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)
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.
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.
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:
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.
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)
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.
The length of the suspension triangle will depend primarily on 3 factors:
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:
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.
There are only two materials used which I am familiar with:
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.
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.
Those who sacrifice freedom for safety, have neither.
Do not dig your grave with your teeth. (Unknown)
Last edited by TeeDee; 05-07-2009 at 20:03..
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