Carbon/Fiberglass

[Superfast Matt McCoy]

So what are you all using for your bodywork? any discussion on the befits of fiberglass/carbon? specifically, can anyone show why using carbon on bodywork is a benefit? any glass teams having problems with weight/stiffness?

[Superfast Matt McCoy]

So what are you all using for your bodywork? any discussion on the befits of fiberglass/carbon? specifically, can anyone show why using carbon on bodywork is a benefit? any glass teams having problems with weight/stiffness?

[Chris Allbee]

what about the added weight of making glass look nice? paint, bondo, etc? a good carbon lay-up can be put on the car without any work beyond pulling it out of the mold.

[Nihal]

We used fiberglass lst year with a male mold and it was a mess. We used two layers sandwiching a cotton hex-cel layer for stiffness. It took ages to get the fiberglass pieces flat and decent with hours and hours of bondo and sanding. This year we are going to go with a CF femal mold setup.

Nihal Murthy
UC Berkeley FSAE

[Superfast Matt McCoy]

Chris,

Three things:

1, I don't know why you would need bondo on fiberglass. first of all, it's a really bad idea, because fiberglass is flexible and bondo is not. This would of course lead to parts of your body left on the ground in light red chunks. If your mold is made so poorly that it needs bondo, then it's going to look bad with either glass or carbon.

B, To get a good finish with carbon, you need to use a gel coat (or perhaps pre-preg with a really smooth mold surface). If you don't, you end up with a hazy finish, possibly with a few air pockets (see: Oklahoma's 2005 car). You could buff that out, but you might buff through the resin. That is unless you have a think enough resin surface, in which case you already effectively have a gel coat. When using fiberglass, you simply use a colored gel coat and buff that out to a shine that looks as good as or better than paint.

iii, The benefit of carbon comes in that it is stiffer (and stronger, but your bodywork shouldn't be carrying any significant load). The only problem you should see with less stiffness is if you have a flat piece that is not constrained well. Rarely are individual flat pieces seen in bodywork and if you have flexing problems due to lack of constraint in fiberglass, then that is likely to show up in carbon as well.

But let's say for the sake of argument that you need to reinforce some fiberglass pieces. You would of course use something like wire loom between layers to add a ridge to the necessary areas. Let's estimate liberally that this adds 2 pounds to your total weight. We'll Assume 489 pounds with carbon and 491 pounds with fiberglass and 70 horsepower. This changes your peak power to weight from 1:6.986 to 1:7.014. This is the same as dropping 0.3 horsepower. This would change your score in the dynamic events by, well, probably nothing.

However, our car was costed for $320 worth of fiberglass. If this had been costed for carbon, it would have come out to $800, subtracting 0.96 total points and moving us down one spot, knocking us out of top 25, and shattering all our hopes and dreams. Good thing we used fiberglass.

And so fiberglass over carbon is justifiable, which is always good for the design event.

Perhaps I'm wrong, and if so I would like to know that. Mostly though, I would like to know (quantitatively) why.

Nihal,

I've had experience with carbon and glass and I can tell you they are very similar to work with, though I grant you carbon is a bit easier in some respects (glass easier in others). Your problem is probably more due to using a male mold than anything else.


-Matt

[Kurt Bilinski]

My advice is to vacuum bag whatever material you choose. I didn't do this and have spent a ton of time filling the 100s... no, 1000s of bubbles and pinholes. I'm sure if it had been squeegied more it would have helped, but I had trouble with the fabric "backing out of corners" in the mold. What was supposed to have a 1/8" radius sometimes had a 3/8" radius. Very time-intensive to make right. Vacuum bag!

[Chris Allbee]

i don't really have any quantitative reasons for using carbon in this case. looking at the point metrics you came up with, it would make sense to go with glass instead. i'm more appealing to the judge's (and everyone elses) senses. carbon just looks impressive. the judges are human as well. if they like looking at something they will generally be more receptive to it.

[Kurt Bilinski]

Good point about the cost/benefit of carbon. Since the FSAE car is supposed to be a weekend car anyone can afford, it makes good sense to deduct points for carbon. For F1 or military applications it's a different story, but that's not what this is about. Besides, if the car is to be "sold" to racers, the carbon can always be an option.

[RKemmet]

Im for carbon fiber. With out male molds and vaccuum bagging we were able to get amazing finishes with very minimal sanding and a simple clear coat. with our female molds we didn't even need the clear coat or any sanding. We are also able to go with much less material when using carbon fiber, much less. For similar stiffness we would use probably about 1.5-2 times the amount of fiberglass, at least, and we would want to paint the fiberglass, adding more weight. With the carbon we can leave it, with almost nothing on top of it, and use even less. In 2005 all our pieces were 2 layers of carbon. Seeing how stiff those pieces were and seeing that stressing them with their fasteners adds rediculous amounts of stiffness, we are going to only use a single layer for our 06 car, effectively reducing the cost and weight in half, and making it cheaper than fiberglass according to our rules. You can see how our stuff came out at http://formula.engr.arizona.edu/05pics.html