But first, a little science....
All climbing or load bearing carabiners have a kN rating either printed or etched on them. If you don't see the rating, it probably means you bought it in the Lowes checkout line and it's designed for holding your keys rather than supporting the weight of a falling person. NEVER use carabiners without ratings.
For those carabiners designed for climbing, the kN rating can usually be found along the spine of the carabiner. A kN, or kiloNewton is equal to about 101.97 kilograms (roughly 225 lbs). A kN is not simply a weight, but a force of gravity. In other words, a free falling 225 lbs man would generate a tremendous amount of force as he falls. The rapid deceleration caused by a carabiner catching his fall would be much greater than 1 kN(225 lbs), since we are dealing with a dynamic fall. Newtons second law of motion comes into play during a fall:
Force= Mass X Acceleration
Acceleration for a falling human body is 9.8 meters/second. So for every second you're in free fall, add 9.8 until eventually due to air resistance and other factors, you hit terminal velocity.
And so on......
Many climbing carabiners are rated for upwards of 20kN (that's 4,500 lbs!). Ratings like these make it sound like they are over-engineered to hold extreme weight, and they are. But after taking into account the amount of force that a fall generates, you realize that carabiners must be rated this high to avoid failure.
To give you some perspective, the breaking force of the human body is somewhere around 12 kN. That's why ropes and other safety features are designed to be dynamic and stretch. By stretching and giving way, a gradual and less violent deceleration happens when you fall.
For the carabiner above, you'll notice that three kN ratings are listed. The first force rating (24 kN) would be for weight that is properly loaded along the spine of the carabiner. This will always be the strongest rating, as this is the proper way to distribute weight.
The second rating (8 kN) is the amount of force that the carabiner can take when it's cross loaded. Cross loading a carabiner is NEVER recommended. This is because the strength of the carabiner is greatly reduced when the weight is distributed over the gate and away from the spine.
The third rating (8 kN) is the amount of force that the carabiner can take when it's gate is in the open position. Not only do you risk the rope slipping out of the carabiner altogether, but you greatly reduce the integrity of the carabiner when the gate is open. Again, this is never ideal and should be avoided!
It's important to note that a carabiner can be reduced to an even lower kN rating when it's tri-axially loaded. A tri-axial load will essentially pull the carabiner in three separate directions, causing weight to be pulled away from the spine. This greatly reduces the strength of the carabiner and can be extremely dangerous.
Keep in mind that the kN rating does not take into account wear and tear. If your carabiners are looking a bit rough due to regular use, be sure to inspect them often for defects. Also, kN ratings assume proper use.
Carabiner Shapes and Designs
In the above section, we brought up the concept of a "properly loaded carabiner." A properly loaded carabiner is one that aligns the rope along the spine of the carabiner. Many carabiners are uniquely designed to "allow" for proper loading. I've listed a few examples below of the different carabiner shapes, and the advantages to using these designs.
Oval shaped carabiners have been around for decades, and are still used in many applications today. They feature a simple design that allows them to be flipped and rotated easily without interfering with other equipment. They are also fairly low cost. The draw back to using oval shaped carabiners is that they are usually heavier and are prone to shifting. Their symmetrical design tends to encourage weight transfer towards the middle of the carabiner as opposed to the spine (where the carabiner is strongest). Unintentional weight transfer can lead to a dangerous cross loading scenario. For this reason, they typically have a much lower kN rating than other shaped carabiners.
D-shaped carabiners have an asymmetrical shape. Their shape transfers weight away from the gate, aligning the rope towards the spine of the carabiner. D-shaped carabiners are designed to clip into protection easily, and their high strength rating allows them to be extremely versatile.
HMS carabiners (hemi-spherical) or "pear" shaped carabiners have an oversized opening. This design makes it easier to clip in and out of ropes. Its shape also makes it ideal for belay use. Like the D-shaped carabiners, HMS carabiners allow for proper rope alignment and loading.
Locking vs. Non-locking
The above picture demonstrates the difference between non-locking and locking carabiners. The carabiner on the right features a "screw-gate" locking mechanism, which allows users to manually lock the gate closed. Locking carabiners are an essential piece of gear for rappel/belay devices and for any other critical protection point. When using a locking carabiner, orient the screw-gate downward to ensure that it doesn't become loosened by gravity.
Wire Gate vs. Solid Gate
While wire gate 'biners may appear to be less strong, they are actually safer and more ideal than solid gates for a number of reasons. Wire gate carabiners take up less space, allowing for a larger gate opening and lighter overall weight. They also have a much lower mass than solid gates, which prevents the gate from slamming open during a dynamic event such as a fall. To test this, hold both versions of the carabiner in one hand with the gate facing up. With your other hand, strike the bottom of the carabiner. You should hear an audible click only when you strike the solid gate carabiner. This is actually the sound of the gate rapidly slamming open and closed. You can imagine that on a fall where a lot more kinetic energy is involved, a gate slamming open could inadvertently allow your rope or piece of protection to come out of the carabiner. A potentially deadly consequence.
Opposite and Opposed Carabiners
Mountaineering methodology emphasizes redundancy. The picture above demonstrates two non-locking carabiners that are both opposite and opposed to one another. The theory here is that you are eliminating a single point of failure by adding both strength and redundancy to your attachment point. Even in the event that both gates were to slam open, they would cross over one another and greatly reduce the odds of the rope slipping out. Opposite and opposed LOCKING carabiners should be used at master points. In the event that a locking carabiner is not available, two non-locking carabiners that are opposite and opposed are thought to be the equivalent of one locking carabiner.
The above picture again demonstrates the concept of opposite and opposed locking carabiners on a snow anchor. Mountaineering rules and acronyms exist to ensure a wide margin of safety. One of the most important acronyms out there is known as ERNEST, and is used when building anchors.
Equalized- Every component of the anchor should carry an equal amount of the load.
Redundant- A single point of failure should not exist anywhere on your anchor.
Non Extending- Meaning if one component breaks, the anchor shouldn't jolt forward and shock load another component.
Strong- The anchor should be as strong as possible.
Timely- All of the above features should be accomplished as efficiently and timely as possible.
We hope that this blog hit on all of the questions you may have had about carabiners. For additional information, visit the link below.