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For flexure, BB30, will not increase stiffness by 3

Agreed

For torsional stiffness, I dont know, you can work that one out!

I have

Something like a BB axle should not be composite

Also agreed. Any time you're constrained by external dimension, your first port of call is likely to be the material with the highest stiffness by area, not by weight. I think this might break down with BB30, since my calculation indicates that making a 30mm axle which is as stiff as a square taper axle (presumed stiff enough, given that they are still used by track sprinters) yields a wall thickness so thin that buckling could become an issue. Moving to a low density material such as aluminium or CFRP could start to look favourable in that case.