dog Posted August 17, 2009 Share Posted August 17, 2009 Say I'm building an anchor and want to shorten a sewn runner. One way to do this would be to tie an overhand knot in the runner and clip into the shortend sling above the knot so that a pull would pull on the knot. I am concerned that with skinny spectra/dyneema/dynex materials the knot could pull through or the strength of the material could be significantly reduced at the knot. Is this considered safe practice or is there a better (stronger/ faster) way to shorten a runner? thanks Quote Link to comment Share on other sites More sharing options...
Rad Posted August 17, 2009 Share Posted August 17, 2009 IMO, knot in dyneema doesn't sound good. Options include: double it. triple it. set up a sliding x between pieces. if suitable (e.g. multipitch) use clove hitches or figure eight in rope to allow rope to help equalize pieces leading in to a power point. Equalizing is most important for less than perfect placements. Bomber pieces need not always be equalized. Quote Link to comment Share on other sites More sharing options...
goatboy Posted August 17, 2009 Share Posted August 17, 2009 Is this a top-rope anchor (relatively low, predictable forces in a known direction of pull) or a multi-pitch anchor (potentially high forces (could be factor 2), less predictable direction of pull (could be up or down, or lateral)? In general, tying a knot in a sling (especially one as relatively weak as dyneema) weakens it appreciably (up to 50%). So, consider the first question (TR or Multi-pitch) and the corresponding implications for force when deciding whether weakening your attachment point is warranted. The options Rad mentions for shortening the sling in a more functional way are usually better choices than knotting the sling. Another option I've seen is TWISTING the sling (i.e. turning it without knotting it) to shorten it, but I have not seen definitive tests about the effect this has on its overall strength (though intuitively it's a stronger method than knotting the sling). Quote Link to comment Share on other sites More sharing options...
froodish Posted August 17, 2009 Share Posted August 17, 2009 New Dyneema sewn runner: 22KN. Take half that strength for a knot: 11KN. 0.5 Camalot in a perfect placement: 12KN. Perfect placement for a #1 Tricam: 12KN. Sure there's better ways to shorten a sling (doubling, etc.), and Dyneema slings do appear to lose significant strength with use, but if it was a newish sling, I wouldn't lose any sleep over a knot. Quote Link to comment Share on other sites More sharing options...
rocketparrotlet Posted August 18, 2009 Share Posted August 18, 2009 An overhand knot would cause a sling to lose 30% at most. Even after this loss, it's still stronger than a bomber placement. You could also just double it. -Mark Quote Link to comment Share on other sites More sharing options...
snoboy Posted August 18, 2009 Share Posted August 18, 2009 An overhand knot would cause a sling to lose 30% at most. Are you sure about that? My understanding is that rope tends to lose about 30-35% due to knots, but webbing loses closer to 50%. Quote Link to comment Share on other sites More sharing options...
RuMR Posted August 18, 2009 Share Posted August 18, 2009 YOU'RE ALL GONNA DIE!!!!!!!!!!!!!!!!!! Quote Link to comment Share on other sites More sharing options...
rocketparrotlet Posted August 19, 2009 Share Posted August 19, 2009 An overhand knot would cause a sling to lose 30% at most. Are you sure about that? My understanding is that rope tends to lose about 30-35% due to knots, but webbing loses closer to 50%. I've heard less- webbing loses 20-35% (an overhand knot would be on the lower end), and rope loses 15-30%. Also remember the strength webbing is rated for, compared with the strength of a bomber placement. That's what I was taught in a Mountaineers climbing course, anyway. If you do not feel comfortable with this approach, then just do what makes you feel comfortable. More redundancy is never a problem. -Mark Quote Link to comment Share on other sites More sharing options...
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