Page 29 - Rappelling

Tensile Strength vs. Loop Strength

trength ratings are often given as tensile strength and loop strength. Tensile strength is tested by a
Sstraight pull on a single strand of the material with no knots, done by wrapping the material around
a smooth bar (4-inch diameter gives the most accurate test) on both ends and pulling until it breaks.
Loop strength is the material tested in a loop configuration, either tied with a knot (in the case of web-
bing, usually the water knot) or sewn with bartacked stitching. In general, webbing loop strength when
tied with a water knot is about 80 percent of twice the tensile breaking strength, and bartacked sewn
webbing loop strength is generally about 15 percent stronger than the same material tied with a water
knot, depending upon the quality and number of bartacks.

minimum of 3 inches in length. It is important to for pound, is stronger than wire cable. Most experts
tighten the water knot carefully and firmly, as it has say that the manufacturer of Dyneema consistently
a tendency to loosen when not tightened properly produces more high-quality fibers than the manu-
in a sling that is being used over time. A double facturer of Spectra material, and most of the climb-
fisherman’s knot is hard to tie neatly with webbing ing and rappelling slings on the market today are
and requires a much longer piece of material, but it made from Dyneema.
is very secure and will not loosen. A disadvantage is Both Spectra and Dyneema slings are con-
that it is difficult if not impossible to untie once it structed from parallel fibers—very strong but with
has been heavily loaded. high lubricity, which means the material itself is
Why would you even use nylon webbing tied inherently slick. That is the reason you can only buy
with a knot as opposed to a sewn runner? A sewn it in sewn loops—it does not hold knots well. Do
nylon runner is just as strong and more secure than not cut a Spectra or Dyneema sling and re-tie it with a
the same material tied with a knot, since you don’t water knot!
have to worry about the knot loosening or com- Both Spectra and Dyneema have a lower melt-
ing untied. The answer is for use in rappel anchors ing point than nylon (around 300°F for Dyneema/
when tying slings around a tree or through bolt Spectra compared to nylon’s melting point of around
hangers. It is also sometimes useful to untie the 480°F). The lower melting point, along with the
knot, thread the webbing through something (like a inherent slipperiness, make Spectra and Dyneema
tunnel), and re-tie it. slings a poor choice for tying friction hitches like the
prusik, klemheist, or autoblock, compared to 5mm or
Spectra and Dyneema Slings 6mm diameter nylon cord.
Spectra slings, introduced in the late 1980s, were In a pinch, if you need to use a sling to tie a
lighter, less bulky, and stronger than nylon. friction hitch, use a nylon one over a Dyneema or
Dyneema is a more recent innovation, typically sold Spectra sling, as nylon will grip better. The newer,
in various-length loops sewn with bartacked stitch- thinner (10mm width) Dyneema slings will work
ing in 10mm width. Dyneema and Spectra both for friction hitches, and they do possess some
have almost the exact identical chemical makeup of nylon in their construction, but if they start to
high-molecular-weight polyethylene, which, pound slide on a rope when under load, the friction will

18 RAPPELLING

Rappelling_i-174_3pp_CS55le.indd 18 7/24/13 10:15 AM