Carbon fiber and plastic have similar weights. Carbon fiber is much more expensive and should be used where weight AND strength are required, whereas plastic is fine when only weight is a consideration.
How strength needed for the Roadster's skin? Yes, the density between the two (plastic vs CF) are very similar. And people use CF on skin areas where there's lots of tight corners and tight angles where strength and a good structure is needed, but I don't see any of that on the Roadster. But I'm no expert there. A Roadster done in plastic would look cheap and feel cheap. A Roadster done in fiberglass would be very similar to the Elise and have a extra pounds added. I have a CF hard-top and fiberglass Elise top which is shorter and lower than the Roadster's. However it feels like a lead weight in comparison. I know Elon pushed for the more expensive CF very hard where Martin wanted to go fiberglass due to lower cost in comparison. With that I'm sure Elon was thinking more on aesthetics and feel of a 100k price of the car. I believe he also wanted to go CF due to weight if I'm not mistaken, where others argued it was no worth the cost for the amount of pounds being shed compared to the overall gross weight of the car.
As for the hood, I'm looking at pics of Roadster crashes and the hood area is quite strong. When it breaks it breaks like a potato chip and is a very clean break. Usually CF or fiberglass when it breaks shows / pulls of some of the mesh with some resin, its not all too clean of a fracture. This weekend I'm going over my my friend's who has a hood that was involved in an accident. If its broken I'll take a close look and pics to see what the material is.
Here're some interesting clips about Elon & the CF construction during the early days:
"They decided to go with a carbon-fiber body instead of a polyester glass composite. At Musk's request, they lowered the doorsills — the lowermost part of the door — to make it easier to get in and out of the car. They switched out standard headlights for bespoke ones. Musk thought that the seats were uncomfortable, so they were retooled. Musk didn't like the material of the dashboard, Eberhard recalls, "and wanted something less cheap." Then there was the transmission, which got delayed again and again. As Musk
put it, the transmission "is not an inherently difficult item, but if you have two suppliers screw the pooch on you," then you're looking at some tardiness.
"Each of these is a reasonable decision," Eberhard said. "You have to consider that it's going to cost more money and cost on the schedule, and that was never accounted for.""
Tesla: The Origin Story - Business Insider
"Tesla employees soon got to witness the same Musk that SpaceX employees had seen for years. When an issue like the Roadster’s faulty carbon-fiber body panels cropped up, Musk dealt with it directly. He flew to England in his jet to pick up some new manufacturing tools for the body panels and personally delivered them to a factory in France to ensure that the Roadster stayed on its production schedule. The days of people being ambiguous about the Roadster’s manufacturing costs were gone as well."
https://www.quora.com/How-would-you-describe-Elon-Musks-way-of-managing-people
"Elon Musk was determined to build “not just the best electric cars, but the best cars.” This was a grand strategy, and a risky one. Every added goodie meant more cost and more time. Among the controversial changes was the lowering of the door sill, which made getting in and out of the car easier, but sacrificed much of the cost savings from using Lotus’s existing chassis. “I was very insistent on things during the design phase, and it is true those things cost money,” Musk told CNN in a 2008 interview, “but you can’t sell a $100,000 car that looks like crap.”
One major decision concerned the material that would be used for the body. Again, the boys ended up deciding to go all the way, and use the best material available – carbon fiber composite (aka carbon fiber reinforced plastic or CFRP). As strong as steel, as light as aluminum, and much more flexible to work with than either, this space-age stuff is emerging as the ultimate material for automobile exteriors. It still isn’t cheap, and in 2004, deciding to use it was a bold move. Musk was particularly adamant that the Roadster should use carbon fiber, and he eventually convinced the others.
Once this decision was made, of course, the team was no longer building a modified Elise, but a completely new car, and they had the freedom to design the body however they wanted."
Charged EVs | New book excerpt: The early days of Tesla
