Quilt / Sleeping Bag -- Differential Cut?

[andrewa]

Orion, lovely project. Looks good.

As to differentially cutting the baffles for one thin, it depends what it is. Ideally a diifferentially cut sleeping bag will use baffles cut on the curve from baffle fabric. If using flat baffles they will need to be deeper to allow for potential collapse around the curve of the bag. The balance here is wt, vs stuffing around vs actual loft.

For sleeves on jkts, to get around that degree of curve with a differential cut requires the baffles cut to it. Straight oil loses too much. This is why we are now seeing the spiral baffle approach. Much easier to sew with a straight baffle, and only one baffle per bag.......

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[Orion]

Forest -- thanks for the suggestion for a reinforcement. I had something simpler in mind but that looks pretty good.

Andrew -- who uses spiral baffles besides Montbell?

[andrewa]

Just Montbell, as far as I've seen! It's a lovely progression in design concept, and clever. Less sewing for them. A new design that no one has thought of. Quite a clever concept in terms of making a flexible sleeping bag shell. It's. omething none of us have really thought about, isn't it?

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[Orion]

How does it save them sewing? There isn't just one baffle since the baffles terminate at the zipper. There should be roughly the same number of baffles in a zippered spiral design as in a conventional one, for a given tube width. I counted at least ten in a photo of one of Montbell's spiral bags.

[blacksheep]

The primary design concern is down control. Minimizing heat loss, providing the optimum volume chambers in core areas. Almost everything else is marketing, which is fine, so long as primary design concerns are not compromised.

[Orion]

No question that the primary concern is minimizing heat loss. But everything else only marketing? I was under the impression that the spiral design was to provide greater volume flexibility. Montbell's spiral bags replaced their Super Stretch models a few years ago. The stretch bags had elastic in them and were marketed to people who sleep with one knee up or are claustrophobic in standard mummy bags. Just guessing here, but maybe the spiral approach weighs less yet still provides the stretch that these people desire?

[andrewa]

All interesting intellectually, but not sure of practical outcomes.

When I was a med student in the 80s, I worked part time in Mtn Designs. I spent a lot of time thinking about design, and arguing design aspects with customers. I started making my own gear. As a result of my thoughts about the " ultimate sleeping bag design" at that stage, I made 2 differentially cut bags, with vertical chest panels and goretex outers. I also made 2 fully differentially cut down jkts with goretex outers.

Lots of planning went into these items, and they were warm and provided what we needed then, but they were heavy. The down was 550loft, the nylon 1.3oz, plus the goretex 2 ply shell. Each sleeping bag was about 2kg, and the jkts 800g.

Now I sleep in the snow wearing all my clothing, and a quilt made with straight baffles, no differential cut, no goretex outer, no side block baffle, no down muff, no spcially designed hood, no special big dick foot box ( just a drawer cord at the end), and I'm very comfortable.

My conclusion is that whilst fabrics and down quality have improved a fair bit, and llow a weight reduction, the design hype is just marketing. It isn't that hard to make a useful sleeping bag.

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[Orion]

You're probably right. It's just bag full of feathers.

I have a friend with a back problem. He has to sleep with one knee up and the other leg extended so he can't be in a zipped up mummy bag. For him it's either a really wide bag or he leaves his bag unzipped, quilt-like. And that's what he does. It works but he carries a heavier bag to compensate. I've never seen one of these stretch bags that MB sells, but when I think of my friend I wonder about them.

[andrewa]

I also like to bend one leg up, and stretch out the other ( also because of back probs) . The quilt approach therefore suits me v well, and my single experience this winter with an exped down mat ( in snow) was also vv worthwhile. Outside of snow, I normally just use a simple air filled torso ngth mat, plus or minus some 2-5 mm closed cell foam, dependent on forecast temps. Same quilt for both uses. Just more insulated clothing under it in winter.

A