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Mad Scientists: Why not a carbon fiber tube frame chassis?

I've asked this question on various forums - probably including this one a few times - and never gotten a really clear answer. I see folks welding tube and semi-tube frame chassis for 911's...but I always wonder why no-one bothers to get some carbon fiber tubing and use structural epoxy to build a chassis. In theory it should be easier to build and be ultralight. Granted, it would be difficult to repair in case of a crash - but that is also true of nearly all carbon fiber parts.

So why not build a tube frame chassis from carbon fiber?
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Old 06-20-2005, 02:40 PM
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Well, cost for one. I don't have numbers handy, but it would set you back a serious grip to do it right. I don't think that carbon tubing is the proper material for a tube chassis; I think a carbon monocoque would be better. You're talking about aerospace and formula 1 technology, and the average joe on the street just doesn't have access to that kind of tech and cash.

Not that I haven't fantasized about it...
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Old 06-20-2005, 02:53 PM
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When race cars went from steel to CF, they also went from tubes to Monocoque. More efficient use of the material.
Old 06-20-2005, 03:52 PM
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I did see a carbon fiber 996 tub at Laguna seca a couple years back. The whole tub with doors, hood, and decklid only weighed a liitle over 100lbs.
Old 06-20-2005, 04:11 PM
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Dantilla has a good point.

I saw info on the wt. of the C-GT tub somewhere - IIRC is was a lot more than 100 lbs.

BTW, the early 911 tub weighs about 640 lbs.
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Old 06-20-2005, 04:13 PM
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Carbon tubes are strong, the joints aren't. Not malleable enough. It can hardly be pulled off on bicycles, let alone a car.
Old 06-20-2005, 04:32 PM
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Many of the SCCA D sport racers are now built using a steel tube frame with carbon sheets bonded/riveted to it. Makes a very stiff and light frame.
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Old 06-20-2005, 06:07 PM
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if you want to do tubes, use Ti. It doesn't fail catastrophically like CF does. And is almost as light. Only problems are machining and welding, but it can be done without huge technology leaps.
Old 06-20-2005, 06:13 PM
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Ti is a little flexy, depending on the alloy. Most more common alloys aren't really suitable.
Old 06-21-2005, 05:26 AM
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4130 steel tubing is pretty tough to beat in the tube frame dept.

Carbon tubes could not be just "epoxied" together at joint clusters while 4130 steel tubes can be quite readily coped and welded into very strong
cluster joints. With steel tubes, generally speaking it is either broke or is not. With epoxied gusseted tube joints, partial failures would be difficult to detect until it was too late.

Also, the stongest carbon structures are typically made from pre-peg
materials, then oven cured. One would need refrigeration for storage, then huge ovens for curing. Hand laid carbon fiber components are strong, but not as good as the pre-peg made oven cured components.

I have built aircraft fuselages from 4130 and have also built composite wing structures using composites. Both technologies are used in different structures on the same high performance aerobatic aircraft.

Indy cars use carbon for the tubs and other components that are monocoque type structures, yet still use 4130 for things like suspension arms and links.
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Old 06-21-2005, 05:52 AM
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I priced out filament wound carbon tubes for this application several years ago. The tubing would have cost about $1000.00 but that was cheep compaired to tye cost of the nodes required to tie it all together. Just bonding the "clusters" together would not produce a strong enough joint; the way to go was to bond the tubes into hollow cast titanium nodes. That is where the real expense would be. I can't remember, something like 30 investment cast nodes would have cost about $4500.00. But still, this is something I think about from time to time.
Old 06-21-2005, 08:08 AM
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Quote:
Originally posted by John Druckrey
I priced out filament wound carbon tubes for this application several years ago. The tubing would have cost about $1000.00 but that was cheep compaired to tye cost of the nodes required to tie it all together. Just bonding the "clusters" together would not produce a strong enough joint; the way to go was to bond the tubes into hollow cast titanium nodes. That is where the real expense would be. I can't remember, something like 30 investment cast nodes would have cost about $4500.00. But still, this is something I think about from time to time.
Clearly I came to the right place to find my "Mad Scientists"!

May I ask why you went for titanium joints? Could you have used traditional 4130 welded steel joints and then connected them with c/f tubes to gain the big weight savings?

And makaio, you saw a c/f 996 TUB???? That is a sick (and yet utterly beautiful) idea. Now I want a c/f '72 tub for the ultimate RS clone! Did you get any pics or see who owned/built it?
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Old 06-21-2005, 09:14 AM
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c/f 996 TUB -- IIRC, there is a post about it on here somewhere -- and/or an article in one of the mags.
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Old 06-21-2005, 10:21 AM
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I was going for the greatest weight savings and strength. When I started the conceptual design cost was not a consideration; only weight and stiffness. Welded-up 4130 nodes would work fine - only drawback might be distortion during welding requiring some "truing-up" machine operation which would most likely cause the wall thickness to go undersize, weakening the node. If I remember correctly the compleated frame (DSR) would have weighed somewhere around 25 pounds.
Old 06-21-2005, 10:29 AM
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TWENTY FIVE POUNDS???????

Oh my dear lord...$1000 + $4500 = $5500 seems almost modest considering how much some folks will spend to shave off a few pounds...have you priced c/f hoods and fenders lately? I have and OUCH...
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Old 06-21-2005, 10:33 AM
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There's no distortion when welding steel tubes, you use a jig to hold them in place, and are adding welding metal to make the weld. It bridges any uneveness in tube lengths, that's why it's one of the best methods to use in local, low-tech shops.
Old 06-21-2005, 10:36 AM
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Even F1 teams with their seemingly unlimited budgets have determined that if you're going to use Carbon Fiber, you might as well create a tub. I don't know for sure, but I suspect that the pound or two of material that you'll lose will be more then offset by the loss of regidity and strength by doing away the the panels. Keep in mind, (I'm no Aerospace engineer by the way, but I have played around with some FEA software), the trade-off is either a relatively lightly stressed "skin" or more highly stressed tubes. If you go with the tubes, you get to save on the weight of the material between them. In the case of carbon fiber, that material is pretty light, so the savings will be pretty small given the context of a ~1100 lb formula car, or even less in the case of a bigger car.

Then you also need to factor in the loss in intrusion protection which needs to be replaced some how. Even tube-framed FF cars have unstressed aluminum panels on the sides to protect the driver from intrusions.
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Old 06-21-2005, 11:53 AM
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Investment cast lugs aren't the only solution for joining CF tubes, Cafree bikes (among others) makes custom frames with bonded CF lugs. The thing is, if you were to make a chassis from CF, why would you use tubes at all? One of the greatest advantages of using composites is the ability to create any shape you want with any lay-up orientation you want. You are essentially creating the material and the form at the same time. I think it would be the same amount of work and much better to build an entire chassis from CF than it would be to try to build some funky CF tube subframe; either way it would be a major science project that would require a lot more time and money than it would be worth (in my opinion).
Old 06-21-2005, 11:57 AM
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OK - add another $5000.00 for some very accurately machined fixtures; as I remember there were about 12 different node configurations. Filament wound carbon epoxy does not like being "tweeked" into place. These tubes would need to slip and be bonded into the nodes without any distortion. A very eligant way forward; however fraught with pitfalls.
Old 06-21-2005, 11:59 AM
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Quote:
Originally posted by Cory M
The thing is, if you were to make a chassis from CF, why would you use tubes at all?
I hope someone can correct me if I'm being ignorant, but I would assume the benefit of tubes would be cheaper production. The engineering would be simpler since the stresses are unidirectional (i.e. relatively uncomplicated) and the only "mold" required would be a tube of some sort. It would also require a MUCH smaller autoclave...all making it seem (perhaps falsely) like a more reasonable project for a talented DIY'er or fabricator with prior c/f experience. Maybe I am grossly simplifying the challenge...again...
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Old 06-21-2005, 12:06 PM
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