e36 vs e46 "rear subframe" issues

[Beej]

First, I hate that everyone calls these rear subframe issues. The rear subframe is always perfectly fine - it's the goddamn frame that has issues - let it get bad enough, and you're replacing the trunk floor, not the rear subframe. If only.

Anyway, I bought my e46 blissfully unaware they had this issue. Even worse, I laughed at e36 owners for having a rear end that fell apart. You idiots! I knew of the generational similarities between the e36 and e46, but just assumed that BMW, like any respectable entity, would've fixed the issue.

Soo...you can't swing a dead cat on e46 forums without hearing about a failure. And finally...to my question.

Is the issue the same on the e36 chassis as it is on e46? Cracks at the front right, rear left rear subframe mounting points before eventually they collapse/fail? If so, why are there any e36's on the road anymore? I remember reading/laughing about it back in my e30 pride days, but I can't find as much info on e36 failures as I can on e46 ones.

For those who aren't familiar with our dirty little non-secret - the beginning:


And the end:


There are aftermarket "solutions", and this thread may get there eventually, but I'm genuinely curious the nature of the issue first, and why the fuck BMW didn't fix it. I still don't know why e36/7 and e36/8 cars have their own issues, since they were more e30 than e36 back there.

[JPolen01]

The issue is real. We are getting ready to do the subframe upgraded in our shop e46 m3. Right now it's ok but with 180k miles on it and adding the extra grip from the sticky tires its only a matter of time before it goes.

[D_Eclipse9916]

First of all, yes, wtf BMW why did you not fix it?

Second. Not all E36s were affected. E36 M3s actually came with a reinforcement built in from the factory. E36 M3s do NOT need any additional "subframe bracing". They do need additional bracing when pushed with stiff suspension and bushings for the rear trailing arm pocket (with sphericals or delrins) and sway bar reinforcements when you put in big bars.

The E36 325i/328i can simply weld in the factory part number reinforcements that were put in the M3. Its like ~$30 straight from the dealer and any easy weld on.

Z3, it's the floor ripping, remember, doesnt matter the suspension, it is the load between the subframe and floor. Don't know that much about it.

E46s including M3s have the issue pretty badly though. I *think* we see it a lot more with e46 m3s because of the higher power of the car/bigger/heavier differential (210mm case instead of 188mm). My mom's does not have it currently, but I will be under there checking again soon when I do the valve adjustment and diff flush. Heck it's almost to the point where I need to pull out the subframe and replace allt he bushings/ball joints.
E46 M3s have pretty stiff suspensions and bushings, are abused pretty regularly, and have weak chassis metal back there. Unsure of why BMW didnt fix it. Crazy that the chassis can be that stiff, but then just rip apart.

[SlimKlim]

First of all, yes, wtf BMW why did you not fix it?

Second. Not all E36s were affected. E36 M3s actually came with a reinforcement built in from the factory. E36 M3s do NOT need any additional "subframe bracing". They do need additional bracing when pushed with stiff suspension and bushings for the rear trailing arm pocket (with sphericals or delrins) and sway bar reinforcements when you put in big bars.

The E36 325i/328i can simply weld in the factory part number reinforcements that were put in the M3. Its like ~$30 straight from the dealer and any easy weld on.

Z3, it's the floor ripping, remember, doesnt matter the suspension, it is the load between the subframe and floor. Don't know that much about it.

E46s including M3s have the issue pretty badly though. I *think* we see it a lot more with e46 m3s because of the higher power of the car/bigger/heavier differential (210mm case instead of 188mm). My mom's does not have it currently, but I will be under there checking again soon when I do the valve adjustment and diff flush. Heck it's almost to the point where I need to pull out the subframe and replace allt he bushings/ball joints.
E46 M3s have pretty stiff suspensions and bushings, are abused pretty regularly, and have weak chassis metal back there. Unsure of why BMW didnt fix it. Crazy that the chassis can be that stiff, but then just rip apart.

That explains why I've never had the issue with the E36. I will need to drop out my subframe this winter and replace all the bushings though, pretty sure some of them are original. hock: If its any consolation E36 shock towers have been known to catastrophically fail, especially on northern cars. I think mine has the reinforcement plates in addition to the RE mounts. I think...

BMW tried to fix the E46 issue in MY2002 with a different trunk floor and adding a cross brace to the front of the diff, but it was about as effective as their attempted HPFP fixes on the N54 motor.

If yours is starting to crack there are tons of reinforcement kits out there. I'd imagine the sooner you get it done the easier it'd be. Turner has a pretty good looking one for $140. http://www.turnermotorsport.com/p-91-e46...t-kit.aspx No idea what labor would be on it though.

Dj this one applies to you, I've heard of some guys doing V8 swaps in an E36 and straight up ripping the diff right the fuck off the chassis, especially in the 318 cars. I assume you already know that, but adding a cross brace or some sort of reinforcement might not be a bad idea when you do your swap.

[ispoonwithmugen]

So E36' and E46's have this problem. What about E30's? I think I've read on r3vlimited (an e30 forum) about something like this and I think they linked the turner motorsports solution also. Is this something I should be concerned about? I'll definitely check it out now just to make sure.

[D_Eclipse9916]

Dj this one applies to you, I've heard of some guys doing V8 swaps in an E36 and straight up ripping the diff right the fuck off the chassis, especially in the 318 cars. I assume you already know that, but adding a cross brace or some sort of reinforcement might not be a bad idea when you do your swap.

Really? First time I have heard of this issue. The 318is had much weaker diffs, so I could see that? but reinforced subframe M3s?

This car has brand new E46 m3 RTABs/limiters being put in with Delrin diff mounts.

[CaptainHenreh]

So E36' and E46's have this problem. What about E30's? I think I've read on r3vlimited (an e30 forum) about something like this and I think they linked the turner motorsports solution also. Is this something I should be concerned about? I'll definitely check it out now just to make sure.

E30's had a totally different rear suspension geometry (semi-trailing arm, shared with the 318ti and the first-gen Z cars).

This shouldn't apply to you. I've seen a ton of SpecE30's on track, and never seen a non-wrecked one with any kind of torn frames or subframes. Doesn't mean it doesn't happen, but it's probably something you don't have to worry about.

[ispoonwithmugen]

Sweet, that's a relief :thumbup:

[Beej]

Well that's pretty interesting. I'm still shocked that it's worse on e46's when e36 M3's had special reinforcement, but I guess that explains it a bit. What kind of reinforcement is it exactly? If it's weld-on, is it like the aftermarket plates available for e46's?

As for "solutions", I'm not really thrilled with any of them:

• BMW recommends "structural foam" be injected into the rear floor, in an apparent attempt to make everything solid. Seems decent, but can only be done if any existing cracks are less than like 20mm or something. Tubes are like $150 a piece or something, you need 2 (some people do 4).
  
• Aftermarket plates: welding them on (can be done before structural foam, but not after). I'm just not really thrilled about having so much welding done back there - too much of a chance for rust, and if the welder sucks, just a bad job in general.
  
• Aftermarket plates: epoxying them on. There's much debate as to whether this is less or more effective than welding. I like the idea that the load is spread out over the entire contact surface of the plate (as opposed to welding, which only secures them at the edges) and that there's no risk of rust. Plates are like $120-140.
  
• Tying the rear floor into a cage/bar. Mason Engineering makes a fancy rear strut bar that takes up half of your trunk, but it only seems to attach to two mounts.

Despite the fact that you have to play with heating liquid things that become solid, and that being an easy thing to screw up, I'll probably go with foam + epoxied plates. AKG also makes "street" (no increased NVH, apparently) urethane bushings that have more surface area on the chassis side of things - probably throw them in at the same time.

Should probably do it soon, too, since I can't think of a replacement car.

[BLINGMW]

So I haven't read threads on the e46 issue, is this a "track" thing or is this something to think about for a street car too? I remember hearing about it for e36, owning one myself at the time and seeing so many on track, and always figured the chance of incident was exagerated (internet forum's specialty!). I too ASSumed BMW figured out whatever it was.

FWIW, I second the vote for epoxy plates. Easiest, cheapest, least intrusive, and seems like it would help a lot. :dunno:

What about E30's?

Nope, when the last e36 is crushed, there will still be e30s on the road. :wink:

[D_Eclipse9916]

What about E30's?

Nope, when the last e36 is crushed, there will still be e30s on the road. :wink:

[BLINGMW]

psh, you won't have those cars in 10 years. But it's likely I'll have an e30! :lol: Maybe one with a Tesla drivetrain.

[Beej]

In getting ready to tackle this, I've probably been doing too much research for my own good. However, after only poking around and looking at other people's diagrams - I finally came across a cut-away of the danger area:



There's also a relatively new player in the market. UK-based Redish Motorsports speaks much more intelligently on the issue, and makes older solutions seem like a "spray and pray" kind of thing. Here are their plates:



Compared to the usual:


Aside from being massive in comparison, they included those holes for rosette welds. If you scroll up, they recommend taking a bit of metal away to weld all 3 layers as opposed to slapping thicker metal on the outer part:

Inner "box" that has threaded receiver for subframe bolt (#4 in the diagram).
Outer frame skin (#2 in the diagram).
Reinforcement plate.

If not brilliant to tie those 3 pieces together, it's at least one of the more innovative ideas.

Only thing is, I'd still only let a welder I really trust to touch anything down there - I just think it's too easy for rust to accumulate back and down there, or for the welds on such thin material to not be done correctly. Because of this, I'm leaning towards foam to hold the insides together and if possible, epoxying the big Redish plates. They're apparently investigating the epoxy approach these days.

[D_Eclipse9916]

Hmm let me know what you find out. Just bought a welder, going to get some practice welding up the exhaust on both my M3s, and then will be tackling my mom's E46 subframe. I don't like the idea of foam because it "traps" moisture.

[Beej]

Ha, I've wanted to learn to weld for years. I don't think the back of this car makes for a good first project.

In all the e46 discussions I've read, I can't recall a single mention of moisture being an issue. A quick googling turns up some conversations about it, but...is that really an problem? The stuff is as hard hell, it can't possibly absorb moisture. Is it because there might be small gaps and air pockets for the moisture to go?

[D_Eclipse9916]

Is it because there might be small gaps and air pockets for the moisture to go?

Exactly. It's why Tundra Frames like to rust, and one reason why car sunroofs have drains. That said, it may not be an issue, but I like good ole fashioned welds.

[JustinG]

When you are ready to do this hit up Delta V or PAS, you shouldn't have any worries if they tackle the job for you. May not be the cheapest places, but it will definitely be done right.

[Beej]

Might as well document the nonsense.

So I decided to go with epoxying the plates as "additional" metal instead of the "weld" route, which would've consisted of welding the "inside" (#4 in diagram above, part that gets loose) to the additional reinforcements, at both the top and bottom. Reason being is that I wasn't comfortable with any welding back there - it's hidden, exposed to a lot of moisture (and salt, etc), and I just haven't had great experiences with welds. They all rust.

I'll keep the text minimal and just describe the steps with pictures. If taking on the job, assume you need to take a shower between each step.

1) Remove all the things.


2) Clean off any loose dirt in a vain attempt to minimize the shit that gets in your eyes.


3) Wire wheel the undercoating and remainder of the shit that, despite masks and goggles, will still get in your eyes. If you look at the lower left hand spot weld, you'll see a small crack that couldn't be seen with the undercoating on there. I was disappointed to find that, but at 98k "enjoyed but not abused" miles and in comparison to a lot of other horror stories out there...it's really not too bad. BMW's bulletin on this thing basically says that up to 20mm ain't no thang, and they inject "structural foam" in such scenarios. Mine was 4-5mm, and was just the one.


4) Scuff both the chassis and the reinforcement plate, clean the hell out of them, and then epoxy them up (I used 3M 08115 "panel adhesive"). Let it cure for 24 hours (I waited 36).


5) A lot of people - even those who weld - just use spray paint over their work. I really wanted something a bit more than that, and so I went with POR-15 (photo is of a different mount).


6) And on top of that (except for where the bushings touch), I used some rubberized undercoating.


7) I decided to go with Powerflex subframe bushings - theory being that it'll spread the load over a bigger surface area than the OEM ones did.


I haven't the foggiest how some people claim shops do this for $1k - I'm pretty sure I hit that in parts alone.

I sleep a little better at night, but I'm not 100% satisfied - though I don't think any solution has fully proven itself yet. They seem to be hard to source, but I'll eventually get 4 kits of BMW's structural foam (2 per corner) and "fill" the cavities. If it's not bomb-proof after that, then I'm getting a new hobby.

[BLINGMW]

so did you end up doing the foam too? Looks like those plates are pretty nice.

Having inherited my wife's E46, and looking to put a little trailer hitch on it... I'm wondering if I should do something like this first. The hitches I've found mostly attach to the spare tire well :evileye:

[rherold9]

I should have never stepped in and read this thread