Differential Cut Question, Please help.

Blake,

I think the conventional wisdom says to increase your shell width the same height as your baffles. So, 4in baffles would mean a 54in shell in your case.

Ryan

If you had a circular cross sections, it would be 2 * pi * 4 inches = 25 inches bigger circumference = 75 inches

If you assumed the bottom is flattened because you squish the insulation and it was a half circle, it would be half that, 12.5 inches bigger circumference = 62.5

Thank you both for replying. Both answers give me viable options, I appreciate the feedback. I think I might do a differential fill and put only 2 or 3 inch loft on the bottom, since I might use a Klymit sleeping pad which will allow minimal loft below me.I am thinking for the bottom I wont do a differential cut, just for the top. I'm doing a middle zip bag; so I think this might be viable, and not tricky at all to do. Thanks for the help!

I think that will actually work much better! So I can just at the extra loft circumference of the top half of the bag and make that part slightly larger. If I match the bottom at 25in. then make the top part 37in to offset for the differential cut, and it will come out to 62in. Or I could just make it 33in so that it comes out to 58in. total. That way I don't have to buy extra fabric to make it work, since my fabric will be 58in. wide. Of course I will have to give allowance for seams. Thanks for the help; I think this will work out quite nicely, and allow me to optimise the loft.

Not only do you need to allow for the seams, you also need to allow for the fabric not being the claimed width. It's not unusual for fabric to be an inch or so narrower than the claimed size. (It's also not uncommon to be wider than claimed.) Depends on the sourcing of the people selling the goods.

Thank you David,
that is something wise to consider. While I was waiting for help I did take the time to figure out if I would still be able to make the design I want to, if I had to make the outer shell 72in in circumference. I found that I would be able to do that without having to buy any extra fabric than I was already planning on purchasing. It would just require more work, since I wouldn't be able to keep it as one piece. So Either way I should be able to manage the project, just hoping I can do it the less difficult way :)

Blake,
IMHO, you need to consider how much inner girth you really need.

Unless you are way smaller than the rest of us, 50 inches for an inside girth is going to be tight.
I don't think you'll be able to wear anything substantial in there.
You may have a hard time bending your knees and bringing your thighs up. (sleeping on your side.)

Western Moountainering's Lynx is 62"
Marmot's Lithium is 58"
Zpack is 56", and is ok if you are 5'10 and 140#, and naked.

I suggest a "mock up". Take a sheet, sew it at 50", and try to move around in there.

Along the edges of the fabric, there are the holes that were created when the fabric was made. Normally I cut those off. If I really want to get the most width, sometimes I'll use the holes for the seam allowance.

Good idea about mock-up. Do you have a sleeping bag you already have you can use?

I follow the math you are doing with the increase in diameter.

You won't be a circle when you sleep, though. You will be more of an elongated oval. The bit over your chest will be mostly flat and then you will have a curve around each shoulder. The curves around the shoulder will have much smaller radius than if you look at the thing as one large circle. Even those shoulder curves won't be full… The bottom will be flat.

So, I don't think you will need anything like diameter+(2*loft)… I think this will give you a lot of volume to fill with down.

Volume of 50" Circumference = 199 * 72" High = 14,328
Volume of 75" Circumference = 447 * 72" High = 32,184
32k – 14k ~ 18,000 / 850 cui Fill = 21 ounces…
This is before 'overstuffing' around 20% (or whatever amount you like) to make things not so loose.

Maybe think of the bag as a completely flat thing with your baffle height. You pick a point about 1/4 way in from each side and that needs to curve around you. The shell in that area needs to grow and stretch a bit but the top/middle and what wraps around to become the bottom will not need to grow so much…

If you side sleep a lot, it is worth thinking about a bit extra in the middle as the bag curves around the one shoulder there. I have thought about it, anyway.

I don't know any rules about this, myself. So, I can't tell you how much. I will not even tell you how much I have planned to use because I don't know if I am right. I think you will have a lot of extra material and this is the way I rationalize it. I was interested to hear the rule of thumb regarding add baffle height to shell.

I agree 50" is not very big. When I read that, I figured you were making a quilt and just called it a bag. You may have already settled on 50 with reason, but I would also recommend either the sheet idea or looking at store bought bags.

> … it will come out to 62in. Or I could just make it 33in so that it comes out to 58in. total. …

I got a little lost here, but you are coming up with 58" and I think that sounds more better. Think about where the extra girth needs to go, where the curves will be.

– – – – Here. This is a helpful thread I found.

This fellow made a quilt and a pretty detailed pattern is described in the PDF file stored as his 'gear list'.

Here is the thread with pictures: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=63726

This is for a quilt. He used lengthwise baffles but where the extra girth needs to go can be visualized for crosswise baffles.

He cut the top/bottom in three sections and then sewed them together. This is in place of one great sheet then curved around the body and I think it helps things curve/work better. Since this is a quilt, he had top + 2 sides. You would have a top to sew to a bottom. This uses more material and might not be interesting to you. The graphics are good, though.

Looking at the graphics, it appears he has an extra 40mm (check a PDF for exact measurements) on each side of his shell for a total of 80mm or ~3" girth. But the top is superimposed with the bottom so the extra girth is doubled, I think… I believe this fellow used about an extra 6" girth. As I say, it was a little hard for me to understand, so check my estimate and see what you think. [His baffles are not as high as yours so you might need more girth]

He has a detailed, pattern as a PDF stored as the 'gear list' in his profile. It is of interest just for more precise measurements; the interesting graphics are in the thread.

The pattern is really a grid you create following his measurements and then connect the dots.

Sounds like you have accepted the mission. Good luck.

The first thing that strikes me is the lofting at 4". I believe you want a little more than 4" on a side, like around 4.25" to 4.5" to hit -10F.
This gives figures of around 70" for total girth at your shoulders. Note that with cold weather bags you will need to figure draft collars, both around the zipper and your neck. The baffles need to be fairly loose to allow the down to loft. I would calculate down fill based on 750fill to insure you have enough. The extra girth will allow the draught collars/sleeves to loft a bit.

I expect that you will find 50" very tight. I use a 56" bag in 3 seasons and find it to be totally filled with a down jacket/long johns. This will actually add about 4" inches to my girth at 28F. I also have a 44" chest. Tight fitting will actually make you a bit colder by pulling everything tight. This will reduce the lofting, hence warmth. A larger bag is generally better, provided you don't "swim" in it.

"Differential Cut" actually refers to the difference between the upper and lower lofting of the bag, ie, the part above you and the part below you. The upper loft does more than the part you are laying on. So, the bottom 1/3 or so is filled to about 3" with the rest filled to about 4.5"(an example only.) It should be filled fully around the sides and upper. For the same weight, you get a slightly warmer bag. I don't know what they call the difference between the inner and outer shells.

Much of the difference between inner and outer shell will be determined by what you fill with. And, it will matter what you have for a pad, and how well you sleep at colder temps. The ideal 900fill will seldom loft fully. As many others have pointed out, a 10-20% overfill is likely needed.

At -10F you are looking at frost build up in the down. Keeping the down dry, and condensation free for 5-6 nights is a real problem. You might want to check on a vapor barrier inner. The problem is that in warmer temps, say 20F, you will build up large amounts of perspiration in your sleep wear, reducing the versitility of the bag. Extra venting at the feet and shoulders needs to be considered.

"You won't be a circle when you sleep, though. You will be more of an elongated oval…"

Doesn't matter

This is more of a math problem than designing a differential cut, although maybe they're related : )

If it's a half circular cross section, the outside is 4 * pi bigger as already discussed.

Consider a cross section with 8 inch flat sections and 8 inch radius arcs on both corners:

The outside is 4 * pi bigger than the inside

It doesn't matter that the sides are partially flat

I see. The distance around needs to be paid whether it is through an arc with greater or lesser radius.

Thanks for all the comments. Looked at a sleeping bag I have now and decided on going with 56-57 inches at the shoulder. Thanks for questioning me there, it would have been very snug!

I figured out all the math I needed thanks for all the help. I also ordered the materials! I got M50 from Thru-Hiker for both the inside and outside shell. I figured it will be so cold that the shell feel isn't that important feel since I will be layered, so I didn't go with the M90T. I am going to use 850fp down from Wilderness Logic, buying in bulk, its $100 per 16oz. w/free shipping.

I ended up figuring I could make the inside shell out of 3 yards of material. The outside liner I will have to use 4 yards since, I will have to sew two pieces together so it is wide enough and long enough to work. I think that will be the simplest way, so I can just use the same pattern. I estimated the bag to be around 24oz. when all said and done. That's my goal anyways.

I am doing 2in baffles on the bottom, and 6in for the rest. Essentially the design I decided on will have the typical bag design in the middle with a hood and then two flaps that fold over. They are identical to the middle just cut in half, that way I will have a center zip. I got the main idea on the design from http://www.backpackinglight.com/cgi-bin/backpackinglight/MYOG_down_quilt_bag.html#.Ugg4x6yRM9k By Roger Caffin and then made tweaks to fit my needs. Its going to be a closed hood with a circle for the face. Similar to the Sub Alpinist by Nunatak. http://www.nunatakusa.com/site07/other_bags/subalpinist.htm

I did work baffles for the neck and zipper into my design. I decided for the neck I will make the fabric some where around 6 inches longer at the neck and then make a bite out of it and sew it up to use as a baffle. It is the way that Roger Caffin described how to do it in his article. Seems like I can make it work for my project.

James,
I looked into the vapor barrier, seems like a valid point, since I will be in below freezing weather. I hadn't thought of it so thanks for bringing it up. I decided I will make a suit to wear instead of making the inner liner, or having a sleeping bag liner. I'm going to make Pants, Shirt, Socks and a hat out of Polycro. I figured this would be a better option worn over my base layers so the moisture that gets trapped wont get all my layers wet. I have some neoprene gloves that I will wear so, don't need to make those.

Thanks for all the help! I will keep you all posted on how the bag turns out!