Need advice on type of metal for MYOG pack frame

[Hiker 816]

I need some input from you engineering or metalworking types. I am planning on making an internal frame for a pack I'm constructing. I want to create the frame by bending a metal rod into a U shape, and then attach the hipbelt to the bottom part of the U. With food and water, I expect that the max weight of the pack might hit 50 lbs on rare occassions, but not more, so the frame needs to be strong enough to support that. I presume that aluminum is the best choice for the metal rod, but what alloy, temper, and size? From what I've read, the 7075-T6 is much stronger than the more common 6061-T6, but I've also heard it is much harder to work with. How difficult would it be for me to bend it permanently into shape? Is there another alloy that would be better?

And what is the largest diameter rod that I can realistically bend at home without any special equipment? I could probably buy a tube bender like this, but that's about all I'd have at my disposal. Thoughts?

Thanks!

[Colin Krusor]

Aluminum and titanium rods, in a handful of different alloys, are not difficult to find in many diameters, but I think a single U-shaped rod will probably give a fairly heavy and flexible pack frame. U-shaped tubing, or even aluminum sheet, could give a much lighter and more rigid result.

[Scott Nelson]

I am thinking about a similar pack frame. I bought some aluminum tent poles from Quest outfitters. They sell the tubing in various diameters. Some of the sizes have 90 degree elbows. I was able to bend the poles to contour to the shape of my back by filling them with sand and using a jig. The hard part is keeping the poles from twisting at the elbow connector. I am thinking I might epoxy them or use a cotter pin. Wtih the elbows it is easy to make a rectangular frame. Another challenge is making additional cross bars. I have not found "T joints". I am thinking of sewning a wide webbing joint to hold the cross bars in place in the middle of the pack. I was inspired by Roger Caffin's pack design website. I got sidetracked on this project when I found a cheap Jansport pack and was able to modify it to only weigh 3 pounds.
Let us know if you figure something out. Scott

[Roger Caffin]

Hi Christofer

You can bend even Easton tent poles and arrow shafts in 7075 T9 alloy with care and the RIGHT sort of bender. But the bender you referred to won't work with 7075 alloy as it is MUCH too aggressive – too small a radius. You need a roller bender and multiple passes.

The cost is a stack of failed bends while you experiment!

Cheers

[Jeremy Greene]

Perhaps Gossamer Gear, Six Moon Designs, ULA, etc. would sell you a frame for $10-$30.

[Hiker 816]

Thanks for all the info! Since what I have in mind is going to be an internal frame (not external), I want it a little bit flexible, and not too bulky, but I still need it to be strong enough to affix a hipbelt directly to it along the bottom of the U-shape. I've been leaning against using tubing since I think it would have to be of a pretty large diameter to be strong enough. The large diameter would make it hard to bend, rather inflexible, and bulky. That's why I've been looking at solid aluminum rod (or any other metal you suggest that would be better).

Something like the Gossamer Gear frame is exactly what I have in mind, but inverted relative to how they use it. I think it would also have to be a bit stronger than the Gossamer Gear frame because I want to connect the hipbelt directly to it (think of the way an external frame pack, or a McHale Critical Mass pack, attaches the frame to the hipbelt). I don't know for sure, but I suspect that Gossamer Gear uses the 6061 aluminum alloy. I am hoping that by using 7075, I can get more strength in the same size and weight frame.

If I got a 7075 alloy rod of a 3/8" diameter thickness, do you think it would be strong enough? And on the other side of the coin, would I still be able to bend it by hand or with a reasonably priced tube bender?

Thanks!

[Roger Caffin]

> a 7075 alloy rod of a 3/8" diameter thickness, do you think it would be strong enough?
Ah – do you really mean a 'rod', as in solid?
If so, how many cars are you trying to lift?
No, you could not bend it cold.

Cheers

[Hiker 816]

"> a 7075 alloy rod of a 3/8" diameter thickness, do you think it would be strong enough?
Ah – do you really mean a 'rod', as in solid?
If so, how many cars are you trying to lift?
No, you could not bend it cold."

So what diameter would work to allow me to bend it, and still be strong enough?

Thanks!

[Sam Farrington]

Christopher,

Have done a lot of this using a Rigid brand ratchet 5/8" tube bender. Used to be able to go to used ski gear sales and pick up lots a raw material cheap, but now the poles are all made of an alloy that is too stiff to bend – so it shatters instead. Unfortunately, the Rigid benders now go for several hundred dollars – mine was purchased long ago for <$100. There are cheaper ones from the Tool order companies, but they are of much lower quality, & don't ratchet.
Recently made a LaFuma style folding camp chair, tho. Found a hunter's folding stool at a Brookstone outlet here in NH with alloy that looked bendable, but it was 3/4" inch tubing. So bought a cheap mail order bender for under $100 since only 4 bends were needed. It worked, but only with much more care and much less facility than the Rigid, and I had to bolt the main bending plate to a wooden post in my basement. The bender kit weighs a ton, but handles a number of different diameters of tubing.
The earlier post about Quest tubing might be a good source of tubing. But don't think anything less than 5/8" tubing will be robust enough to hold its shape unless you really baby it. Maybe you can borrow a Rigid from a machine shop just for your 2 bends, or have them do it.
A suggestion: Invert the U, and buckle a hip belt band to the lower legs. You may need a bent cross piece at the bottom of the U for enough tension on the band. Attach the hip belt to the band at points not too far from the band's center, so that as you walk, the belt can rotate a little with your hips. You can put cups on the leg bottoms to take some of the abrasion when you set down the pack, and the frame top will be rounded at the bends so as not to catch on stuff, and look nicer. I think that is the essential design of the frame on the new pack BPL is selling – lots of pix on this site.
Have made a lot of fancy bent tube frames for panel loaders and the like, but for lowest weight, finally settled on old reliable, the top loading pack, but with a more flexible ABS tube frame suppporting a stretched mesh back panel. (Design marketed by Alpine Designs years ago). But you could probably do the same thing with the inverted alloy U.
Have found stretched mesh to be the most comfortable back panel design.
Hope this is helpful.

[Roger Caffin]

Hi Chris

What you haven't specified yet is what sort of pack frame or pack load you want. It really does matter.

I would imagine that I could bend 3 mm 7075 rod with care. 4 mm rod might be a bit tricky unless a large radius was desired.

But the frame design specs have to come first.

Cheers

[Hiker 816]

Thanks for all the info everyone! My plan is to create a frame just like the one used by gossamer gear (scroll down to see photo of frame), but out of stronger metal, inverted in the pack, and attached directly to the hipbelt with webbing. I'll purchase several different diameter rods in both 7075 & 6061 alloy (yay ebay!), and start experimenting.

[Bill Fornshell]

You might get a few constructions ideas from my External Pack Frame Thread:

SUL External Frame / 1-1-2009

[David Olsen]

I have bent aluminum tubing on conduit benders and also
over a plywood curve I cut with a jig saw.

For really small stuff you might get some help using these too– Just $1.99

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=65794&xcamp=google&utm_source=googlebase&utm_medium=cpc&zmam=13262200&zmas=12&zmac=112&zmap=65794

[James D Buch]

Lightweight structural materials selection involves both material properties and geometric properties. In pure tension, a solid straight rod and a hollow tube of the same metallic cross sectional area (meaning same weight per unit length) made of the same material will yield (stretch) at the same load. [In axial (or lengthwise) compression, however, the hollow tube will generally way outperform the solid rod in resistance to buckling failure.]

In bending, buckling or torsion, however, this equality of behavior in axial tension is not true. The engineering calculations for yielding for both rod or beam bending and buckling involve the moment of inertia of the rod/beam shape. "Formulas for Stress and Strain" Raymond J Roark. an engineering classic reference.

For torsion, the appropriate geometric parameter of the rod/beam cross section is the polar moment of inertia.

These geometric factors end up making the performance of a solid rod in axial compression (buckling failure), bending or torsion much worse than a hollow tube of the same cross section area (same weight per unit length) [of the same material] would be.

Put simply, the fact that the most common frame geometry of the older exterior frame backpacks was a hollow metal tube was no accident.

If you take into consideration putting the reinforcement metal into a confining cloth tube or slot, the buckling conditions are radically modified by the lateral support. While this may be difficult to calculate, there can be substantial increases in the buckling load and the original aluminum flat (shaped) bars in the early internal frame backpacks can perform quite well.

There are internal frame backpacks which use small diameter hollow metal tubing placed inside confining cloth tubes which perform quite well. Some of the Osprey packs come to mind.

If you are going to use solid rods in a backpack frame, it would likely be a good idea to give them lateral support (anti-buckling) by confining them into well fitted cloth tubes attached firmly to the pack walls. This appears to be well known to manufacturers.

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