Thursday, 14 April 2011

All Things Equal? Musings on Equalising Anchor Systems...

Last night i managed to get myself into a discussion about the usefulness of a piece of climbing equipment know as an Alpine Equalizer which is made by the American company Trango (demo video from Trango here). Simply put the Alpine Equalizer (AE) allows a climber to quickly equalise a 3 point anchor system at a belay stance. Its potential advantage over conventional belay set ups is that it allows for dynamic equalisation between anchors, meaning that if a climber should fall in a direction the belay is not rigged for then the AE automatically readjusts the lengths of each leg to equalise the belay and distribute the forces between each anchor as evenly as possible.

 All very nice but the major flaw i see in this design is that the only way to make each anchor point independent, and thus remove the possibility of shock loading the whole system should 1 anchor fail is to clove hitch the sling to each anchor, which means it can no longer readjust the equalisation and acts in the same way as the set up would if rigged with a standard sling.

At first I dismissed the AE as a useful piece of equipment for these reasons but some of the points brought up in the discussion made me begin to question this, so i did a little research of my own. From the day i started climbing i was taught, as i'm sure most people were that anchors for a belay should be Independent, Equalised and at the right angle (i.e under 90degrees) but some of the reading i've done is making me question this.

Its particularly the Equalised part which i'm questioning. Not so much the need for all your anchors to be equalised but the best way to do this. For equalising 3 anchors using a sling my first reaction would be to link all three anchors with a long sling, bring all the arms together and tie an overhand (Fig. 1). This gives independent anchors and i always assumed fairly evenly equalised ones.
Fig. 1
A lot of the reading i've done however suggests that this system doesn't distribute the load to the 3 anchors nearly as evenly as you might think. Indeed a report entitled 'Multi-Point Pre-Equalised Anchor Systems' (available here) shows that the forces on the center arm of this set up will be almost double that of either of the side anchors. Unfortunately the copy of this article i have is read only so i cannot copy their results into here but they're easy enough to find in the article.

Their findings also suggest that the length of each arm of the sling is inversely proportional to the forces exerted on it. E.g. if the middle leg in Fig. 1 was shortened to half its length, the forces exerted on it compared to the side anchors would double. I've attempted to demonstrate this in Fig. 2 but i'm not sure how clear i've made it. Imagen the side anchors are in solid rock set back from the belay and the yellow sling represents another piece of rock protruding between them, thus the middle leg is considerably shorted... Make sense?

Fig. 2

So if the usual systems used by climbers don't actually equalise the anchors nearly as well as we though then is it time to consider a new approach? Perhaps.

A report titled 'A Look at Load-Distributing and Load-Sharing Anchor Systems' published in 2007 looked at this (available here), studying the load distribution between different Load Distributing (Alpine Equaliser style setups) and Load Sharing (normal setups as shown above) setups and the forces experienced when one leg of the system should fail. Their results are interesting to say the least. They experimented with 7 different load distributing setups, using different materials and rigs and one standard load sharing setup.

Their results show that the different materials used to rig the load distributing anchors have a great impact on how evenly the forces are distributed between the anchors.For load testing onto all three anchors, seemingly the less friction there is in the system the more evenly a load distributing anchor will spread the forces. The systems which used higher friction materials such as 1" nylon tubular webbing result in significantly higher forces being exerted on the center anchor than either of the side anchors. Slicker materials seem to be able to very nearly evenly distribute the force, although a slightly higher force is always exerted on the center anchor.
The Load Sharing anchor on the other hand, experiences higher forces on the center anchor than all but the highest friction Load Distributing systems. Fig.3 is, as far as i can tell, the Load Distributing setup used in this report, although the diagrams a a little hard to decipher so it may not be an exact replica. Mine in made using 5mm dynamic chord.

The conclusions which can be drawn form this are that if rigging a belay with 3 or more questionable anchors then, while it seems impossible to truly equalise them in the real world, a Load Distributing setup as shown above could almost certainly distribute the load most evenly, thus reducing the chances of any one anchor failing.

Fig.4 & 5 show another 2 options for Load Distributing belay setups.
Fig. 4


Fig.4 is my attempt at creating a Load Distributing setup which would minimise the shock load on the other anchors should one fail. In practice if the center anchor fails then a very small shock load results but if either of the side anchors fail you are still left with a large shock load as the system readjusts.
Fig.5 is essentially a home made Alpine Equaliser. 2 crabs could also be used (one for either strand of the sling in the center) which might reduce friction in the system. Using a DMM Revolver style crab would also help.

Coming back to the original point however, none of these systems manage to create a belay which is both equalised and independent, meaning while they reduce the chances of any of the anchors failing if one does, the rest of the system is shock loaded.
'A Look at Load-Distributing and Load-Sharing Anchor Systems' explores what exactly happens should this situation arise.

Obviously the Load Sharing system comes out of this study best as no shock load occurs when one anchor fail and the force is shared fairly evenly between the other 2 anchors. The results for the Load Distributing systems is more interesting however.

When one of the side anchors failed all of the Load Distributing anchors experienced high Maximum Arrest Forces and poor equalisation, i.e larger forces are transfered onto one of the two remaining anchors. Interestingly it seems to be the higher friction systems which equalise worst under normal circumstances which equalise best when a side anchor rips.
When the center anchors rips the forces are shared far more evenly between the remaining 2 anchors but the Load Distributing anchors still experience much higher forces than the Load Sharing system.

My Conclusions?
It seems to me that you have three options.
1) use a Load Sharing anchor system, knowing that it will most probably exert disproportionate loads through some of your anchors but also that should one anchor fail it will exert the least force on the remaining anchors. It is however, worth remembering that when setting up a Load Sharing system that keeping the length of each leg of the system equal will help to minimise the differences in force being exerted on the anchors.

2) use a low friction Load Distributing system to truly distribute the forces as evenly as possible and thus minimise the possibility of any one anchor failing but accept that if one does your other anchors will be subjected to greater forces than if any other system were used.

3) use a high friction Load Distributing system. This will share the loads between anchors better than a  Load Sharing system but not as well as a low friction Load Distributing system. Its will also exert more even forces on the remaining anchors should one anchor fail than a low friction Load Distributing system but still subject them to a much greater force than if a Load Sharing system was used.

So which system should be used where? As with so many things in climbing i think there are too many variables to really come up with a defiant answer for this. It each system has its pros and cons and argument could be made for and against each on a number of situation.

Thoughts on setting up low and high friction Load Distributing systems:

DISLAIMER: This part is largely based on my own experiences and a healthy chunk of guess work.

It seems to me that the most important factors influencing the total friction of a Load Distributing systems will be the material used to join the anchors and the crabs used. This of course excludes the idea of adding additional equipment such as fig8s for simplicity of setup.
From the results it appears that thiner, slicker materials create less friction. As such i would imagen that narrow dyneema slings or webbing would be best for reducing friction and wider, stiffer sling (nylon tubing for example) would create a higher friction system.
As for the crabs i would assume that large diamiter, round topped crabs would cause less friction, or even better a DMM Revolver with integrated pully. While conversly smaller crabs which would result in sharper angles being crated in the webbing should produce a higher friction system.

For some more reading Will Gadd has a few thoughts on it as well which are worth reading. These can be found herehere and here.

These are my personal opinions on the matter based on what i have found through a little research. If anyone has any comments on anything i have said please feel free to say. Also if anyone has come across anything along related lines please let me know! Every days a school day after all!


  1. Interesting stuff, thanks for posting this up. Will you be making any changes to the way you set up belays now then?

  2. Hi, glad you found it interesting.
    Im still undecided if the benefits of greater equalisation outweigh the hazards of potentially shock loading the system. A friend of mine may actually be able to run some of the systems through some simulation software so i'll see if anything comes back from that but for the moment i'll probably carry on as i always have, using load sharing systems.
    I'm certainly more open to the idea of using load distributing systems than i was though.


  3. Can you email some info on Fig.3 please. What kind of knot, how to tie it, possibly links to more info, etc.
    Thank you