Lets talk body rigidity

[rbg]

I took rear seats off in both my cars: the m and e90 335 to do overall cleaning rejuvenation. Moving them around I started thinking about seat deletes. For example, bottom parts are very high in both cars and not worth deleting as they provide a decent amount of sound insulation from the diff. However, back parts are ridiculously heavy (again on both platforms). Honestly, looking on the construction it doesn't need to be that beefy. I started looking id the seats have any structural purpose. What I found is that for e46 (non-m) rear seat being not foldable actually adds quite a bit to the torsion rigidity. BMW claims they redesigned the chassis and for e9x platforms and folded seats do not reduce the rigidity. So my question here, if you do a delete on e46 m3 how much rigidity you lose and do people who delete rear seats go half/full cage or if the do some additional bracings?

[Obioban]

I don't think removing the seats will meaningfully impact your chassis rigidity. The CSL used non metal back seat backs and is marginally stiffer than the M3 (because of the CF roof-- 19,100 nm/deg vs 18,500 nm/deg for the M3, IIRC). In the document where BMW talks about the chassis stiffness benefit of the CF roof, they only mention the small increase from the roof, no mention of also compensating for the back seat not being metal.

I believe they're as beef as they are because they have to protect you from anything you have in the trunk during a crash, and block the heat from the trunk, and block the noise from the trunk. I took some apart to see how viable recreating the CSLs weight savings there would be, and by the time you get to the metal structure, there's not actually that much weight left. Plus the lower seatback rotation pins are bushed in plastic, which would make them pretty terrible as a chassis stiffener.

The non fold through e46s have one continuous sheet of steel welded across the opening. If you did something similar and deleted your back seat, I believe you'd gain a significant amount of chassis stiffness and lose a significant amount of weight. Though, add sound deadening and heat insulation back in and you'll be getting close to stock weight again...

I've thought about doing the above a couple times, but believe I benefit more from the fold through seats than the additional chassis stiffness I'd gain. The M3 isn't nearly as floppy as people think, as people always look at the non M e46 numbers. The M chassis bracing added up to a pretty significant increase in chassis stiffness.

[bigjae46]

A half cage would add a lot more rigidity.

I think the rear seats have minimal impact on body stiffness.
I have a full cage and I cut out the rear shelf/deck. Haven’t noticed any issues like the rear window coming loose from body flex.


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[rbg]

thank you! What do we guys think about something like this: https://www.ecstuning.com/b-irp-_-in...irprsb-46~irp/ I really like the fact that you can take it off in those rare cases when you need to fold the seat and carry something (wheels/tires for example)

Actually looking to this list more carefully that what I found http://youwheel.com/home/2016/06/20/...ehensive-list/

To my first post: e46 sedan w/o folding seats 18000, with folding seats 13000, coupe -- 12500. Actually doesn't sound right to me.
More general observation that torsion rigidity doesn't necessary goes alongs with general perception of a vehicle: Pagani Zonda, Porsche 911 Turbo, McLaren F1 ~ 26-27000, Volvo S60 -- 29400, VW beetle -- 26000, Mazda CX-5 - 27000. So not what you expect. However, I doubt anyone did any data curation for that list lol :-)

[Obioban]

A half cage would add a lot more rigidity.

I think the rear seats have minimal impact on body stiffness.
I have a full cage and I cut out the rear shelf/deck. Haven’t noticed any issues like the rear window coming loose from body flex.


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The fold through seats probably have a minimal impact. But, the fixed seats have a huge impact, according to BMW’s published numbers.

[Tbonem3]

Few people, surprisingly, run a rear strut bar.

[rbg]

Surprisingly that run or that only few? :-)

[Tbonem3]

Surprising to me that few run a rear bar ya.

[Obioban]

Few people, surprisingly, run a rear strut bar.

They make the trunk rather significantly less useful... and part of the beauty of the m3 formula is the practicality of it.

[rbg]

They make the trunk rather significantly less useful... and part of the beauty of the m3 formula is the practicality of it.

Ok I see. I agree on that. Thats why I was thinking of something that you can remove with undoing a pair of nuts (sort of gem front bar) like I posted above maybe.

Totally off topic: Is p0lar on the forum or is there any way to contact him? I'd like to ask him couple of things about his tremendous work he did on brake bias.