shorten sewn runner

[dog]

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

 

 

[Rad]

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.

[goatboy]

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).

[froodish]

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.

[rocketparrotlet]

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

[snoboy]

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%.

[RuMR]

YOU'RE ALL GONNA DIE!!!!!!!!!!!!!!!!!!

[rocketparrotlet]

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