QC Lab: Sling Strength in 3 Anchor Configurations

Now that's one of the most loaded questions I've ever heard, because, of course, there is no real definitive answer. There are so many factors involved, including quality of the placements, quality of the rock or ice, materials available, etc. For the sake of the discussion, however, we narrowed it down to assuming two "perfect" bolt placements and using one equalized sling. My immediate answer was that such a set-up would be plenty strong for most climbing applications no matter which way you slice it, but any time you knot a sling it undoubtedly weakens it.

Remember I'm not a guide and don't pretend to be one, and I'm not suggesting which anchor equalizing method is better or worse. All I'm providing is some data based on a very few (i.e., one) data point for each scenario.

Testing

My crack crew of QA engineers and I decided to check out the three most common equalizing methods using a single 48" runner: Sliding X, Sliding X with Knots, and Figure 8. Again, I'm not going to get into the merits or negatives of each situation (e.g., shock loading if one anchor placement blows, how "equalized" they actually are, etc). This is just an apples-to-apples strength comparison of the three configurations.

Results

Configuration
Peak Load (lbf/kN)
Failure Point
Sliding X
8000/35.6
none (machine limit)
Sliding X with knots
4760/21.2
webbing @ knot
Figure 8
5272/23.5
webbing @ knot


Sliding X


Sliding X with knots


Figure 8

So what do these numbers mean?

A couple of things to remember:

  • CE-certified slings are rated to 22 kN (4946 lbf)
  • Typical CE-certified carabiners (e.g., lockers, wiregates, bent gates, etc) in closed gate are rated 20 kN minimum (4496 lbf)
  • CE-certified cams are rated 5 kN, but most are over 10 kN

Using a Sliding X anchor, our tensile tester couldn't even break it. Now that is BURLY. And both configurations with knots were more than 20 kN in ultimate strength. So just as we've seen in previous sling-on-sling girth hitch experiments, knotting slings, etc, knots reduce the ultimate strength by anywhere from 40-60% and the failure mode is always at the knot. However, even though that seems like a big reduction in strength (which it is) the bottom line is that the anchor is still plenty strong for most any typical climbing scenario thrown at it.

Climb safe —

KP


Kolin Powick (KP) is a Mechanical Engineer hailing from Calgary, Canada. He has nearly 20 years of experience in the engineering field and has been Black Diamond's Director of Global Quality since 2002. Kolin oversees the testing of all of Black Diamond's gear from the prototype phase through continual final production random sample testing. If you have a technical question for KP, please email him at askkp@bdel.com and he will TRY to respond.