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    Senior Member TeeDee's Avatar
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    Wink Bridge Design Principles

    Bridge Hammock Design Principles

    I've been investigating the Bridge Hammock for many months now and have learned a few things about the design principles.

    Thought I would share what I have learned with the members of HammockForums.

    There are several parts of the Bridge Hammock that affect the comfort and usefulness of the Bridge Hammock. These are: (not necessarily an exhaustive list, but those I think are most important)

    1. fabric length
    2. fabric width at head and foot ends
    3. fabric width in center
    4. arc depth
    5. spreader bar length
    6. spreader bar material
    7. suspension triangle side length
    8. ridge line length
    9. material used for arc
    10. Bridge Hammock fabric


    I will discuss most of these topics. Probably not exhaustively, but enough to give those who care to design and make their own Bridge Hammocks an insight into the trade-offs involved and what they can do to increase their comfort and decrease the time and materials spent in false starts.

    Fabric Length

    I'll start with a simple topic right off the bat: Fabric length. This is easy, just decide the length of a mattress on which you are most comfortable and make the finished fabric length the same. Most people I know are very comfortable on a Queen size bed, so a finished fabric length of 80" is fine. If you are extra tall, then add to the length accordingly. Usually 4" to 6" above the head and below the feet is usually plenty and more than enough. If you like to sleep with one or both of your arms above your head, then you may need more room above your head.

    After deciding on the fabric length you are then confronted with the decision of what to make the fabric width, the arc depth, the suspension triangle length and the ridge line length. All of these design parameters are intimately related and choosing one pretty much guides you to setting the other parameters, but it is necessary to start with one.

    It is probably easiest to start with 2 of the following 3: end fabric width, center fabric width and arc depth.

    Choosing 2 of the three will define the third.

    There are design principles to consider in choosing all three.

    Center Width

    The center width influences the comfort and usefulness of the Bridge Hammock. If the center width is too much, it makes getting into and out of the Bridge Hammock harder than necessary, especially for those with a handicap. Think of laying down in a bathtub, swinging your legs over the side and trying to get out. Not easy. I have found that a center width exceeding 32" starts to hinder exit of the Bridge Hammock.

    A center width of about 26" makes getting into and out of the Bridge Hammock exceedingly simply, even for handicapped people. At that width it is more like laying on a plank, swinging your legs to the side and then to the ground and standing up. You do not have to get up and then out of the hammock. The problem with a 26" center width for most adults is that it is just not wide enough to adequately contain a top quilt or blanket. They tend to spill over the sides at the center and allow cold air to get under the top cover. If you have an over cover or a rectangular under quilt that closes in the sides and contains any top cover, then a 26" center width becomes more usable. Also, draft stoppers on the sides of any top quilt will help considerably in this regard.

    My experiments have shown that a center width of approximately 32" is a very good compromise. It provides enough side wall to cradle the user and also to keep any top quilt or blanket within the hammock. A center width of 32" allows enough of a side wall to keep any top cover contained within the Bridge Hammock, while not greatly impeding entry/exit. A good viable compromise. I highly recommend this center width.

    Fabric Width, Arc Depth and Center Width

    So once you have decided on a center width, you then have to decide on the head/foot fabric width or the arc depth. Choosing one determines the other.

    First I should say that I have tested on people with varying shoulder widths from slim to somewhat wide, i.e., compared to myself. I measured myself using the Ray Jardine method of laying on my side on the floor and measuring from the floor, up and over my shoulder and back to the floor. I measured 50"/51" so that will give you some idea of what I consider slim and wide.

    Here I'll deal only with Bridge Hammocks which have equal fabric widths at the head and foot ends. I tried unequal widths and decided quickly that I didn't like that option. I dislike asymmetric hammocks of all kinds and so I am not a good source of information on asymmetric hammocks. Grizz has made at least one Bridge with unequal head and foot widths and seemed to like it, so he would be a better source of information on that option.

    If you choose the fabric width, then the arc depth is computed simply by subtracting the center width from the head/foot width and dividing by 2.

    Conversely if you choose the desired arc depth, then simply multiply that value by 2 and add to the center width to compute the end fabric width. Simple. The computation is simple, making the decision is the hard part.

    I have experimented with head/foot fabric widths from 56" to 32" and with arc depths from 12" to 3" and center widths from 36" to 26".

    First a too large head/foot fabric width means several things:

    1. the spreader bars need to be longer to prevent "shoulder squeeze"
      1. longer spreader bars cause more interference with a tarp and make it harder to make a storm pitch
      2. longer spreader bars are heavier and more cumbersome to carry
    2. it takes more fabric to make the Bridge Hammock, hence more weight
    3. it takes a wider under quilt to secure to the corners and provide insulation, hence more weight and cost
    4. it takes a deeper arc to obtain a usable center width. if the arc is too shallow on a wider head/foot end, the center is wider than necessary making it harder to enter/exit the Bridge. Also a deeper arc uses more webbing or cord on the arc, which increases the weight and cost. The weight increase is marginal, but still there.
    5. the extra spread of the fabric on either side of the head decreases the visibility out of the hammock. The extra fabric on the foot end also decreases the visibility out of the hammock.


    Making the head/foot end fabric unnecessarily wide does not increase the comfort level of the Bridge Hammock. Thus, ideally, we want the fabric to be only just "wide enough" and no more.

    In choosing the fabric width, you have to consider the length of the spreader bars you will have to use. My investigations have shown that for all the people with which I have tested the Bridge, it is desirable to have the head spreader bar length between 0.70 to 0.75 times the end fabric width and the foot spreader length between 0.50 and 0.60 times the end fabric width. The longer head end spreader prevents "shoulder squeeze" and the shorter foot end spreader increases the side-to-side stability of the bridge, i.e., reducing the side-to-side tipsiness.

    Reducing the fabric width while maintaining a usable center width, reduces the arc depth. You do not necessarily want a fabric width that reduces the arc depth to a small value. In my experiments, an arc depth that is on the order of 3" is not good for the average Bridge Hammock user. Assuming that you make the spreader bar long enough to prevent squeeze, a shallow arc depth feels more like a flat cot than a hammock in which you are cradled. The combination of a small fabric width and shallow arc raises the body so that the upper arms are level with the arc or above it. For some purposes this can be good. But for a general purpose camping Bridge Hammock, this exposes the upper body to more predation by mosquitoes and makes the use of top quilts or blankets even more problematic.

    If you are making a spring/fall hammock in which mosquitoes are not a problem and it is not overly chilly, then a shallow arc, on the order of 3", and a small center width on the order of 26" will yield a head/foot width of 32", and provide a really nice and very light Bridge Hammock. That Bridge Hammock combined with a head spreader of 0.70 x 32 == 22" and a foot spreader of 0.5 x 32 == 16" will make transporting the Bridge Hammock very easy and easy to hang under a storm pitched tarp. I have made such a Bridge Hammock and find it to be very comfortable. The completed hammock with end panels, corded arcs and suspension weighs approximately 5 oz. However, since the upper arms are above the arc and pressing against it, there is no practical way to protect them from mosquitoes except heavy clothing. A Bridge Hammock of this size is of very limited seasonal use. I would use only for late spring and early fall or indoors.

    My experiments have also indicated that everybody likes an arc depth of approximately 6" to 8" better than shallower or deeper depths. A shallower depth makes one feel that you are closer to laying on a flat surface and a deeper depth requires a wider fabric which blocks too much visibility out of the hammock and doesn't add to the comfort.

    So working from a 32" center width and an arc depth of 6", we compute that the head/foot fabric width should be: 32 + (2 x 6) == 44"

    I have settled on a head/foot end fabric width of 44", an arc depth of 6" and a center width of 32". I will probably make Bridges in the future with a narrower center width and/or more or less depth on the arc and or more or less head/foot fabric width for unique and special purpose uses. But for general camping or home use, these dimensions have proven to be the best all around compromise.

    Spreader Bars

    The next question I recommend you consider is the spreader bars used for the head and foot ends of the Bridge Hammock.

    As I wrote above, once you have determined the desired head/foot end fabric width, the length of the spreader bars can be easily determined. Simply multiply the fabric width by a factor between 0.70 and 0.75 to determine the header bar length. The footer bar length is similarly determined by multiplying by a factor between 0.50 and 0.60.

    Thus, for my 44" fabric width, I use a head end spread bar length of 32" and a foot end spreader bar length of 23". Those are the values I used for a 46" wide fabric and haven't bothered to change since they are well within the desired range of values and allow me to switch between already completed Bridges without changing the spreader bars.

    I have also experimented with using the desired spreader length to compute the head/foot fabric width. The minimum collapsed length of my hiking poles is 22". That yields a head/foot fabric width of: 22 / 0.7 == 31.5" or 32", which then yields a foot spreader length of 32 x 0.50 == 16". Using a 3" arc depth, we have a center width of 32 - (2 x 3) == 26". That is what I call my Minimalist Bridge.

    Spreader bars can be made from several things. A short list would be: (I have arranged the list from my most desirable to my least desirable)

    1. AL hiking poles - my favorite. Very strong and no extra weight to carry. When used properly as a spreader bar, you will have 3 nested lengths of AL providing a very strong bar. To set the proper length of the pole for a spreader bar, simply insert wood dowels, poplar or oak work well, into the sections of the pole to limit the amount the pole can collapse. This makes it easy to set-up, simply collapse against the dowel stops.
    2. wood dowels - oak dowels work very well. I have used 5/8" diameter dowels on both the head and foot ends, and still do, with no problems and use a 5/8" oak dowel for my foot spreader bar all the time. I have used the 5/8" diameter oak dowels for a head end spreader up to spreader lengths of 32" with no problems. They can be easily notched on the ends and the suspension triangle ropes slipped into the notches. I suggest a notch depth of at least 4 times the diameter of the rope to keep the dowels on the ropes during entry/exit (yes, I learned that rule of thumb the hard way). The notches make the method of attaching the spreader bars to the suspension ropes moot. The oak dowels are approximately the same weight per inch as the AL tent poles sold by Quest Outfitters online. The oak dowels can be easily purchased at Lowes or Home Depot for less than $4.00 and can be easily cut to the desired length. The longest oak dowels available at either is 36" which is plenty long enough for either head spreader or foot spread bar. All things considered, I think that oak dowels are the lightest and simplest spreaders bars to make and use.
    3. tent poles - AL is probably better than carbon fiber which is much weaker laterally. 5/8" == 0.625" OD tent poles have been used successfully. Quest Outfitters has the tent poles available in 18" lengths. I personally dislike the AL tent poles simply because there doesn't seem to any viable option for carrying them other than attached to the outside of the backpack. I also do not like AL tent poles because you will usually have to use 2 poles with joint in the middle. The joint introduces a weak point.
    4. branches picked up along the trail - chancy and you are gamboling on the sturdiness of the wood used. Not a choice I would recommend except in an emergency or if your regular spreader bar is lost or broken.
    5. metal tubes. Two pieces are usually used for a spreader bar to keep the length manageable. The 2 pieces are swagged so that the end of one fits neatly into the end of the second. The ends may be notched to fit over the suspension triangle ropes. This choice will be the heaviest and most cumbersome. I would not recommend using metal tubes for several reasons:
      1. weight - the heaviest alternative
      2. notching the ends on the metal tubes is more work than for the oak dowels
      3. tarp interference - the ends of the notches in the metal tubes will have some very sharp and hard points unless more work is done to file them down and smooth them out, requiring a lot more metal work. These points are far more likely to damage any tarp they come into contact with than any of the above alternatives. A plastic cap of some sort may be provided for the end of the tube. The plastic cap is simply an extra piece which is probably to going to get lost a lot.


    My recommendation: a hiking pole and 5/8" oak dowels. The oak dowels are readily available, easily worked with wood working tools and easily carried on a hiking pole as described below.

    We use a hiking pole for the head end spreader and an oak dowel for the foot end spreader. We use short lengths of 1/8" diameter shock cord to attach the oak foot spreader to the hiking pole. I use 4" cable ties to form the shock cord into loops, which are snug against the pole shaft and hence captured and cannot be lost. The notched ends of the dowels are then inserted into the shock cord rings securing the dowel to the hiking pole. The shock cord loops keep the dowel secure and readily available without trying to find room inside or outside the pack. When using my Bridge, which requires 32" and 22" spreaders, I simply use one hiking pole completely collapsed for the 32" bar and the oak dowel for the other spreader bar. I insert poplar or oak dowels in the hiking pole used for the 32" bar to prevent it collapsing more than the required 32". Simply loosen the sections and collapse fully against the dowel stops, tighten slightly and you have a spreader bar.

    Suspension Triangle

    The length of the suspension triangle will depend primarily on 3 factors:

    1. spreader bar length,
    2. spreader bar material strength,
    3. occupant weight, and
    4. desired ridge line length


    It is desirable to keep the sides of the suspension triangle as short as possible. Longer sides of the suspension triangle require a longer ridge line and hence a longer tarp ridge line to completely enclose the Bridge Hammock and the suspension triangles. In this regard it is desirable to keep the Bridge Hammock ridge line less than or equal to 10' so that a reasonably sized tarp may be used.

    In opposition to this rule, as the triangle sides become shorter, the compression forces on the spreader bar increase. Thus, it is beneficial to make the sides of the suspension triangle as long as is practical and as short as practical.

    What is practical?

    Well, as in most things it becomes a competition between reducing the compression forces on the spreader bars and reducing or eliminating the interference of the bars with the tarp and keeping the ridge line within the confines of the tarp coverage.

    First consider reducing the compression forces. The compression force on the spreader bar is a function of the following parts of the Bridge Hammock:

    1. hammock and occupant weight,
    2. the length of the spreader bar,
    3. distance from the middle of the spreader bar to the ridge line, and
    4. the length of the suspension triangle side.


    The compression forces are directly related to the first 2 factors, i.e., as the factor increases the compression force increases and conversely as the factor decreases, the compression force decreases.

    The compression forces are inversely related to the last 2 factors, i.e., as the factor increases the compression force decreases and conversely as the factor decreases, the compression forces increases.

    For example, if we keep all the other factors constant and increase the occupant weight, the compression force increases. To compensate, we could then increase the length of the suspension triangle side to reduce the compression force, thus keeping the force balanced at an acceptable value.

    So we are trying to juggle the above factors to keep the compression force on the spreader bars at or below an acceptable value for the spreader bars being used.

    Practically speaking, the first factor is fairly constant, unless you go on a drastic weight reduction program or really gorge on the Thanksgiving Turkey and pumpkin pie. So we'll consider the first factor constant.

    That leaves us with factors 2 through 5. The problem is that these factors and the ridge line length are all inter-related, changing one, changes the others. For example, leaving the side of the suspension triangle unchanged and increasing the length of the spreader bar, shortens the distance from the spreader bar to the suspension triangle apex and the distance from the spreader bar to the ridge line.

    Since spreader bar length is largely determined by the head/foot end width of the Bridge Hammock, it becomes apparent why the choice of hammock width is very important and why it becomes necessary to keep the width as small as comfortably possible.

    My recommendation for a fabric end width of 44", and head spreader bar length in the 0.70 to 0.75 range, a suspension side length of 25" to 28" works well.

    I could give you the formula for computing the compression forces, but I have found that most people are bored by such things. So, my best recommendation is to keep the fabric width within a reasonable value as outlined above, thus keeping the spreader bar lengths reasonable. The only other factor you can overtly control is the length of the suspension triangle. Here unless you are extraordinarily heavy, a hiking pole and oak dowel are probably going to be able to withstand the compression forces with the triangle dimensions recommended above. This is especially true if you make the spreader bar length short enough that all 3 sections of the hiking pole overlap so some extent. If you use the dimensions that I have given above, then the fabric will most likely fail before the hiking pole or oak dowel.

    Ridge Line Length

    Now to decide on your ridge line length. Here I advise experimenting for what feels comfortable for you. The length can vary by 2 or 3 inches without changing the feel of the Bridge by much.

    So you don't have to be overly accurate in setting the length.

    Start with a length of approximately 114" and vary it longer and shorter.

    If the length is too short, then you get the proverbial banana with the butt hanging lower than the head and feet.

    If the length is too long, it pulls the ends out and the arc up which pulls the butt up higher than the head or feet, resulting in what I call the inverted banana.

    You want something between those two extremes.

    The other consideration to be considered for the ridge line length is the ridge line length of your tarp. Ideally you want the full suspension triangle within the tarp. This makes it easier to close off the ends of the tarp if wanted.

    If the Bridge Hammock itself is too long, then keeping the ridge line length under the tarp becomes more difficult and may not be possible.

    In the end, what you really need is for the Bridge itself to be under the tarp even if the suspension triangle extends past the edge of the tarp.

    Arc Material

    There are only two materials used which I am familiar with:

    1. webbing, either polyester or polypropylene The webbing is available in widths ranging from 1/2" and greater. If weight is a concern I would recommend the 1/2" polyester tubular webbing. It has a rated strength of 1200 lbsf which is adequate.
    2. rope. Here you have a wide choice of available materials in the new high tech ropes and in rated breaking strengths ranging from 1200 lbsf to over 2000 lbsf. The higher the rating, the higher the price and the greater the weight. Generally the small diameter, high strength ropes will be costlier, but lighter than webbing. Polyester webbing will be approximately 3 times heavier than equal strength dyneema/spectra rope of approximately equal cross sectional area. The weight penalty for equal strength polypropylene webbing is even greater. I personally prefer 3 mm dyneema rope, either Amsteel Blue or the AS-78 from New England Ropes. The AS-78 is the stronger of the two and the costlier. Either work equally well. I would stay far away from ropes that are fairly rigid such as the sheathed Spyderline. Some of the coated ropes can also be very rigid when stretched under load. The rigidity of the rope makes for a very rigid arc that bears uncomfortably on the legs on entry/exit from the Bridge Hammock. I have used the 2.8 mm Spyderline on the arc of at least one Bridge Hammock and learned not to use it for that purpose again for this reason. I prefer the AS-78 as the "softest" small diameter rope I have used. I cannot tell much difference between the AS-78 and the 1/2" tubular polyester webbing. 3 mm rope has a cross sectional area almost equal to the 1/2 tubular polyester webbing and thus when flattened by pressing on it with the legs, the AS-78 feels much the same as the webbing. Of course, using 1" wide webbing would be much easier on the legs, but much, much heavier. Heavier not only due to the webbing, but also due to the extra material needed to encase the webbing.


    There is another consideration in choosing either rope or webbing for the arcs: the suspension triangle.

    If rope is used for the arcs, the rope can simply be extended past the fabric and naturally becomes the suspension triangle. The use of a simple Zeppelin Bend to join the 2 arc ropes for the desired suspension triangle length is all that is needed. If the Zeppelin Bend is toggled, then changing the suspension triangle length becomes as simple as pulling the toggle and re-tying the Zeppelin Bend.

    If webbing is used on the arcs, then consideration must be given to the method for transferring the suspension forces from the webbing to the suspension triangle. The method used here will depend on the material of the suspension triangle, webbing or rope.

    If rope is used on the suspension triangle, then the 2 methods I know of to transfer the forces to the rope from the arc webbing is either sewn loops in the webbing or metal rings. Either will work with the metal rings being the heavier choice. Sewn loops may suffer from abrasion between the webbing and ropes though. So far not enough experience has been obtained to determine if this is a problem.

    If webbing is used for the suspension triangle, then the webbing may simply be extended equal amounts for both arcs with a loop sewn in both ends and a metal ring sewn inside both loops. This method suffers from a simple problem: the size of the suspension triangle cannot be altered without a major modification. Also, simply extending the webbing leaves the problem of how to attach the spreader bars to the webbing. One solution to this problem is the method used by the Jacks in their BMBH, i.e., to use rings sewn into loops at the arc ends and also into the loops sewn into the suspension triangle webbing. For a commercial Bridge Hammock using webbing on the suspension triangle is a more viable option than for a DIY Bridge Hammock since it is unlikely that the user will want to modify the suspension triangle anyway.

    One note of caution: The apex of the triangle must be firmly secured to the suspension rope to the tree. If the suspension rope to the tree is simply looped through the apex of the suspension triangle, the suspension triangle rope can then slide from side to side and it will do so, dumping anybody attempting to get into the hammock.

    I highly recommend using rope for the suspension triangle since it avoids the problems associated with the webbing and because of the weight advantage of rope over equal strength webbing.

    Bridge Hammock Fabric

    Most people will simply use ripstop nylon for the fabric of their Bridge Hammock. The use of ripstop nylon is used by most people for their hammocks simply because that is what most people are using for their hammocks and because it is so readily available. 1.9 oz per square yard DWR ripstop nylon is readily available from both JoAnne Fabrics and Hancock Fabrics, two national fabric retail chains. It is probably also readily available from most local fabric shops, especially those that are in areas with a lot of backpacking DIYers.

    Nylon, as a hammock fabric has one great disadvantage though: stretch. Nylon stretches and has a "memory", i.e., once the load is removed from nylon it returns to it's original shape and then stretches again when the hammock is used the next time. Now the stretch of nylon fabric isn't a lot, but it does alter the use of Bridge Hammocks.

    I prefer polyester fabrics, especially microfiber polyester fabrics when I can get them. I have used a microfiber polyester ripstop fabric which weighed 1.92 oz per square yard, actual delivered weight. It remains my top choice for the Fabric of Bridge Hammocks. All of the other microfiber polyester fabrics I have been able to find are slightly heavier than this though, about 2.2 oz per square yard. This weight is approximately the same is the actual weight of 1.9 DWR nylon ripstops.

    The microfiber polyester fabrics are super soft and silky feeling. In comparison, 1.9 DWR nylon ripstop feels hard, crinkly and rough. Everybody I have asked to compare Bridges made of DWR 1.9 nylon ripstop and a microfiber polyester fabric, much prefer the microfiber polyester. For a spring, summer and early fall Bridge Hammock use, this is a big difference.

    Besides the feel of the fabric on the skin, there is another aspect of nylon and polyester that affects the Bridge Hammock. Probably even more than any gathered end hammock.

    Stretch.

    For the Bridge, the stretch of the nylon means a longer ridge line length. This is due to stretch of the nylon under the butt, thus accentuating any banana shape. To overcome the stretch the ends of the hammock must be pulled further out, raising the arcs and hence the butt.

    I compared two Bridge Hammocks, identical except for the material used: 1.9 DWR nylon ripstop and microfiber polyester. To obtain the same lay in the nylon ripstop as in the microfiber polyester, the ridge line had to be about 4" to 6" longer.

    This added ridge line length uses up some the adjustment possible and still remain under the tarp.

    The problem with the microfiber polyester fabrics is simply supply. Not many people use it and so the supply is much more limited than for 1.9 DWR nylon ripstop.


    That pretty much summarizes what I have learned to date about Bridge Hammocks and their design.

    I hope that this is useful to others in their DIY Bridge Hammocks.
    Last edited by TeeDee; 05-07-2009 at 20:03.
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  2. #2
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    Many thanks Tee Dee. I slowly becoming intrigued by the bridge concept.

  3. #3
    Member I Splice's Avatar
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    TeeDee -

    Thanks for the design notes.

    I've been looking at your Awk script but I can't figure out what a "belly band" is. Google gives me a lot of hits for a pregnancy aid. Somehow I don't think that's the right meaning.

    I also coded up some simple-minded asin and acos functions. I didn't even use Newton-Raphson. The version of gawk that I have (Cygwin) doesn't have those as built-ins. My versions almost certantly have some bugs but they give the same answers as your test output, so I hope they are good enough. I can post those if anyone is too lazy to do a good job on them.

    I started playing around with bridge hammocks after I came across the Oz Heritage Fleet page in 2004(?). My first attempt was Tyvek and didn't last long. I haven't done from then till I started reading this site. The discussions on this group have me on my 2nd working prototype. I just rediscovered the dart in the head end of the Oz hammock.

  4. #4
    GrizzlyAdams's Avatar
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    Quote Originally Posted by I Splice View Post
    TeeDee -

    Thanks for the design notes.

    I've been looking at your Awk script but I can't figure out what a "belly band" is. Google gives me a lot of hits for a pregnancy aid. Somehow I don't think that's the right meaning.
    belly-band : piece of lightweight elastic that goes attaches to opposites of a bugnet or over-cover, and passes under the hammock to pull the sides of the bugnet/over-cover down and flush with the hammock body.

    I also coded up some simple-minded asin and acos functions. I didn't even use Newton-Raphson. The version of gawk that I have (Cygwin) doesn't have those as built-ins. My versions almost certantly have some bugs but they give the same answers as your test output, so I hope they are good enough. I can post those if anyone is too lazy to do a good job on them.
    I've done all the calculations I need with Perl scripts; also I always use parabolas and not cat curves (can't remember if the awk scripts are solving parameters for cat curves). For iterative calculations (e.g. find the parameter of a centered parabola that intersects vertical lines y +/= S/2, where S is the spreader bar width, and has a given arc length over that range) I just do a binary search over the parameter space. No inverse trig functions involved, just some piece-wise linear approximations to the arc length.

    As long as I'm rambling here, I'd say that my own experiences with the bridge are congruent with TeeDee's. All of our early versions used the Australian model, with deep cuts and high walls. We all learned that less is more, and narrowed the design down.

    I like center widths in the 32" - 36" range. Being a mere child of 52 years in reasonably good shape I have less concerns about getting in and out of one with the wider width.

    I'd try to use lightweight polyester micro-fiber fabric if I could find some in muted colors. The only stuff I've seen is on the Rockywoods site, haven't pulled the trigger on any of it yet.

    One point TeeDee did not address on the cord versus webbing for side suspension question is keeping the hammock body from slipping down the cord when you get in. I know a couple of people that have sewn the cord through to the fabric, we'll see how that solution holds up. I have another that I've posted on, TeeDee has one that has IP restrictions on it.

    I like a narrow foot bridge. I cut fabric to give me this shape. When TeeDee uses a significantly shorter spreader bar at the foot than at the head then it seems to me he gets essentially the same effect, except that the curve of the hammock at the foot is deeper than the curve at the head. I cut mine so that the depth of the hammock at the foot and the head is the same, even as the hammock body narrows. I don't know that any of this makes a significant difference, except that cutting the hammock to be narrow saves a little weight.

    Something else TeeDee did not touch on are bugnets. You can easily make a big bugnet cocoon like the ENO or other DIY versions people do. That's a lot of unnecessary netting though. TeeDee's system has a bugnet like an over-cover. The challenges there are fastening the thing down, hence the bellyband and something at the corners. A couple of times I've sewn in netting, which is a cleaner design I think, uses less netting, but adds the weight of a zipper, and leaves you with a bugnet all the time. The latter part doesn't bother me much because if I don't need a bugnet I can carry a hammock that doesn't have one. Takes more work to sew in a bugnet though. My current working version is of the over-cover design, with a belly-band. 'cause I'm in a hurry to have a working version for a hike I'm doing in a week!

    Grizz

  5. #5
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    Teedee I learn so much from your posts. If you ever want to go hiking together, it would be like school for me. Seriously you don't live that far away.

    Where do you get your polyester micro and how wide is it?
    Peace Dutch
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  6. #6
    GrizzlyAdams's Avatar
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    yeah...gonna try out that Zeppelin Bend this weekend. I was just thinking about how to lose a bit of hardware (for me, normally a ring) where the sides of the suspension triangle come together. Zeppelin Bend looks like the ticket.

    Grizz

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    Senior Member TeeDee's Avatar
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    Quote Originally Posted by Dutch View Post
    ....
    Where do you get your polyester micro and how wide is it?
    Rockywoods.

    They used to have a black microfiber polyester ripstop. The ripstop wasn't important to me, but the microfiber and polyester are. It's the lightest polyester I have found at 1.92 oz per sq yard. They have other microfiber polyesters at 2.2 oz per sq yard which is essentially the same as DWR 1.9 nylon ripstop.

    They sent me a sample on another microfiber ripstop they have and for which they are not sure of the fiber. I've done the burn test and I'm sure it's not nylon. It doesn't burn anything like the other nylon ripstops I have. Compared it to the microfiber polyester in the burn test and the results are the same. They list the weight the same as their previous stuff at 1.92 oz per sq yard. I've ordered 14 yards and will measure and weigh when I get it to confirm the weight. From the small sample, the 1.92 seems to be right.

    All of the microfiber stuff I have gotten from them is right at 60" to 62" wide. Really nice stuff. They did have a Gold Dust color. I liked that color also - very subdued. They still list it, but don't have enough to fill any of my orders. I think they only have 1 or 2 yards left. Still like the black better though for color and weight.
    Those who sacrifice freedom for safety, have neither.

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    Senior Member TeeDee's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post
    yeah...gonna try out that Zeppelin Bend this weekend. I was just thinking about how to lose a bit of hardware (for me, normally a ring) where the sides of the suspension triangle come together. Zeppelin Bend looks like the ticket.

    Grizz
    Yes - I learned of the Zeppelin Bend here on the forums - it's become my favorite bend. Easy and fast to tie and holds really well. I've even stopped tying safety knots for it.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

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    Member I Splice's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post
    yeah...gonna try out that Zeppelin Bend this weekend. I was just thinking about how to lose a bit of hardware (for me, normally a ring) where the sides of the suspension triangle come together. Zeppelin Bend looks like the ticket.

    Grizz
    Thanks for explaining the term belly band.

    Trying to run TeeDee's Awk script reminded me why I dropped it for Perl in the early 90's though using Awk provided by someone else is a lot better than starting from scratch.

    For my suspension triangle, I'm using a single piece of Dyneema with a small loop spliced in and eye splices on each end. Think of a capital 'A' with a really high bar. I've spliced in the bar of the 'A' so that there is about a half-inch hole to attach the suspension triangle to the support line.

    I'm attaching the suspension triangle to loops sewn in the side webbing using an eye splice. Actually, right now I'm tying a lark's head on one side and a toggle on the other. Permanent attachment will be with eye splices.

    I played around with a single line suspension some last night - I used one a couple of times 3 or 4 years ago but I decided to save the weight of the carabiners that I was using.

    The configuration that I used several years ago had carabiners clipped the tree huggers. Then the support rope ran from the end of the hammock through one carabiner across the top of the hammock (like a ridge line) through the other carabiner then back to the other end of the hammock. The idea was that I could get my butt off of the ground using a trucker's hitch or a sheepshank in the line. And I could adjust how level I was by sliding the line through the carabiners. In the version I used last night, the line was stretched between the supports and the hammock was hung from it.

    However, last night the stress was too high on my hammock supports. One post looked like it might lift out of the ground because of the way it's braced. I'm planning on burying a big chunk of concrete on the side away from the hammock and chaining the post down.

    In the wild, the extra stress from a single line suspension won't bother a 5 foot diameter redwood tree but it might affect an 6 inch madrone.

    Once the post is safely anchored, I'll do some more experiments to see if loosening the main line will make it work better. If it works, the plan is to do some more splicing so that the suspension line and hammock are a single unit.

    I think the 'final' version will use polyester fabric. In a bridge hammock, that should be strong enough to support me and be lighter than the 3.5 oz nylon in this prototype.

    Light nylon stretches too much. I had trouble for a long time with the hammock sagging during the night and putting my butt on the ground. That's originally why I started using Dyneema lines.

    I'm about one good dinner away from 250 lbs.

    It turned out that the 1 inch webbing was stretching as well as the hammock body. Yes, I know polypropylene isn't supposed to stretch but getting 1 1/2 inch polyester webbing from Seattle Fabrics and changing from Ed Speer's 1.9 oz nylon ripstop to his 2.5 oz Supplex nylon means that my but stays off of the ground all night long.

    This prototype uses the 1/2 polyester webbing from Sail Rite for the side webbing. I think the final version will use 3/16 Dyneema, maybe with a two foot section of webbing in the center to save my delicate thighs.

    There isn't a lot of force pulling the fabric down the side ropes, so a little stitching should hold it just fine. The previous prototype used both hand stitching (about a foot total on each side) and just tying the points of the hammock body to the support ropes. Both techniques seemed to work just fine.

    I made the foot a few inches smaller, just like you did.

    I want enough extra length (3 or 4 inches at each end) so that there is room for some bulky insulation in the end caps. Playing around it looks like I can get away with a shorter head spreader if I move it down about 9 inches along the curve. The pole just clears my head when I sit up. With my head being slightly beyond the pole when I laying down, it looks like I'll need a dart so that the fabric is taut under my head.

    In this version, I sewed some webbing loops to lash the pole to. When I do one using Dyneema, I can just splice in some loops.

    I might let the poles stick out a few inches past the edge of the hammock and see if I can rig the tarp to them I'm using some unmodified garage sale ski poles because I'm not going to risk my one piece carbon fiber poles.

  10. #10
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by Dutch View Post
    .......
    Where do you get your polyester micro and how wide is it?
    Dutch - the black microfiber ripstop from Rockywoods arrived today.

    I did a burn test on it. I have other polyester microfiber fabrics from Rockywoods and lots of nylon. For me nylon always melts and forms small brownish/black globs before it will burn.

    The known polyester microfiber just burns with black smoke. It leaves a black crinkly residue on the burnt edge.

    The burn test on the new black microfiber ripstop duplicated the polyester microfiber results. It definitely did not melt like nylon. I think that pretty much rules out nylon for the new material. Not totally conclusive that it is polyester, but it duplicates the polyester results. Any other fabric that would do that??

    I ordered 14 yards, received 15 yard 1". It is exactly 60" wide. Weighs 47.50 oz, which computes out to 1.90 oz per sq yard.

    One side is matte black and the other side is shiner. On the matte black side, the hand does not slide easily on the fabric. I wouldn't call it tacky, but not real slick either. The shiner side is much slicker. That's the side that is going inside the Bridges. The matte black side is probably the treated side, whatever the treatment is.

    The ripstop squares are a little smaller than for all of the nylon ripstop I have.

    The Rockywoods web site describes the stuff as:

    Microfiber Ripstop Water & Wind Repellant, Breathable Fabric
    60" wide. Super lightweight fabric at 1.92 oz/sq yard

    PN EMR-108

    This fabric is from Nextec, the makers of EPIC. Great for ultra light weight jackets, pants or tarps.
    Nominal Fabric Width: 56-60"
    They are right on the weight per sq yard. When I called and asked whether it was polyester or nylon, they didn't have any documentation stating one way or the other. They did a burn test, but with nothing to compare to, they weren't too sure. They sent me a sample and I performed a burn test to satisfy myself that it very probably is not nylon. Then I ordered. So far I like the stuff very much.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

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