Increase frame stay stiffness – width or thickness?

[Chris L]

I’m currently using .5” wide by .125” thick 7075 aluminum flat bar stays for a MYOG backpack. Like typical frame stays, they contour to the shape of my back with the wider axis flat against my back. I’d like to increase their stiffness so they flex less under heavier weight

I’m embarrassed to ask this since once upon a time I graduated with a degree in engineering, but should I increase the width to .625” or the thickness to .16” (those are the next options up)? My gut tells me to increase the thickness, but maybe either increase would produce the same result?

[Jon Fong / Flat Cat Gear]

You need to increase to Moment of Inertia. Fundamentally a function of the base * height cubed.  So the best band for the buck is to increase the thickness and not the base.  My 2 cents.

[Mike B]

Thickness, Increase the material in the direction of the deformation.

[Chris L]

Thanks gentlemen!

[Brian T]

It might not be a reasonable point to make, depending on your setup, but you’d get a lot more stiffness with a different shape than flat bar.
For example you could run a carbon tube from hipbelt to the top of the harness outside where your back contacts the pack and gain a lot of load stabilization for just a few grams. can you post pictures of your application? we might be able to help you accomplish your goal another way.

[Chris L]

Thanks for the reply Brian. I’m working on a high volume, 90L pack for packrafting. I’ve done lots of experimenting, and currently prefer a design that places the stays against the pack where they need to have some contour. Also, the hip belt bolts to the stay, so the stay needs to have sufficient size and strength to allow that

I’ve had decent luck with some .433” Easton tent pole aluminum tubes. They are very light for their strength and are probably the best option. I’ve managed to bend these, but it’s a bear and tough to do accurately. The aluminum alloy used in those poles also seems somewhat brittle, and I worry about breaking one with heavy loads or with some sort of impact. That fear is probably unfounded…

The question about the aluminum flat bar stems from looking for an option that is readily available and bombproof for this application, if a bit heavier.

I also found this company that sells carbon fiber pack stays, although they’re quite expensive: https://www.swcompositeworks.com/the-sherpa/

[baja bob]

You should get in contact with Dan Ransom of https://sockdolagerequipment.com/

He works with bending aluminum tubing for pack stays. I have a set from him. From what I understand, those tubes can snap and send shrapnel flying. Be careful.

[Chris L]

Thanks Bob. Dan and I have discussed pack details a fair bit. His stays are too thin to bolt through, but that’s where I got the idea for my larger diameter tubular stays. He has the bending down to a science, which I don’t!

[Brian T]

Chris, have you thought about adding additional components to the suspension in addition to the stays?
For example this old pack (mid 2000’s) from Osprey did something clever on their 90L pack. They used two titanium rods (very thin) and some straps to keep them under tension in an arch. This stiffened the pack a huge amount, laterally, and load transfer wise. It also had flat aluminum stays against the back.

This is the pack.

Here I highlighted the helper stay’s I was describing.

You could also try adding an HDPE frame sheet to the stays, depending on the performance characteristics you’re looking for they could both make a difference in carryability. 90L is a BIG bag, so I’m guessing it get’s heavy. Hope you figure out how to make it super stable. Please post more about it, interesting project!

[David Thomas]

Definitely add thickness before width.  Doubling the width will make it twice as stiff.  Doubling the thickness will make it eight times as stiff.

Or, if you want it to be twice as stiff: add 100% to its width or add 26% (1.26 = cube root of 2) to its thickness.

Or go to a different design concept than solid stays.  For instance, foam-core stays with very thin skins of metal, wood plys or composite.  But unlike solid metal stays, foam-core can’t be bent to a custom shape.  If you want a particular curvature, you need to build the skinned/foam-core member that way from the start.

Most of my foam-core stuff is optimized for cost and availability versus lowest-possible weight by using door skins (bought as a 4′ x 8’sheets or scavenged from old interior or closet doors) and the “blue board” used under concrete slabs.  If you play with metal stays first to find the desired curvature, you could then recreate with foam- core stays skinned with 1/8″ plywood (cheap and easy) or carbon / fiberglass composite (lighter, but more tedious).

[Chris L]

Lots of good ideas here, thanks!

Brian, I recall that osprey pack. I’ll give that concept some thought. There are some size constraints – the pack needs to fit length-wise inside a 9” packraft tube. But that design meet that.

David, that’s SUPER helpful to know about the increase in stiffness. As it turns out the next size up in thickness would be a 28% increase, so sounds like that would about double the stiffness.

I’ve actually thought pabout prepreg carbon stays, but not sure I’m ready to disappear down the lamination rabbit hole just yet. I have more interest in the sewing side of the equation.

[Jon Fong / Flat Cat Gear]

The problem with stiff stays are that they are great, but forming other than a straight bar can be problematic.  You might consider looking at the Dana Design LoadMaster.  This was a prized backpack for Hunter to carry out game as they would really load those packs up (like close to 100 pounds).  I bought one new for $100 and sold it 10 years later to a hunter for about $300: they’re that good.  There was an extrenal load lifter attached to the hipbelt that was great.  I know, just what you need, another cook stirring the pot!  Best wishes.

[Sam Farrington]

Have made several hourglass frames that support sturdy suspended mesh back bands that protect the back, as seen in a number of Ospreys (although I think they make the bands too rigid).  The frames were made of Jansport 5/8″ diameter alloy tube from old packs, and bent with a heavy 5/8″ Ridgid hand bender.   These served well for years; but nowadays prefer something lighter. With Easton .340 inch diameter tent pole tube, much lighter mesh and parts can be used. (Note: Anyone who has sat on a woven fabric chair seat should appreciate the use of a ventilated strong mesh seat in a heavy rainstorm).

The tubing was bent by Roger Caffin, well known at BPL for his ingenuity and fabric experience.  Using his home made tube bender dubbed the “Rolling Jenny,”  he made several tubes that were beautifully done for an hourglass frame.   And with the reduced weight, even open opposing sidearms can be added to cinch the waist band snugly around the waist with no buckle or belt over the belly.  Now that the total carry weight can be kept under 3 lbs, a much more comfortable frame is possible.  Kudos to the cottage manufacturers for their efforts.

The Easton .340″ flexible alloy tube has been used on a number of high quality tents and packs, and have no concerns about its strength.  If it can support a tent in severe windstorms overnight, it has proven its reliability.  Nevertheless, for dome tent purposes, the tubing will be crossed at both peaks with two strong elbows, and cross at both sides of the tent about halfway up to insure strength and stability.  Although I’ll now use best quality carbon fiber tube on the tents.  Taking it easy now.

[Chris L]

Thanks Sam! I’ve followed some of your hourglass designs with the sidearms. Super interesting.

I tend to agree that the Easton tubular stays are likely the best option. I’ve been using the .433” diameter versions. I just haven’t mastered the bending. The one place I worry about the Easton tubing when used in packs is impact/crushing resistance. They are quite brittle. This is not something that is likely to be encountered when used as designed for tents. It’s probably an unfounded fear and can be mitigated with care, but dropping a 50# pack on the ground when tired could result in some unusual forces. Maybe.

Packrafting provides some unique constraints. When empty, the pack needs to fit into a 9” diameter tube. This limits the use of cross members or elaborate stay shapes. When packing, it needs to carry 50#+ comfortably and withstand the forces larger loads can produce. I have a design that carries 60# loads relatively comfortably and weighs around 2.75# depending on pack material, features, and frame stay weight. Nailing the stay material feels like the final hurdle.

[Roger Caffin]

When empty, the pack needs to fit into a 9” diameter tube.
I am genuinely curious: why a 9″ tube?
I have seen quite a few photos of ordinary (ie wider than 9″) packs strapped to the bows of a packraft. That seemed to work.

Cheers

[Chris L]

Sorry Roger, my previous comments were probably cryptic for those unfamiliar with modern packrafts. Most boats these days have an airtight zipper that allows gear (including a backpack) to be stored inside the boat tubes. Boat tube diameters range from 9-12” or so – hence the constraint on pack size.

In non-whitewater situations, putting your pack inside the boat (rather than on the bow) keeps it dry and reduces entanglement safety hazards. In whitewater, it greatly improves boat handling and reducing the entanglement potential is even more important.

[Roger Caffin]

Thank you. Consider me now educated.
I guess I would not have even noticed a pack inside a tube!

Cheers

[David Thomas]

Packrafts – the new preferred smuggling tool!

I’m often struck by the trade-off of pure inflatables and RIBs – tremendous floatation, but so much lost volume in those tubes compared to a skiff / canoe / dory of similar beam.

[Sam Farrington]

Figure that if we are going to use ultra light side arms, the pack has to be lowered gently.  Same for most of our gear.  My choice is to do this and go ultra light.  But see from this and other threads, that many prefer the heavy duty approach.  For me, ultra light is necessary due to aging (me, not the gear).

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