"Alpine Equalization" Don't clip the thumb loop
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I first heard of this potential issue from AMGA Certified Rock Guide Ben Wu on this Instagram post. Connect with Ben on Instagram, @benwooster, or on his website.
While the “textbook” way to equalize a gear / trad anchor is to use slings or maybe a cordelette, there are some alternatives.
One of them is so-called “alpine equalizing”. Here, you try to share the load between two pieces of gear, typically cams, by clipping one directly to the other. If the crack is cooperative and you can move the cams up or down the crack slightly, you can often get pretty decent load sharing.
However, there's a right way and a not-so-right way to clip the cams to each other.
In the photo on the right, the top cam is clipped to the thumb loop of the bottom cam. If the bottom cam fails under load and transfers all the load onto the top cam, you’re now cross loading the thumb loop in two opposite directions, which it’s not designed to do.
So how bad is it?
Gear-breaking mad scientist Ryan Jenks from HowNOT2 and I tested this. We found that the thumb loop can start to deform at around only 2 kN! The ultimate failure point was around 12-ish kN, which means you're probably not gonna die. Check the video below for all the details!
But you ARE likely to mess up your cam, it's not good practice and there's definitely better ways to clip, so don't do it!
To quote Ryan in the video below, “It's not dangerous, it's dumb.”
The photo on the left shows a better way to alpine equalize: Clip the top cam into the SLING of the bottom cam, not the thumb loop. Another alternative, clip one carabiner into another.