Line failure from excessive load? Calculation of load.

edited September 2009 in Safety
While my 200lb line is out of action (new hoop winder to be ordered), I am thinking of using my lower rated 80lb line that came with my FF8 for use with my FF16.

I have done a search for cases of line failure but the results I have found would suggest that any failures are as a result of some outside influence, e.g. line cut or severed due to contact with some kind of object.

So has anyone experienced a pure and simple line failure when the line has been in seemingly good condition?

As the topic suggests, I'm also angling towards failures through excessive line load. Last week I anchored my FF16 to my bicycle while setting up because the ground was too hard for my dog stake. The wind was sufficient for me to hang my SLR, so between a Bft 4 and 5, and my bike didn't move an inch (I guess it weighs around 28lbs).

Has anyone got a way of calculating the line load? A simpleton's theory using the above would suggest that the load on my line last week was less than the weight of my bike and my SLR rig - maybe 30lbs. Of course, I'd assume that you would need to add in some kind of ground friction on the bike. If this theory held up then I would start to wonder why we use 200lb line.

Ultimately, I am trying to gauge the margin of safety I would have in identical conditions but with the lower rated line.

There is an opportunity here for the geeks to unleash all kinds of science and maths on me, so get stuck in!!


  • I have had several experiences of line failure. Thankfully none involving rigs and cameras.

    First one was some years ago when I bought some line from a local kite shop that should have been strong enough for my needs but failed sending the kite gliding to the ground. Thankfully not into trees or someone's garden.

    I had access to University test faculties at the time and to put my mind at rest tested the breaking strength of the line. It was about 20 % of the stated strength. The line was from a well known kite manufacturer, although I can't remember exactly which, but it was brand new and failed well away from any knots. The shop exchanged it but the next one was just as poor. I can only think that a batch of line had been mislabelled and I was unfortunate to get two reels form the same batch. Anyway shortly afterwards I came across good old Dacron Blackline on sale at a kite festival and invested in some heavy duty line.

    My second incident involved that same Dacron Blackline which I probably used for much longer than I should have. Hence the next incident was entirely my fault. I was flying my FF16 in what were probably not ideal conditions. It was already a very windy day when a big gust snatched the kite, shock loading the line that I was running through a dog stake. The line parted at a knot that I had noticed but had not removed about 3m from the end of the line. The kite floated away and ended up caught in a tree about 250m away. This is where it stayed for nearly a month until I could mount a rescue mission, taunting me every morning when I walked my dog on the field. When I got it down the kite was fine which is a testament to how well built they are.

    Naturally I replaced the line and now do so at the first signs of wear and tear. I also make sure that I don't get unwanted knots in the line.

    Its through these little incidents that we learn.

    I now know not to underestimate the difference between normal loading and shock loading.
  • As Martin says, unnoticed knots (often the result of careless looping of the line when attaching or detaching the line from the kite) in the line are weak points, as are the small abrasions resulting from the line being trodden on when lying on stony ground such as a beach or getting caught in thorny undergrowth in fields. I rarely use my 110lb line for my FF16 - the wind where I live can pick up very suddenly - preferring the safety of 250lb line. A sudden gust can result in strong shock impulse that will pull out a ground stake (so I don't use mine any more either).

    If I remember, the wind on Guernsey can be almost as fickle as it is in South Devon, so I'd be careful using 80lb line with an FF16.
  • edited September 2009
    Kev, absolutely don't try to fly your 16 with that cheap 80 lb line.

    Make yourself a temporary winder for the 200# line: a simple wooden bobbin, made out of a short bit of timber or better yet plywood, say 1/2" to 3/4" thick by 6" wide by 12 to 14" long. Cut a big notch in each end. Simple as that. If you pull the line off an armspan at a time, it's pretty easy to launch from, but don't try to wind back on the bobbin directly coming down -- haul the line hand-over-hand and wind it on the bobbin after the kite is down. If you want to keep it twist-free, figure-8 it on one side of the bobbin, like this:

  • Seems like a unaminous 'no' to a FF16 + 80lb line!!

    I'll have a look around home and see what I can use for a substitute winder.

    The load calculation question is still open though.........
  • Sort of in this same line.... I always here about what a safe wind speed is to fly kites, but I was wondering if there is a safe line loading by pounds that you should be flying a kite By this I mean you have a kite up 500' you can't really tell what the windspeed is at that height. I have measured my line under load with a fishing scale and it was pulling at about 12 lbs, (felt like MUCH more) what I would like to know is, how many pounds of pull is safe? (not counting gusts) I have a 10' Delta Conyne with 150 lbs dacron line.
  • edited September 2009
    To test my concepts with you: isn't it pretty kite independent? You will fly a small kite in heavy winds, and a big kite in light winds, both not with a line pull harder then you can handle. So a 80lb line wouldn't fit for any kite used for KAP: a person can pull harder then 80lbs, so that is on the light side. Certainly if it is cheap line, and not for certain 80lbs. 150lbs would be more on the heavy (safe) side, e.g. able to pull a person up (okay, depending on person!!), and more like able to absorb shocks. Shocks happen with small kites in heavy winds, and big kites in light winds.

  • This came up in a thread a few years ago. I'd have to search on it to find it, but a sort of consensus was to do a 1/7 under-rating for the line strength. I use closer to 1/10 under-rating. That is to say if I'm flying with #200 line, I don't want more than #20 of line pull. Since #20 of pull is seriously iron-bar flying, I typically don't go above #10 of pull on the line, if I have any choice in the matter.

    All of which leaves you plenty of room for the unexpected, like sudden gusts, weak points in the line, and other things that really do happen almost every time we go out.

    Hey, I've got a fun safety margin story!

    We've got a new piece of outdoor equipment being installed at work. The design was more or less ramrodded past the design review process on the assurance, "Hey, we hired a professional for the job. They know what they're doing. Stop second-guessing them." The design came back, the parts were fabricated, and everything arrived. And it was FLIMSY looking. So a bunch of us looked at it and decided we had to see the calculations for wind loading, ice loading, etc. Turns out there weren't any. So the designer did some back of the envelope calculations and arrived at the conclusion that we had a 2:1 safety margin. Which sounded great until you looked at his initial assumptions. He missed the max wind speed by a factor of four, the weight of the equipment being bolted to his hardware by a factor of two, the cross section of the equipment (despite being supplied with drawings) by a factor of two, and NO calculations for ice loading. The upshot is the thing is under-designed by a factor of between 10:1 to 20:1.

    All of which goes to show that having a seriously generous safety factor to take care of the things you DIDN'T expect is a good way to design things. Like kite line.

  • In line with Tom, one missing part in the calculation: if the dSLR is 1 kg, the pull of the line has to be more than 1 kg because of the angle of the line: only the vertical component of the line pull (from the rig to the kite) is lifting the dSLR upwards. Same for the bike with even lower angle. So, it the line pull could have been more then 30lbs. All that is without shocks taken into account.
    So some adjustion to my previous post: 150lbs can pull certain persons in the air, but not when KAP-ing (or it should be a kite straight above your head, pulling with 150lb ;). But, I guess 150lbs will give someone a hard time.
  • Well, first of all that rated strength is just average strength that of type of line (i.e. average static load at which that type of line breaks), particular pieces of line will differ, and variance could be quite high. Just remember that this is not some certified equipment, so manufacturers can be (and often are) optimistic in their rating. Then remember that simple overhand knot will reduce line strength by half, fortunately this is pretty worst case scenario, other typical knots being in the 60-70% range (from popular knots only blood knot and double or triple fisherman's are in the 90% ballpark). So anything above 1/3 of stated test strength is asking for kite flying freely away immediately. As Tom wrote safety factor 7:1 or even 10:1 (especially if kite flying away could pose danger to 3rd parties -- for example power lines which while being well* beyond kite on the line range could be contacted if kite broke loose) is the way to go.


    * anyways one should be well away from high tension power lines, but kite which got broken loose could fly significant distance - esp. low wing kites and deltas. And even not conducting line hanging from high tension (>25kV) line is extremely dangerous. Some years ago there was fatal accident in my country when some kids playing under 220kV line started throwing sticks at it. This caused an electric arc to come across 6-8m(sic!) distance killing 4 of them instantly. Lower tension (i.e. mid tension 1-20kV and low tension <1kV) lines are less dangerous but unless you're a specialist you might not distinguish them (I know some 110kV lines which are hard to distinguish from 15kV line for a layman), so do not even attempt.
  • Hey, I found that knot book with the estimated breaking strength for a bunch of knots!

    "The Knot Book" by Geoffrey Budworth

    Half blood knot is rated at 80% line strength.
    Bowline is 60%

    Figure of eight loop is listed, which is how I've had my line tied to my clips, but no breaking strength is listed with the knot. (DARN!) In any case I re-tied mine using half blood knots. Thanks, Brooks!

    The alpine butterfly is also listed, but as with the figure of eight loop, no breaking strength is listed with that knot.

    Apparently this book is out of print, but Geoffrey Budworth has written a new book on knots, "The Complete Knot Book".

  • Gee, Tom -- I didn't know you were looking for that book. It's right here on my desk.
  • Safety in kite flying means that the pull of the kite is known. To measure the pull, use a scale such as for weighting fish.
    Once the maximum pull of the kite is known, the line strength can be set:
    - just flying a kite in normal wind conditions, apply a factor x3
    - flying a kite in gusty winds or flying socks on the line in normal winds, apply a factor x6
    - kapping, apply a factor x10
    When in doubt, better measure again the maximum pull before the rig is on the line.
  • Well, it should've been sitting on mine, too. It was on the bookshelf not with my other knot books, but with my sailing books. Go figure!

  • I had my 10' Carbon sparred Delta Conyne out Sunday Kapping on 150lb Dacron. The wind was picking up but I really believe I was still under or just at the 20mph top end of this kite. Unfortuneatly the Carbon Spreader bar snapped (it wasnt recovered so I dont know if it was the ferrule). I was able to tretrieve the kite and camera but no spar. My question is this, I realiezed the wind had picked up and was bringing the kite in, but I really didnt think I was that far over the 20mph limit of the kite. We always talked about wind speed but for Line loading, but for a newbie lke me that is pretty tough to accurrately determine, especially at hight. I would like to use my very accurrate fish scale to determine line load and from that come up with a max line load that I should ever fly with on that kite to not avoid line failure but to avoid a frame failure. Now this would entail coming up with an engineering figure of what load on a particuarly kite will overstress the frame to a point of failure. Has anybody every looked at that for the various common kites used for KAP i.e. Delta Conyne ROKs and various flowforms? I would think that this would be much more helpfull than wind speed, though it would have to be particuarly to a specific sail pressure. I really want to figure this out because it has me nervious, I have been out in very close to the same conditions several times and know I am a little nervoius hanging my rig out there again. I am looking into gettting a FF8 as the next kite in my arsenal for windy days, but funds arent available right now so i'm stuck as a 1 kite act.
  • Hello,

    Hmm, it's not that easy, as different kites get overpowered different ways, and the effect of overpowering isn't always some spar snapping. Well, some kites do not even have spars :) And even then things could vary from kite to kite -- as for example some asymmetries due to manufacturing imperfections play a role (sometimes significant). Some kites get bent too much effecting change in geometry causing instability (many Roks behave that way), some get unstable due to aforementioned asymmetries, then some simply "develop a broken spar". Then such things like birdle settings and tuning, tails, etc will affect high wind capability to large extent. Now add turbulent nature of wind itself, and all local conditions differences... So I'm affraid the only viable option (I don't expect complex structural analysis, wind tunnel testing etc, are used for normal kites) is trial and error and building more and more experience... But that's fun :)

  • I can see that, but I would be looking for just some specifics for a small group of popular kites, I would think that the manufacturers of FlowForms for instance would be able to come up with a safe loading factor for each size i.e. 8, 16 30 etc.. The fudge factor could handle the differences between individual kites of the same design/size. I wouldn't expect that to vary that much. Bridle changes would take care of themselves, if you modify the bridle for high wide, dont you change the angel so that there is less pressure against the kite? That would mean the pull would be lower.
    I would even be happy to see some of the more experienced Kiters provice some feedback, i.e. "How many pounds of pull on a FlowForm 16 are you are comfertable with before you expect a material failure of the kite? Maybe that way some of the experience from others can be used in the begining for us Newbies until we get better at estimating the wind at altitude or learn what a reasonable pull on a line was. The first day I went out with some moderate wind I couldnt believe how hard my kite was pulling, I would have estimated it at about 20-25 pounds, I was astonished when it was only about 10-12 lbs when I put the actual scale on it. The thin line really compounded the perceived pull.
  • I don't think you'll find many manufacturers who have this kind of data for their kites. And if there were such data, I'm not sure how much good it would do for the newbie. There really aren't any shortcuts for the beginner -- you gotta fly a lot and then fly some more.

    Next time you fly and feel the tug, you'll know it isn't pulling as hard as you expected it was the first time -- but what that means to you is a purely personal reaction. David Hunt tried years ago to get performance data for a variety of kites, and didn't get much help from the KAP community. Most people just fly em and don't try to quantify em.
  • edited November 2009
    Check Chris Becot site, there are both kite plans as well as their wind range, and for many there is also min/max line pull, and similar data.

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