"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.