How is this subframe crack?

[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%.

[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.

[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.

[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.

[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.

[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?

[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.

[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.

[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.

[dl.m3]

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.

I plan to add a Mason style rear X brace to my existing Redish plates. Also have 95A poly subframe bushings. Is this going to significantly stress my front 2 subframe mounting points? Car is mostly weekend use and 1-2 track days a year. Not sure if there's a easy way to address the front mounting points without dropping subframe haha, will probably do that eventually though..

[ZHPizza]

I thought I had read, and maybe it was specific to non-m's, that if you secure the rear mounts via topside anchoring, then the front mounts won't be able to flex around and tear.

[poss]

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.

Unfortunately that may seriously complicate matters and limit your repair options if you have BMW's cavity foam gunk band-aid, as it puts off toxic fumes when heated by welding - so many repairers may refuse to touch it.

I suggest you confirm this before purchasing anything.

That aside, if you put in rail plates you have to cut a section of the boot floor away anyway - so you might as well put a brace in and extend the rear subframe mount bolts up through it while you're there. This is an M3 not some lesser-powered E46 after all. A cavity brace is easily manufactured out of some square section and pipe.

The CMP rear bar is actually 2 bars - one in the cavity and another on top directly between the chassis rails (that protrudes into the boot) to provide maximum possible reinforcement. A single bar between the rail plates is likely more than adequate to prevent RACP cracking, but if you want no compromises rigidity and you don't mind loosing boot space then the CMP bar is the way to go.

[ZHPizza]

poss

All good I riveted the popped spot welds on the wheel well and made my own brackets to tie the subframe mounts to the chassis rail via rivets and panel bond:



And through bolted the front mounts to eliminate any concerns with the top welds giving out:



Then went all belt and suspenders by adding an x brace too:



Haven't out the side panels back in but I'm happy with how it's shaped up so far:

[Jersey_M3]

You may wanna have a look at this option if you don’t want to lose boot space. It looks very well built. If you have Instagram you can go to @weldtechbristol. He is the one who installs them all for the supplier of PSdesigns

http://www.psdesignsglobal.com/_shop/e46---m3/weldtech-bmw-e46-hidden-rear-axle-carrier-panel-reinforcment-brace-kit/

[AussieE46M3]

I thought I had read, and maybe it was specific to non-m's, that if you secure the rear mounts via topside anchoring, then the front mounts won't be able to flex around and tear.

I've seen a few brace manufacturers claim this to however, it's illogical.

There's two moment in action. One about the prop and the other about the wheels. This moment is then broken down into point loads at the subframe mounts.

Looking at it I'd imagine that the rear mounts support the largest portion of the prop shaft torque given the mounts are more eccentric to the axis or rotation. There's still the moment about the wheels axis which the front mounts will be experiencing as well as a portion of the spring force (~1/3) if running a divorced rear setup.

The front mounts admittedly don't seem to be anywhere near as much concern as the rear mounts however, are still a problem. All the M3's I've seen have had plenty of broken spot welds and at least 1 crack beneath the rear seats in addition to the top welds failing.

I recall doing some basic numbers on how tucking a 1" bar beneath the spare wheels well skin impacts the second moment of area and I recall it not being significant and only increased rigidity by a fraction.

I personally perceive the end plates as the most significant part of the puzzle and having a flat plate welded in avoids putting any eccentricity on the plate while having a end plate bolted to the subframe mount and nothing else then introduced bending into the slender bar.

My thinking is a significant reinforcement plate beneath paired with chassis rail plates is the best approach. The bigger plates offer a larger increase in rigidity to the hollow profile much like a slender bar on top and the chassis rail plates stop the panel from being able to pull away from the chassis rail and thus prevents all the subsequent failure patterns.

You can then always add a bolt in brace or additional weld in structure if you want/need more rigidity.