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Carabiners like this,

are marked with the force rating. The one in this picture is rated,

  • 20kn the long way
  • 8kn the short way
  • 5kn with the gate open

Why are carabiners so much weaker when the gate is open?

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Being metal carabiners are ductile and deform under load. When the biner is under load and gate closed, the gate of the biner is captured in the nose, and forms a closed loop. When the gate is open, this loop is not closed and the nose is free open beyond its designed limits. If you load a closed biner to its gate open rating, and try to open the gate, it will probably be jammed (although due designed over engineering it may not be).

This does two things - the obvious, but less important part is the gate shares some of the load with the spine. However, more importantly, the gate stops carabiner opening further and deforming beyond its design limits. The shape of the biner means a majority of the force is transmitted in line with the spine, but the gate must be closed and locked to the nose to achieve full design strength.

  • 3
    Good answer. An it would be worth adding that the this is why there are screwgate or locking carabiners. The screwed sleeve of itself adds no more strength but ensures that the gate of the carabiner is no accidentally opened. – Paul Lydon Jan 10 '17 at 13:07
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    @PaulLydon: The main reason we have locking carabiners is not to prevent the loss of strength associated with opening the carabiner. The main reason is to prevent the rope or cordelette from coming out of the carabiner, in situations where that represents a point of failure with no redundancy. – Ben Crowell Jan 10 '17 at 15:47
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    @BenCrowell I agree with you completely. Locking carabiners could be used to eliminate gate flutter/shutter, but I don't think that is what Paul was talking about in his comment. I've never heard of anyone using locking carabiners for this in the field. People use wire gates instead. – Erik Jan 10 '17 at 16:11
  • @Erik: But these days carabiners, or carabiner-like devices, are used for lots of purposes other than climbing. Just offhand, we use them to fasten the gates of the horse corral, one with a screw lock fastens the safety wire for the trailer, I clip my dog whistle to my pace with one... – jamesqf Jan 10 '17 at 18:36
  • @jamesqf I agree but most of those uses aren't load bearing applications so strength isn't an issue which is what the question is about. The biner is a nicer cotter pin for the corral, or a loop to dangle the whistle. Your trailer biner might become load bearing, but in that case Ben's comment is spot on. I think Paul wanted to say the same thing Ben said, but Paul said it more ambiguously. In other words, locking biners make it harder for the contents of the biner to escape, but don't increase the strength. – Erik Jan 10 '17 at 18:53
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Let's go to the source and watch some carabiners getting broken!

While these videos show wire-gate carabiners, the idea is the same for a solid gate carabiner like the one pictured in your question. When watching the video, note the distinctly different failure modes: an open carabiner "bends" until breaking, while a closed carabiner "stretches" until snapping. This partially explains why, all else being equal in terms of construction and materials, a D-shaped carabiner is stronger than an oval: the D-shape better aligns the forces along the spine of the carabiner, leading to "stretching" instead of "bending."

These distinctly different failure modes can even be used to autopsy a broken carabiner and determine how it was loaded when it failed. For example, see this Black Diamond QC Lab warning about the even more problematic scenario of loading a "nose-hooked" carabiner.

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