Couple of points...
I really don't think it would cost that much more. GenRight and Nemesis already make aluminum fenders that will cost you about the same as steel fenders from other vendors. With GenRight the increase in cost for aluminum over steel is modest. The real difference is in materials cost and production velocity (MIG vs TIG, etc).
Why not Cr-Mo -
Racing vehicles, drag cars etc are built around a tube frame to begin with, so the cage is a fully supported structural element. There is no inherent weak link in these designs and they are designed to provide protection in incredibly nasty high-speed wrecks.
JKs are very different. The cage doesn't tie into the frame (become structural) unless you do some serious cutting and fab. So when you add to the factory cage, you're strengthening it but also really just moving the weakest point somewhere else in the cage. It's never going to be a fully integrated structural component. Also, you're likely to roll slowly, and a few rotations at best, when wheeling. What you need is more protection, not the ultimate protection like race cars. What I'm saying is that a full CrMo cage attached to the JK factory cage is overkill for our needs....and weighs too much.
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