas for reinforcement.

There are some modifications that need to be completed for the reinforcement of the front and rear mounting points of the RACP. For the front mounts a hole saw with a guide pin is used to cut away the sheet metal and reveal the top of the RACP. For the rear mounting points the ‘T’ impressions need to be flattened smooth to be even with the rest of the panel to provide the rear reinforcement bar a flat surface to mount.

This tool was provided in the tool kit by VinceSE2 and comprises an arbor, hole saw and custom guide pin which centers the hole perfectly above the front RACP mounts. The picture below shows it drilled half way. We discovered it was easiest to drill halfway through and then bend the flap down before finishing the hole.