MYOG 120L (7400ci) Optimal Shape of bag for external frame pack?

[Jens W]

Hello Guys,

I’m thinking of making my own roll top bag for an old  Haglöfs Skarja X backpack. What is the optimal shape for such a large bag? Looking at the sherpas they seem to maximize height and minimize circumference. Looking for suggestions. Commercial bags like HMG Porter seem pretty cylindrical at around 110 cm circumference? Some of them have a slight bulge at shoulder height?

The Skarja X has a “shelf” at the bottom of the external frame that is around 30cm by 30cm. I have noticed for skiing a very wide pack or a pack with large side pockets can interfere with elbows during poling but have not noticed this during hiking with poles.

[Sam Farrington]

Am a fan of the butterfly, or hourglass, shape for internal, external or hybrid packs because:

– They adapt well to using a suspended mesh backband.
– They do the job with the least amount of tube length, so are lighter.
– They need far less T-fittings than conventional external frames, so are lighter still and require less work
– They work well if a shelf is preferred at the top of the the pack for carrying food and/or stashing wet gear.  This keeps the center of gravity more forward than stashing in a big rear mesh pocket.
– Even when made with very light tubing, like flexible hoop tent tubing, they are strong and flexible.
–  They protect the back with less weight, and with a couple twists, the center of the frame can be recessed to assure pack contents do not protrude against the back.
– They can be designed to work just as well or better as internal frames, or for me, as hybrid frames with only the bottom of the frame protruding outside the pack, and reinforced with tough flexible electrical sheathing over the portion of the frame that rests on the ground.
– They work well if side-arms are desired on the frame.  A bit more vertical tube is required at the bottom of the frame for the side-arms to hinge on.

They do require a little more finesse with the tube bending, though.  I first tried kite fittings, found them way too flimsy, and use plumbers’ hose barb fittings for cross bars.  Since I’m using mostly Easton or other tent poles and elbows, the built-in ferrules take care of holding the butterfly together.  Inexpensive JB Weld epoxy is far better than most for bonding alloy metal parts because it contains fine metal particles.

Also, learning what-fits-what with the tubes and fittings takes a while.  Watt makes very rugged, light nylon hose fittings that are stronger than polypropylene.  I try to avoid boring out the fittings as much as possible so as not to weaken them.

[Jerry Adams]

cylindrical – big enough to fit bear canister sideways

but I’m into pretty basic pack, probably makes sense to have it bigger around on top or whatever

[Author]

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