"In this Special Series, I explore the E46's famous rear subframe issues - and why it isn't really about the subframe. I document the causes, the available solutions, and my selection of CMP Engineering's chassis reinforcement kit.

In this entry, BMW gets sued. I touch on the original design flaws, BMW's attempt to fix it and how they fell short until the M3 V8s came around. I used my experience on the Phoenix's restoration to provide insights. " 
- Matt

We all used to care. 

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. 

This was BMW's PR response back then. 

"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 https://www.e46subframeclassactionsettlement.com/."

It wasn't rare. 

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. 

Don't be afraid to go deep. 

There are various ways to deal with problems in life: avoidance and wishful thinking is not a solution on this generation of M3s. It's the fastest way to the E46 shadow realms.

Dealers and shops often report no cracks following an inspection. This is often an incorrect finding based on lack of knowledge. This 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.

You will most often than not discover cracks once the rear subframe is removed, dirt is cleaned off, and the rear axle carrier panel is thoroughly examined.

With the underbody fully sanded down after removing even the fuel tanks, my chassis was in good shape. 

What's a subframe?

Subframe: This is the actual Rear Axle Carrier on which all suspension arms are mounted along with the differential. 

On this chassis, the rear subframe is comparatively small versus modern cars. It was designed this way as suspension design of this era had massive rear trailing arms that were bolted directly to the chassis, and relied heavily on sway bars for chassis control. 

The rear trailing arms' chassis bushings commonly fail as they bear a lot of force. 

While subframes can crack, or have welds fail, it is not the core catastrophic failure. 

Mine was replaced in the original mechanical restoration to fit solid subframe bushings, and put to new condition and beyond in the paint & body restoration. 

The welds were still in great condition, with no visible rust prior to sanding it down. My previous subframe wasn't so lucky. I sold it used and the buyer found cracks.

Time was wasted, money was lost, but CanadaPost got paid. 

Subframe Mounts: The subframe mounts are the mounting point for the Subframe bushings to the RCAP on the E46.

What's a RACP?

RACP: Rear Axle Carrier Panel commonly referred to as the "floor panel"

It's all that, in a nutshell. 

What are frame rails?

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. 

At the front, they connect to the front bumper carriers. 

At the rear, they also connect to the bumper carrier. They are visible by the 3x bolt holes. 

The frame rails run across the entire length of the chassis. 

The original M design

Load Path: Rear axle torque load distribution to the frame rails.  

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.

This is the culprit of the infamous subframe issue of the E46 - few of it is actually related to the subframe itself. 

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. Both front mounts are attached to the RACP rather than directly to the frame rails themselves.

The front subframe mounts are bolted upwards by the rear seats. 

The RACP itself is too weak: the panel is inadequately attached to the rest of the chassis.

You can see the layers of sheet metal work. If this fails, it's catastrophic - a new trunk floor is required. E46s used to be parted out over this. 

 

BMW M's Foam was to fix it. 

BMW's official fix was to inject structural foam between the two layers of the rear axle carrier panel - essentially, below the spare tire.

When the foam is fully hardened it adds strength between the sheetmetal layers and prevents them from flexing. The foam did help relieve the stress at 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 frame rails. 

Aftermarket plates aren't enough. 

Reinforcement plates will resolve the issue of the RACP cracks that develop. Unfortunately, if the panel is cracked, it needs to be repaired and welded before plates can be applied. 

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. 

My E46 M3 had reinforcement plates properly welded, but uncoated as we got to the underbody during its restoration. 

The welds were still great. We kept those and coated them right. 

How we can fix it good. 

In order to avoid bending the connection and/or the vertical wall of the frame rails one should create something that is rather rigid such that it applies a clean vertical force to the frame rails. 

 

Taking a page from the future. 

BMW learned from their mistakes and fixed these chassis issues for good with the E9x.

The E9x M3 moved to a multi-link front and rear suspension anchored off subframes. 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.

While it doesn't crack anymore, the subframe and differential coating is prone to surface rest and degradation. I had to restore mine entirely around 100,000km. 

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 became standard on the F8x M cars and G8x M cars, along with optimizing the subframe's weight by using a tubular construction. 

With the problem-solution gap framed, it's time to permanently fix it. 

Up next: I chat about my selection of a complete subframe reinforcement kit and unbox a rapid delivery from down under.