Build My Own FeedQC Lab: Extending a Cam Sling
When I rack for a multi-pitch trad climb, I rack like this:
- Cams (with a biner on each cam), set of Stoppers on my harness
- Quickdraws and a few spare biners on my harness
- Over-the-shoulder runners with one biner on each—over my shoulder (well duh...)
If I need to place a cam, I grab one, place it, clip it and go.
If I need to place a cam and extend the sling, I place a cam—then either use one of my quickdraws and clip through the sling like this:
Or grab a shoulder sling and extend all the way like this:
Not rocket science.
(FYI: if I place a Stopper I either use a quickdraw, a shoulder sling with the biner that’s on it PLUS a spare biner, or a quickdraw AND a shoulder sling for extra length.)
So a while ago when I was down in the desert climbing a tower with a buddy, and I came up to several cam placements like this—I was confused.
I saw him fiddling at all of these cam placements and was wondering what in the world was going on up there—he’s taking forever farting around with gear—maybe it’s because he’s so strong he doesn’t get it—but for me, I need to place the piece and keep moving before I flame out. So as I’m seconding his pitches and having to deal with this unfamiliar conglomeration of slings on slings on cams, etc—I’m wondering:
- Why is he doing this? and
- How much does this affect the strength?
- And, I am getting pumped out of my mind.
I mean nylon on nylon or Spectra on Spectra, etc—sounds like bad juju to me—and I’m not talking about girth hitching anything here—just looping it through…
I get to the belay and ask my partner what’s up with the method—and then it all clicks—ahhhh "old school alpinist," light is right, save a biner, etc, etc—and old habits are hard to break. Regardless, I got back from the weekend, explained the situation to the crew in the QA lab and we proceeded to do a few quick tests.
The Tests
We slung some 8 mm Spectra through a typical cam sling and did a few pulls in the tensile tester and a few drops in the drop tower. We compared the results to a cam sling only.
Here are the results:
Tensile Tests:
15.6 kN
16.2 kN
15.5 kN
Average: 15.8 kN
Actual historical average for cam sling ONLY: 25.5 kN
Therefore sling on sling method provided results 61.8% of historical average—or another way to look at it, it reduced the strength of the cam sling by almost 40%.
Drop Tests (note: these are NOT UIAA drop tests)
In both test configurations (i.e., cam sling only and cam sling threaded with 8 mm Spectra), the rope broke after more than six successive Factor Two drops (80 kg mass) with peak loads of over 10kN. The cam sling or sling-on-sling method was NEVER the failure mode in drop test scenarios.
Conclusions
The sling-on-sling method of extending a cam sling does save the use of a biner but in my opinion is cumbersome for both the leader and the second. It also appears to reduce the ultimate strength of the system, however, in most cases not so much as to be the weakest link in a real-world climbing situation.
Bottom Line
When you’re extending a cam sling, use a biner and make everyone’s life a bit easier. If you’re a super light-and-fast type of guy, the sling-on-sling method works, but know that it does weaken the system. Also note that none of the tests we performed took into account possible wearing from rubbing and friction—possibly even reducing the overall strength of the connection even more.
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.
United States / English 
21 Jun 2011, 8:52PM
No one has mentioned the big advantage of hooking one sling to another. One less Biner to unclip or load in a strange direction. Is the sling on sling method better or worse than a Biner weighted over an edge? What about if the sling twists, could it come unclipped? Just playing devils advocate here, I almost always do it the way recommended above but wondered if there was any merit to the "old school" method.
6 Dec 2009, 10:16PM
Thanks for the post. I'm wondering what difference there would be if the slings were girth-hitched rather than just looped through. Would this significantly increase or decrease the strength of the system?
4 Dec 2009, 8:22AM
Brian, your approach might be a little bit overkill, although safer when compared to a normal one carabiner setup (and as such, nothing wrong with that!). Just mean with this, that I can imagine myself building some pump with this approach! I'm not so strong to double-clip every time I have to extend. :)
I just try to make sure that the sling is not twisted (as in your picture), and that the sling will be pulled properly when I climb past it (so that it is not too long for the purpose, that is, which could allow for the carabiner to change it's position - when pulling the rope at the next piece for example).
I guess it's just a matter of approach and taste. I find it more faster to adjust the length of the sling if necessary (and to carry two or three different lengths of slings, depending on the climb), than to clip two carabiners. Also, I try not to use wire gates, as these can sometimes have "smaller teeth" to hold the gate (although being a little less heavier) which can allow for the gate to possibly unclip more easier.
2 Dec 2009, 2:32PM
I, and many of my firends, carry cams on a single biner and have 2 ft runners slung short with two biners on them which are easier to get to then when they are around your shoulder. If I need to extend the runner I clip it to the biner on the cam and run the rope through both biners flipping one so that the gates are in the opposite directions. This give a little additional insurance that the gates won't unclip. Photo: http://www.climbri.com/AMC/Instruction/Lead-Climbing/image013.jpg
1 Dec 2009, 7:13PM
My reason for suggesting this is I belive 10mm Dynex, all Dynex has considerable less strech to it and thus acts cable like eliminating a key components ability to absorb loading stresses.
1 Dec 2009, 10:23AM
This makes me question girth-hitching two long runners together. Do you have other tests or postings on that? Is it always better to use a biner to connect two slings? How much better? This is great info!
30 Nov 2009, 5:41PM
Cool section. Are there any ideas of what specifically caused the reduction in cam sling strength by using the Spectra runner vs the historical tensile tests (i.e. do the "sharp" edges of the Spectra runner cause increased localized stress compared to the larger radius pull-test pins, does the Spectra runner cause uneven loading across the width of the cam sling due to its edges being pulled tighter, etc.)?
30 Nov 2009, 4:24PM
Andrey --
• For sure you can connect a quickdraw or runner (using carabiners) to the thumb loop of the cam, but note (per the BD instructions), that the strength of the unit is reduced by approximately 2kN in doing so:
http://www.blackdiamondequipment.com/uploads/black-diamond/files/MM5865_D%20Camalot%20IT_WEB.pdf
• As far as girth hitching a sling to the thumb loop on a cam… there is a definite significant reduction in strength of the sling, and a future QC Lab posting will cover this in more detail. Once again, the best way, in my opinion to lengthen a cam sling is to either clip a quickdraw into the cam sling, or use a shoulder runner and clip to the biner on the cam.
30 Nov 2009, 4:23PM
John --
• We only tested 10mm Dynex on standard cam sling (ie. 11/16” supertape)
• My guess is there would still be a reduction if testing supertape on supertape, but not quite as much of a reduction as when using the thinner Dynex.
30 Nov 2009, 6:38AM
Why not to place your sling to "eyelet" of a cam (not to it's sling) and avoid sling-to-sling connection still not using a biner? (sorry for my English)
29 Nov 2009, 7:51PM
Did you test std nylon with std nylon, pictures show 2 different nylons, correct??????
27 Nov 2009, 2:35PM
Nice little section, keep these coming, There are a lot of either bone head climbers or simply unaware climbers out there. A major key issue in this situation is the old "soft on soft" rule, avoid it. I know the chances of them rubbing so much and the friction adding up to a level that would break the sling connection is slim, but possible, and if you do this and assume its an ok method and then apply it to other situations like top rope set ups and anchors then there could be a problem.