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Thread: Bridge Bag

  1. #1

    Lightbulb Bridge Bag

    Inspired by other members' DIY bridge hammocks and tents, I have had an idea for a very light-weight hammock suitable for Hong Kong winter or North American Fall weather.



    Rationale: To me a bridge hammock is essentially a replacement for a sleeping mat. Therefore, whereas a hammock is a suspended tent, we could have a suspended bivi-bag. There is the added advantage that I could pack a bed-roll and bivi on the ground if I turned up at an untried camp site and found it devoid of trees. The best part is that the bottom of the sleeping bag is uncompressed and you get the full loft effect.

    Design: My idea above is intended to make use of a minimum of materials and be easy and quick to construct. Essentially, you take a standard sleeping bag (plus bivi if you want) and using a sewing machine, stitch in a series of buttonholes along the side on the bottom side near the zip. If you have a bivi as well, the holes should approximately match up.

    Next, cut and hem a sheet of breathable rip-stop material to the approximate shape of the item below. The caternary cut outs between the attachment points are not fundamentally necessary. This sheet should be tapered to fit the inside of the sleeping bag and the attachment points should match up nicely.

    Next, make an attachment sleeve cap for your hiking poles. This should fit over the ends of the pole and there should be a tab so they can be tied one to the other like mittens and hold on the ends of the poles.

    Next, either attach webbing straps or cord from the poles to the attachment points of the support sheet and caps through the buttonholes, or instead use a sewn and hemmed sheet panel cut in a catenary curve to replace the straps. The attachment should be removable ideally to facilitate cleaning.

    The Result: Once the assembled system is hung, you can get in onto the support sheet and zip yourself in or just lay the top cover over depending on the weather. You can also use a sleeping bag liner, which would go around you like normal and you would lay in it over the suspension sheet. The suspension sheet and straps would weigh less than, and pack smaller than ,a sleeping mat.

    Let me know your thoughts. All I need to do now is get hold of a sleeping bag to test it out on.

  2. #2
    GrizzlyAdams's Avatar
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    thoughts

    Three thoughts-
    a) that's an interesting idea. I'm looking forward to some pictures and related experiences

    b) anticipated related experience is challenge getting in and out.

    c) I did a back of the envelope calculation of the amount of webbing needed, assuming
    - 6 ft long platform
    - webbing lines all start 18" above the attachment plane
    - 7 evenly spaced webbing lines

    comes to 42 ft of webbing. The 1" polyester camo stuff from OWF weighs 0.3 oz / ft; the tubular 1/2" stuff I use on my hammock body is about the same.
    So using this kind of webbing brings the weight to 12.6 oz, which is about an ounce and a half more than the weight of my DIY bridge's body (including the webbing). That's before you add in whatever hardware is wanted for the attachment points.

    Now 2.8mm Spyderline weighs 0.07 oz / ft, for a total of 3 oz. , which would seem to be the way to go to make this a win weight-wise. I suppose you'd use loops of webbing on the extensions of the "body" where you have connections. Say 4" each, there are 14 of them, add an ounce and a half, and then the weight of the body. Est. 2 sq. yrds of 1.9 oz fabric. Throw in 4 SMC rings for the suspension corners.

    rings 4x 0.4 oz = 1.6 oz
    body 2 x 1.9 oz = 3.8 oz
    tabs 1.5 oz
    line 3.0 oz
    -------
    9.9 oz

    I suppose thread weighs something.

    By comparison my (larger) DIY bridge body + webbing + rings comes in at (est) 13.5 oz.


    Grizz
    Last edited by GrizzlyAdams; 10-12-2007 at 12:55. Reason: added an ounce for endcaps neglected the first time.

  3. #3
    Senior Member NCPatrick's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post
    I suppose thread weighs something.
    Muuuuch better.


    "Civilization is the limitless multiplication of unnecessary necessities."
    - Mark Twain
    I go to nature to be soothed and healed, and to have my senses put in order.
    - John Burroughs

  4. #4
    Thanks for the benefit of your experience. I'm new to this.

    I suppose I had better rethink the webbing. I could use fabric panels instead of webbing. I could even change to a suspension bridge structure instead of the current cable stay structure. This would make it easier to enter.

    I think 3mm spectra is over engineered. I may try 3mm for the main cables and less for the suspension verticals. I could use 2mm but if out-and-out weight saving is required, I can use 20lb test line, which is about as thick as two hairs. Very thin lines could be drawn straight through the sleeping bag with a needle.

  5. #5
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    I agree that you don't need super strong lines, but I'd be tempted to go with something stronger than 20 lb test. I have 60 lb spectra fishing line that is .016" diameter and 200 lb that is .030" diameter. (You can get Power pro cheap off Ebay.)

    I'm willing to bet you could "sew" both right through a sleeping bag with the right size needle. Weight??? really low... Strength???? pretty good.

  6. #6
    with that many support lines, (if they are all properly equalized), the weight is split between 32 different strands of line, you could get away with some pretty light line, although, when entering/exiting, weight may be alot more localized. i would go with the 200# or more.

    one problem i see is the sides are completly blocked off by the lines, would you have to crawl in through the end of the sleeping bag?

  7. #7
    I made a little progress today. It took so long because I had to go buy most of the parts. The design has evolved a bit in the process. Partly, I followed your advice and partly, I cut some corners in the production process.

    Sheet:
    I traced out the footprint of a sleeping bag in some $1 per yard ripstop nylon and tacked 3/4" of the edge down incorporating a line of 3/16" polyester webbing using iron-down interfacing. I did not incorporate a catenary profile between attachment points as it's probably over-engineered as it is. Then I stitched through the webbing with good quality polyester thread in a tight straight lock-stitch. This has left me with a sheet that fits inside my sleeping bag with a reinforced edge.

    Vertical suspension lines:
    I plan to make the suspension lines soon. I only have 20lb test braided spectra fishing line at home, so I'll need to get some 50lb+ line tomorrow. I will tie buttons to the end of the fishing line. The suspension lines will be drawn through the polyester webbing with a needle and the buttons will act as a line stopper so they don't pull through. I may stitch them down, or I may not.

    Tension cables:
    My sleeping bag is 6' long, so it requires 7 tie-off points on the perimeter. Today I took two lengths of 3mm spectra line and tied 7 linesman's loops in each, with one loop every foot from the centre. These will be the attachment points for the suspension lines.

    Spreader bars:
    I will use bamboo staves. I can use hiking poles, but I only carry one and bamboo is light and easy to find in most wild places in Hong Kong. I could tie a hitch in my lines, but I'll probably manufacture a light end-cap from 1" tubular webbing.

    Assembly:
    First, I will set up the tension cables and spreader bars in my workshop suspended from attachment points on the wall. Then I will tie on the suspension lines. The centre suspension line will be very short, so I will be able to sit in the hammock in the middle with my feet dangling off the side. I don't know at this stage how long the others will be. The other suspension lines will be tailored in pairs so that I can lie flat and in comfort.

    Weight estimate:
    I estimate the total weight excluding spreader bars and sleeping bag to be around 6oz. This comprises almost 4oz for the already completed sheet, plus the weight of the two lines with some excess (because I'd rather not cut my line) is about an ounce. The weight of the tubular webbing, shirt buttons and fishing line will be negligible, but we'll call it an ounce for now.

    I'll be leaving for Thailand on Tuesday, so won't be able to make much progress for a week, but I'll keep you posted of developments.

  8. #8
    I forgot to write that I have checked the sheet fits in the sleeping bag. There is a 1" gap to the zip/seam as expected. This means that I will stitch the suspension lines through the bottom panel and when I climb in, I will be able to zip up.

    When this is done, I have some 30D ripstop polyester that I could make into a bivi-bag. This is like the stuff Go-Lite uses for their ultralight rain jackets and should keep light rain off. As I don't need it to be good enough to use as a ground sheet, it ought to do as a bivi-bag for the Bridge Bag. I am still not 100% decided if I want a tight bivi-bag or a tent or a sock.

  9. #9

    Prototype 1st and second hang

    I assembled the Bridge Bag this evening.

    The sleeping bag hangs fairly flat when not loaded.

    Empty


    However, when I'm in there is just a bit of banana-ing. Not so you'd notice when you're in it. In fact, it's a bit more comfortable like this so I didn't adjust the vertical lines to compensate.

    Fully Loaded


    This is what it looks like from inside with the cover over. I can zip it up or unzip it while lying there. I can sling the cover over the side (it doesn't touch the floor) and I can lie on the closed hammock. I can also untie it and assemble it without the sleeping bag.

    View from Inside


    The liner takes all the load and there is a little space below so the loft is not compressed. It's warm all over and under.

    Top Open


    A quirk of using the suspension lines instead of a sheet of fabric is that I can get in by stepping through the lines astraddle the bag and sit down into the hammock.

    Getting In


    In a previous picture, you can just about see that I had temporarily drilled holes in my spreader bar. I plan to source this at site and wouldn't want to have to drill holes in the field, so I made these end caps from tubular webbing.

    Spreader Bar


    Below is a closer look at the end cap. You can see how I stitched over the end providing a tunnel for the line to go through and how I locked the end cap in place with a figure-8 stopper.

    End Cap

  10. #10
    That was the second hang. The first hand was a lesson in humility - I was using 120lb test dyneema braid and stupidly hung the thing at chest height. I actually had to climb in using a chair. Anyway, the line broke like it was made of cobwebs or something. Suddenly I was hanging upside down at the knees from the remaining line. I had figured that 170lbs divided by 14 was way less than 120lbs, but the reality was that all 170lbs was focussed on one line tightly knotted and therefore much weaker... then the next and next and all that happened in under a second.

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