Don't put rubber bands on an “open" sling
For those with a short attention span, here's the short version:
Do NOT put a rubber band on a sling like in the photos below, you could die.
It also applies to any kind of a rubber band (or tape) on your personal tether - do NOT do this.
Who would ever do something like this?
Here's an article (perhaps behind a EVIL PAYWALL) about a woman who had tape on a sling she was using as a tether, similar to the rubber band in the photo below. She had it clipped incorrectly, leaned back at an anchor, the tape broke, and she fell about 60 feet. She was extremely fortunate to be partially spotted by her climb partner, who was able to partially cushion her fall. Neither of them were seriously injured.
Here's another article on a similar accident: “On June 12 Karen Feher (33) and her partner were climbing Rico Suave (5.10a). Upon reaching the anchor, she clipped in. Her setup: She had two thin dyneema slings girth hitched to her harness. At the end of each sling was a locking carabiner held in place with a rubber Petzl keeper (called ‘Petzl Strings’). The “string” is designed to fit on the end of a Petzl runner in order to keep the lower carabiner on a quickdraw in place for easy clipping and to protect the webbing from abrasion. She clipped a locking carabiner to each bolt and probably called, “Off belay.” It is unclear if she was going to rappel or be lowered. It doesn’t matter. She took a fatal fall about 50 feet to the ground.”
Related to this: the tragic accident in 2013 of then 12 year old climber Tito Traversa, who died after taking a ground fall high on a sport route. An investigation showed that many of his quickdraws had been assembled incorrectly, with the rope only clipped through the rubber portion and not through the nylon sling. So, even having the rubber band device on the bottom carabiner is certainly not foolproof.
Even if you’d never think of rigging something like this yourself, you might see someone else doing it; maybe you can prevent an accident. I once saw a climber do this exact thing on the end of their personal lanyard/tether. At the time I saw it, I didn’t know about this potential problem, or I definitely would've pointed it out.
If you want to keep your tether carabiner from rotating or being cross loaded, simply put a girth or clove hitch in the end of your tether (right side photo).
The clove or girth hitch gives two benefits: 1) Secures the carabiner for optimal loading along the spine, AND 2) creates redundancy in the (highly unlikely) event one strand of the sling fails.
Here's the longer version.
First off, some definitions.
An “open” sling is one that’s sewn with bar tack stitching and makes a loop / circle. The other type of sling is known as a “dogbone”, typically found on a quickdraw for sport climbing. Slings for sport climbing are usually a piece of tripled webbing, with multiple rows of bar tacks for strength and rigidity.
Open sling (top) and sport climbing dogbone sling (bottom).
example of “open” sling (top) and sport climbing dogbone sling
On a typical dogbone, the bottom or rope clipping carabiner may have a small bit of rubber on it to keep the bottom carabiner from spinning and preventing cross loading.
Below are two examples. The top is a device from Petzl that has the rather unusual name of “String”. The bottom is a more generic quickdraw that has a thin rubber band between the dogbone and the carabiner. (In my opinion, this is a slightly safer set up, because you can clearly see that the dogbone is properly clipped to the carabiner, unlike in the Petzl example. More on that below.)
Left: Open sling with rubber band on bottom carabiner, dangerous!
Right: Standard quickdraw with rubber band thingie on bottom carabiner, totally fine.
Side note: This “String” / rubber band should only go on the BOTTOM carabiner. If you put it on the TOP or bolt clipping carabiner, the carabiner can rotate with rope movement and perhaps be loaded in a way that causes a large reduction in strength, and possible failure. See dramatic warning diagram from Petzl below.
image: https://www.petzl.com/US/en/Sport/STRING--installation-and-precautions
So, you might be thinking, if it's a good idea to do this for a sport climbing draw, how about I do the same thing for my alpine quickdraws (or even personal lanyard) made out of a 60cm (single) or 120 cm (double) sewn open sling? Just add a thick rubber band, and no more worries about cross loading. Should be fine, right?
Do not use a rubber band (or tape) on an open sling.
Why? If the sling gets clipped back to itself, it can result in the carabiner ONLY clipped to the rubber band and not on the sling at all, YIKES! This is easy to do, especially when you're doubling or tripling up a longer sling to rack it as an alpine quickdraw, or clipping a lanyard back onto itself to tidy up your harness.
This diagram from Petzl shows how this can happen.
image: https://www.petzl.com/US/en/Sport/STRING--installation-and-precautions
Here’s a similar warning from Black Diamond:
image: https://blackdiamond-web.cdn.prismic.io/blackdiamond-web/2294b9da-d748-4742-9837-cc2c8be53285_MM5824_K_Runners+and+Dogbones_IS-WEB.pdf
On sport climbing quickdraws, having some sort of rubber band on the bottom carabiner is generally safe (but not always, see accident report below). This is because the bar tacks and the stiff sling prevent this from happening. It’s a bit hard to describe in words, much easier to see in the video below.
I highly encourage you to get a sling, a rubber band, and see this mechanism of failure for yourself. It might take a few tries to figure it out. (This is similar to the reason why we avoid using sewn pocket daisy chains intended for aid climbing as a personal lanyard, because if you clip two adjacent pockets, you’re only held by the bar tacks between the pockets.)
Finally, here's a simiular video from UKClimbing.com that shows how this can fail.
Safety Video - The Danger Of Open Quickdraws
