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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?
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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.