[The Nightman]

Originally Posted by Simbl

As a bit of a half way house between m3 arms and monoballs. Powerflex make an offset caster bush. This allows for additional caster that would give subjectively better steering feel, more self centering, straight line stability and the most important part more dynamic camber about an extra -0.25 when cornering. Something people don’t realize about the m3 is that the additional caster and some camber is build into the m3 hub geometry not just the control arms. This gets closer to m3 geometry.

The only reason I have not gone with them myself is the roads are awful in the UK at the moment m. The M3 Arms are just about bearable.

Neat, I wasn't aware of those. Here's a link for those who are interested and attached are the installation/adjusting instructions.

https://www.powerflexusa.com/shop/pf...?category=1787

[D989]

Originally Posted by Simbl

I don’t think so the rear subframe holes don’t line up. There is some post on this forum or baby bmw forum with what in the rear looks like it might be possible..

The eps issue is with front half. On e92 the is an additional brace that obstructs the m3 belly ban.

Does this mean that on an e90 with non-eps that the only hurdle is changing the rear mounting holes. Also are the v braces necessary on an e90?

[Simbl]

Originally Posted by D989

Does this mean that on an e90 with non-eps that the only hurdle is changing the rear mounting holes. Also are the v braces necessary on an e90?

Yes just the rear holes. They only need slight adjustment to line up. The front mounts line up fine.

I’m not completely sure the standard undertray fits perfectly with the full M3 belly pan installed. The OEM solution is to adapt the M3 undertrays.

Now regarding the braces, it’s important to separate what they actually do from what the M3 belly pan does.

The M3 belly pan reinforcement plate is a shear panel. It closes the open firewall and front subframe window and significantly increases front-end shear and torsional stiffness. It works in-plane and effectively turns that area into a partial torsion box. That’s why it’s such a meaningful structural upgrade.

The V-braces are not doing the same job. They primarily add longitudinal stiffness and local bending resistance. They improve the load path between structural pickup points and reduce local compliance under braking, acceleration and cornering, but they do not act as shear panels and they do not create a torsion box.

BMW used them differently across the E9x range:

E93 convertible
This has the longest V-brace plus an additional front brace behind the bumper. With no fixed roof, bending and torsional stiffness drop significantly, so BMW added longitudinal and triangulated reinforcement to compensate for the missing roof structure.

E92 coupe
This has a V-brace under the front subframe because the span between the A and B pillars is longer than the E90 due to the longer doors and larger aperture. That reduces side structure continuity, so underbody triangulation helps restore stiffness.

E91 touring
This uses additional front reinforcement because the long roof and open rear load area reduce torsional continuity. There is no fixed bulkhead behind the seats like the saloon.

E91 and E93 rear braces
These tie the rear floor and shear panel area together. On the e92, braces run from the rear subframe area toward the centre tunnel.

E90 should inherently be the stiffest non-M shell because it has a fixed roof, shorter door aperture and a closed C-pillar and rear bulkhead structure. That’s why it doesn’t rely on the same level of underbody triangulation.

So no, the V-braces are not necessary on an E90 in the way they are on the E92 or E93.

If you’re comparing impact, the M3 belly pan reinforcement is the bigger structural upgrade. The V-braces are incremental stiffness improvements that improve load path integrity and reduce local compliance.

Different jobs and very different magnitude of effect.

[D989]

Wow what an incredible answer. Thank you

[D989]

Since I have to change the holes anyway I'm curious if it would be a better/cheaper idea to just make my own out of aluminum sheet. Someone on here mentioned it with the use of cardboard for a mock up. Unless they were referring to the f80.

[ornicar]

Originally Posted by D989

Since I have to change the holes anyway I'm curious if it would be a better/cheaper idea to just make my own out of aluminum sheet. Someone on here mentioned it with the use of cardboard for a mock up. Unless they were referring to the f80.

My opinion is that if you find an E9x M BP cheap, it is better to use it than start from zero.

[ornicar]

" E92 coupe
This has a V-brace under the front subframe because the span between the A and B pillars is longer than the E90 due to the longer doors and larger aperture. That reduces side structure continuity, so underbody triangulation helps restore stiffness. "

Incredible answer a good synthese.

In your opinion is is the same with the E82 / E92 ?

[Simbl]

Originally Posted by D989

Since I have to change the holes anyway I'm curious if it would be a better/cheaper idea to just make my own out of aluminum sheet. Someone on here mentioned it with the use of cardboard for a mock up. Unless they were referring to the f80.

It depends what material you can actually get hold of and the price. In my case the used BMW panel cost about a third of what an equivalent aluminium sheet would have cost.

Also, the OEM M3 belly pan isn’t just a flat sheet of aluminium. The material BMW uses feels different to standard sheet, and most of the strength actually comes from the design rather than the thickness.

The panel has rolled edges, swages and embossed sections which dramatically increase stiffness. Those features increase the section rigidity of the panel by a large amount without adding much weight.

If you make one from a flat sheet you won’t achieve the same stiffness unless you can recreate those formed features, which usually requires proper press tools or bead rollers. Without that, a flat plate of the same thickness will be noticeably weaker and more flexible.

Because of that it’s usually much easier to start with the OEM M3 undertray and modify the mounting holes or shape as needed rather than trying to fabricate one from scratch.

[D989]

Originally Posted by Simbl

It depends what material you can actually get hold of and the price. In my case the used BMW panel cost about a third of what an equivalent aluminium sheet would have cost.

Also, the OEM M3 belly pan isn’t just a flat sheet of aluminium. The material BMW uses feels different to standard sheet, and most of the strength actually comes from the design rather than the thickness.

The panel has rolled edges, swages and embossed sections which dramatically increase stiffness. Those features increase the section rigidity of the panel by a large amount without adding much weight.

If you make one from a flat sheet you won’t achieve the same stiffness unless you can recreate those formed features, which usually requires proper press tools or bead rollers. Without that, a flat plate of the same thickness will be noticeably weaker and more flexible.

Because of that it’s usually much easier to start with the OEM M3 undertray and modify the mounting holes or shape as needed rather than trying to fabricate one from scratch.