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  1. #11
    Quote Originally Posted by Just Bill View Post
    My apologies as I misspoke slightly: my use of 'you/your' meant to mean more of a general sense of the baffles in anyone's particular design rather than the baffles you personally laid out.
    I think that misled you slightly and caused some confusion.

    For example:
    "Your ideal design holds a uniform thickness and density of down into the shape you will use the piece of gear."

    If I had said "An" ideal design rather than 'your' I believe we are on the same page mainly.
    You mentioned synthetic batts being nearly ideal earlier in that they are easier to work with vs. loose fill.

    Really if one is looking to optimize... and generally agrees with a loft chart (or converted to R-value) for a given target rating:
    The goal should be to deliver that loft uniformly with the least material.

    I believe that was the point cruiser was driving at in discussing baffle spacing (width) and what I was getting to regarding the whole quilt rather than a single baffle in isolation.

    If height is the driver that dictates warmth in the finished quilt... we need to choose a width that creates the least amount of deformation possible to maintain uniform loft. This is balanced against the need to use the least amount of shell material.

    Generally speaking (in real world) the lower the loft the tighter the baffle spacing.
    If you wanted a 2" thick piece of gear one might think to increase the baffle spacing to 10" and eliminate baffle material.
    Because we all agree that the 2"hx10"w rectangle would quickly deform towards a circle... we'd have to keep pumping in down to keep it filled and our average height would quickly rise beyond our goal of 2" thick at the center. A 2 to 1 ratio is decent starting place, but many push that 2"hx 4"w to 2"x5".

    In some sense what you are modeling is a typical down puffy jacket. A mid weight model frequently has a 3/4" high by 1" wide baffle design.
    The next level up would be a 1" x 1.5" baffle spacing.
    I think we are on the same page for all of this. The only way my baffle shape is quantifiably "ideal" is that it minimizes down shifting. One can certainly use baffles wider than they are tall, and it may even be a good idea in most circumstances to save weight. The tradeoff is a greater potential for down shifting, but that can be minimized if one pays close attention to how things like tension and gravity influence the baffles, factors I ignore. My only argument is that these tradeoffs need to be acknowledged for other baffle shapes, and be done deliberately, instead of just pretending you can throw some conic sections on the top of a rectangle, overfill by 10%, and still wind up with the shape and loft you predicted.

    Even when designing other baffle shapes, it is a good idea to keep the truncated circle shape in mind. By determining the area of a given baffle shape and comparing it to the area of a truncated circle shape with the same wall height and perimeter, you get a useful proxy for the potential for down shifting. The closer the areas are to eachother, the lower the potential for shifting.

    A few tricky variables are missing that make what you are trying to do difficult, so in some sense you've thrown out a few variables already.

    Lets consider a baffle as a balloon:
    Or ideally as a soap bubble which would want to form a perfect sphere as that is the ideal volume to surface area shape.

    Adding down is like blowing up the balloon... which might be measured in PSI of air or in this case a variable we don't know.
    PSI- Expansion power of down fill (based upon fill power) is one; tied tightly to real world fill power vs laboratory conditions.

    From there- we might note that we require additional PSI to inflate said balloon based upon the material properties of the balloon itself. If the balloon is very thin or very thick, if it's under tension already or not. So the actual weight of your shell material will allow or limit the expansion power of the fill to achieve it's ideal shape.

    In theory we have Cubic inches of volume per ounce but that is calculated under a specific conditions test in a specific vessel.
    In a long tube shaped baffle there is less resistance over the length of a baffle as opposed to in cross section. So in real life we often find that a well filled baffle will do a better job of eliminating dead spots in the LENGTH of the tube rather than expanding in cross section. In cross section we can discuss the x and y axis expanding... but in the z axis there is almost zero resistance to expansion present... certainly much less than in x and y.

    So in reality, even in a single baffle condition, we find that we don't really have a spherical balloon but what we have is the relatively fixed diameter hot dog shaped balloon that gets longer, not fatter as it is inflated. It takes exponentially more force to increase the diameter of this type of balloon as opposed to filling it's length. You must inflate to the maximum length before you even begin to have enough volume of air to attempt to build enough pressure to increase diameter.

    How would one calculate this I am not sure.
    While the balloon comparison makes sense on its face, it is not really analogous to how baffles actually work. This is because the elasticity of the balloon is the major factor in determining how much it will grow, a factor we have little intuitive sense of. While typical quilt fabric is somewhat elastic, it is a very minor factor, and can be quite safely ignored, at least for rational levels of overfill. The major factor in determining the shape a baffle ultimately takes is just the lengths of the different fabric pieces.

    Again we have not even considered things like shell material weight, gravity, humidity, etc.
    These are indeed factors I am not considering, and of those gravity is the biggest one. I hope to tackle it eventually.

    It is true that the rectangular baffle does deform... but within limits. Much like that first puff of air into a balloon animal balloon does alter it's shape from a listless shell into a tube... However once going the balloon grows in length, not diameter. At some point though... you would have to increase PSI... or in our case density of fill to a point where it becomes inefficient to do so.

    It is this balance of cross sectional baffle shape in the x and y axis we shoot for so that the length of our baffle (z axis) is fully filled with no cold spots. We want to get in just enough down so that we puff up a bit past a true rectangle that is filled end to end... but not so much that the density increases and down fill is wasted. That is why it's a bit unrealistic in the real world to achieve the more dramatic perfectly circular shape. I think at best you could think of cutting out your baffle from a football rather than a basketball.


    Again... we are still isolated to a single baffle.

    Let us go to two baffles... or ten. Each of these competing or pulling against each other for the adjoining shell material if one is looking at an outer and inner shell joined by strips of baffle material.

    If your 5" wide baffle deforms to 4" wide... it needs to pull a half inch from each adjoining baffle to do so.
    Yet each of those baffles is fighting for the same thing. In much the same way if one were to push enough circles together they are forced into a honeycomb shape no matter how much you'd like each individual cell to remain a circle.

    Not that could not be overcome with enough fill as we are only building one layer of cells thick... but if efficiency is the name of the game it's not ideal to create this condition.

    If 10 baffles at 5" wide make up your 50" wide quilt (and we agree you need 50" wide to cover you)...

    If these baffles deform to 4" we now need 12.5 baffles to cover the same 50"

    Without getting fancy- we need more shell material and more fill to do this.

    It is wrong to calculate a simple 2"x 5" rectangle.
    And eye opening how down is needed for as little as 1/8" in depth and width due to deformation towards a circle.
    However I'd argue that it is also wrong to keep pumping in down once you've achieved a full chamber.

    Many here have not understood this concept and tried making a 50" wide quilt only to find it ended up 45" wide after they kept shoving in an extra bit of down into it 'just in case' and inadvertently overfilled the bag.

    Just as many do the opposite and figure down for a non-expanding fixed geometry 2" x 50" rectangle and wonder why they have a listless and useless piece of gear.

    Understanding fill, overfill, and overstuff is the lifetime of work. Not necessarily the geometry or calculus or thermal calculations. The nuts and bolts of it... the material properties and realities that would require decades of study to quantify or identify into something useful... or even measurable.
    I think we are in agreement here as well. My point about how the truncated circle is the shape you will eventually end up with if you add enough down is not meant to be an invitation to do so. It's just meant to reinforce the idea that this is "the" reference baffle of sorts, the one that other baffles should be compared to in order to understand them better. Given the common mistakes made in predicting the final shapes of quilts, it is definitely a useful reference.

    I joked on a bridge thread not long ago that I like science, rules, ratios, and math too.
    But a good bit of 'voodoo' is often needed as well. Or perhaps better stated a bit of art or bad poetry.
    I'm smart to a point- but a bit of intuition, artistry, and simple craftsman's instinct for how things really work is needed to get you from a sweet idea on paper to a satisfied customer in the woods.

    I'm all for optimization... nothing the natural world likes better really.

    'Mitakuye Oyasin' is what the earth has to say on the subject and remains the most useful.
    'We are all one, we are all related' is roughly what that translates to.

    All things in balance more or less.

    Don't mean to shart in your cheerios. Sleeping gear needs more science applied to it, but that science does have some limits and dead ends to it as well. Or perhaps you've cracked it and it's flown over my head.

    My comments are not to discourage you, but to point out a few of the holes so you can plug them if you're able.
    No worries, I think it is just a misunderstanding of what I am setting out to achieve. What I am doing is creating a solid model baffle, one that can be easily studied and well understood. A reference baffle of sorts. I probably should have been a little clearer about that before I got too cheeky on my optimization comment. While it is easy to optimize a model, real life remains quite stubborn.

  2. #12
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    Quote Originally Posted by Bindle View Post
    I think we are on the same page for all of this. The only way my baffle shape is quantifiably "ideal" is that it minimizes down shifting. One can certainly use baffles wider than they are tall, and it may even be a good idea in most circumstances to save weight. The tradeoff is a greater potential for down shifting, but that can be minimized if one pays close attention to how things like tension and gravity influence the baffles, factors I ignore. My only argument is that these tradeoffs need to be acknowledged for other baffle shapes, and be done deliberately, instead of just pretending you can throw some conic sections on the top of a rectangle, overfill by 10%, and still wind up with the shape and loft you predicted.

    Even when designing other baffle shapes, it is a good idea to keep the truncated circle shape in mind. By determining the area of a given baffle shape and comparing it to the area of a truncated circle shape with the same wall height and perimeter, you get a useful proxy for the potential for down shifting. The closer the areas are to eachother, the lower the potential for shifting.
    Yar... I suspect if you were to add some dimensions to your sketches it might come together better.

    The truncated circle is actually in pretty common use in apparel and would likely make more sense if we were talking jackets rather than quilts.

    That's a bit what I was trying to get at regarding a 1:1 ratio. It does carry the advantage of reduce shifting which is a much bigger issue in apparel than sleep gear.
    And on the flipside... since you are talking a smaller piece overall... the impact of the added baffle material has less impact on finished weight.

    In sleeping gear... the 2:1 ratio or 2.5:1 is more common.
    In this case it's a bit more of a truncated ellipse or oval than a circle unless you're pushing for that 1:1 (ish) ratio.
    Though you do have a little more shifting... this can be addressed and you do reduce shell weight. It's a more static piece of gear too so the shifting isn't as amplified as in clothing.

    Basically- square or rectangle as your base and deform from there... sounds like you were talking the former and we were assuming the latter?

    At some point some place- I think I mentioned taking a rectangle- then using it's perimeter to get an equivalent circle.
    You could then calculate the area of the rectangle, and of the circle- and average them for your cross sectional area... which you can then multiply by length.
    Perfect, no. But gets you much closer than simply guessing on how much % to add to your rectangle.

    My only point with the balloon was more to point out that your down tends to fill into the length rather than deform the shape of your rectangle much beyond that middle shape.
    The forces seem to balance out a bit and you end up with more density than deformation, especially if trying to push that rectangle into a circle. Much less an issue if pushing a 1:1 square into a truncated circle.

    There are creative solutions to all of this for sure...George at Loco Libre's zig zag baffle creates something pretty close to a clean 3 dimensional box at very even fill for example with little or no distortion.
    To an extent if talking turning loose fill down into a 'batt' of insulation his method is pretty close.
    Other folks do a varied cross section in the outer shell to help ensure down doesn't settle, or alter baffle direction or length.

    And when you do put 3 dimensional differential cuts in there you get a base line shape that is more pentagon shaped than rectangle for an UQ... which is sorta what the catsplat calculator was shooting for even if it didn't graphically display well as such.

    So yes it's hard for some of us to follow what you're looking to do in the context of MYOG/DIY quilts- though if these factors aren't that critical to your reference baffle then it's not a concern really.
    We may be too nuts and bolts for your current exploration perhaps.

  3. #13
    Senior Member Cruiser51's Avatar
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    The idea of the "truncated circle" makes logical sense, however as Bill noted ( I think i did as well), sometimes math/logic only takes you so far on problems like these .... at some point that shape will be harnessed to a hammock, the goal there will be to minimize the air gap between the UG and the hammock under load. This is typically accomplished via shock cord and some type of flexible suspension to hold everything up snug .... that is going to play hell with your assumptions about the shape of the inner portion being circular, as this will likely get flattened a lot by contact with the occupant.

    The shape defined in the calculator I posted is likely closer to reality in a loaded hammock with a typical suspension system, the shape also seems to assume that the quilt will lay flat and the discussions in that previous post pretty much agreed that a differential between inner and outer shells really needs to be addressed.

    I would suggest incorporating baffle spacing for both the inner and outer shells, they are not usually going to be the same for a good/robust design ... the spacing would be dependent on the baffle height and quilt dimensions ... this would result in angled baffles and a circular shape at the top ... and a smaller circle at the bottom, but I would also suggest that the bottom (part against the hammock) be considered as closer to flat that circular when under load.

    Brian

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