What was BMW thinking?
BMW left us hanging on a misunderstood issue
BMW was famously sued in 2006 in a class action by BMW E46 owners of all models that were suffering from major structural failures. The lawsuit was ultimately settled in 2009 and a quick fix was offered by BMW.
From what we now know today, BMW knew that fix wouldn't last as they properly engineered the rear end of the E9X 3 series chassis and these issues were never to be heard of again.
This entry to my Build Journal is a deep dive into BMW's rear chassis shortcomings on the E46: often misunderstood cracking issues in collaboration with Vince @ PracticalPerformance.
In collaboration with Vince @ PracticalPerformance, he provided most of the data, knowledge and sketches to shed light into often misunderstood cracking issues found on E46 3 series.
Concepts & Definitions
- RACP: Rear Axle Carrier Panel commonly referred to as the "floor panel".
- Frame Rails: commonly referred to as "chassis legs", these are the sturdy rails that go along the chassis and create the solid backbone of the E46.
- Subframe: This is the actual Rear Axle Carrier on which all suspension arms are mounted along with the differential.
- Subframe Mounts: The subframe mounts are the mounting point for the Subframe to the RCAP on the E46.
- Load Path: Rear axle torque load distribution to the frame rails.
Rear subframe inspections often don't cut it.
Dealers and shops often report no cracks following an inspection. This is often an incorrect finding based on lack of knowledge.
It's unfortunately very uncommon that there are no cracks present once the rear axle is removed, dirt is cleaned off, and the RACP is thoroughly examined.
This extremely unfortunately leaves owners unaware of a mission critical failure that could have been addressed at a reasonable cost, but will be very costly down the road.
The Original BMW Design
The E46 M3 rear subframe mounts are almost all the way out to the chassis leg, but the fronts are very close to the center of the car.
The cross section of the vertical plane going through the rear mount threaded receivers only connect to the outer most part of the chassis leg. This puts a lot of stress at the hotspots.
However, both are attached to the RACP rather than directly to the frame rails themselves.
The design of the E46 chassis forces the load path to take a detour from the subframe mount to the frame rails. It is taking that detour via the flimsy, 0,75mm sheet metal of the RACP.
The RACP itself is too weak and the panel is inadequately attached to the rest of the chassis.
This is the culprit of the infamous subframe issue of the E46 - none of it is actually related to the subframe itself :)
How BMW tried to fix it with foam
BMW's official fix was to inject structural foam between the two layers. When the foam is fully hardened it adds strength between the sheetmetal layers and prevents them from flexing.
"BMW has agreed to a proposed settlement of a class action lawsuit concerning the Sub-Frame structures on 3 Series ("E46") models produced from 1999 through 2006. Under rare conditions the attachment points of the Sub-Frame may develop a fracture or crack. BMW has prepared an inspection, approved repair procedure, and reimbursement policy in keeping with the terms of the proposed class settlement.
This settlement will only pertain to US residents and is not a recall of any kind. Details will be forthcoming, pending the court's final approval of the proposed class settlement. In advance of the final settlement, BMW customers with questions should visit http://www.e46subframeclassactionsettlement.com/." (broken link)
The Foam does help relieve the stress at hotspot #1 (by stiffening the "beam" within the area enclosed by the cavity highlighted in grey).
However it does nothing to improve the load path from the mount to the chassis, nor does it improve how the load is dissipated to other parts of the chassis leg.
How the aftermarket tried to fix it.
Reinforcement plates can resolve the issue of the RACP cracks that develop. While better than foam, it unfortunately does not fix the load path issue.
E46 M3 with reinforcement plates will eventually also suffer top side RCAP failures.
Vincent @ Pratical Performance deep dives into a demonstration to show the cracks into the top mounts of the RACP.
How E46 M3 owners can actually fix it too.
In order to avoid bending the connection and/or the vertical wall of the chassis leg one should create something that is rather rigid such that it applies a clean vertical force to the frame rails.
Vincent's initial idea fix this issue is conceptually sketched below.
VinceBar: the Optimal Solution by Practical Performance SE
Vincent @ Pratical Performance refined the conceptual design above with the final sketches for the VinceBar.
The Simplified VinceBar directly couples the rear mounts to the chassis rails, thereby solving the chassis design flaw that relies solely on sheet metal to do the job.
Apart from providing further vertical force, the VinceBar also strengthens the factory beam and the bolt through design also relieves the beam.
The bolt through design relieves the spot welds in the vertical plane as well as the bending.
Very similar to the Cross Braces available on the market, the VinceBar is however nearly completely hidden.
It doesn't negatively impact the usability of the car's already small rear trunk compartment. It not affect the luggage space or the ability to load larger items with the rear back rests folded down.
It's a solution that addresses the root causes of the BMW E46 rear chassis failures.
How BMW actually fixed it with the E9X M3 and future generations.
BMW learned from their mistakes and fixed these chassis issues for good with the E9X.
The subframe mounts have been positioned directly under the sturdy frame rails with a wider subframe instead of in the RACP. No reinforcement to the RACP is needed to relieve it from all, or parts of the load.
Pictures showing the restoration of my rear subframe. While it doesn't crack anymore, the subframe and differential coating is prone to surface rest and degradation.
Subframe issues have been a thing of the past since. BMW M grew confident enough in their rear subframe design to offer the M3 GTS with solidly mounted subframe - a first for them.
It has since become a standard design feature of all BMW Ms in F series M cars and newer generations.