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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Fri, Aug-12-2016, 03:56:34 PM   #1
VinceSE2
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Default ("Subframe") RACP cracking and separation - How common, what models?

First of all, as you probably already know, it's not the subframe that fails on our cars. It's the chassis/floor panel the subframe is attached to.
This panel is commonly referred to as the Rear Axle Carrier Panel, or abbreviated, the RACP.

That said, I went hunting for a used but undamaged RACP for a couple of projects I'm planning.

I found out there is one of those old fashioned car junk yards not far from where I live. The kind where you go out yourself and remove the part(s) you need (the more modern ones don't keep the chassis, just the loose parts).

So I went to the BMW section, found the row of 3 series, and a number of E46's. Knowing that the sedan (and even the wagon IIRC) and coupé, including the M3, share the same RACP I started looking under the E46 closest to me and worked my way down the row.

The first one was a 2000 320i sedan, milage unknown (as with all these examples).

Here's what I found at the left rear subframe mount:



As with many E46's built before February 2000 the threaded receiver had been pulled out of the RACP. What surprised me was that this was a lousy 320i, not a very torque-laden model!

The second E46 was another 320i sedan, this time a 1998.

Here's what I found at the left rear subframe mount:



At first this one looked good. But looking a bit closer I could see it had been welded.

The third one was a 2003 330i coupé. Knowing the 2003 has the updated RACP (with the added spot welds on the vertical surface), that even the 330 has limited torque, and that it has a "soft" drive line, I was certain this one would be in good shape.

At first glance it did look good, but here's what I found:



Right above the mount you can clearly see the extra spot welds I previously mentioned. These have, as seen in the picture above, successfully stopped the threaded receiver from being torn out of the RACP. But as BMW never reinforced the load bearing part of the RACP it has instead cracked the transitional area where the sheet metal is also stretched thin.
It's the spot we have become familiar to look for the first cracks, further inbound from the mount as seen in this picture:



That's a pretty nasty crack for a 330i.

And sure enough it also had the separation of the RACP from the wheel well (chassis leg) panel as seen in this picture the thin line in the seam sealer following the contour of the wheel well):



I'm not sure if it was also cracked at the front right mount as it wasn't clearly visible due to a rust flake I didn't bother to remove:




So is this a scientific basis of, and proof that, three out of four E46's will crack their RACP sooner or later?
No, of course it's not. Perhaps these where scraped because of the damage, who knows?

But it is proof that even the E46's with "lesser" engines crack their RACP's. And that relieving the RACP from most of the load (for example via a cross bar connecting the rear mounts to the chassis legs) is a very good idea. And investment.


After these three damaged RACP's I did find an un damaged RACP in a 2003 320i sedan.
Most likely I'll be cutting off it's rear end, and bringing it home as a development platform.

Thanks for looking.



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Old Fri, Aug-12-2016, 04:11:18 PM   #2
lmtfi
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Thanks for posting. There is a LOT of rust on those cars.
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Old Fri, Aug-12-2016, 04:22:13 PM   #3
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

another thing is that it's hard to 100% inspect this without removal of the subframe and taking a closer look.
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Old Fri, Aug-12-2016, 04:35:01 PM   #4
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Great writeup!

I do not have any Subframe support in and was look at vincebar from you, but should I get the Masonbar for cross support? I so far could not find any cracks on mine
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Old Fri, Aug-12-2016, 07:17:17 PM   #5
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

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Originally Posted by natethegreat View Post
Great writeup!

I do not have any Subframe support in and was look at vincebar from you, but should I get the Masonbar for cross support? I so far could not find any cracks on mine


Thanks

If you are referring to torsional rigidity, the Mason bar+xbrace (the latter is also needed for RACP load relief) offers this while the stealth VinceBar does not.

The main reason to choose the stealth VinceBar would be to preserve usability of the trunk space.

Having them both is however a bit redundant as far as the RACP load relief goes.

But thats one of the projects I'm planning for once I get the rear end (which will include the rear shock towers and more) home.


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Old Fri, Aug-12-2016, 07:32:04 PM   #6
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Quote:
Originally Posted by VinceSE2 View Post
Thanks

If you are referring to torsional rigidity, the Mason bar+xbrace (the latter is also needed for RACP load relief) offers this while the stealth VinceBar does not.

The main reason to choose the stealth VinceBar would be to preserve usability of the trunk space.

Having them both is however a bit redundant as far as the RACP load relief goes.

But thats one of the projects I'm planning for once I get the rear end (which will include the rear shock towers and more) home.


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In your opinion, is the xbrace worth the extra money over the standard subframe support kit they offer?
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Old Fri, Aug-12-2016, 07:38:32 PM   #7
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Thank you vince for the very informative post.
I need to reinforce my car really bad.

Quote:
Originally Posted by lmtfi View Post
Thanks for posting. There is a LOT of rust on those cars.
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Old Fri, Aug-12-2016, 07:39:43 PM   #8
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Dear lord
..now I'm gonna have nightmares
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Old Fri, Aug-12-2016, 08:04:11 PM   #9
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

Quote:
Originally Posted by silvergray545 View Post
In your opinion, is the xbrace worth the extra money over the standard subframe support kit they offer?
IMO, the only Mason product worth buying is the full rear x brace (which includes floor bar and shock tower bar) or the GTR bar which connects at the same points. Both are right under $800 I believe.

Vince's front mounts are also a great idea and are the only front solution that I'm aware of.

I'm leaning towards Mason because I'm about 10 minutes away. Shipping kinda kills Vince's package for me, personally.
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Old Fri, Aug-12-2016, 08:05:12 PM   #10
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Default Re: ("Subframe") RACP cracking and separation - How common, what models?

I think based on all the evidence and given how old the cars are getting its safe to assume you have cracking until proven otherwise. Its just crazy that something this serious was missed in the engineering and development of the car. I mean the ability for the RACP to withstand the demands of the car should be a pretty low level requirement for any car company.
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Discussing ("Subframe") RACP cracking and separation - How common, what models? in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)