Anchors and loadings

This page gives some background theory on equalising anchors and describes, with illustrations, how to equalise anchors using either slings or the rope.

Anchor angles

Equalising with slings

Equalising anchors with the rope

Anchor Angles

You’ve got to the top of you’re climb and you’re about to set up a belay. Alternatively you might want to set up a top rope. The instruction book says to use two anchors and to equalise them. It also says that the angle between the two anchors must be less than 120º. Why?

Lets look at the basics. If you have two anchors, one above the other, equalise them, then load them by trusting your life to them, each anchor will obviously be holding half of your weight. In this arrangement (the ideal), there’s a downwards force on the each anchor of half a climber’s weight and no horizontal force.

Now consider a more usual configuration of two anchors side by side and equalised.

Again you load the anchor by hanging in your harness. Unless you’ve studied physics or engineering then you’ll have to trust me on this bit. If the angle between the two anchors is “a” then the load in the sling going to each anchor is

Now this time there’s a horizontal force pulling the anchors inwards of

There’s still of course a vertical force on each anchor of ½W

So, what’s this telling us?

Firstly, the load in the slings connecting you to the anchors has gone up. For two vertical anchors it was

L = ½ weight

For the case where there is an angle, it’s now

but cos(½ a) is always less than one so the load always goes up.

Lo and behold, if a = 120º,

L = weight.

This means that despite you having two equalised anchors, each anchor is now taking your entire body weight. If the angle is greater than 120º then each anchor will be taking more than your body weight! It’s no longer worth having two anchors any more as each one is more likely to fail.

OK, so where is this increased load actually coming from? Well its not out of thin air, if it was then we’d have solved the world’s energy crisis. What happens is that there’s always a downwards pull on the anchors of ½W which remains constant regardless of the angle. As soon as you move away from two in-line anchors you get a horizontal force pulling the two anchors in together. It’s the combination of this inwards force together with the vertical force that creates the increased load in the slings and hence the anchors.

If you keep increasing the angle past 120º all the way up to 180º (horizontal anchors) something weird happens. Although you’re only loading the rope/sling with your weight the load on the anchor is

but cos(90) = 0

Dividing anything by zero gives infinity. i.e you’ve put an infinite load on the anchors which will guarantee failure! In practice this won’t happen as the rope will stretch which will reduce the angle to a fraction less than 180º so you’ll never get the mythical infinite force.

With two 180º anchors you’ve got a Tyrolean traverse. These loading calculations show why Tyroleans are so bloody dangerous! You’re putting HUGE loads on the anchors and the rope. It also explains why a bow will shoot an arrow a hell of a lot further than you can throw one, and why you shouldn’t three way load a karabiner.

You can use this 180º loading to positive advantage in other ways. If you get a vehicle stuck, rather than trying to tow or winch it out directly, fix a rope from the vehicle to a tree, then go to the mid point of the rope and pull at 90º rather than along it. Hey presto, one huge load is applied to the stuck vehicle.

So, when equalising anchors don’t have more than 120º between the anchors as you’re actually increasing the likelihood of anchor failure. Unfortunately 120º isn’t that easy to visualise so try and keep the angle to 90º, a right angle. Then you know you’ve got a bit in hand if need be. To reduce the angle between any two anchors, lengthen the connecting slings/rope

Equalising anchors with a sling

You're at the belay station and the second is not going to lead through, this means you can't use the rope to tie in and equalise the anchors, you've got to use slings. There's several methods of doing this. Firstly, lets assume you've got two anchors to equalise. There are two ways to do this, both work perfectly well.

Firstly the simple method. Take a sling and tie an overhand knot in the middle of it. Now clip each loop into an anchor, with the knot at the bottom. Take a screw gate karabiner and clip it through both loops either side of the bottom knot. It is absolutely vital that the karabiner goes through both loops. To equalise the loads between the two anchors, move the position of the knot. This method is simple to set up, but is a little fiddly equalising the loads.
The second method is a little more complicated but uses less sling and is easier to adjust. Clove hitch a sling to one anchor. Clove hitch the same sling to the second anchor but leave a slack length of sling between the two anchors. You now have a single loop joining the two anchors. If you were to clip this loop directly and one anchor was to fail, then slack would appear and you would shock load the anchors. To equalise them properly tie either an overhand knot or a figure of eight in the bottom of the sling and clip the karabiner into the new loop that forms. Fine tune the adjustment by adjusting the clove hitches.
Some people do not feel safe unless they've got three bomber anchors. Unless they're very close together you're going to need a big 16 foot sling to equalise these in one go.
The quickest way is to start off by clipping a loop of sling through each anchor in turn and then just pull on the slings to roughly equalise them. Now grab all the loops and tie an overhand knot in them (a figure of eight will be too bulky). Clip the karabiner through the resulting loops.
If you need to equalise even more anchors then you'll need to start knitting. You might well need to do this when you've only got crap gear placements, lots of crap placements all equalised together can make a good belay. Work methodically, equalise two or three bits of gear. This point can then be regarded as a single anchor point, equalise it to others in exactly the same way as described above.

Equalising anchors with the rope

You're either at the top of a climb or at an intermediate stance where the second is going to lead through. You can now use the rope(s) to tie in. This is much easier than using slings but the disadvantage is that you use rope, generally not a problem if you're already at the top but it might be if the next pitch is a long one. There's also another disadvantage that I'll come to later. However, personally, I always tie in with the rope.

We'll start off with the simplest solution - one bomber anchor that's in reach. Simply take the rope from your tie in loop and clove hitch it to the anchor. (The harness isn't shown on the diagrams for clarity, however, the loop at the bottom of the diagrams goes to your harness). Also note that I've used quick draws in the illustrations, the official method is to use screw gates on a belay, not snap links.

The next method is if you can't reach the anchor from your stance, in which case you can't adjust the clove hitch on the anchor. This is also the method you have to use if you belay off a tree by walking the rope around it rather than fixing a sling to it.

Clip the rope through the krab on the anchor (again, the official line is to use a screw gate), go back to the stance and using a screw gate through the tie in loop, preferably a HMS type 'cos they're bigger, clove hitch the rope to it. You can fine tune the adjustment now from the comfort of your stance.

Unless you've got a convenient tree or a big boulder, chances are you're going to have two anchors. If you're climbing on double ropes then simply use the methods illustrated above with a separate rope to each anchor. I haven't got a diagram of this, well I have, but with two ropes its a bit messy and you can't see everything.

If the anchors are out of reach then you only need one screw gate on the tie in loops and clove hitch both ropes to this. You'll definitely need a HMS krab as a D shaped krab is too small. Don't hold both ropes together and clove hitch them in one go, clove hitch each rope independently.

The diagram illustrates how to equalise two anchors with a single rope. Assuming you can reach the anchors from your stance, clove hitch the rope from your tie in loop to the first anchor. Now clove hitch the rope to the second anchor but leave this joining loop slack. From this second anchor take the rope back to your tie in loop and clove hitch to it with a screw gate krab. Voila.

If you need to equalise even more anchors then you'll need lots of spare rope. You could of course use slings to equalise two anchors, thus bringing them to a single krab which the rope then goes to. The following description assume you've got lots of anchors and you're using a rigging rope, (more applicable to a top rope scenario).

Tie a figure of eight in one end of the rope and clip it to the first anchor. Walk back to where the stance is and thread the rope through a krab, however, make the tie in point further away from the anchors than you need. Now go to the second anchor and thread the rope through the krab. Don't clove hitch it. Now walk back to the tie in point and loosely clip the rope through the krab that you left there. Keep on like this until you reach the last anchor. Clove hitch the rope to this anchor. Go back to the tie in point. Pick up the krab that has all the bights of rope through it and by pulling on the ropes you can adjust the lengths so that all anchors are equalised. Now you'll find out why you wanted extra rope and the tie in point was further away from the anchors than you needed. Remove the krab, take all the bights of rope and tie a huge figure of eight knot with all the bights. The resulting loops do not all have to be the same length, in fact it will be a miracle if they are. You only need to clip in to one of the loops, not all of them.

There is one big problem with using the rope to tie in. A big advantage is that you can use anchors a long way back from the crag edge without having to carry up loads and loads of slings. This is also the disadvantage. Climbing ropes are dynamic and they stretch. If you've got a heavy second and they fall off then the ropes from you to the anchors will stretch. You will be pulled a couple of inches off your stance and due to the way nylon behaves (visco-elasticity in technical terms) when the second gets back on the rock you will stay where you are in your new belay position. If the inept second falls off again then you're going to get dragged forward again as the ropes stretch yet again. The best advice I can offer on this is if you think there's a good chance the second will come off then belay a little further back from the edge than you normally would. I've ended up about 12" further forward from where I started when the second spent a considerable time dangling on the ropes. It doesn't take a genius to figure out that the thinner your climbing ropes the worst this problem is going to be.

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Disclaimer

Climbing is dangerous and setting up a good belay is a vital safety task. If you don't understand what you're doing, don't climb. Before you use any of the techniques described here, make sure you actually understand what you're doing. If you don't then seek professional instruction, or at the very least, practice in a safe environment until you do. Think what will happen if an anchor fails, in a safe environment, unclip an anchor to simulate a failed anchor - the belay point shouldn't move if you've got it right (do not go unclipping anchors half way up a cliff!).