The RBS knot (pronounced "arbs") is named after Richard B. Schroeder, its inventor. It was quite popular before "everyone" switched to mechanical ascenders. The knot is tied by simply clipping the carabiner to the sling, spiraling the carabiner around the standing line twice, clipping the sling again, and locking the carabiner. This can be done in only a few seconds. The relative orientation of the carabiner and the standing line can take on a number of forms, but they all seem to work. The RBS knot is easily moved, but it tends to fall if unloaded. As with all carabiner knots, a locking carabiner is recommended.
The inverted RBS knot also works, but it never became popular, probably more because it was perceived to be "incorrect" than for any real disadvantage.
Recently a highly skilled young caver and I were talking about emergency ascenders when I mentioned the RBS knot and received the deer-in-the-headlights look. Do we not teach these things anymore? How can this travesty happen? There are things lurking in our ancient history that merit preservation in our collective memory, for someday that knowledge may save us some grief.
Richard B. Schroeder first described the RBS in a Nittany Grotto Newsletter in 1963, and several more newsletters reprinted his article. Bob Smith tested the RBS and compared it to other knots in a 1964 Guacharo article. His article, later reprinted in the 1964 Speleo Digest, concluded, “This knot seems to be the only one of the newer knots which is worthy of consideration.” The RBS knot soon became quite popular and only lost its following when everyone in the known universe switched to using mechanical ascenders.
Tie the RBS knot by clipping a carabiner to the sling, spiraling the carabiner around the standing line twice, clipping the sling again, and locking the carabiner. This takes only a few seconds, and does not require removing one’s slings.
The relative orientation of the carabiner and the standing line can take on a number of forms, but they all seem to work. The inverted RBS knot also works, but it never became popular, probably more because it was perceived to be "incorrect" than for any real disadvantage.
The RBS knot is easily moved, but it tends to fall if unloaded. Because it loosens so easily when raised, there is some excess slack to take out when the knot is subsequently loaded. This is a common tradeoff among various knots, and the RBS lies toward the “easy to move” end of the scale. My suggestion? Just throw the knot upward and set it with a snap of the wrist.
Even more so than with other carabiner knots, I recommend using a locking carabiner, since the carabiner moves around and sometimes rides against the gate. As with all ascending knots, different slings and standing ropes can give different results. As always, a slipperier sling holds better. Practice on a stiffer standing rope like PMI Pit Rope since those types of rope tend to provide less grip.
I like knowing the RBS knot and how I can use it as a backup. I often have a carabiner with me when vertical caving, and I don't need to bring anything special. Some people like the Petzl Tibloc. All you need is a carabiner, a Tibloc, and some care so that you don't shred the rope’s sheath. The Tibloc is light, but not bringing a Tibloc is lighter. With only a carabiner, I can use an RBS knot. If I want to carry more, well, that’s another story for another time.
We continued our conversation and I showed the RBS knot to my anonymous young friend (you know who you are). I even provided a copy of Prusiking to read as a homework assignment. The knot made a favorable impression on my friend. Have I made a convert? Time will tell.