I still don't understand why the CF tub isn't significantly lighter than
the Elige's AL tub.
This has yet to answered. Does the Elige's 150lb "tub" include the passenger
compartment and the front suspension pick-up points, steering rack et al.
US safety standards ruin our fun.
The answer to that depends on a whole host of factors, a couple of which have not been mentioned:
1) While composite structures can provide higher strength/weight than metallic, the major advantage is typically more in marketing than engineering.
2) The reason for this is that metals are MUCH easier to quantify than composites. I can go to lowes and buy a piece of 6061-T6 bar stock, and am guaranteed that the strength, hardness, density, and stiffness are within a couple of percent of book values. Even more so, I can weld, glue, or bolt several of them together and have very high confidence in the performance of the assembly.
This is completely untrue for composites. Composites by definition are not a 'bulk' material, so the fiber orientations, core materials, weave types, etc are all critical to determining the strength of the resulting part. Even more important, processing has a huge effect on the performance of composites. If a slightly different resin is used, or the fibers are not quite wetted all the way, or cure temp is a few degrees off, the difference in performance of the resulting part can easily be 10-20% off, or even much more.
3) Composites are also MUCH harder to design when it comes to deformable crash structures and energy absorption. This is for the same reasons as above: a metal will bend, fold, and tear fairly predictably because it has the same properties throughout. A composite can do any of those things as well, or the plies can separate, or it can pretty much disintegrate without absorbing much of any energy at all.
The end result is that while it is possible
to design a composite tub that is much lighter than a metallic one for the same performance, it isn't easy to do it. When you're talking about a low priced sports car without huge sales volume, the easy way out is just to over-design it to be roughly the same weight as a metallic chassis, but with the marketing advantage of carbon fiber.