Guide to Making a Bridge Hammock
Guide to Making a Bridge Hammock
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This Guide was slightly updated on Feb. 8, 2008 to reflect some changes in the recommended basic design. Text added in this revision is denoted in a blue color.
So you've seen them on the Internet, all your hanging friends want one, their product placement in Hollywood block-blusters entices you to want one: you have to have a bridge hammock, but you're afraid of what it takes to build one. Fear no more, the Guide to Making a Bridge Hammock will take you through it, step by step. It's easier than you think.
A bridge hammock hangs differently than most other hammocks. The hammock body hangs suspended between two ropes, or lengths of webbing. The long edges of the hammock body are cut in a curve to which the suspension webbing is sewn. The curve's shape is based on the same principles as lie behind suspension bridges, hence the name. A key distinctive of a bridge hammock is that one can lay flat in it. So flat that sleeping on one's stomach is possible.
Most of the material described here was published first on the Hammock Forums site (www.hammockforums.net) by contributors TeeDee, schrochem, and GrizzlyAdams. You can find many photos and postings by schrochem and GrizzlyAdams in the forums depository. TeeDee removed all his contributions when he stopped participating in the forums; many of the techniques documented here were first demonstrated by him.
See also TeeDee's multi-volume article "My Bridge Hammock System" published in December 2007.
A lot of the activity on the Bridge Hammock thread at hammockforums.net was motivated by questions about, and experiences of people that had seen the JRB Bear Mountain Bridge Hammock at the 2007 Trail Days exposition. This product has come to market, and can be purchased here.
The Guide has five main sections. The first one describes how to make the main body of the bridge hammock, and the second one describes the suspension system. These two sections are all you need for a functional bridge hammock. The third section describes an underbody that you can use to support an underquilt or use as bug protection. The fourth section talks about bug-netting, principally describing my approach to putting in a fully removable zip-in bugnet. The last section discusses weight.
My main purposes in writing this Guide are to bring together in one place the key ideas needed to build a bridge hammock, and to show the curious but hesitant that putting together a basic model is really quite straight forward. I got into it, without knowing how to sew, when TeeDee showed how to do it and I saw that I could learn enough to get started. All of us that have been exploring this hammock have marveled at the richness of design possibilities, and the opportunity for creative thinking and solutions. So jump on in, the water's fine!
The body of the bridge is basic, and easy to make. For indoor use, the body (and suspension) is all you need.
First, the materials you need.
My later versions of the Bridge hammock first cut the width of the hammock fabric to 54" in width, and 82" in length. Using narrower width allowed a flatter lay for a given spreader bar width. With use I found that a hammock that is significantly longer than my body allowed me to move too far to one end or the other, and upset the distribution of weight that helps the hammock lay flat. With hems, an 80" hammock leaves me 5-6 inches of space when occupied, and so I end up always where I need to be to distribute the weight properly.
Cutting the Hammock Body
Next you cut the fabric. You can find a variety of approaches documented. TeeDee and schrochem just used the Aussie design, which provides a diagram of the fabric cuts. For this diagram, or any other specification, you can create a poster-board template; mark out the curve, cut out the poster board, align the flat edge of the poster board with the long edge of the fabric, mark the fabric, then cut.
An easy way to create the kind of curve used in the bridge hammock is to take a long flexible pole---like a tent pole---bend it, and mark the poster-board (or the fabric itself). For example, using the Aussie site's diagram you'd arrange for the ends of the pole to intersect the corners of the hammock when the center of the pole is pulled (e.g., by placement of weights, and pull the center out using an attached cord, as shown here.
Another way is to use math to compute the curve. I got interested in what makes the design yield a flat lay, and researched the physical principles involved in suspension bridges. I learned that for a suspension bridge, the curve is parabolic, and that essentially any parabolic curve will do. Below are links to two tables that describe specific curves, and one table that can be used to create any parabolic curve that may be desired. Each curve is described in terms of a set of two columns, where the first one gives the distance (in inches), left-to-right, along the edge of the fabric, and the second column gives the height (in inches) above that where the cut is to be made.
This curve is for a hammock 72" long, with the maximum cut depth of 12".
This curve is for a hammock 90" long, with the maximum cut depth of 12".
This curve is for a hammock 80" long with the maximum cut depth of 6". This has become my standard.
This curve lets you create a parabolic curve for any length, and any cut depth you like. Simply multiply each number in the first column with the desired length, and multiple each number in the second column with the maximum cut depth.
In this picture I've copied a parabolic curve onto a piece of fabric that is doubled over, so with one cut I get both sides, and they are cut identically.
The Aussie design is not precisely parabolic. The fabric at the head end bends up at a steeper angle than the foot end, to account for the larger amount of body mass above the waist than below. One way to accommodate this is to choose an off-center point for the curve to reach its highest value, then fit one parabola from that point to one end, and a different parabola from that point to the other end.
I experimented a lot with the shape of the cut, and decided in the end that it didn't matter. So I'm sticking with a symmetric parabola from here on.
Sewing the Body
Hem the short ends of the hammock body any way you like.
The seam attaching webbing to hammock body is going to be bearing your weight, so you really want to get it right. I followed TeeDee's suggestion to start by aligning the edge of the webbing with the edge of the cut edge, and lay down one stitch to hold the alignment. Now roll the webbing into the fabric twice, and sew in an upper stitch, and a lower stitch. Should like like this.
I'm cautious (and slow) when sewing. I first pinned the webbing to the edge, illustrated by dramatic re-enactment below.
Upon rolling twice, I snugged up the fabric around the webbing tightly, and pinned again
The (real) finished product looks like
This image has one more stitch in the middle, holding a zipper that is tucked away out of sight for this shot.
At each end of the webbing thread in a steel ring, then sew it in using bar-tacks.
I'm more likely to use an SMC descending ring these days, as it better supports my use of hiking poles as spreader bars. More on that later.
A bar-tack is a short tight zig-zig, run back and forth 3-4 times.
In both the bridge hammocks I've made, I threaded the ring and sewed the bar-tacks after the webbing has been fully sewn into the fabric. This has the rings extending beyond the hammock body by 1.5"-2" . TeeDee appears to have attached his rings before the final inches of fabric and webbing are joined, for his rings were essentially flush with the end of the hammock body.
I've come to bring the rings in as close as I can, also. This brings the spreader bar in closer to my shoulders, so I get more of the benefit of the spread.
The bridge hammock suspension system is made up of two spreader bars, four spectra lines (one to each corner of the hammock body), two load rated points where the two lines from a spreader bar come together, and a ridge-line.
The materials I have used include
In current versions I use hiking poles as spreader bars, of equal lengths. The list above becomes
I don't intend to carry these spreader bars onto the trail; what I describe is a backyard / car camping solution.
I drilled 1/4" holes directly through the PVC, into which the 1/4" bolts are pushed and nuts attached. Two bolts per pipe, separated at the distance I want for the hammock ends (for me, 36" at the head, and 30" at the foot). The corresponding hardwood dowel is inserted before the second bolt is attached. This proves some stiffening against the compression forces that otherwise try to bend the PVC. The rings attached to the hammock corners slide over the ends of the spreader bar, and are held in place against the bolts by compression force.
Ideally one would be able to use hiking poles as spreader bars. The forces on the spreader bars are pretty significant, and so some caution needs to be employed here. There's not a great body of experience to relate. TeeDee reported a way of rigging particular couplings at the hammock corner rings, and modified his hiking poles to fit them, so that the compression force was delivered directly down the long interior center of the pole. Not all poles can be modified the way he did.
Since I first published this article a number of solutions have been offered on HF for spreader bars.
I have found you can lash branches picked up on the trail to the hammock corner rings, such as
The details of the lashing look like this.
I found by experiment that an important component of the lashing is that the stick be lashed twice, with the connecting cord passing through the ring. You can also do this using a hiking pole, but want very much to do as much as possible to reduce the compression force on the pole. I'll say more about this shortly.
You tie a length of spectra to the ring at each corner. I use the full turn with two half hitches. The two lines associated with one end of the hammock are tied at a joining point, where also is tied the end of a line used to control the flatness. Descending rings are used in the image below
where the center line is for the flatness control line. TeeDee used an ordinary ring there, because his suspension to the tree was more spectra through a tree hugger, tied off with a cleat. schrochem has been talking about using the cinch buckle.
Whatever the device used to bring these lines together, you are forming a triangle. Note the angle called alpha. It plays an important role in thinking about compression.
Intuitively, the weight of the body in the hammock will be working to force the ends of the hammock together, this is the compression force. There are two parts to this force. One part comes from below, due directly to the occupant's weight. The other part comes from the hammock transmitting this weight force along the suspension lines to the ring buckles. The second part of the compression force depends on the length of the suspension lines. For a given spreader bar length, the shorter the line from corner to ring buckle, the larger is this component of the compression force. In fact, this component grows in proportion to weight*cos(alpha)/sin(alpha). Just to give you a feel for this,
Awareness of compression affects what length of suspension line we choose to go from corner to ring buckle. The longer it is, the larger alpha becomes, and the smaller the compression due to the suspension lines becomes. But the longer it becomes, the farther apart must be the trees we hang from, and the higher up. By using knots that can be loosened after use, you can adapt different hangs to local conditions. I suggest providing 4' of suspension for each corner-to-ring line, which is enough to give you a value of alpha=68 for a spreader bar that is 36" long. You should not tie the suspension lines so that the distance from the center of the spreader bar to ring buckle is less than 1/2 the spreader bar length, (e.g., alpha < 45 degrees) .
My how times have changed. I've moved towards using shorter suspension triangles than I recommend above, after seeing TeeDee's recommendation in his large article, and after (gingerly) experimenting with a shorter triangle myself. Since my hiking poles at 36" seem to be up to the compression that is on them, I'm now enjoying the benefits of a shorter triangle. These principally are that I can hang between trees that are closer together than before, and all of the suspension can fit under a tarp with a 10' tarpline. Right now, with a 36" spreade, the sides of the suspension triangle are 28". I computed these so that when hung, the distance between my suspension rings (i.e. length of the ridgeline) is exactly 10'.
Another change I've adopted is to connect the two cords from the sides of a suspension triangle to a device that I then connect to the suspension rings. Here's an example. The two suspension lines come together in a coin, with a loop that goes through the suspension ring. The orange line is the ridgeline. Imagine instead of that ring that the lines come together at a carabiner. Then the carabiner just clips onto the ring. This makes putting up the Bridge hammock really easy.
Just as the sag in a normal hammock is affected by the distance between the hammock ends when suspended, so too is the flatness of the bridge hammock. If the ends are too far apart, then the center of the hammock lays higher under tension then do the ends, and you have an inverted banana. If the ends are too close together, the center lays lower than the ends. You can set the flatness by tying a line between the ring buckles; you can adjust the length of that line by tying separate lines to each ring buckle, then tie both to an intermediate device such as a ring or micro-biner. You just affect the overall length of the line by changing the length of one of the lines between the tie at the ring buckle and the tie at the intermediate device. The length that gives a flat lay will depend on a variety of factors. You'll have to experiment once you have a working bridge hammock hung.
In my current set-up the ridgeline is set to 10', with no adjustment. That's short relative to prior ridgelines with bigger suspension triangles.
Since writing this article I've experimented with different underbodies, and for my next Bridge intend to make it a double body hammock, with the edges sewn together at the webbing. In that hammock the approach to the bugnet will also be different.
The materials used for the underbody and support system include
schrochem had the insight that a second (lighter) hammock body could be suspended beneath the bridge hammock and be used to support an under-quilt. Furthermore, with a little rope magic, that underbody could be raised and lowered from inside the hammock. In my implementation I used no-see-um netting for an underbody, because I wanted to be able to use it just as a means of bug protection on the bottom. I cut an 80" length from a 54" wide piece, then finished all four edges with bias tape. On the corners I put re-enforcement fabric, and D-rings. Lightweight cord is attached to the D-rings; the underbody is raised and lowered entirely at the corners.
I rolled the short edges to create a channel for shock cord, and so make it possible to have the underbody "cap" whatever it contains.
Being suspended entirely at the four corners, the underbody hangs in a straight line between its corners on a long side, while the hammock body bends in towards the center when someone gets into the hammock. I fixed this by sewing small O rings at the center of the long edges, stretched a piece of shock cord between and through the two O-rings, and attached a shock cord clip at each end.
The shock cord can lie between the underbody and main body to allow the underbody to hang loose, or be positioned outside of the underbody to pull it up close to the hammock body.
The picture at the top of this article illustrates the underbody supporting an under-quilt.
It is very handy to be able to raise and lower the underbody from inside the hammock. This ability is easy to engineer. Consider the figure below
A lightweight cord (black and yellow) is tied to a clip (like the ones used to keep a pair of gloves or mittens together). The clip is attached through the underbody D-ring. We have to make sure this line has clear passage to the interior of the hammock. In the figure the line passes through a micro-biner, and then rises up towards the ridge-line. It passes through another ring to bring it in-line with the support line holding up the bug-net :
You can see the underbody line passing from the micro-biner up to a ring tied to a black line (which is itself tied to the flatness setting ridge-line. (I'll say more about this secondary ridge-line shortly). Now there is one piece of cord, at the ends of which are those clips. What passes through the black-line ring and enters the hammock space is actually a loop, so there are no knots to constrain the movement of the cord between ring and micro-biner.
Coming into the hammock body, that loop is attached to a micro-biner with a lark's head knot. To the black line is attached a prusik knot onto which the micro-biner is clipped.
The end of an underbody is therefore adjusted up or down by sliding the prusik knot towards the center (to raise) or the end (to lower).
I thought it important to not depend on the main flatness control line for the underbody support lines and (more importantly) the ridge-line supporting the bug-net. The main ridge-line can be quite high, and varies with the length of the suspension lines attached to the hammock corners. With a second ridge-line hanging from the main one, I could design bug-net at a known height above the hammock body, and attach exactly the right length of cord for the underbody control. So the secondary line is attached to the main line using a Klemheist hitch. You can adjust the gap between the ridge-lines by adjusting the position of the hitch on the main ridge-line. The tension of the secondary ridge-line can be adjusted by using an intermediate ring, just as does the flatness control ridge-line, as shown above.
The ring through which the underbody lines pass is rigidly affixed with respect to the Klemheist hitch. The length of the underbody support cord is selected to have it be at full tension when the prusik knots are moved as closely as possible to the center, as shown above.
My next version will have a slimmed down version of the bug-net. Stay tuned!
The Hammock Forums contain a great deal of information about building bug-bivies that enclose a hammock. It should be straightforward to adapt some of these designs for a bridge hammock. The key difference has to do with the suspension cord passing from inside to outside of the net. In a bridge hammock you'll need to make provision for two suspension lines and a ridge-line to pass through the bug-bivy. Cuts, re-enforcement, and some velcro ought to be enough.
I tackled putting in a sewn-in but removable bug-net, and will describe that.
The materials used include
In this design the bug-net has three pieces. A main piece hangs from the secondary ridgeline, and is sewn to the hammock body on both sides using 2-way detachable zippers. Each end has a bug-net end cap.
After sizing and cutting the three pieces, I edged each with bias tape. On both non-zippered ends of the main piece I sewed two opposing strip of Free Magic velcro. The effect is to have an edge with velcro on both sides. I similarly edged the short end of the hammock body.
The end-caps are attached to the main bug-net piece and to the hammock body using Free Magic velcro.
I edged one side of each bug-net end-cap with Free Magic. The bug-net end-cap is then like a cork, fitted entirely in the interior of the opening formed by the end of the main bug-net piece, and the body of the hammock.
There is a ring of Free Magic on the outside of the end. This is used to afix a breeze-stopping cap, such as is illustrated in the figure at the beginning of the article. Like the bug-net endcap, the breeze-stopping endcap has Free Magic on one side that lets one cap it over the end onto the outer ring of velcro.
I confess that a prime motivation for using velcro is that it is very much more forgiving of sizing mistakes than would be a zipper or sewing the ends in. As we will see, there is a price in weight to be paid for this insurance.
The geometry of the main piece is interesting. Imagine a hammock with an occupant, and envision how the webbing moves in three dimensions from corner to corner. It goes down towards the center, because of the suspension cut. It also in towards the center, because the hammock body is narrower at the middle than at the ends where it is held apart by the spreader bars. We need to cut a piece whose edges follow this trajectory. There is a way to mathematically work out a good approximation. I did this to get a rough estimate of the shape first. Doing this I found that the widest part of the main piece is at the middle, and that the width needed to put the bug-net peak 12" above the spreader bar is less than 54"---the width of the bug-net material. This means the main piece can be cut as one piece of no-see-um.
Fortunately, you don't need to go the mathematical route for determining the main piece geometry.
I separated both zippers, and sewed the lower portion to the hammock body, along the webbing. Here I placed the zipper inside the hammock, so from outside only the zipper coil and sliders are visible. For fitting the bug-net I zipped in the zipper's upper piece. Getting into the hammock, you start pinning an edge the upper zipper portion on one side, at the middle. With the no-see-um piece draped over the support line positioned at the desired height above spreader bars, you now pin the no-see-um to the upper portion of the zipper on the other side, at the opposing position. You want the tension tight enough to define good lines up to the support line, but not so much tension as to tear the no-see-um from the pins. Now you work out from the middle, in both directions, pinning the no-see-um to the zipper to define the shape. Once done you carefully unzip the fitted piece, and cut, leaving margin for a bias tape edge. The zipper is longer than needed; before detaching the cut piece from the zipper, mark (using thread) on the bug-net and zipper the spots where they join at the beginning of the zipper, and where the bug-net ends.
After the main piece is completely edged with bias tape, align the edges with the zippers using the markers, and sew the zipper to the edge of the main piece. Remember that the zipper back is on the inside of the hammock. When you're done, the outside looks like this
and from inside the zipper looks like this
The geometry of an end-cap is simpler. It has an upper component that is a triangle. If the spreader bar separates the corners by S inches, and the height of the secondary ridgeline is H inches above the spreader bar, then the triangle has a base of S inches, and two sides, each of length square-root( H*H + S*S/4 ). The lower portion of the end-cap is a bit different. What I cut for my version was an ellipse of dimensions commiserate with the length of the fabric (laid flat) between end-points, and the width of the spreader bar. This works well enough when the secondary ridge-line is pitched a little lower than the design assumes, so there is slack in the fabric. The point is that an ellipse is fine when there is no-one in the hammock. But with an occupant, the edges of the end-cap are under tension due to weight, and follow a straight line from hammock corner to the occupant's body. This is seen in real life as
I should note here that the breeze-blocking endcap does not actually have velcro all the way around its edge. At the bottom there is just one piece at the center. This, in part, is why we see daylight at the corners.
The true geometry is modeled by the picture below :
We can measure the length of fabric between corner points (2*E + B) and we can estimate B empirically---it will on the order of 12" at the foot end, and on the order of 20" at the head end. In principle we could cut the end-cap material to these dimensions. I'm not so sure that's necessary though. The sewing is harder I think in making the corner, particularly if one were to sew the end-cap into the hammock body rather than use velcro. With an ellipse there is a nice continuous curve for the sewing. The geometry will adapt when there is an occupant. It won't be a perfect fit
Going still with the ellipse then, how do you do the right thing? This gets a little technical, but not greatly technical. Let's say that the length of the fabric between corners is L inches, and that the spreader bar separates those corners by S inches. x=S/2 is the length of an "axis" of the ellipse. An ellipse has two axis, the length of the other one is approximately
y = square-root( 2*L/3.1415 - S*S/4 )
Given the two axis lengths x and y, there is a simple way described here using just a yard stick and a pencil to mark off one quadrant of the desired ellipse. The definitions of x and y above correspond to the x and y axis used in that explanation.
So trace one quadrant of the ellipse on a bit of cardboard, then use the cardboard to trace out the lower half of an ellipse on some newspaper. Draw above that the triangle that defines the upper portion of the end-cap, and then on each edge add margin for the bias tape. This serves as your end-cap template. After cutting, mine (the bug-net one) ended up looking like this :
When we're done with all of the bug-netting, the hammock looks like this
Making the wind-stopper end-caps is entirely similar, we just add a bit more material uniformly around the edge to account for it going over the end.
After this first version I moved towards sewing in end-caps onto the hammock body. A good shape for this is a parabola, but cutting it to be exactly right is a bit more work. The problem is to find the parameters of a parabola such that the length of the curve is exactly the length of the fabric under the spreader bar. This turns out computationally to be more complex than the average HF reader wants to deal with. For this I offer different solutions.
There's a lot of stuff on the bridge hammock I built. Here's a run-down on the weight distribution:
There is a weight penalty here for flexibility.
There is a lot of activity going on still with the bridge hammock, with more forum members either jumping into the fray, or expressing interest in doing so. While there is a lot of material here, I hope to have shown that anyone who is really interested can at a minimum build the main body and suspension system without too much effort.
I will refine this article as material for that becomes available, and will add sections as we gain more experience with other design options. Please feel free to add suggestions, either through the forums or through the PM mechanism.
Finally, I want to express my particular thanks to TeeDee and schrochem for the stimulating ideas and discussions we've had about the bridge, and to others that have tracked that thread and chimed in with their own bits of wisdom and humor.
August 18, 2007
My next version is aimed at summer use, and will be lighter weight. Details to follow...
Since writing this article, almost a half dozen people have reported building a bridge hammock and have shared ideas about it. There's lots of experimenting still to do...this DIY bridge thing will be going for a while, I'm sure.
Grizz (February 3, 2008)
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Great instructions! Thanks!
I will be starting my own DIY bridge hammock this summer, and will be using your instructions. Thank you for taking the time to make them. As i am a big man, 275 pounds and 6'2, i will be making one more suited for my size (as i can't find any commercially).
what do you think about .625 aluminum t*nt pole from quest outfiters as spreder bars? to heavy?
Guide to Making a Bridge Hammock
Weight is a factor, but I have carried those poles backpacking for at least, lemme see, 150 miles over the last five years or so.
I've found now what may be the almost perfect hiking poles to do dual duty as spreaders and so now the tent poles are relegated to backyard and scouting outings
I was reading The Ultimate Hang and it mentioned bridge hammocks and I thought it would be perfect for my kids, just so I don't have to worry about them falling out or anything. I think this write up is just what the doctor ordered. I'll probably make one that's child sized so I can get them out hanging with me. Great write up!!
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