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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 Mon, Apr-15-2019, 08:31:57 PM   #1
bimmerfan08
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Default Nearly 6 years after my RACP underside reinforcement, I found a crack.

You can put a tally in the crack column for my E46 M3. Nearly 6 years ago I reinforced the underside of my car's RACP with plates and epoxy, which was well documented in my extensive DIY. No cracks were found at the time...at least on the underside. This past weekend I decided to remove the rear interior and hack up the metal in order to access the rear and front mounts from the topside. The main reason was preparation for topside reinforcement soon, but I was also really curious as to what the condition of the mount locations were from the topside.

The first mount I exposed was the driver's side front mount. I was relieved to see that it was in great condition and appeared undisturbed. When I removed the makeshift access panel for the passenger side front mount, I discovered what many before me have - a large crack, propagating across one of the welds. Lastly, I removed the top layer of metal from the rear to access and assess the rear mounts. From what I can tell, all looks in order in the rear. I guess I can say I'm not surprised I found a crack given this is a well-known issue with the E46 chassis and the fact that I do track my car. It could be worse based on some of the RACP monstrosities I've seen come across this forum over the years.

So my next order of business is reinforcement. I have a welder lined up that's willing to go with whatever design I best see fit for the car. With that said, I have some time to mull over a design. I have a few reinforcement design options in mind laid out below.

1.) For the rear mounts I'm considering a simple brace across the upper RACP layer that ties into the chassis rails and uses extended bolts.
2.) I'm also considering (based on what I've seen in a few race cars) welding a frame directly into the rear factory mount locations and tying it into the chassis rails - which would utilize the factory hardware.
3.) For the front mounts, I'll just have simple reinforcements fabbed up that use piping.

Has anyone considered or tried option #2? Any other inputs or suggestions for designs? Ideally I'd like to keep the reinforcements as concealed and clean as possible.

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Last edited by bimmerfan08; Mon, Apr-15-2019 at 08:44:02 PM.
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Old Mon, Apr-15-2019, 08:54:56 PM   #2
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Default Re: Nearly 6 years after my RACP underside reinforcement, I found a crack.

My car looked about the same.

Iíd consider going with a bolt through design and then use grade 8 washer.


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Old Mon, Apr-15-2019, 09:38:04 PM   #3
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Default Re: Nearly 6 years after my underside RACP reinforcement, I found a crack.

for front mounts, a thru bolt with a washer or a thin plate under the head to distribute clamp load should be adequate.
for rear mounts, drill out the existing thread and mount your preferred rear mounting brace (gtr xbar, cmp etc) using longer bolts.
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Old Mon, Apr-15-2019, 11:06:47 PM   #4
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Default Re: Nearly 6 years after my RACP underside reinforcement, I found a crack.

6 years ago did you also do similar surgery to check for cracks from the top? If not, could they have already started there?

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Old Mon, Apr-15-2019, 11:26:11 PM   #5
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Default Re: Nearly 6 years after my underside RACP reinforcement, I found a crack.

Did you check the spot welds along the chassis rail beneath the rear bench seat or for cracks around the far left bolted in bracket?

Your damage is very minor. I'd be curious to see if those areas are showing any signs of failure.

Regarding your reinforcement proposal, if you want it to remain concealed you are limited in options. I would feel extended hardware would provide negligible benefit if you introduced a structure that adequately integrates into the existing structure and sufficient links (with welds) to the threaded insert within the RACP.
As you mentioned, many race cars have done it this way with additional tubes added to a cage structure rather than depending on an extended bolt and am yet to see them have any issues with that method. Utilising the factory hardware would make life easier for you.
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Old Mon, Apr-15-2019, 11:49:12 PM   #6
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Default Re: Nearly 6 years after my underside RACP reinforcement, I found a crack.

"
1.) For the rear mounts I'm considering a simple brace across the upper RACP layer that ties into the chassis rails and uses extended bolts.
2.) I'm also considering (based on what I've seen in a few race cars) welding a frame directly into the rear factory mount locations and tying it into the chassis rails - which would utilize the factory hardware.
"
1) If something like Vince-bar is used then the beefy transverse rectangular tube is stiff enough to hold the panel from moving up/down
2) but on the bottom due to the tight space between the mounting point and the subframe bushings, I don't see how you will add a beefy frame going left/right here. A thin frame only stabilize the bottom panel from left/right shear force when cornering, but doesn't help stopping up/down motion on the panel.
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Old Tue, Apr-16-2019, 10:15:56 PM   #7
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Default Re: Nearly 6 years after my RACP underside reinforcement, I found a crack.

Quote:
Originally Posted by bimmerfan08 View Post
I guess I can say I'm not surprised I found a crack given this is a well-known issue with the E46 chassis and the fact that I do track my car. It could be worse based on some of the RACP monstrosities I've seen come across this forum over the years.
Sorry to hear about this OP, hopefully you get it sorted soon.

However I do have a couple of questions:
  • How many miles have you done since reinforcement?
  • How often has your car been tracked since reinforcement?
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Old Wed, Apr-17-2019, 01:14:25 AM   #8
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Default Re: Nearly 6 years after my RACP underside reinforcement, I found a crack.

Quote:
Originally Posted by mrgizmo04 View Post
6 years ago did you also do similar surgery to check for cracks from the top? If not, could they have already started there?
same question
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Old Wed, Apr-17-2019, 03:11:44 AM   #9
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Default Re: Nearly 6 years after my underside RACP reinforcement, I found a crack.

Thanks for the replies so far guys.

Quote:
Originally Posted by mrgizmo04 View Post
6 years ago did you also do similar surgery to check for cracks from the top? If not, could they have already started there?

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Originally Posted by Dirty Elf View Post
same question
Negative. This is the first time I've seen the topside of the mounts. That question popped into my head as well. But without seeing the condition of the mounts at the time of reinforcement, it's inconclusive on whether the crack began before or after the underside reinforcement. Regardless, it reinforces (no pun intended) the notion that just inspecting and reinforcing the underside isn't enough.

Quote:
Originally Posted by R60BBA View Post
Sorry to hear about this OP, hopefully you get it sorted soon.

However I do have a couple of questions:
  • How many miles have you done since reinforcement?
  • How often has your car been tracked since reinforcement?
I've put approximately 26k miles on the car since reinforcing the underside with plates in 2013. And 3 track events (5 total days) within that period.
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Old Wed, Apr-17-2019, 03:15:47 AM   #10
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Default Re: Nearly 6 years after my underside RACP reinforcement, I found a crack.

Quote:
Originally Posted by bimmerfan08 View Post
Regardless, it reinforces (no pun intended) the notion that just inspecting and reinforcing the underside isn't enough.

No doubt this is true. I just really wonder if those cracks were already there or if they formed even with the reinforcement from the bottom. Six years seems like a long time and they didnít look too bad.



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Discussing Nearly 6 years after my underside RACP reinforcement, I found a crack. 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)