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Ultra-Light External Frame Pack
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BPL Subscriber
(BobOne) - F
CF tubing connectors on 05/17/2006 00:58:46 MDT Print View

Oh, BTW, I just had email with Steve Ferrel over at Kite Studio ( and he says he can do his fittings in aluminum as well as his usual ABS. Ti would be lighter and have better fatigue resistence, but it seems like some judicious overbuilding with the aluminum may cause the stress risers created in the tubing by this simple sort of joint to fail before the fitting.

Several of their fitting configurations look like they might be useful for a backpack frame construction, and being able to buy these off the shelf makes it a lot easier to play with assemblies than coming up with your own joints.

Bill Fornshell
(bfornshell) - MLife

Locale: Southern Texas
ideas - pack frames on 05/17/2006 03:16:11 MDT Print View

This picture shows one of the fittings from kitebuilder. I call it a bushing with a set-screw. I bought 6 or so in two different sizes but had to drill them out a little. kitebuilder has several things that I use and I have talked to Steve a couple of times about other fittings. The one you can see easy is on the center tube and has what looks like a white round screw top.

That frame has been changed some and I removed the center tube. It has four of those bushing/fittings on it. They make adjusting the Thermo Plastic cross piece easy.

I am about to make the next version of a wing frame. I have a new hip belt and shoulder strap design that will go on it and should be a little lighter. I am waiting for some new material and might get it in a day or two.

Image hosted by

Getting the right fittings and other little stuff necessary will always be somewhat of a problem for these frames. I talked to Bill Stephenson (the younger) about their old frames and he said they had a problem getting the necessary fittings and stop making the frames.

I also had a chance to talk to Skip Yowell at Jansport about a year ago. I was able to ask him some questions about his "Wing" design and suggested that the time for the "wing" idea in todays lighter material might be ready for a come back. I told him what I was doing with his idea and said it was time for a well designed, really light external frame. He said Jansport was coming out with a couple of new items for there 30th, I think anniversary, this year or next. I might like them but they were not light by what I call light.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: ideas - pack frames on 05/17/2006 07:44:21 MDT Print View

Bill said, "He [Skip Yowell] said Jansport was coming out with a couple of new items for there 30th, I think anniversary, this year or next. I might like them but they were not light by what I call light."

Jansport made some really light packs for the time. I have one that is about 3500ci for the pack bag and there is enough room to strap on at least that much more on the frame extensions, top and bottom. The whole rig is about four pounds.

Compare the Jansport to the welded h-frame competition and you can see their good design. The Jansport tubing is much smaller and thinner and the frame is easily adjusted. The pack, waist belt, and other hardware is light by the same degree.

If the same design were build using titanium tubing and UL concepts and materials were applied to the pack bad itself, the weight would easliy drop over 25%. Trimming down some of the large arcs of tubing on the top and bottom would drop the weight further. A 3 pound pack is easy to imagine and I'm sure something close to 2 pounds is possible.

Those side struts on the Jansports always looked to complex and heavy to me. I understand the engineering advantages, but all that metal and plastic jutting out didn't appeal to me.

I think a laminated carbon fiber frame could go a long way towards a design using the side strut concept and keep the hardware to a minimum. It is not beyond the resources of a back-yard builder to make an epoxy and carbon fiber frame. Prototype molds are not terribly difficult to make.

I'm thinking of a frame with large openings that looks like a squared-off figure 8 with the bottom wrapping around to form the side struts. The shouler straps could be mounted directly to the frame and a typical mesh back band could cushion the top section. The hip belt would be a full circle and mount on the side struts or another mesh panel could suppport the back with two separate straps for the hip belt. Pad to suit.

If the frame bows too much, it could be stiffened with a couple arcs of carbon fiber. I've seen paper tubes used for stiffening-tube molds on fiberglass prototyping and the same could be done with carbon fiber-- a half round of thin plastic tubing, etc would form the arc and the cloth is laminated over the top. With a little brainstorming, it may be possible to remove that mold material after the epoxy is dry. The resulting space could be handy for mounting purposes or a spot to put survival items or tent poles.

The pack bag could be mounted on the other side of the frame with fabric straps running through slots machined in the frame material and conventional Delrin fittings. With some fiddling, the pack could be attached with hardware and reinforced panels in the pack bag fabric.

My choice of a pack bag(s) would be Sea-to-Summit silnylon waterproof bags strapped on through slots in the frame. That makes for total customization and eleminates the need for other waterproofing and more layers of fabric. It makes me crazy to buy an UL pack and then add expensive bags to go inside! The bags could be swapped at will and easily replaced if damaged or technology changes. The slot method of mounting actually lightens the frame and uses no hardware other than the straps. And I wouldn't need to make the bags. Of course, I think we could find someone to make a cuben fiber pack bag :)

Bill Fornshell
(bfornshell) - MLife

Locale: Southern Texas
ideas - pack frames on 05/17/2006 12:50:52 MDT Print View

Hi Dale, Glad to see you jump in here with you thoughts and ideas.

I agree with you on the Jansports packs "for there time" comments. When I talked to Skip Yowell most of my comments were about using titanium and some type of composites. I mentioned I was using some Thermo Plastic in my packs and he did say they were looking into that material. I really enjoyed talking to him and listening to his comments about the early days. I am sure Jansports could come up with a good 3 pound pack if they thought there was money in it.

For the big companies making packs as one of their products I am afraid it is all about the "bottom" line $$.

There are a few pack makers I think might be able to come up with a 2 to 3 pound light external/hybrid external of some type. The first is a company that really jumped into the pack market this year in a big way - Mountain Hardwear. I really want one or both of the different "EXODUS" monsters. I don't want the packs to use as they are really heavy. I want them to re-engineer and drop their weight down to around 2 pounds. I was about to buy one of the Exodus - Harrier model with this last discount from REI but they were on a sales hold while the hip belts are being replaced. I think I could re-make the frame using lighter material and sew up a new and a little smaller pack bag out of Cuben material and get down to 2 or 3 pounds. 2 pounds would be my goal. I have made a set of shoulder straps a lot like the ones they use and would use a different strap setup on the hip belt. I have had both the "Motive and the Cruiser" models fitted to me and weighed both the bag on and off the frame and the frame alone. I guess I have about 3 hours flight time logged in those two packs. I talked to one of the guys at the MH customer service about those packs till he knows my voice. They are using some type of composites in that pack and if the $$ were there could just add a really light version to the Exodus series. Will they, I would not bet on it.

That leaves GoLite as my next best pick. Coup has said something about looking into the different mylar materials. This I would take to mean Cuben Fiber but there maybe others. It is however one thing to add packs made out of Cuben type material to your pack lineup and another thing to add a light weight external frame to your line up. Cuben packs are not that hard to make.

Then you have several other pack makers such as Ron Monk at Six Moon and the McHale. Both make really well made packs. To heavy for me but well made. Of the two I would bet of Ron Monk as McHale seems to be stuck on "Heavy" and "Old Design". The McHale packs maybe the BEST out there for large packs and I really like Ryan's smallish size Summit pack, but I don't see McHale adding a very light external of some kind to his product line.

The next two companies who could/might be able to pull it off are Gossamer and or ULA. I would expect both to add a Cuben pack to their product line sometime but I don't think they are up to the development of a light weight external frame/pack bag like we are talking about.

I am sure there some others that I didn't list who might/could also do this.

Then last, there are the leaches lurking out there ready to steal any good idea they read about or see pictures posted at this MYOG site and others like it. They do very little or NO research and development and just steal ideas and get them into production as fast as they can.

For those that are still reading I want to make one more comment:

Dale, I make all my pack bags to attach easy to my frame and in the same way. Or you might say I make all my frames to hold my pack bags the same way. I can then change out the bags from smaller to larger as the hike requires in a very easy and quick way. I also have started using an attachment system on my pack bags to add different pockets or remove the pockets as I need to carry different sized things on the outside of the pack. That way I only have the extra weight of the outside pockets when I need them. I agree with your last couple of paragraphs that this is the way to go. I also think getting pack bags made out of Cuben will get easier in time. Or you can always just make your own.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: ideas - pack frames on 05/17/2006 13:55:45 MDT Print View

I got on the carbon fiber frame kick when one of my co-workers came in with a megabucks bike with a carbon fiber frame. I could easily balance it all on two fingers and one with some pain :)Of course the stuff looks sexy too-- a marketing department's dream.

The frame I have in mind is much like the old WWII plywood pack frames, with larger holes and some reinforcement in the right places to control flexing. The bottom would have the wings/struts to hold the belt. The back band would be much smaller-- 6" or so tall. I'd love to see some sort of twist-to-adjust and lever-to-lock tighteners for the back band. Most of the old back frames use ladder buckles and it is hard to get them nice and tight.

In production, it may be easier to mold long sections of the u-shaped frame and just chop them to length. The struts could be bolted or epoxied on afterwards. That may be a good idea anyway, as they could be replaced if broken and several sizes could be offered. That would allow some vertical adjustment for torso length too. All said, that frame should only be a few ounces.

Edited by dwambaugh on 05/17/2006 21:00:31 MDT.

BPL Subscriber
(BobOne) - F
Packs still... on 05/17/2006 15:20:18 MDT Print View

Hi Dale, nice to meet you too.

I'm hoping we'll find somebody with the right combination of time and skills to jump into backyard carbon fiber to inspire us and create a movement. It's really not that hard, and the only thing that requires a lot of labor is getting things to look pretty like those high-end bicycles, but that barrier is also surmountable. Some pretty good work can be done without the vacuum pump, which brings the cost of entry down.

Bill, I hope you won't worry too much about the people who steal designs and bring them to market. I wish they'd compensate the people who develop stuff for their own and community use, but at least if they get something on the market, other firms will have to compete with their probably shoddy implementations and after the market does its magic we'll all have access to better commercial products as a result. Have you looked into open source licensing modeled after software projects to get some protection for community-developed projects?

Here's something a I ran across that conceivably help us out: 3D modeling software has tended to be quite expensive, and could help both for our own visualizations and for communicating visual ideas...and now Google has something for free, with a more fully featured version available for some fee.

I haven't caught up with rapid prototyping (solid 3D realizations from 3D software) in a few years) but I see the price of the machines is coming down, which should make the price of work at service bureaus cheaper too. At some point this will leap to HP making a home machine; the price should come down a whole lot then not only due to economies of scale but because current manufacturers are probably protecting their high-end business by keeping their entry-level machines at $15,000 and above. RP could be a good way to go for a backpack mold. But I like the idea of custom molds for epoxy/CF for the parts that touch the body, and it's probably easier to do that for now with plaster or such than by scanning, although scanning is also not an impossible hurdle and there might be a service bureau around that would make it simple and cheap. If you look into service prices, be aware that the one-off prices are higher than if you find one that will let you gang your work into a group of other projects and let the turnaround time be a couple weeks or so instead of a day or two. When they put a lot of projects in the machine at once, they can get more out of the machine, so the cost of the output can be lower. This site can help you find service bureaus:

There might be significant yuppie appeal to a CF backpack frame, don't you think? Maybe someone will jump into this commercially.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: Packs still... on 05/17/2006 16:42:11 MDT Print View

"There might be significant yuppie appeal to a CF backpack frame, don't you think? Maybe someone will jump into this commercially."

If there is a market for $50 Ti one liter pots, and people will stand in line 20 minutes for the priviledge of buying a $3 cup of coffee, there should be no problem cooking up a marketable CF backpack.

You mentioned vaccum pumps-- you could probably rent one. The ones used for refrigeration and air conditioning have plenty of power. I wonder if you could fake it with a column of water-- provided you live on the edge of a bluff or other drop:)

Hey-- I forgot the water bottle holders molded into the struts.

Edited by dwambaugh on 05/17/2006 17:32:15 MDT.

BPL Subscriber
(BobOne) - F
Vacuums on 05/17/2006 18:14:24 MDT Print View

Just quickly on the vacuum issue, the cheap way to fake a vacuum is an aspirator pump that attaches to a sink faucet; they're quite cheap. You don't draw as much head and it's not as fast, but it might work out for smaller vacuum bag layups. I actually have one that I was using for pulling the bubbles out of silicone molding materials for investment casting using a plexiglass vacuum chamber. That chamber probably has over a quart of volume, so maybe it would at least be fast enough for a small vacuum bag layup. I'm not sure it would work out but it would be worth looking into for someone doing small layups.

As I mentioned previously, DIYers brought the price of the vacuum bags down...they used to be pretty pricey.

Bill Fornshell
(bfornshell) - MLife

Locale: Southern Texas
ideas - pack frames on 05/17/2006 20:19:30 MDT Print View

I went back to the Aircraft Spruce site and ordered one of their "Composite Materials Practice Kits". It comes with a book. I will read over the book and see how to use the stuff and play with it. They have other books and if this one is not enough I will check out some of their other books.

I also looked at the free 3D software program that was mentioned. They don't have it for the Apple/Mac computer yet. I will check back there from time to time and watch for the Mac version to come out.

I am open to debate on several designs to try and see how they work out. If you have specific ideas post them and when we get ready to model them I will give it a shot.

This needs real imput or it will not work.

We can do this public here or as a closed group someplace else.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: Vacuums & layups on 05/17/2006 20:57:20 MDT Print View

For a one-off this small a good dry layup and some weights would do the trick. Vacuum bagging works grea with odd shapes like a canoe or kayak hull.

If I were to make a frame like the WWII plywood frame, flat weights or some sort of big water bladder would give the layup a good squeeze. You can get most of the extra out with a squeegee on something this flat and small-- reaching across and covering the whole layup with one stroke of the squeegee wouldn't be a problem.

You know, with a mold shaped like the WWII plywood unit, it would be easy to make a clamp mold-- layup over the postive bold and clamp the negative one right over it. Put the whole works in a jig with a hydraulic auto jack and squeeeeeeeze. A set of pony clamps, ratcheting bands, or even a Spanish windlass could put enough pressure on the mold halves to to a pretty good job.

Daniel Goldenberg
(DanG) - M
Re: ideas - pack frames on 05/17/2006 21:02:12 MDT Print View


You might check out this site:

In the upper left corner look at the
"Carbon Q&A" article and the "How I made a Carbon Fiber Frame in my Garage" articles.

There is some good general info there about how to layup the carbon fiber. The application is bicycles, but of course the concepts are applicable to lots of other projects.

You pack frame project is very interesting. I myself have been experimenting with framed packs (even for light loads under 15 lbs) after using frameless packs and I think they are a valid option with benefits even for light loads.


archeopteryx .
(archeopteryx2) - F
cut and fold panel construction on 05/17/2006 21:32:25 MDT Print View

For making something like the plywood pack frame you could use cut and fold construction with prefabricated sandwich panels. This would eliminate the need for a mold. To get the weight down to BPL standards you'd probably have to cut a lot of holes though. Here's a pdf that explains the procedure on page 10 of 16. Rather than just gluing the joint as they show in the diagram you can also reinforce it with some glue soaked cloth. If you find a machine shop that cuts these they might be able to give you some scraps for experimentation.

BPL Subscriber
(BobOne) - F
More pack design stuff on 05/17/2006 22:38:14 MDT Print View

If anyone feels the need for a private group please speak up and we'll see what we can do.

Wow, Bill, this is great. It's wonderful that you've set off in this direction. I think improving the external frame backpack may be the "lowest-hanging fruit" of longer-distance hiking equipment development.

I'd suggest thinking "triangulation" in developing a frame instead of "ladder". It's easier to get strength with less materials with triangulation and molded materials allow the pieces to be arched back out of the way of physiological elements that they might interfere with if they were straight tubes. Also, it might make sense once due to the same capability for producing arched structures to bring the vertical side pieces of the frame at least somewhat around towards the front of the body, at least in spots, and then blending way forward into the wings...think of it more as molding a sheet around the back of your torso, then removing the parts that aren't triangulated...of course in the actual layup, there's no need to waste materials by actually cutting the holes.

As to the points of contact, with a better fit from molding than other methods allow, less padding may be needed. Bill has some experience with this with his thermoplastic webbing so will probably have some important input on that. I wonder if padding might be done away with entirely, in fact.

But for the sake of other people being able to carry the pack, and for fewer molding steps at least in an early attempt, it might be sensible to develop a universal mounting setup that could allow either padded fabric or harder but more shapely body contact points to be hung from the frame. This universality might come at some cost in closeness to the body, though.

The closest way, with the least arm-swing interference, might be to bring the frame wings very close to where they need to be for the individual the pack is tailored to at the hips, with molded contact pads with whatever the optimal pad thickness might be. Of course, the closer one gets to hard contact points, the more the pack will move with the body, and it might be desirable to build motion back in somewhere to allow for fluid movement.

That designed in flex would be an interesting project of some size in itself but it may be productive to imagine you're replacing the flexibility of the spinal column and hips in connecting the top and bottom body-connection points of the pack.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: cut and fold panel construction on 05/17/2006 22:58:00 MDT Print View

Interesting stuff, but I'm thinking of a shaped CF sheet that is 1/16"-1/8" thick with a couple large holes-- as I said, a squared off 8 with the sides curved around to make a space for the back bands to stretch across and mount the wings near the bottom for the waist belt to swing from. The top, center and bottom cross-bars of the 8 might need some arched half-tube girders to stiffen them up a bit as most of the stress would be the back bands trying to collapse the frame-- rolling it up so to speak. The pack load should be pulling backwards from the top and hopefully, pushing down towards the waist band, vectoring towards your hips. This carbon fiber pack board would be a broad shallow C shape with the legs of the C being 3"-4" deep. Additional strength could be added by arching the broad section of the back panel, creating some resistance against the stress of the back bands.

This is the low tech version. With more carbon fiber construction technology, a ladder frame could be made with the tubes being elliptical in cross section, just like the bike frame I saw. The bottom ends of the tubes would be like a cuved L or J to form the waist band supports. The cross bars could be slightly arched. This would allow conventional shoulder strap and pack bag mounting methods to be used. Torso adjustment could be the usual pin-and-stacked-spacer arrangement. It would look very high tech and be easiliy marketed with a decent silnylon pack bag and simple, Spartan shoulder straps and waist band. I would aim for 2 pounds all up.

BPL Subscriber
(BobOne) - F
Dale's last post and more on 05/18/2006 01:18:24 MDT Print View

I'm not sure where the arched half-tube girders you mention would run, but I agree that a lot of the forces try to flex the pack front to back. This might be dealt with in part by running longitudinal raised ribs from top to bottom near the left and right edges of the frame, placed so as not to interfere much with the packbag or body. This would add a lot of stiffness to a plate-like frame. The more arch there is in the frame, to a point, the more this flex issue tends to take care of itself, but there are obvious limits. Also, the fact of the wings extending forward can be used, by bracing them somewhat up along the pack (think triangular wing with the narrow point towards the belt buckle) to increase the longitudinal stiffness of the frame in the front-back dimension.

The hips have a complex set of motions, and they run somewhat counter to the shoulders. This puts vertical loads into a pack that would, for illustration, tend to collapse a simple rectangular frame into a trapezoid. There is also a rotational moment around the vertical axis of the pack, with force translations on many vectors through those extremes. I'd think any designed-in motion around the hips and shoulders would tend to be desired not in the front-back frame flex direction but in the rotation about the vertical axis and one-hip-contralateral-shoulder-up-other-down modes.

The lowest-material way to cut the tendency of a rectangle to collapse into a trapezoid is to connect the opposite corners with a rigid element, a payoff of triangulation. Because we use something related to a flat plane for a frame in order to make room for a packbag, there are some constraints on containing the rotary forces (arches in the plane would help), but I'm thinking that triangulation from the lower corners to some central point or points along the lumbar region would be the lowest-material way. An idea of the lines of stress might be had by putting a fold-prone semi-resilient material on a torso and watching where folds tend to point when walking. My guess is you'd tend to see them running in a fan shape from the hips into the lumbar much as the muscles in that region do. Adding a rib onto a basically planar frame along those lines of force would seem to increase strength and stiffness in those areas as well...I think I'm probably talking about something similar to Dale's girders here.

Laying up hollow ellipsoidal tubes like a balloony composite bicycle frame would garner a lot of strength from the tubular structure in a material-efficient way by enclosing more space, but seems like it would be a fair amount of extra work and it still might be desirable to flatten the tubes in certain areas, e.g., to get the pack frame closer to the back or to provide more arm clearance. Also, I don't think we'd be twisting most of our tubes very much as those on a bicycle might be twisted, and I think offhand that that twisting force is why the bicycle frame members majorly benefit from enclosing a space with a skin. Aircraft wings, on the other hand, tend to have spars to take the levering force of the wings, rather than trying to get most of the strength in that direction out of skin strength. which would require more material. So if your forces are largest in a plane, a rib in line with the force can be the lowest-material way.

BPL Subscriber
(BobOne) - F
About the prefab panels on 05/18/2006 02:21:45 MDT Print View

Prefab panels with holes cut would be a fast and strong way I would think. Double-skinning a ribbed structure is the next step up in strength once you have ribs on a plane...put another plane parallel to the first one, and it will strengthen the ribs and the points of greatest available leverage for the thickness right in the direction you'd want to reinforce the ribs to resist folding the plane, with minimal material. The hex-cell structure picks up additional strength over the skins alone to resist deformation of the rectangular shape relative to a square cell structure running vertically and horizontally by aligning planes with the lines of force. But for a backpack, the hex panel would also be thicker than might be ideal in some regions, and not as thick as would be ideal in others, and of course doesn't form as closely to the body or pick up the arch strength, and it has pretty uniform strength in resisting bending of the plane while backpack the big need for stiffness in a backpack frame may be to resist front to back folding of the fram, but it sure does provide a quick stiff framework to build something around.

In a more idealized structure that one might build with the extra effort after going past building around hexcore, lessons can still be learned from the hexcore: double skinning the ribs in semi-planar areas of great stress is a great way to pick up strength.

Bill Fornshell
(bfornshell) - MLife

Locale: Southern Texas
ideas - pack frames on 05/18/2006 04:07:26 MDT Print View

Dale, I know that old Army frame. It was used or one very much like it in Vietnam to carry mortar rounds when away from a base camp.

Daniel, I looked at the "how to make bike frames site", interesting and more or less low tech of carbon fiber working. I have used a blue foam material for molds with my Thermo Plastic material. It is easy to work with.

archeopteryx, I looked at the material you referenced. I have used some core type material. You can buy it with several types of outer material. I have some samples with a thin aluminum outer cover. It is very strong and sort of light. I used some as a frame sheet for three versions of a frame I made once. Some is planned as the "deck" material for the Titanium snowshoes I now am now building.

BobOne, I need to read and re-read your last three posts and do some drawing and try to understand visually what you are talking about. I think I need a picture of sorts to see where you are heading with the "triangulation" idea. I think it is just more of a vocabulary issue. I have some Hospital Lab tests and a chest x ray this morning and will print out the last bunch of posts and take them to mull over if I have to wait for my tests. Is there anyway you can do some simple drawings to give me a better idea where you are heading. One of my old frame may be in the ball park with some of what you describe. I am having a little trouble finding the switch to turn on the light in my old brain.
I think it is time to develop a common vocabulary to blend high tech ideas/words with low tech words and pictures. With a few scale models or simple line drawings or we may understand the different ideas faster.

Does this have any merit?

Jim Colten
(jcolten) - M

Locale: MN
Re: ideas - pack frames on 05/18/2006 07:34:41 MDT Print View

a common vocabulary always has merit (unless the objective is to kill a lot of time not making progress)

Bill Fornshell
(bfornshell) - MLife

Locale: Southern Texas
ideas - pack frames on 05/18/2006 08:46:11 MDT Print View

I have 5 to 7 days to wait for the Composite Practice Kit and a couple of books.

If some of us don't understand what is ment by some of the words and terms being used little will take place. Also others who might want to have some imput will just not try.

I am not talking about a book. The first time I/we use a word or term that I/we think some may not understand I/we could take a moment and explaine what it means or give a reference. I am not talking about teaching a MYOG Terms 101. At this point I think the list of words would be very short.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Re: Dale's last post and more on 05/18/2006 13:26:15 MDT Print View

Your raised ribs idea makes a lot of sense. The molds would be a little more complex, but the weight would drop and the whole frame could be laid up in one step. I'm thinking 1/2" to 3/4" high and smoothly transitioned to the main pack frame surface over a 1" to 1-1/2" distance-- like waves. Once you go there, it would be easy to make horizontal and diagonal waves to stiffen the frame. The smoother the transition, the easier it would be to squeegee into the bottom and work the epoxy out. Same advantage with the mold making--- sanding and smoothing the mold itself.

Bill talked about motion and shoulder/hip stresses. I was thinking about a frame made in two peices, both roughly pentagon shaped and one point of each would overlap and have a single bolt with a bushing. My guess is the top unit could be longer than the bottom, with the joint near the lumbar area. That, with the wings, would allow the hips to rotate front to back (like an abdominal crunch)and swing side to side-- like your leg taking a step up on a trail-- a gimballed pack frame :) This would remove a chunk of material from each side too. Take a couple envelopes and open them up and put the tips of the flaps point-to-point to see what I mean.