What size CF tube to match strength of alum tube?

[Nick Larsen]

Could not figure out a good way to word my search on the subject, so I came up empty handed.

My question, for those that might know, how do I convert the diameter and wall thickness of an aluminum tube, to the Dia and wall of a CF tube, if I want to maintain the same strength?
I assume that a cf tube of the same diameter and thickness of the aluminum tube would be significantly stronger.

For example, I have a 5/8" od aluminum tube with slightly more than 1/2 id.
What size CF tube would I need to have the same strength and stiffness?

For perspective: I'm working on an external frame pack design, and I want to start at a control before I start reducing the strength to save weight. I'd like to come up with a sub 2lb pack for normal loads up to 50lbs, then work on a sub 1lb pack for up to 20 or 30 lb loads.

[Nelson Sherry]

Different alluminums and different carbon fibers have vastly different properties. So there is not a simple answer to your seemingly simple question.

[Nick Larsen]

Dang.

Ballpark? Anyone know what type of aluminum jansport and kelty use in there external frames?

I suppose I could just order several sizes of tubing and make a judgement call, but that seems like a cost I could avoid with a little research.

[Nick Larsen]

Ah heck! Just realized this is in the wrong forum!
Well, if any moderators see this, I meant to put it in "make your own gear"

[Gary Dunckel]

Nick, you aren't thinking of bending your frame, are you? You can't bend carbon fiber rods like you can aluminum tubing.

[Nick Larsen]

Say what?! Lol, no, I'm well aware of those properties of CF.

[Nelson Sherry]

Don't they bend cf to fling arrows? This just became a multi use thread if the pack frame can also be used as a bow ;)
Could also be used as a tent pole, a fishing rod, or a splint. Hmmm.

[Jake D]

to be fair, the CF they use for bows is not in tube form and is layed up so it will bend.

carbon arrows also have flex as do aluminum arrows which are tubes but that is not the same as bending into a hoop or something.

pretty sure someone has MYOG'd an external pack using carbon arrows for the frame and plastic connectors.

could probably PM Bill about tips
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=59

[Loki Cuthbert]

daryl clark has a bunch of posts about his external frame packs that he has built for himself, wife, and friends. His packs with no shoulder straps that are counter balanced pouch on the front are pretty cool.

he has several other threads to read up on.

[Nick Larsen]

Yes, I've read all his posts, thank you. I'm looking to go in a different direction however.

[Jason Amick]

Yes the answer may vary person to person but you have select according to your requirements.

[Ben H.]

Sorry I missed this earlier and I am surprised another bpl geek hasn't answered you yet. I will provide an answer but make no guarantees I analyzed this correctly. I am a mechanical engineer but I am specialized in fluid and heat transfer not mechanics of solids.

To maintain the same stiffness you want to maintain the ratio of E*I. Or:

(EI)aluminum pole = (EI)carbon fiber pole

E is the Young's Modulus. It is typically around 10,000,000 lb/in^2 for Aluminum. Carbon Fiber is a little tricky as it can change based on the direction of the grain. http://www.matweb.com is a good resource.

I is the moment of inertia. For a tube it would be: I = pi/64*(OD^4 – ID^4) ~ 0.05*(OD^4 – ID^4), where OD = outer diameter of the tube and ID is the inner diameter.

Trying to match the strength of two tubes is a bit trickier as you have to postulate the failure mechanism. I would match the stiffness and don't get a tube that is too thin and you should be fine. Let me know if you have any questions.

[Nick Larsen]

Ben:
Thank you! That's exactly what I was looking for, a formula, and a way to determine the variables.

I was at the point I had a general idea and was just doing to start with trial and error, but you've saved me.

I do have one more question. Could you do my calculations for me? Ha!
Thanks again

[Dale Wambaugh]

Seems there are a lot of variables in wall thickness, alloys, construction techniques and quality. I don't think you can pull it off with a calculation unless you have comparable data for two exact materials. I saw a lot of tensile strength data, but nothing for compression.

It's hard to do with stuff on the web— the practical solution being to use a scale and grab a piece and give it a good hands-on tweak.

Maybe someone has a recommendation from personal experience? Some of the tent pole suppliers like Fibraplex or Tentpoletechnologies.com might have good info.

[Ben H.]

Young's modulus (Modulus of Elasticity) is usually the same in compression or tension. Matching the stiffness of tubes from dissimilar materials is pretty simple (as I showed above). Trying to predict apriori when a tube will break is a bit more difficult of a task. Carbon fiber is usually stronger than aluminum so if you match the stiffness you are probably better off in terms of strength unless you introduce novel failure modes. That is why I suggested Nick stay away from thin walled tubes.

[Ben H.]

"…Could you do my calculations for me? Ha! …" -NL

OK, I am a sucker for a good MYOG BPL Geek question. I calculated the EI for your tube (Aluminum 5/8" OD, 1/2" ID) = 0.0442 lb-in2. I then went over to McMaster-Carr and checked out the tubes they list for sale (http://www.mcmaster.com/#standard-carbon-fiber-hollow-tubing). It is notable that they do not give the Young's Modulus for their tubes (you might want to call them up and see if they have a product name for those tubes or other information). I then went over to http://www.matweb.com and searched for carbon fiber. I found values around 30,000,000 – 40,000,000 psi (3-4 times larger than aluminum). Based on that info, the 1/2" (0.043 wall thickness) stuff at McMaster will be a bit stiffer and the 3/8" (0.043" tw) will be a bit less stiff compared to the aluminum tube.

[Nick Larsen]

Sucker…

Well, that somewhat confirms that the experimental direction I was going to take was on the right track. I'll be using biaxial carbon fiber sleeving over 3/8" rod which puts the ID, somewhere between the two McMaster-Carr tubes you referenced. Now I just need to find out what the wall thickness is of each sleeve to find out how many layers I will need to get roughly .04"
That narrows down my starting point a lot! From there I think it will be down to good old destruction testing to find out if my tubes need more layers or fewer, or larger diameter, etc.

Lets just hope I can melt out the ABS rod with acetone when I'm done…

[Dale Wambaugh]

"Lets just hope I can melt out the ABS rod with acetone when I'm done…"

Perhaps you can find polyethylene rod that might slide out when the epoxy is dry? I would test with a short section to start of course.

[Nick Larsen]

I found some backer rod that I think is polyethylene, because it didn't melt in acetone :(

Will the epoxy not stick to it?
I wonder though, as I am making a slightly complex tube with 90* turns. Maybe I could start with a short 1ft section of just one 90* turn in it. Pulling it out would likely be a lot faster than trying to melt 6ft. of ABS…

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