"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, a new kit shows up last minute from half way across the world. I unbox Cayn's upper chassis reinforcement kit the Phoenix and chat about its engineering and technical features."
Over the Summer of 2023, I started unknowingly chatting with the founder of CMP Auto Engineering about the E9x M3 rear suspension and its complete conversion to ball joint. It's a cheap conversion using purely Genuine BMW ball joints.
I had been looking for the required shims to do it but instead went complete spherical with my curated Turner chassis optimization kit.
Cayn = CMP.
By then, I realized Cayn was CMP's engineer and founder. I had previously heard great things about his raised solid subframe bushings in the E46 M3 communities, namely from Obioban at NAM3.
Cayn is an engineer by trade, and an enthusiast first. CMP grew from interest in kits Cayn first developed for his own E46 M3: a classic story.
I told him about my dilemma on the E46 M3's subframe reinforcement: my car was intended for the street - a large trunk brace that would block the folding seats was not an option.
He had just released a new street kit! A virtual hand shake later, it was already in the mail. The kit got here in 4 days.
A comprehensive solution.
My E46 being already fitted with reinforcement plates at the subframe mounts, my focus was on upper reinforcement. This is where most kits on the market start, and end, with few offering a basic reinforcement to the upper frame rails.
As a refresher, I highly recommend you read the previous entry "What was BMW thinking?, to get a primer on the fundamental design flaws, and the lingo used below.
The frame rail plates.
The E46 M3's subframe bolts do not connect to the frame rails of the chassis or anything else that could be considered structural. Instead, the bolts are held by single layer of sheet metal towards the wheel arch.
A catastrophic here results in the flappy trunk floors, the most visually painful of all failures. Even with underside reinforcement plates installed you aren't safe from this as you cannot correct one of the biggest design flaws from just underneath.
For Cayn, new plates connecting the chassis rails to the rear axle carrier panel are the start to upper reinforcement. He considers these the bare minimum.
The plates have had multiple design revisions to consistently improve fitment and ease of installation.
The frame rail plates are laser cut from 2mm thick mild steel (zincanneal). It is rated to 250MPa yield and zinc coated.
While the plates could be fitted on their own, I wanted a complete solution. They are designed to fit with CMP's rear structural beam. The table acts as the trunk's floor here.
There's a sense of a well built puzzle to the complete kit.
The rear beam.
This is where CMP starts to separate itself from the pack by its design ingenuity.
The rear beam was designed for owners with my particular goals: I wanted to drive this car like I mean it, but without any visually noticeable bracing in the trunk.
This kit reinforces the rear subframe mounts and creates a direct transfer of stress from the subframe mounts to the chassis rails.
The kit is made of 2 folded sheet metal components that make up the beam when installed. It sits under the trunk's liner.
This kit also increases the height of the trunk's floor for improved structural rigidity.
In engineering terms, the vertical cross section is increased, improving rigidity across the the entire width of the rear axle carrier panel.
The upper and lower section are connected by a roll cage tubing in 1.5" outside diameter and 1/10" wall rate at a stronger 350MPa yield.
The front beam.
The front beam sets this kit apart and completes the "Stealth" kit I have curated. CMP stands on its own here as the final stage to comprehensively reinforce all four rear subframe mounts from the top.
The front kit on its own wouldn't do much, but the combination with the rear kit provides the other 'L' profile needed to make a rectangular section with the 4x subframe mounts.
With this kit, permanent peace of mind is permanently achieved, without sacrificing the rear folding seats.
It works in very similar ways to the rear beam by connecting the front subframe mounts together to create a direct transfer of stress.
It also increases the height of the trunk's floor to improve structural rigidity.
The winglets are designed to contour and reinforcement the chassis, while not obstructing the rear seat folding mechanisms.
The winglets elevate the structure so that it can sit against the chassis rails like the rear beam does.
This is how it will sit over the chassis' sheet metal.
The kit also includes gussets and top caps. These directly reinforce the sheet metal sections of the front subframe mounts.
Notice the laser cut notches on the triangles? They are designed to fit the nose of the round caps. The upcoming installation entry will make this evident.
To standardize installation, Cayn integrated holes in the beams that act as alignment guides during installation. I will cover this in the installation entry.
The threaded sections are for race cars planning to sure the X bracings he has developed. This was a bridge too far for my needs.
All these pieces complete the puzzle for a permanent solution - but I understand it's difficult to visualize.
Up next: it's time to cut up the trunk's floor, assemble the kit and weld it in.
I'll touch on why I don't recommend this is a DIY, and why Cayn's clever approach to design makes this a more affordable, professionally installed solution than anything out there.