Weld steel plate into rear seat pass through?

[Maxhouse97]

I have a local shop that said they can weld a steel plate into the rear seat pass through. This is in attempt to reach the same structural rigidity as a sedan with out the rear seat pass through. I had thought about slonik’s carbon panel but with the war I don’t trust that source.

Thoughts on this? Anything to watch out for? I’m a big fan of structural rigidity and I see this as a low pain way of increasing it (as opposed to a roll bar). Thanks.

[AussieE46M3]

This is a brace that does exactly what you're talking about however, doesn't seal the boot like the sedans or a carbon wall does.

Made in Australia.

https://cmpautoengineering.com/collections/bmw-e46/products/e46-front-bolt-in-brace

[tlow98]

Welding is not the best option - it weakens the metal by making it more brittle. River and epoxy is better.

The slon option seems solid from the reviews I’ve seen.

[AussieE46M3]

Welding is not the best option - it weakens the metal by making it more brittle. River and epoxy is better.

The slon option seems solid from the reviews I’ve seen.

Yes, you can bond the panel in however, this fear of welding is unrealistic.

The cars were welded together when made and to this day BMW's are still welded together. Stitch welding is common practice for GT class race cars among many other forms of motorsport and even done to some road cars when they left the factory such as the rear right wheel arch in all E46's (the one that doesn't fail) and the majority of the chassis for Type R Honda's in the late 90's/early 2000's for example.

Yes, thermal embrittlement is real. No it won't suddenly make something structurally sound structurally compromised especially if you're adding further support to the structure.

If the welding is done correctly and the car was designed correctly in the first place, welding is not an issues.

Only less significant semi-structural panels are bonded in like roofs, spare wheel wells, quarter panels (and yes a partition wall between the cabin and boot) etc and has to be done to a very high standard of quality to be reliable.

[tlow98]

Yes, you can bond the panel in however, this fear of welding is unrealistic.

The cars were welded together when made and to this day BMW's are still welded together. Stitch welding is common practice for GT class race cars among many other forms of motorsport and even done to some road cars when they left the factory such as the rear right wheel arch in all E46's (the one that doesn't fail) and the majority of the chassis for Type R Honda's in the late 90's/early 2000's for example.

Yes, thermal embrittlement is real. No it won't suddenly make something structurally sound structurally compromised especially if you're adding further support to the structure.

If the welding is done correctly and the car was designed correctly in the first place, welding is not an issues.

Only less significant semi-structural panels are bonded in like roofs, spare wheel wells, quarter panels (and yes a partition wall between the cabin and boot) etc and has to be done to a very high standard of quality to be reliable.

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.

[VinceSE2]

I have a local shop that said they can weld a steel plate into the rear seat pass through. This is in attempt to reach the same structural rigidity as a sedan with out the rear seat pass through. I had thought about slonik’s carbon panel but with the war I don’t trust that source.

Thoughts on this? Anything to watch out for? I’m a big fan of structural rigidity and I see this as a low pain way of increasing it (as opposed to a roll bar). Thanks.

Before you go off and do something that could be rather permanent, please consider that the extra structural rigidity of the Sedan is mostly due to the proper B pillars. Most of the torsional movement occurs between the A pillars and the C pillars.

This is clearly illustrated by the fact that the sedans with folding rear seat backs (just like with our coupes) have very noticeably higher torsional rigidity than the coupe.

But yes, the sedan without folding rear seat backs is even more torsionally rigid.

So my point is that you should not expect to gain the same level of structural rigidity as a sedan without folding rear seat backs.


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[bigjae46]

Welding is not the best option - it weakens the metal by making it more brittle. River and epoxy is better.

That is absolutely false...100%. Welding is a time proven method to join two pieces of metal together. Welding does not make the metal more brittle, that is an extremely misinformed statement. Improperly welded joints in certain types of service (ethanol) that with no PWHT can fail due to brittle fracture.

An unskilled welder will introduce more heat into the parts and have an increased risk of warpage. Then you can have stresses in a structure that may break.

A skilled welder will minimize the heat affected zone, ensure there is good gas coverage to minimize contamination, and get full penetration which will exceed the strength, reliability and longevity of riveting and most bonds.

[bigjae46]

I have a local shop that said they can weld a steel plate into the rear seat pass through. This is in attempt to reach the same structural rigidity as a sedan with out the rear seat pass through. I had thought about slonik’s carbon panel but with the war I don’t trust that source.

Thoughts on this? Anything to watch out for? I’m a big fan of structural rigidity and I see this as a low pain way of increasing it (as opposed to a roll bar). Thanks.

What's the goal? If you want to avoid a roll bar and don't need the pass-through then why not. You could just weld two bars in an X in the opening and probably achieve similar results with less weight.

[Fresh1179]

If you are looking for structural rigidity, a proper cage (as referenced above), will give similar results, or the results I think you are looking for. I can understand the desire to have a block off plate if you are running a fuel cell or an elaborate sump setup. If that isn't the case, perhaps I am missing the point. It does look much cleaner though.

[tlow98]

That is absolutely false...100%. Welding is a time proven method to join two pieces of metal together. Welding does not make the metal more brittle, that is an extremely misinformed statement. Improperly welded joints in certain types of service (ethanol) that with no PWHT can fail due to brittle fracture.

An unskilled welder will introduce more heat into the parts and have an increased risk of warpage. Then you can have stresses in a structure that may break.

A skilled welder will minimize the heat affected zone, ensure there is good gas coverage to minimize contamination, and get full penetration which will exceed the strength, reliability and longevity of riveting and most bonds.

yeah l, I clarified a bunch of rational further below. Forums are hard to convey things sometimes. I see I did a poor job here. This is me practicing “think before I post in the future”.

[Maxhouse97]

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.

[bigjae46]

yeah l, I clarified a bunch of rational further below. Forums are hard to convey things sometimes. I see I did a poor job here. This is me practicing “think before I post in the future”.

[PetrolM3]

Before you go off and do something that could be rather permanent, please consider that the extra structural rigidity of the Sedan is mostly due to the proper B pillars. Most of the torsional movement occurs between the A pillars and the C pillars.

This is clearly illustrated by the fact that the sedans with folding rear seat backs (just like with our coupes) have very noticeably higher torsional rigidity than the coupe.

But yes, the sedan without folding rear seat backs is even more torsionally rigid.

So my point is that you should not expect to gain the same level of structural rigidity as a sedan without folding rear seat backs.


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This is wrong and misleading. Non-m Coupes and folding sedans are almost identical in rigidity where the non folding Sedan is in a league of its own. Search around and you’ll see… but please do not spread misinformation on this.

[PetrolM3]

I have a local shop that said they can weld a steel plate into the rear seat pass through. This is in attempt to reach the same structural rigidity as a sedan with out the rear seat pass through. I had thought about slonik’s carbon panel but with the war I don’t trust that source.

Thoughts on this? Anything to watch out for? I’m a big fan of structural rigidity and I see this as a low pain way of increasing it (as opposed to a roll bar). Thanks.

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.

[VinceSE2]

This is wrong and misleading. Non-m Coupes and folding sedans are almost identical in rigidity where the non folding Sedan is in a league of its own. Search around and you’ll see… but please do not spread misinformation on this.

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?

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