What size CF tube to match alum tube strength?
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Nick Larsen
(stingray4540) - F

Locale: South Bay
What size CF tube to match alum tube strength? on 02/17/2013 16:27:12 MST Print View

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 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.

I suppose I need to know what type of aluminum is used in kelty and jansport frames first. Anyone?

ETA: I originally posted this in the wrong forum, so I'm reposting it here.

Samuel C. Farrington
(scfhome) - M

Locale: Chocorua NH, USA
CF tube to match ALU on 02/17/2013 18:53:59 MST Print View

"How do I convert the diameter and wall thickness of an aluminum tube, to the Dia and wall of a CF tube?"

Nick - Maybe not such a good approach because:
- Quality of carbon tube is all over the place. It is not a constant.
- Carbon has very different qualities. With most layups it is stiffer for the same weight and thickness, and while stronger, won't bend under stress like even the most highly tempered ALU. For the same reason, it is more prone to crushing.

If you are working with 5/8" OD ALU tube, and want something equivalent in carbon as your post suggests, please note:
- Not sure what is Jansport's, but it is a step up from 6061T6, and weighs around 1.5 oz per running foot. It is stiffer than 6061T6 but still bends to small angles in a Ridgid bender.
- You cannot bend the carbon, so assume you have a tube-based design that doesn't require bending, and have figured out the connectors to use for 5/8" material. Would not suggest J-Sport ALU tube connectors, as carbon will tend to crush, crack or break over the rigid lip. Use a flexible, but strong connector, and even consider reinforcing the carbon tube with a smaller tube inside around the point where it enters the connector.
- The best quality and light weight for the money carbon tube in the 5/8" range I've seen mentioned on this forum comes from golf club shafts, from Dallas Golf and others. There are also carbon tubes for quadcopters on eBay. You could do a search and read some of the BPL threads. There is a very recent one posted this year. Look under MYOG tent poles and trekking poles.
- The only problem is that the golf shafts all taper to a lower OD at one end (as you would expect if you'd ever seen a golf club). If you can design to accommodate that, then you're in business.
- While good carbon tent poles run around half the weight of ALU, carbon tube gets heavier in thicker walls and diameters. But it may still be possible to get the pack under 2#. IMO, though, you will not find a simple formula that will tell you what diameter and wall thickness to use, for the reasons stated above.
- I have found it necessary to start first with what materials are available that are the best for the money that appear to be strong enough, and then design around the materials, not vice-versa. It is in that respect, I think, that MYOGers may differ most from professional engineers (beside a lot of us being less technically trained, of course). After wrestling many months with a flexible frame design, I finally opted for a .156" OD solid pultruded carbon rod telescoped into a .230" OD wrapped fiberglass tube, both from Goodwinds kites. It was the strongest arrangement I could find for the size and weight range, and would never have guessed the outcome when I started with .21" OD pultruded carbon rod that broke every way for Sunday. -- Above all, stay away from pultruded carbon tube if you can. It never fails to disappoint.
-To fit the human back, you're going to want something prebent for the cross-braces on a conventional frame. Carbon won't do this (unless you've found a source for bent tube. If so, many want to know). Take a look at Roger Caffin's designs on the FAQ/MYOG sections of http://www.bushwalking.org.au/FAQ/FAQ_Index.htm
You may need to use elbows of some kind. All of which takes one back into the issue of sourcing flexible connectors for carbon. Again, designing around what materials are available seems to work best.

I hope all that is of some use to you. With some more specific info, I might be able to do better. Good luck.

Edited by scfhome on 02/17/2013 19:08:00 MST.

Nick Larsen
(stingray4540) - F

Locale: South Bay
Re: CF tube to match ALU on 02/17/2013 19:22:51 MST Print View

Thank you for the in depth response. I think I will just have to pick a place to start then go from there.
I was just hoping there was an easy answer from someone who deals with CF on a regular basis.
Of course I could always take the proper engineering route, but it's easier to spend money, ha!

Daryl Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Re: What size CF tube to match alum tube strength? on 02/18/2013 09:15:47 MST Print View

Nick,

I can't answer your question directly but my experience with carbon fiber external frames might help you get started. Look here:

Frame


Skyshark 400s (wrapped carbon fiber) have been more than strong enough for loads up to 40 lbs with my pack frame. Samuel (previous poster) has located a couple of brands that are are even stronger. I don't have any hands-on experience with those, however.

My days of 60 pound packs are over so I haven't even tried to make one. I've never had a Skyshark frame break, however, so they might be even be strong enough for 60 lbs.

Your 1 pound and 2 pound goals are well within reach. My lightest pack capable of carrying 30 lbs, for example, is under 1/2 lb (frame, bag, straps, waist belt).

Daryl

Edited by lyrad1 on 02/18/2013 09:17:44 MST.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: What size CF tube to match alum tube strength? on 02/18/2013 13:57:05 MST Print View

> a 5/8" od aluminum tube with 1/2" id.
Woo - way heavy!

Thing is, different Al alloys have very different strengths. VERY different. The strongest alloys (eg 7075 T9) cannot even be bent without very specialised equipment and skills.

I used CF arrow shafts like the Victory VForce 300 (from memory) for my pack frame. 0.244" ID (ground down 1/4" rod obviously for the mandrel) and ~0.295" OD. That H-frame pack (pics at the FAQ) has carried loads of >24 kg for ski touring.

Do NOT, repeat NOT, using any pultruded CF tubing. It splits brilliantly when bent. Use either wrapped or 2D wound.

In short, the strength of the CF rods is not an issue. But the joints, the way the harness attaches, and the design are.

Cheers
www.bushwalking.org.au/FAQ/

Nick Larsen
(stingray4540) - F

Locale: South Bay
Re: Re: What size CF tube to match alum tube strength? on 02/18/2013 19:43:24 MST Print View

Thanks guys.

Daryl:
thank you, I have already read about every thread regarding the externals you and another gentleman on here have come up with. I have gotten a lot of ideas, but it's nice to hear what kind of strength you are getting.

Roger: thanks for the link. I haven't come across yours yet. This gives me more food for thought with my design.

I may have found a way to manufacture my own tubes that allows me to incorporate bends and curves. If it proves to be viable and cost effective, well lets not get ahead of myself...

I think I have an idea now of where I'm going to start. It's just going to take some experimentation now.