How strong are rappel rings? It only needs to be about 2x more than your weight to hold you during a rappel (in case you don't rappel smooth). Add a little safety ratio for good measure and it's super good enough. The problem is when it wears down from use.
Kevin Heinrich pulled this rolled aluminum SMC rappel ring off a popular route in Yosemite and it broke at 1.5kN instead of it's 14kN it is rate for, which is legitimately dangerous. SMC says you can use these rings for "slacklining" and if you are only doing something 10m or shorter, it's fine, but for any long slacklines it can break around the force you can achieve with a bunch of pulleys. Know your gear and it's limitations so you can be super safe enough.
I'm shocked rolled aluminum broke as high as 15kN, however the machined or forged aluminum rings were no surprisingly at least 10kn stronger. You can see in the video that some of them stretched more than others but only a little bit before snapping, as is the nature of aluminum.
If you have titanium anchors, you need a titanium ring attached to them. We test the ENDURA RING from Titan Climbing who made the world's first certified titanium anchors. They make high quality products and definitely under rate the MBS on the ring at 50kN because it broke at 90kN! They claim it still meets a 25kN standard even when 60% worn. The Krabi Bolting Fund is limited on their funds so they buy in bulk uncertified titanium rings from a Chinese supplier that we jokingly call Chitanium Rings. Those rings broke at 80kN and 79kN. Super good enough though we recommend Titan climbing's rings. Titanium stretched a LOT before breaking and sparks when it does break.
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The Fixe steel rings are welded and that is where the ring broke. When we had the weld against the shackle it broke a 35kN ring at 89kN! This could be from a better weld than the others or the way we pulled on it. The next one had the weld in between the shackles and it broke at 47kN. It's all above MBS but what a range. The 50kN PLX ring broke at 56kN also at the weld. The forged ring "Tecforge" (now part of Colin Milas) is an uncertified 316L stainless steel forged ring from France that my Portugual slack friends sent me to test. It broke... my spanset at 94kN. Not sure if you NEED that for a rappel ring, but it is bomber!
On single pitch climbs, if a lot of people will be lowering the climber or even top roping through the anchor itself, it will wear out eventually. In the Bolting Bible's "Book of Metal" chapter, you can see Jim Titt's experiment where he shows titanium wears down twice as fast as steel. Obviously rolled aluminum rings are NOT super good enough for durability and if you see a worn out one, don't rappel on it without it backed up. Install Titanium hardware if the environment is harsh for bolts, and use stainless if it's not a popular route and doesn't need to be replaced often. You can get away with normal steel like Mussy Hooks if it will be worn down before it has a chance to corrode. Learn a lot more about that in the Bolting Bible