May 20, 2010 at 10:24 pm #1259212
As some of you might know. I built and tested several cuben fiber hammocks over the last few months. I love how cuben fiber hammocks lay (flat) and felt like it was the perfect hammock material. I had even planned to offer them for sale in the future.
I however have decided to abandon this idea after a hammock failed while being used by a beta tester last weekend. The tester was moving around in the hammock after spending a night in it when it failed. Luckily he was only 12" off soft ground and was un-injured. The hammock was built using CT2K.08 which is a .75oz a yard material and has a break strength of 105lbs/inch.
The beta tester was testing the difference between the channel end and the gathered end. The failure took place 18" down from the gathered end which signifies this was a material issue and not a manufacturing issue. At first I thought it might have to do with how the end was "gathered" but after seeing pictures of the failure I realized that a failure like this could occur with a channel end as well.
I sent the hammock to a Materials Engineer and a Materials Scientist and they concluded that the CT2K.08 might not be an appropriate material since prolonged overloading could cause a failure. They suggested that I use a heavier/stronger material but due to the nature of the material this would not guarantee anything.
We are all fascinated by this awesome material but I think it has limitations and one of those limitations is being used as a hammock.
I am not going to discredit the use of the heavier material at this time because I cannot say whether a failure would or could occur. I just want to offer this post as a disclaimer. If you decide to build a cuben fiber hammock please use the hammock with care.
Here are some photos of the failure.May 20, 2010 at 10:33 pm #1612039
@joshuaLocale: Santa Cruz,Ca
That wasn't an average tear.How about breathe ability?May 20, 2010 at 10:38 pm #1612040
Breathabilty was never an issue and I personally believe the no stretch nature of the hammock keeps airflow circulating much better then a "breathable" nylon hammock that wraps around you. I am never hot in my cuben hammock while I am always hot and feel like a hotdog in a bun while sleeping in a "breathable" nylon hammock.May 20, 2010 at 10:40 pm #1612041
oh lots of cuben for new stuff bags :)May 21, 2010 at 12:27 am #1612062
Thanks for sharing this. This is good info that benefits the community. I wonder if 'high bias' cuben with additional spectra threads at +/- 45 degrees (in addition to 0 and 90 degrees) would be a more efficient use of weight than going to a heavier version of regular cuben. Ie. would 1.5oz/yd regular cuben (CT5K.08) be stronger than a 1.5oz/yd high bias cuben (CT2HBK.18). Cubic Tech spec's indicate the regular cuben would win, but in the real world the high bias stuff might hold up better to twisting or uneven loads etc…you never know.May 21, 2010 at 12:37 am #1612064
@antigLocale: Pacific Northwest
You left out an important bit of information…how much did the beta tester weigh?May 21, 2010 at 1:22 am #1612072
Exactly. If it was a pretty heavy guy he could have shock loaded it by bouncing. A bounce is like a fall and the body weight of the person could easily have tripled. Have you done any load testing like this? Sounds like it would be a good idea to set your mind at ease. You need to try and duplicate what happened. Are you sure 105 lbs/inch is the correct # for that weight fabric? If the end cords are static and don't stretch, it might be easy to pop such a light fabric since there is nowhere for the overload to go to be absorbed. Heavier fabric would certainly be better since there would be more dyneema. .75 is pretty light and obviously all the weight is not the dyneema. I have seen dyneema weekened by too much heat in lamination processes but I don't know how the fabric is made. In the Cuben fiber I have there appear to be weld zones that cross the fabric and if these are caused by heat then too much heat can cripple the dyneema since it has a lower melting point than nylon.May 21, 2010 at 6:16 am #1612110
@kieranLocale: Seattle, WA
I noticed that some manufacturers will double the silnylon material if you're over 200 pounds or so. perhaps that could be done here?May 21, 2010 at 8:18 am #1612143
>>"You left out an important bit of information…how much did the beta tester weigh?"
Before I divulge anyones personal information I would need his approval. In my opinion he was within the accepted weight range. I feel that a hammock should be able to have a 300-400lb weight rating since shock loads are hard to calculate. It's the same reason climbing ropes are rated so high. It seems as weight is less of a concern then the size of your "footprint" and how you distribute the load of your body. Technically a 200lb back sleeper would put a less concentrated load on the individual fibers then a 150lb side sleeper.
>>"Are you sure 105 lbs/inch is the correct # for that weight fabric?"
Those are the specs given by Cuben Tech.
Heavier cuben could work but why would anyone buy or build a hammock out of a material that is more expensive yet weighs the same amount as a polyester or nylon one?May 21, 2010 at 2:05 pm #1612269
@geistLocale: Smoky Mountains
> I just want to offer this post as a disclaimer. If you decide to build a cuben fiber hammock please use the hammock with care. Here are some photos of the failure.
Thanks for the photos and story of the failure. Looking at the photos of the triangular piece of Cuben tied to the tree, it appears to have a stripe running about 7 inches above the tear where the fibers appear to have a much lower density.
I can see additional stripes running straight across the Cuben in that same triangular piece. (I know it is not really triangular but rather bunched together by the rope).
I am guessing the point of tear was right across such a "stripe" (of weakness) in your Cuben.
Can you confirm? Could be useful info for others building Cuben MYOG to be sure to look for such regions in the material and not orient them across high stress areas.May 21, 2010 at 2:08 pm #1612270
@antigLocale: Pacific Northwest
From what I could understand, this might still be a viable option for people in the sub 200# range…May 21, 2010 at 6:36 pm #1612352
Have you considered the different weights and, apparently, grades of cuben?
I saw a list of cuben offered, and there was mention of more strength on the diagonal of one version. If true, this might be helpful.
It appears, to me, the lighter color bands were not flaws but actually show the stretch and failure.
I also wonder if cuben wouldn't do better in s bridge hammock.May 21, 2010 at 9:08 pm #1612417
The stripe is actually a concentrated area of Dyneema. Those stripes run on the longitude and latitude forming 14" x 14" squares. Those people that have had to cut cuben fiber can attest that the hardest areas to get through are the stripes. Also as noted the failure occurred down from the stripe signifying the stripe is not a weak spot.May 23, 2010 at 7:58 pm #1612975
@obxcolaLocale: Outer Banks of North Carolina
Just got back from my trip out west and saw this thread. Looks like the hammock was suspended with dyneema cord…. 1/8 or 7/64ths amsteel?
If that was what was also used around the tree (and it appears from what you can see) then there was no "stretchy" material used in the entire set-up.
Static shock seems like a genuine consideration. Maybe one of the fabric scientist types that seem to frequent the site can chime in.
Hope you're doing well and sorry for this set-back. Hope the tarps are coming along as you'd hoped.
ColaMay 23, 2010 at 8:33 pm #1612995
@orangebananasLocale: San Francisco East Bay
I was super excited about this too.May 24, 2010 at 8:13 am #1613092
I'm the gentleman Lawson is trying to protect…I was in the hammock when it failed and unceremoniously dumped me to the ground.
I'm a big guy – 6 feet tall and 260 pounds. I had slept in the hammock all night and was pretty comfortable but I think I would have preferred a little more width because my shoulders are broad. I was in it about 7 hours before it failed, including getting up twice during the night. I wasn't bouncing in it when it died – I was simply rolling onto my left side for a better view of the wildlife on the lake right next to me and whoop – I was down.
The hammock suspension was indeed 7/64 Amsteel – Whoopie Slings. As Lawson mentioned, one side of the hammock had a channel and the other side was gathered. The failure point couldn't have had anything to do with the method of attaching the suspension. When I removed the slings to ship the material back to Cubic Tech I noticed some additional stress on the gathered end of the hammock…
I apologize for the lousy quality of this photo – I only had my iPhone with me, but you can see that there were other issues with the material and my weight as well.
What else can I say? I really wanted this to work, but was very glad that I had nice soft grass under me when it failed. Usually I have rocks & roots under me in the rest of Pennsylvania.
Mountainfitter was great to work with on this project – I just wish that it had turned out better in the end, but I'm very comfortable that it wasn't anything in the workmanship. I believe it was just the wrong material for the application.May 24, 2010 at 8:32 am #1613108
@jamespatsalides-comLocale: New England
These hammocks are beautiful. Hate to see such a lightweight option taken off the table for the appropriate tree hanger…
FYI, my ENO hammock has (I think) a 250lb rating on the hammock itself and a 225lb rating on the "micro slap straps" (they make a heavier slap strap and hammock combo). I suspect it can handle a little more weight safely (I'm 200lbs and I have napped with my 40lb daughter in the hammock, putting us pretty close to the 250lb rating).
Maybe the trick is to figure out the breaking point and rate the hammock accordingly – specify a (make it up) 225lb weight limit? This way for smaller/lighter folks, this kind of material option would be available, but for heavier folks, we'd need a tougher option… at least everyone would be forewarned about the material and can make informed judgements – just a thought.
Cheers, James.May 24, 2010 at 10:52 am #1613167
@geistLocale: Smoky Mountains
> The stripe is actually a concentrated area of Dyneema. Those stripes run on the longitude and latitude forming 14" x 14" squares. Those people that have had to cut cuben fiber can attest that the hardest areas to get through are the stripes
The 14"x14" squares are what puzzle me about this Cuben failure. The squares are supposed to form a sort-of rip-stop, but in this case the rip went straight across the fabric right through all these concentrated Dyneema stripes without ever turning or stopping.
It also sounds like the rip had a very high propagation speed. It had to get all the way across the fabric before the pilot (Kevin) started flying to the ground.
This raises a concern about failure modes of Cuben. You have run a lot of bonding tests on it. Does it creep and fail or does it "pop" suddenly?May 24, 2010 at 11:35 am #1613188
In the sailing industry its called bursting. Its a combination of "creeping" and "poping" and is known to happen with all types of material.May 24, 2010 at 6:07 pm #1613343
@tallblokeLocale: DON'T LOOK DOWN!!
I'd like to see a doubled over version which makes a more generous tarp.Dec 27, 2010 at 9:12 am #1677913
Maybe i shouldn't bring up an old thread, but with this problem i would add longitudinal strips [could be taped in place] ,wouldn't add much weight but lots of strength.
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