Acceptable cord diameter for anchors

Through various text I've found differences in what is deemed acceptable diameter for cord being used in anchors. Range differing
between 5-9mm. Given the wide range weight becomes an issue. Question out to the MP sages - what diameter to use?

7
mountaineers.org/seattle/cl…

I've floated between 5.5 tech to 10something nylon (the rope).

My typical is 6 nylon for mid-wall.

jaypg wrote:Through various text I've found differences in what is deemed acceptable diameter for cord being used in anchors. Range differing between 5-9mm. Given the wide range weight becomes an issue. Question out to the MP sages - what diameter to use?

There was a good discussion about this on MP, but I can't find it, and unfortunately I don't have time today to search for it. How dare work interfere with my MP time!

Personally, I use 6mm. From everything I have seen, read, and talked to people, 6mm just wears out faster than 7mm, so you have to inspect it more often.

For my climbing friends and me, 7mm seems to be the consensus as "best bang for your buck" because it's light enough, strong enough, wear-resistant enough, etc.

Of course, like all climbing rules-of-thumb, it's important to point out the many other things that may even be more important than rope diameter... Besides things like age or exposure to sunlight of the rope (a new 6mm rope can be stronger than an older 8mm rope), there's the placement of the pro, the angle between the ropes, equalization (difficult to achieve when a single knot is tied)

Below is a nifty image and analysis of the different forces for a simple, two-bolt anchor... At a wide angle, the force per bolt (therefore the rope tension) is about twice the load, e.g. if you fall and exert 3X body weight, the bolts/rope see 6X body weight, so keeping your angles between ropes around 20-ish degrees may be as important as considering a 7mm vs. 8mm rope.

southeastclimbing.com/faq/f…

Incidentally, I created and observed a physics project that connected tension meters at different points of a cordelette and had high school students rig a gazillion (ok, only a million) different anchors to study geometry and force distribution. The end result was that rope angle and slight changes in equalization dominated the cause of tension in ropes, and would therefore would dominate the failure mechanism for ropes within a mm or two in size.

Happy Climbing

a 20 angle is a bit too conservative; I'm fine with 60 and under, you don't really get too much in the difference. You can even eyeball 45 pretty well and be fine.

The real big increase in force changes occur around 125 then increasing dramatically through to 180. Something like this range will be a noticeable difference and probably shocking the conscious of any experienced climber if they saw it in a rig.

You are also assuming everyone is a steel plate with no ability to absorb energy. As well, you hypothesize about exacting equalization in anchor behavior, as has been beaten to death; but really, it's just not that critical in maintaining the entire system. As long as you get some load distribution using solid pro, and yes good rig angles, climbers will be fine at the anchor rig; even more so with dynamic materials & belay.

Yeah, 20ish degrees is conservative. Maybe I should have phrased it "shoot for a small-ish angle and you should be fine" Thanks for the clarification, and I agree that less than 60 is fine. (Though I've had worse angles when I was low on gear and rope length, and the cracks in the rock weren't being cooperative.)

As for hypothesizing about equalization, it's tested, proven, and can be shown mathematically in about 15 seconds, even accounting for rope stretch providing some equalization and the dynamic rope dampening the loads, and is a common aspect of mechanical and civil engineering design whenever ropes/cables/wires are used and a fun project to do with kids so that they learn how testing eliminates the need for a lot of theoretical discussion. The bigger, more practical point is that I've never heard of an anchor cordellete breaking regardless of the diameter (has anyone?). The last anchor-related accident I heard about was discussed ad-nauseum last year following a two-person accident at Suicide Rock, CA, and if I remember correctly, of the many mistakes there was a two-piece anchor that failed when the belayer 'caught' the leader on a hip-belay and then fell herself after the leader ran out the climb and poppped two pieces of gear during his fall. I don't think even an 8mm (or 108mm) anchor would have mattered. Personally I'd probably be ok using a shoelace if it were doubled-or-trippled and a directional piece were placed on a multi-pitch to prevent hip-belay loading, so it goes back to proper gear placement and safe climbing.

The short answer:
6mm nylon
7mm nylon
8mm nylon

My choice is 7mm nylon. No tech cords (spectra, dyneema, etc). Even though they show much higher breaking strenghts, they have little ability to absorb energy. Energy absorbtion and load distribution are critical aspects to safety, imo.

Rule #1: never fall directly onto an anchor (factor 2)
Rule #2: Build an anchor that can withstand a factor 2. This includes solid primary pieces, energy absorbtion (nylon), load distribtion (equalette is very good for this).

In the real world the ideal anchor isn't always available. Every situation is different. Ok, maybe this wasn't such a short answer after all.

Just thought I'd show the forces on an anchor as a function of the angle. This is assuming that the direction of the force bisects the angle of the protection.

Idealized fractional force as a function of angle.

even at 90 degrees you are reducing the load on each anchor by 29%. Reducing the load on each piece at the anchor is secondary to just having the redundancy of multiple pieces in my mind.

Greg D wrote:The short answer: 6mm nylon 7mm nylon 8mm nylon My choice is 7mm nylon. No tech cords (spectra, dyneema, etc). Even though they show much higher breaking strenghts, they have little ability to absorb energy.

The link I posted above has a good analysis of why tech cord is inferior to nylon in other aspects as well.

David Appelhans wrote:. Reducing the load on each piece at the anchor is secondary to just having the redundancy of multiple pieces in my mind.

I would say that but edit for: "Reducing the load on each piece at the anchor to a point of exactness is secondary to having the adequate load distribution between multiple solid pieces."

When I first started trad climbing, I built anchors with 20' of nylon webbing, cheap and iron-clad. Then I was mad at the research I was doing one frigid day, and participated in the almighty conspicuous consumption: I bought a Trango equalizer.

The day I got it I almost sent it back thinking it was unnecessary (glad I didn't). This thing is quick, and perfectly equalizes 2-3 pieces without any guesswork. The bite is reinforced. My only wish (other than having a second) is for it to be of nylon, because I agree that nylon, though heavier, is almost always a better choice because it stretches, and distributes energy then less violently.

If your anchor is A0 the difference of nylon anchor may be negligible, but anything above and it could be the margin you need. Although, with the equalizer pictured, you are assured even distribution from a variety of angles, which could outweigh nylon's stretchyness.

Forestvonsinkafinger wrote:When I first started trad climbing, I built anchors with 20' of nylon webbing, cheap and iron-clad. Then I was mad at the research I was doing one frigid day, and participated in the almighty conspicuous consumption: I bought a Trango equalizer. The day I got it I almost sent it back thinking it was unnecessary (glad I didn't). This thing is quick, and perfectly equalizes 2-3 pieces without any guesswork. The bite is reinforced. My only wish (other than having a second) is for it to be of nylon, because I agree that nylon, though heavier, is almost always a better choice because it stretches, and distributes energy then less violently. If your anchor is A0 the difference of nylon anchor may be negligible, but anything above and it could be the margin you need. Although, with the equalizer pictured, you are assured even distribution from a variety of angles, which could outweigh nylon's stretchyness.

Yep. True. But an equalette made of nylon does all that, is a fraction of the cost and is made of....yes, you guessed it...nylon, and has little extension which an alpine equalizer can only claim after you tie off the arms.

I use 7mil.

I like 7

I like 7mil or the 48" slings

In my AMGA class they told us that 7 was a good amount. one student had 6 and asked "why 7" he told us that the strength went up almost 1000 pounds just from 6 to 7mil. We also had to use static line about 10mil to build certain anchors as well.

It is an interesting question.

While 7 nylon is increasing the perceived safety factor of the anchor, is it necessary?

Is 6 nylon completely adequate for a recreational climbing trad anchor?

one key thing to keep in mind would be to ensure that each arm in the equalization is at least as strong as the gear it's attached to.

if the application is just for equalizing anchors and there isn't danger of the cord being cut in some way, 6mm cord with a breaking strength of around 1600 pounds would do the job if each arm is doubled.

i prefer 7mm nylon because i like its utility beyond equalizing anchors.

What would be the true test of the 6 considering the anchor in entirety?

While a single component can easily be measured, the anchor is a load distributor. What can the entire anchor take if loaded in the direction intended?

7 mm