It. Is. Time.

I couldn't see anything in the pics either. I will state as I always have that welding on plates is a bandaid to cover a crack, but won't do anything to address the actual problem. The forces will always take the worst possible path until you give them a better path to the frame rail via a Vince brace, x brace, etc.

I would only weld plates onto a car with the mounts torn away. Small cracks I would just repair weld.

The pulled spot welds on my wheel well seam were drilled out and replaced with rivets. Then I addressed the load path so it wouldn't happen again.

Quote:

---

Originally Posted by ZHPizza

I couldn't see anything in the pics either. I will state as I always have that welding on plates is a bandaid to cover a crack, but won't do anything to address the actual problem. The forces will always take the worst possible path until you give them a better path to the frame rail via a Vince brace, x brace, etc.

I would only weld plates onto a car with the mounts torn away. Small cracks I would just repair weld.

The pulled spot welds on my wheel well seam were drilled out and replaced with rivets. Then I addressed the load path so it wouldn't happen again.

---

So what would you recommend I do here? I don't see anything when looking. I have this kit ordered, but I'm not looking forward to welding this at all. Right now I'm tempted to simply do nothing. I don't track this car often, but now it'll bother me more than it did if/when I do.

I just looked, I don't see ANY signs of cracking in the subframe. People with cracks must drive their cars *really* hard.

I get what you're saying ZHPizza, if I weld plates in, I'll just redirect it the bad forces elsewhere and that could cause cracks to appear there.
Now I'm kind of at a loss what to do. Maybe I'll see what my welder friend can do for me. If I can get in touch with him, it would be stupid to not take advantage of his help.

Man I thought I had posted this stuff somewhere but I can't find it. Here we go.

So this is a cross sectional drawing of the left rear subframe mount location (where the fun usually occurs) from Vince of the bar fame. This is looking from the rear of the car:

https://uploads.tapatalk-cdn.com/202...d5114d6c5d.jpg

That Chassis leg is everything. That's where we want the force to anchor. But it doesn't because the early 90s chassis designers missed a crucial flaw.

So when you accelerate, the rotational force from the diff pulls down on that LR mount. This is my trunk with the top sheet metal (1 in the drawing) cut away:

https://uploads.tapatalk-cdn.com/202...d3619cf7d6.jpg

https://uploads.tapatalk-cdn.com/202...40368b718d.jpg

Now you can really see the gap between the chassis leg and the subframe mount. Because of that gap, the force is redirected down through the thin sheet metal, up through the wheel well seam, and finally to the chassis leg (yellow is load path, green is where you want it to end up):

https://uploads.tapatalk-cdn.com/202...3eb6ffe15a.jpg

Given that, here is the only cracking I found on my car - right on that seam at the wheel well (hotspot #2 in the diagram):

https://uploads.tapatalk-cdn.com/202...613e79b4d9.jpg

Enhance:

https://uploads.tapatalk-cdn.com/202...ed70ef653e.jpg

Had to clean pretty hard to find that, so make sure you're cleaning everything real well down there, especially at the hot spots.

Vince's solution is to add a hidden bar here:

https://uploads.tapatalk-cdn.com/202...77e66fcd16.jpg

This, I think, is the best solution as it gives a direct load path to the chassis leg:

https://uploads.tapatalk-cdn.com/202...22ea7c645a.jpg

I also support the idea of an X brace moving the load path into the shock tower that is connected to the chassis leg:

https://uploads.tapatalk-cdn.com/202...f08d23f1cd.jpg

https://uploads.tapatalk-cdn.com/202...fe7a4c79df.jpg

The x brace is nice because you don't have to cut the whole sheet metal cover off like I did, just some holes to drop it down (the above pic was from an early test fit, just to illustrate this point).

So, I do not have my PE and am not signing off on anything, but I am an ME and had extensive discussions on this with both Vince and the guy that makes the SME X-Brace where we exchanged several sketchups of load paths and were all on the same page about this thing.

TL:DR
1. Plates are band aids for cracks. I did the same as you - bought plates, cleaned tf out of my underside and didn't find any cracks (around the mounts), and returned the plates. They don't add anything structurally.

2. The best solution is the vincebar type. It connects the mounts directly to the chassis leg, stiffening up the rear and eliminating the load path issue.

3. An X brace is an acceptable solution. Not the direct load path of the vincebar, but still adds the top level support that's really needed to fix the issue and is much easier to install.

Oh and here's how I fixed my seam crack. Drilled out the pulled spot welds, ground out the seam, replaced the spot welds with rivets, and sealed everything up real nice.
https://uploads.tapatalk-cdn.com/202...1039318cff.jpg

https://uploads.tapatalk-cdn.com/202...cccd38ac3e.jpg

https://uploads.tapatalk-cdn.com/202...69772caab6.jpg

https://uploads.tapatalk-cdn.com/202...5728ba7c8f.jpg

Those drawings are fantastic. This is a fantastic writeup. I'm so glad someone else has put the time and effort into thinking about this.
This is more convincing than the plates. Now I understand why the welds on the side come apart.
I'll actually clean underneath the car looking for cracks. I was expecting things to be more obvious and less subtle. I don't see myself sticking an x-frame into my trunk, but I also have no desire to remove the top layer to install a vince brace.

This really is a tricky problem.

I'll post more questions as I think of them. Does this cracking only affect the left side?
What's that black stuff you put on the rivets?

Quote:

---

Originally Posted by Fried_Chicken

I don't see myself sticking an x-frame into my trunk, but I also have no desire to remove the top layer to install a vince brace.

This really is a tricky problem.

---

SME has space saver braces if that's what you're after: https://www.s-m-eng.com/product-page...e-saver-braces

They also have a hidden solution similar to the Vince bar: https://www.s-m-eng.com/product-page/E46-Weld-in-Brace

I have the X-Brace as well (thanks to the Pizzaman) and I highly recommend it. Even BMW themselves used a X-Brace for the GTR.

Attachment 39178

Those space saver braces look really nice.

Would they alone be sufficient? It looks like they only mount to one of the subframe bushing points, what about the fronts?

Quote:

---

Originally Posted by Fried_Chicken

Would they alone be sufficient? It looks like they only mount to one of the subframe bushing points, what about the fronts?

---

There are mixed opinions on that. Locking in the rear mounts will reduce stress on the front mounts as well. Whether that's enough for you is your choice. I've also read that non-folding sedans and convertibles have extra reinforcements on the front mounts. Not sure how true that is. ZHPizza has front cups like on the Vincebar (maybe he can post some pics) if you are looking for a 100% worry-free solution.