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  1. #1
    Senior Member TeeDee's Avatar
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    Scaling the Bridge Hammock

    Scaling the Bridge

    Lately, I've been making Bridge hammocks both narrower, 46", and much narrower (minimalist), 32", than what I started with, 56". From that I learned that as long as I stayed with the 0.70 to 0.75 spreader to head end fabric width ratio and 0.50 to 0.60 for the foot end spreader, the original, narrow and minimalist Bridge Hammocks all are the same in terms of comfort (fantastic) and shoulder squeeze (none for me).

    So I decided to make a short Bridge for a friend. She is 4" 9' and I decided to make her a Short Narrow Bridge. I decided on a Bridge length of 70" for some very simple reasons:

    1. I wanted 6" from the top of her head to the head end and 6" from her feet to the foot end. That gives her plenty of room for her arms and feet. That gave a length of 5' 9" == 69"
    2. I rounded the 69" to 70" to get a simple figure. Then I could use 1/2" hems on each end and cut from exactly 72", 2 yards, of fabric.


    I ran my Bridge design program, using a 46" fabric width and 7.5" deep arc. In less than a second, the output gave me the parabolic curve coordinates and the expected square yardage. It also gave me the end panel dimensions, which are the exactly the same as my long Narrow Bridge.

    Took me about an hour with poster paper and a meter stick to make a body pattern and about 15 minutes folding the fabric in half, adjusting the pattern and cutting the fabric. It then took me another hour to mark, iron the end hems and arc folds and sew the end hems. Another 30 minutes to complete the cords in the arcs. It actually takes me longer to mark and iron than to do the actual sewing, but by ironing first, sewing the hems and arcs is fast and easy. Another 30 minutes sewing the 4 web pockets for the spreader bars.

    Another 15 to 30 minutes tying arc cords together to form the suspension triangles using the Zeppelin Bend and a minute or 2 to hang the SLS with a 98" ridgeline.

    I tested the Short Narrow Bridge and found that it has exactly the same feel as my regular length Narrow Bridge Hammocks.

    In thinking about my various sized Bridge Hammocks today, I have learned that scaling a Bridge Hammock to a different size in either length or width or both is a trivial matter.

    The Bridge Hammock design decouples the width and length of the hammock, i.e., the two dimensions are independent. I can change one without affecting the other.

    When I made my bottom loading ULBA type and then my bottom loading Safari type, I found that changing either the length or width of a regular hammock affects the other. In changing the length of the ULBA type to the Safari length without changing the width, everything changed, the feel of the hammock, the bug netting pattern, the side tie out positions, the sag needed. Everything. It was pretty much the same as starting from scratch in doing the Safari type after doing the ULBA type.

    For my Bridge Hammocks, in scaling both the width and length, I have to make both the new body pattern and the new end panel patterns, but making the patterns is simply busy work and easy to accomplish while listening to my favorite music. After cutting the fabric, the sewing necessary to make the Bridge is exactly the same.

    In scaling the length only, I compute a new body parabolic curve and make the pattern, an hour or so to accomplish. I can use the end panel patterns from another Bridge Hammock of the same width. The differing length has no effect on the suspension triangle size, but requires a new ridgeline length which can be easily computed before hand.

    In scaling the width only, I can use the same pattern for another Bridge of the same length and center width and adjust the distance from the fold for the desired width. Then make new patterns for the head and foot end parabolic curves for the end panels. Another hour or so for the end panel patterns. The same computer program outputs the end panel parabolic curves. The differing width has no effect on the ridge line length, but requires a new suspension triangle size.

    For the over cover, if the width changes there is no change in the over cover. If the length changes, I can simply add or subtract the length change in the center of the pattern and everything else remains the same. A simple adjustment. Since bug netting is essentially an over cover with end panels, scaling the bug netting is done once the over cover and end panels have been scaled which is easy.

    Under quilts and under covers are made of rectangles with a width equal to the fabric width and a length equal to the fabric length. Simple rectangles are easy and they fit Bridge Hammocks perfectly. It is possible to get more complex with under quilts with 1/2 or 3/4 length models and/or models that try to follow the hammock arcs, but that isn't really necessary. Also, I have found that I would rather not follow the hammock side arcs with under quilts or under covers, since the raised sides that are above the hammock arcs provide more wind and thermal protection.

    Thus, the width and length of Bridge Hammocks scale independently. This makes the scaling job extremely easy. Also, scaling the Bridge Hammock does not change the lie of the hammock. You can get in any of my Bridge Hammocks and it will feel the same as any other of my Bridge Hammocks no matter the length or width. This assumes that you follow the simple fabric to spreader bar length ratios that I have laid out before.

    In short, I can custom make a Bridge Hammock of any reasonable length and/or width by simply changing the Bridge Hammock end width, center width or arc depth and length parameters in my computer program, make any necessary new pattern which will appear exactly the same as any other of my Bridge Hammock patterns and get a Bridge Hammock which will feel exactly the same as any other Bridge Hammock I make.

    This is a far cry from what I went through in making the ULBA and Safari types.

    Makes life much, much simpler so that I can concentrate on that which is important, like contemplating the MOND conjecture.

  2. #2
    Senior Member Heber's Avatar
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    Cool stuff. I guessed that it might work this way but I wasn't sure.

    One thing you don't touch on but which I've been wondering about is choosing arc depth. I can imagine scaling it with length to that in cross section every bridge looks like the same parabola. But I'm not sure why we would prefer one parabola over another.

    One possible consideration is that a shallower arc (a flatter parabola) puts more force on the suspension lines for a given weight of occupant. I'm not really worried about the suspension lines breaking, that's easy enough to handle. But more force on the suspension lines means more squeeze on the spreader bars which means we need a more robust bar. Am I right here? I'm trying to remember the diagram Grizz did with the angles. I'm not an engineer so I could totally be wrong here.

  3. #3
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by Heber View Post
    Cool stuff. I guessed that it might work this way but I wasn't sure.

    One thing you don't touch on but which I've been wondering about is choosing arc depth. I can imagine scaling it with length to that in cross section every bridge looks like the same parabola. But I'm not sure why we would prefer one parabola over another.

    I pick the arc depth based on the desired center width. Subtract the center width from the desired fabric width at the ends and divide by 2 for the arc depth. I like the center width to be approximately 32". No special reason other than I've gotten comfortable with that width. My minimalist Bridge center width is 26" with an arc depth of 3". I used the 3" arc depth to keep the center width from getting too narrow.

    The parabola is then dictated by the length and arc depth.

    With use I've gotten to like the 26" center width on my Minimalist Bridges. The narrower center width makes it easier for some people to exit the Bridge Hammock - you have a shallower Bridge to pull yourself up and out of. For this reason I recommended my Minimalist Bridge to a person that has some difficulty getting out of hammocks due to some physical problems. That person said it is the easiest hammock to get into and out of they have used - they're sold on the Minimalist Bridge - or at least any Bridge I make with a narrow center width. I'm thinking of trying them on a Narrow Bridge with an arc depth of 10" to get the 26" center width and seeing if they like that one also.

    Another advantage of the 26" center width is that the arcs intersect the upper thighs or the buttocks when entering and exiting. Since most people have more fat and muscle padding in that area than lower down on the legs, they have more to cushion the arc. With a 32" center width, the arcs intersect just above the knees on most people which puts the pressure on the tendons there with little fat or muscles to supply cushion. That in conjunction with the dynamics of the shallower depth of the hammock, make for less pressure on the underside of the legs during entry/exit, especially exit. You can position the height of the Bridge so that the feet are flat on the ground when sitting, but with a wider center, the arc is higher and your butt is still well below the arc for a 32" center width Bridge. With a 26" center width Bridge, your butt is now more on a level with the arc height and so less of an upwards pull to exit.

    For a comparison, imagine getting out of a bean bag chair. You literally have to pull yourself up and out. Now imagine a regular chair. You are no longer pulling yourself up to exit the chair. That is something like a 32" center width Bridge and the 26" center width Bridge.

    There are, of course, disadvantages to a narrower center width. In picking the center width, the pros and cons of wider vs narrower have to be traded off.

    Quote Originally Posted by Heber View Post
    One possible consideration is that a shallower arc (a flatter parabola) puts more force on the suspension lines for a given weight of occupant. I'm not really worried about the suspension lines breaking, that's easy enough to handle. But more force on the suspension lines means more squeeze on the spreader bars which means we need a more robust bar. Am I right here? I'm trying to remember the diagram Grizz did with the angles. I'm not an engineer so I could totally be wrong here.
    I don't think the depth greatly influences the compression on the spreader bars. For the Bridges that I have done the measurements and calculations on (my original Bridges, 12" arc depth, and my Narrow Bridges, 7.5" arc depth), the compression forces are due almost equally to the suspension triangle and the arcs. So my intuition says that changing the arc depth isn't going to change the compression forces much. I haven't done the measurements on my Minimalist Bridges with an arc depth of 3", but my observation of the spreader bars on those indicates that the forces are about the same as for my wider Bridges. Since I have adjusted the suspension triangles on all of my 80" length Bridges of any width to utilize a 114" ridge line, this is what I expected and have observed.

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