How is this subframe crack?

[AussieE46M3]

does the hidden topside reinforcement plate act like the bar connecting the racp to the chassis rail or is it better to go with the topside beam kit?

I'd envision it as an extension of the chassis rail so that it drops down onto the hollow section profile the rear two subframe mounts bolt into.

This will provide a direct transfer of stress between the rigid hollow section profile and the chassis rail preventing the rear half of the RACP from pulling down and away from the chassis rail, wheel arch and rearward.

The underside plates also provide a good increase in rigidity to this hollow section so adding the remainder of the topside beam kit would be for the purpose of adding further rigidity over the rear two subframe mounts (eg for big power) and/or to extend forward and include the front two rear subframe mounts.

The vast majority of secondary failures I've seen could have been prevented by the chassis rail to RACP plates being installed with the underside plates however, spot welds are still prone to pop above the front two mounts on coupes.

[ZHPizza]

By "factory reinforcement" do you mean the epoxy gunk sprayed into the cavity? (which will now make it far more difficult to fix properly 'cos no-one will want to weld with that stuff there)

That gunk was no fix - more likely a band-aid from BMW to get them thru the warranty period.

I believe so. I didn't see anything around the mount but there was this plastic wall of sorts about where hotspot #1 is on the diagram.

[poss]

This hand drawing from Vince is pretty solid



I had the factory reinforcement on my 05 zhp and no cracks at hotspot 1, but it just redirected the damage to hotspot 2 where the spot welds pulled through. Only noticed it because of the crack in the seam sealer.

By "factory reinforcement" do you mean the epoxy gunk sprayed into the cavity? (which will now make it far more difficult to fix properly 'cos no-one will want to weld with that stuff there)

That gunk was no fix - more likely a band-aid from BMW to get them thru the warranty period.

[Egone]

I have the same issue here in Aus where some installers refuse to convert to a superior product simply because they want to get the car in and out as quick as possible and do not care for the longevity of the vehicle.

I've had 3 cars in for reinforcement that had already been attempted by these other shops.

My view is that it is certainly worth the little bit of extra labour.

On a side note, I did recently start offering a hidden topside reinforcement being the RACP to chassis rails plates: https://cmpautoengineering.com/colle...is-rail-plates

It's a cheap, hidden way of addressing one of the critical issues and has been made modular to the topside beam kit if you wish to do more later on.




Not a problem, when you're ready feel free to shoot me an email if there's any questions I can help with.

Regarding the sale, I'm thinking it will go live at the beginning of December as a Christmas sale provided everything arrives on time, can be assembled, photographed and uploaded in time.

There is a total of 20 new products being added. I'm expecting some products to be discounted as much as 30%.

does the hidden topside reinforcement plate act like the bar connecting the racp to the chassis rail or is it better to go with the topside beam kit?

[AussieE46M3]

From the CMP website - this picture illustrates the fundamental design flaw that causes the RACP to crack. See the hole in the raised T - that is where the rear left subframe mount bolt is. Note that it has ZERO direct connection to the chassis rail FFS.

I still scratch my head as to why they didn't just make the formed hollow rectangle profile taller and stitch weld it straight to the chassis rail. It would not have been any significant amount of extra work as the panel you cut away to reveal that is stitch (not spot) welded to the frame rail. The increase in vertical cross section would also have increased rigidity and may have eliminated the issue of the stress concentration at the rear left subframe mount from cracking.

The front two mounts would still have been an issue but I feel like that one change would have resolved 2/3 of the issues.

I find the wheel arch spot welds let go about the same time cracks start to form on the earlier cars that have fewer spot welds. Based on what I've seen they went from 3 to 7 in later years.

The issue is if you do just plates and even stitch weld the wheel arch etc, cracks generally form on the inbound side of the flange and creep up toward the spring perch. This is because the hollow section profile necks down into a single layer of sheet metal.

Most Non-M owners do not want to overcapitalise with a full topside reinforcement on their cars hence why I offered the budget friendly chassis rail plates. They've been fairly popular.

[ZHPizza]

If BMW had put those cheap rail plates in in the first place then this would likely never have become the issue it has.

CMP could you post that cross-section diagram that shows how the RACP structure goes under the chassis rails, joins together into a single sheet, and then goes out to the wheel arches and then finally back to the chassis rails?

After seeing that diagram and then seeing it for real after cutting that part of the boot floor away next to the rails and seeing the void there it became obvious to me that underside RACP cracking was merely a symptom of this fundamental design flaw.

This hand drawing from Vince is pretty solid



I had the factory reinforcement on my 05 zhp and no cracks at hotspot 1, but it just redirected the damage to hotspot 2 where the spot welds pulled through. Only noticed it because of the crack in the seam sealer.

[poss]

From the CMP website - this picture illustrates the fundamental design flaw that causes the RACP to crack. See the hole in the raised T - that is where the rear left subframe mount bolt is. Note that it has ZERO direct connection to the chassis rail FFS.

[poss]

On a side note, I did recently start offering a hidden topside reinforcement being the RACP to chassis rails plates: https://cmpautoengineering.com/colle...is-rail-plates

It's a cheap, hidden way of addressing one of the critical issues and has been made modular to the topside beam kit if you wish to do more later on.

If BMW had put those cheap rail plates in in the first place then this would likely never have become the issue it has.

CMP could you post that cross-section diagram that shows how the RACP structure goes under the chassis rails, joins together into a single sheet, and then goes out to the wheel arches and then finally back to the chassis rails?

After seeing that diagram and then seeing it for real after cutting that part of the boot floor away next to the rails and seeing the void there it became obvious to me that underside RACP cracking was merely a symptom of this fundamental design flaw.

[AussieE46M3]

I did a huge amount of research on this issue when I discovered the same RACP cracking issue in my newly-purchased (slightly-used) M3 with less than 50K miles on the clock - the big difference being that I bought 2nd hand from a BMW Dealer who told me the vehicle had been fully checked. They lied.

After threatening to sue the BMW Dealer we reached an amicable arrangement. I did not go with the CMP brace in the end because it intruded into the boot space (so that it creates the strongest connection to the chassis rails). I did go with the CMP underside plates tho - the installer complained they were too much work to install as they were so heavy-duty/thorough in their design.

I have the same issue here in Aus where some installers refuse to convert to a superior product simply because they want to get the car in and out as quick as possible and do not care for the longevity of the vehicle.

I've had 3 cars in for reinforcement that had already been attempted by these other shops.

My view is that it is certainly worth the little bit of extra labour.

On a side note, I did recently start offering a hidden topside reinforcement being the RACP to chassis rails plates: https://cmpautoengineering.com/colle...is-rail-plates

It's a cheap, hidden way of addressing one of the critical issues and has been made modular to the topside beam kit if you wish to do more later on.

I'm actually putting the car away for winter now, so I don't intend to have the work done just yet. I'm certainly going to collect the material early though, was thinking the end of this month; when is the big sale you mentioned? I'm definitely interested in the near future and would gladly take advantage of that.. I was going to email you, but figured other people might want to know too! Thanks for reaching out, your work looks top notch.

Not a problem, when you're ready feel free to shoot me an email if there's any questions I can help with.

Regarding the sale, I'm thinking it will go live at the beginning of December as a Christmas sale provided everything arrives on time, can be assembled, photographed and uploaded in time.

There is a total of 20 new products being added. I'm expecting some products to be discounted as much as 30%.

[01SG]

Feel free to email me or shoot me a message via Facebook.

I have a bunch of parts arriving for E36, E8X/E9X as well as F2X/F3X and intend to do a big sale in the near future but, if you're wanting to get onto it sooner I can do you a good deal now.

Here's a link to the Facebook page. https://www.facebook.com/cmpauto

Otherwise my Email is: info@cmpautoengineering.com

Look forward to hearing from you!

I'm actually putting the car away for winter now, so I don't intend to have the work done just yet. I'm certainly going to collect the material early though, was thinking the end of this month; when is the big sale you mentioned? I'm definitely interested in the near future and would gladly take advantage of that.. I was going to email you, but figured other people might want to know too! Thanks for reaching out, your work looks top notch.

[poss]

So CMP is truly the most effective design? I did appreciate all the articles on the website; would like to see that eighteen page installation manual.. They have the underside reinforcement, topside bar, and top extension, and also these Subframe/RACP to chassis rail plates; that should cover everything then?

I might throw in the six-point bar for the hell of it, something extra to look forward to after such a pain in the ass.

I did a huge amount of research on this issue when I discovered the same RACP cracking issue in my newly-purchased (slightly-used) M3 with less than 50K miles on the clock - the big difference being that I bought 2nd hand from a BMW Dealer who told me the vehicle had been fully checked. They lied.

After threatening to sue the BMW Dealer we reached an amicable arrangement. I did not go with the CMP brace in the end because it intruded into the boot space (so that it creates the strongest connection to the chassis rails). I did go with the CMP underside plates tho - the installer complained they were too much work to install as they were so heavy-duty/thorough in their design.

[AussieE46M3]

So CMP is truly the most effective design? I did appreciate all the articles on the website; would like to see that eighteen page installation manual.. They have the underside reinforcement, topside bar, and top extension, and also these Subframe/RACP to chassis rail plates; that should cover everything then?

I might throw in the six-point bar for the hell of it, something extra to look forward to after such a pain in the ass.

Feel free to email me or shoot me a message via Facebook.

I have a bunch of parts arriving for E36, E8X/E9X as well as F2X/F3X and intend to do a big sale in the near future but, if you're wanting to get onto it sooner I can do you a good deal now.

Here's a link to the Facebook page. https://www.facebook.com/cmpauto

Otherwise my Email is: info@cmpautoengineering.com

Look forward to hearing from you!

[01SG]

So CMP is truly the most effective design? I did appreciate all the articles on the website; would like to see that eighteen page installation manual.. They have the underside reinforcement, topside bar, and top extension, and also these Subframe/RACP to chassis rail plates; that should cover everything then?

I might throw in the six-point bar for the hell of it, something extra to look forward to after such a pain in the ass.

[poss]

I saw the CMP bushings were for lowered cars, so I might just go OE or Turner's. I wanted to order it all from one place, but since that's out I'll look at the Vincebar too. His kit also has the gussets to accomplish both front and rear reinforcement on top. I'm going to find a shop with experience and see if they have a preference; it appears they will both do the job.

I cleaned things up a bit and poked around some more after dropping the diff. I found another crack, predictably around the front right subframe bushing, in between that and the gas tank.

The gussets will add some reinforcement to the front of the subframe but you will still be relying upon sheet metal to transfer forces to the chassis rails (which is why CMP offers both front and back bars)

I used Powerflex black for subframe bushings and really like the improvement

[clawhamm3r]

You'll probably find a dozen more after grinding everything down. Pay special attention to the spot welds around the mounts, you'll see tiny cracks radiating out like a pinwheel.