If you designed two structures to do the same thing, in many cases a carbon fiber structure would be lighter than an aluminum one.
If you designed one structure to outperform the other, they could still be the same weight but one might be markedly stiffer.
Imagine two fishing poles, identical weight, one made of aluminum and the other made of carbon fiber. While they may weigh the same amount, the carbon fiber pole may be capable of landing a 50 lb fish where the aluminum one may only be capable of landing a 30 lb fish.
If the carbon fiber chassis doesn't outperform an aluminum chassis of the same weight, I would question why it was so heavy. But, if it outperforms the aluminum chassis, I would understand.
(This is all hypothetical, as I don't know the torsional rigidity of either the Lotus or the Alfa.)
Below is a photo that I grabbed off of the 'net, showing a tube frame chassis. Torsional rigidity is the measure of a chassis' ability to resist twisting (as shown in the picture). With some exceptions (karting comes to mind), more torsional rigidity is desirable for reasons I won't go into here. (Including: road "feel", suspension control, NVH (noise/vibration/harshness) considerations, "modes", etc.) In my former life I ran so many of these simulations that I used to fall asleep thinking in color.
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Last edited by Thomasio; 11-12-2014 at 04:52 PM.