Frame Stay alum tubing Sources

Solid rod (not hollow), or tubing (hollow)?

With that narrow a frame, you’re going to have to bend the tubing (or heavier rod) exactly to the shape of your back.  And if the tube is soft enough to easily bend, it is soft enough to bend out of shape. One solution to this is a suspended mesh backband that with the right amount of tension on the mesh molds to your back; but that cannot be done with a U shape, especially such a narrow and small diameter one.  Looking at some Osprey packs at a shop might provide some ideas.

But for your design, would suggest finding a tube of just the right amount of temper to be bent with a Ridgid 3″ radius tube bender to fit your back and also hold its shape as much as possible with a temper that would allow but hold the low radius curve.  Probably around T 7 or 8.  To increase the width of the frame, and to fit your back, bend the U a little bit at a time.

Not sure that such a narrow frame would fit a back, but you can try with softer tube first.  And some foam padding with a cover fitting over the U and sewn to the back of the pack might also  work.

There are a number of cottage pack companies that sell aluminum stays separately. For example for $12 you can get a ULA Circuit stay. Could probably be modified to suit your needs. Available in different torso lengths.

https://www.ula-equipment.com/product/circuit-aluminum-stay/

Been a few years but I bought one at gossamer gear. The curve fit my back nicely

Eric, my idea is probably too thick/ heavy/ wide for what you’re looking for, but maybe it will work for your design: I cut down a baby carrier frame. I see baby carriers all the time on craigslist for $20-ish. Pics in this thread. I still use it with my airbag packbag here. For thinner tubing, I wonder if you could find someone with an old, ripped, perimeter frame Osprey pack like Sam mentions, willing to let it go cheap, and cut it out of the fabric and hacksaw the frame to size.

I’ve used 26” .340 tent poles from Quest Outfitters w/out the inserts. Forming them by hand to fit your back is a bit tricky so go slow. I managed to ruin one pole in the process so buy spares.

I bought some tent poles from Tent Pole Technologies in Vancouver WA

They bent it for me with a jig made out of a circle of plywood.  A piece of galvanized hanger strap at one place to constrain the tubing you want to bend to the circle of plywood.  The circle of plywood firmly attached to a table.  Then bend the tubing against the circle of plywood.

Bend it multiple times until you get the desired curve.

Dan Ransom could maybe do it?

https://danransom.com/store/Ultralight-Frame-Stays-p311250240

If you’re bending Al tube, it helps to fill it up with small particle sand or the like and bend very slowly and carefully.  Even with the sand in, you can still have it buckle in if you don’t do it slowly and carefully enough.  I was able to bend some pieces by hand using this method, but I also ruined once piece by jerking it too much.   I don’t remember where I bought the tubing now–this was some years back.

I would also recommend considering bamboo reinforced with S-glass fiberglass cloth and high strength epoxy.  You can bend the bamboo with heat and/or steam (you also want to fill it with small particle sand or the like when bending it), and once set and dried, roll a couple layers of 3.7 oz/yd2 S grade fiberglass cloth over it w/ high strength epoxy (S grade F.G. has higher tensile strength than regular grade carbon fiber, but lower stiffness strength. But carbon fiber doesn’t work well with bamboo because of the difference in expansion/contraction between the two materials. When carbon fiber gets hot, it very slightly contracts whereas bamboo expands–this can crack/splinter the bamboo as bicycle maker Calfee found out that hard way. They switched to fiberglass, hemp, and/or linen flax for reinforements after).  The combo makes some pretty strong for the weight and durable material. And if you spray or pour closed cell expanding foam in it, it increases the stiffness at not much weight (and further increases durability/longevity because it doesn’t allow any water to get into it).

Thanks for the mention Chris.

I have shaped a lot of stays the last two years for sure, and have gotten pretty consistent at doing anatomically shaped s-curves in 7075 tubing – which is by far the highest stiffness to weight ratio I have found.  Much stiffer than either 7075 or 6061 flat bar and it often comes in at half the weight.  7075 however is somewhat brittle so it has to be done quite carefully, and it’s limited in how tight of a radius you can bend the tube.  I’m using a couple of radius roller jigs to do different degrees of bend.

The style of frame Eric is trying to create is above my paygrade for sure.  7075 tubing just can’t be bent in that aggressive of a curve, and the high quality tubes are hard to source in such lengths.  Of course you could use a more malleable alloy like 6061 or even going to a rod instead of a tube and probably get there.  But then you lose much of the stiffness and you gain a lot of weight.  It could also be achieved easily with the inserts to make the 90 degree bends, but those inserts are much heavier because they have a lot of reinforcing material to make the curve.

I have been prototyping and playing with 3d printed nylon lately to form custom connections between tubes – and made some cool stuff.  But unfortunately you gain a bit of volume at those joints so my guess is it wouldn’t work in this particular design as it looks like the bottom of the U is designed to nest into a hipbelt pocket (and provide some articulation which a nylon coupler would prevent).

Thanks everyone for the suggestions!

Dan, yes, this is to insert into a sleeve for the removeable hipbelt.

On a separate note, Dan, I’ll be in touch soon as I’m working on finishing up two packs and have another in the works that will need stays. As we previously discussed, I still have some flat bar 7075 left over but once that is out I’m all into the tubing!