Weld steel plate into rear seat pass through?

[EthanolTurbo]

look into the mason GTR brace. I have one and it made my M3 feel just as rigid or even more so than the non folding sedan I daily drive.. it also helps with subframe issues and it looks great (exact replica of the street version of the e46 GTR). Cleanest way to hit two birds with one stone - no welding, no epoxy, no mess, completely reversible.

Adjustable or non adjustable? There's two different GTR braces.

[PetrolM3]

Adjustable or non adjustable? There's two different GTR braces.

Non-adjustable

[PetrolM3]

I’m very sorry if that was missinformation in any way or form, I’m simply going by the comparison between my (folding) sedan, touring and my M3 coupe.

Mostly in terms of driving feel and noticeable flexing/creaking when for instance passing a speed bump at an angle). But also in terms of lifting with a garage jack one side at one point.

The M3 coupe is by far the worst of the three.
My convertible is BTW terrible 😣 in comparison.
More so IMHO than the numbers in a post further up suggests.

Perhaps numbers in terms of Nm/deg don’t convey the same as driving experience? I honestly don’t know.

So perhaps I should rephrase my statement to say that one may or may not experience the level of added rigidity one might expect?

And I still believe that addressing the B-pillar issue would provide excellent results.

BTW, and this is a polite and honest question as I’m curious, are the non-M coupes more torsionally rigid than the M3’s?

Sent from my iPhone using Tapatalk

Here are the official BMW numbers:

- E46 Coupe (w folding seats) 12500

- E46 Sedan (w folding seats) 13000

- E46 Sedan (w/o folding seats) 18000

- E46 M3 coupe: 18500 nm/deg


This shows that the coupe vs sedan (B pillar argument) difference is only 500 where as the non-folding sedan difference is 5500 which is 1000% more!

As for the M3, it has additional bracing and welding compared to a non-M coupe which is why it is just slightly more rigid than the non-folding sedan. I'm not sure how you determined that the non-M coupe is more rigid than an M3 coupe... the numbers tell a clear story. The M3 is a decent amount heavier than a non-M car so that's why you might feel more 'flex' when jacking it up. I will say that a stock M3 doesn't 'feel' as rigid as the non-folding sedan even though the numbers say it is. When I first got my non-folding sedan, it felt more rigid than the M3 and had less creaks but that could be due to weight difference and also because the sedan gets the rigidity from the non-folding bulkhead where as the M3 gets it from chassis bracing and additional weld spots so it might feel like it's flexing more but in reality be more rigid.

Now add the Mason brace (replica of what BMW on the street version of the E46 M3 GTR) which ties the strut towers and rear mounting points of the RACP and you're likely looking at around 23000 on an M3 coupe. That's certainly how my M3 felt after I added the Mason brace but this is just feel and not measured in any scientific way. All I can say is the difference in feel was drastic. Mason is one smart dude to have realized that this brace is such an opportunity and replicated the part (many others have too in the past few years) - BMW must have done a ton of testing before deciding to install this on street version of the GTR to try and come closer to the race version GTR rigidity of 46,000!!! (full cage)

[AussieE46M3]

I guess I should have been more clear - there are a lot of ways to go here. Many of them right in many ways.

All of what you said is true, but the chassis was then either dipped or painted after the welding. It’s not a fear of welding, it’s more about applying the right attachment method in each circumstance given the pros and cons of each. I don’t see how anyone is going to spray the inside of the chassis after a modification like this. I also think rivets and epoxy are better from a corrosion standpoint. Which may not be an issue for people in arid climates. Plus, stitch welding by a Motorsport pro sounds great, but how many people are going to get that level of service? I’m assuming BMW and most car manufacturers weld at the assembly line bc it gets the job done… fast. And it works bc they back it up with the necessary processes to make it work. They’re not going to rivet and epoxy their mass-produced cars. That’s for special stuff.

Is welding ok? Sure. Is it the best? It depends.

Lots of ways to do it right, but as always, ymmv.

After DIY installing some of the Vince mods on my car with rivets and epoxy I don’t think I’ll do it any other way in the future. I felt a lot safer in my home garage. But then again, I’m not an experienced welder. If I had a lot of experience welding I’m sure I’d see this differently.

Would I feel good about taking my car to just anyone to have a bunch of welding done by someone I did not know? Not really, bc I want to own the car for a long time. Timelines are different with race cars than street cars.

Anyway, just some thoughts. As ever, I welcome the conversation and hope to learn as much as I can.

I totally agree that going to any joe blow with a welding gun is certainly a bad move. I think quality is always key. I'd trust a great welder who can follow instructions better than a 'specialist' who is improvising with a welder.

I can understand that corrosion is a concern and certainly would be worth considering depending on where you are in the world however, still depends on where you're welding. Some spots you can get to the back of the weld or, the back of the weld is essentially inside the car where it's pretty well protected from the elements.

In Australia this isn't something we often experience unless the car is >35 years old so I cannot give the best tips of rust prevention however, I spent a few years doing engineering work for a steel foundations business. we are in the NE part of the country which is considered a tropical climate and had to allow for 2mm of loss of section over 50 years for raw steel submerged in dry/saturated soils.
With this in mind and assuming some effort was taken to seal the welded areas from the elements structural rust issues shouldn't occur within the life of the car, especially in the cabin or in areas better protected such as the RACP/subframe mounts. The wheels arches would be of concern given fuel, oil and other acidic things that are on the road and flung up onto the body however, the only area welded there is a flange where you can access both side and seal correctly.

From my own research into panel bond it is great for composite structures however, has a low strain failure limit compared to steel so if applied to an area of high strain (flex) the epoxy will fracture through its cross section and suffer adhesive failure. I've seen a few pics float around of early adopters who were convinced to bond in their early style Turner reinforcement plates suffer this kind of adhesive failure. Welding is also necessary to repair cracks & spot welds so IMO i unavoidable.
This is where it works great on panels that do not have a force applied directly to it (roof, spare wheel well, gussets between hollow sections) as the force is very much distributed over the entire structure unlike a point load like the rear subframe mounts and parts of the RACP. A dividing wall separating the boot and cabin should be fine.

Thanks for the great comments here. My objective is to maximize structural rigidity while minimizing cabin intrusion. Therefore trying to avoid roll bars. If you read the Slon thread about their carbon panel, according to their testing, “boxing” out the rear structure by epoxying their panel across the rear seat opening creates a significant improvement in rigidity (I think it was something like 20%).

This aligns with the data around the difference between E46 sedans with and without rear seat pass through … again a ~25% increase in rigidity.

Sedan (w/o folding seats) 18000Nm/deg of torsion
Sedan (w/folding seats) 13000Nm
Sport wagon (w/folding seats) 14000Nm
Coupe (w/folding seats) 12500Nm
Convertible 10500Nm

Interesting comment on epoxy plus rivets vs welding, the shop actually recommended bolting in an AL panel first but I thought that would not be nearly a as rigid a as a weld in steel panel (for the same reason that welded in cages are better than bolt in). But I acknowledge that epoxy can be just as strong (Lotus basically epoxies the Elise together). I’ll go back to clarify.

I like the CMP brace, but again my thought was that it would not be as rigid as a full enclosure of the box. This is an open point for me though. Would love to hear additional thoughts.

Having installed a semi-dividing structural wall into my own E46 Non-M, It certainly feels like that's where the majority of the torsional rigidity gains in sedans come from.

I think aluminium is a good idea for this application to keep weight down and should be sufficiently strong if you get a ~2mm structural grade. I'd use panel bond and rivets if the mating faces are perfectly flat and have a large surface area however, bolts would be a good idea where not.

One comment I'll make about the CMP front brace vs a full panel. Yes, a full panel would be better, however that brace also ties into the front two subframe mounts which are one of the major point loads applying force in that area and thus, I feel the direct link to the point loads may outperform a full panel that doesn't.

[tlow98]

I totally agree that going to any joe blow with a welding gun is certainly a bad move. I think quality is always key. I'd trust a great welder who can follow instructions better than a 'specialist' who is improvising with a welder.

I can understand that corrosion is a concern and certainly would be worth considering depending on where you are in the world however, still depends on where you're welding. Some spots you can get to the back of the weld or, the back of the weld is essentially inside the car where it's pretty well protected from the elements.

In Australia this isn't something we often experience unless the car is >35 years old so I cannot give the best tips of rust prevention however, I spent a few years doing engineering work for a steel foundations business. we are in the NE part of the country which is considered a tropical climate and had to allow for 2mm of loss of section over 50 years for raw steel submerged in dry/saturated soils.
With this in mind and assuming some effort was taken to seal the welded areas from the elements structural rust issues shouldn't occur within the life of the car, especially in the cabin or in areas better protected such as the RACP/subframe mounts. The wheels arches would be of concern given fuel, oil and other acidic things that are on the road and flung up onto the body however, the only area welded there is a flange where you can access both side and seal correctly.

From my own research into panel bond it is great for composite structures however, has a low strain failure limit compared to steel so if applied to an area of high strain (flex) the epoxy will fracture through its cross section and suffer adhesive failure. I've seen a few pics float around of early adopters who were convinced to bond in their early style Turner reinforcement plates suffer this kind of adhesive failure. Welding is also necessary to repair cracks & spot welds so IMO i unavoidable.
This is where it works great on panels that do not have a force applied directly to it (roof, spare wheel well, gussets between hollow sections) as the force is very much distributed over the entire structure unlike a point load like the rear subframe mounts and parts of the RACP. A dividing wall separating the boot and cabin should be fine.



Having installed a semi-dividing structural wall into my own E46 Non-M, It certainly feels like that's where the majority of the torsional rigidity gains in sedans come from.

I think aluminium is a good idea for this application to keep weight down and should be sufficiently strong if you get a ~2mm structural grade. I'd use panel bond and rivets if the mating faces are perfectly flat and have a large surface area however, bolts would be a good idea where not.

One comment I'll make about the CMP front brace vs a full panel. Yes, a full panel would be better, however that brace also ties into the front two subframe mounts which are one of the major point loads applying force in that area and thus, I feel the direct link to the point loads may outperform a full panel that doesn't.

Great points, and really appreciate the 2mm cross sectional loss comment as I was a CE in my first few roles. What did that 2mm represent as a percent, I’m curious? 5%, 10%, etc?

Appreciate you weighing in. All salient points and I just hope all of this information helps people make the choice that works best for them given individual circumstances.

interesting about the epoxy failure with the turner plates! I’ll have to go look for those posts.