Figure 8 block
A figure 8 block is a block using a figure 8 device which forms a contingency anchor, usually allowing a rigger at the top of a rappel to quickly convert the block into a lowering system without any additional equipment or techniques while the system is weighted.
Contents
Constructing
While there are many variations, the underlying construction of a figure 8 block is simply rigging the figure 8 on the bag side of the rope, then securing the rope from sliding through the figure 8 by securing the bag side of the rope to the same figure 8.
By doing so, one side of the rope is blocked and can be safely used for rappelling. At the same time, the other side of the rope (the one with the 8 block) can be pulled down freely when the rope is not weighted.
A carabiner is often used for safety and to facilitate pull down of the blocked rope. The carabiner is clipped to the small hole of the 8 and to the rope that goes to the bag. The carabiner keeps the 8 from coming undone (extremely rare but possible) while it also helps the block slide over rocks and cracks during the pull down.
Common mistakes
- One common mistake when using blocks is rappelling on the wrong side of the anchor ring. This can usually be prevented by firmly pulling on the rope (while maintaining solid footing on the ground) before starting the rappel: if it gives way, then you are holding the wrong side of the rope. Note, however, that the converse may not hold: it is possible to pull on the wrong strand and not feel it give way; this can happen on long rappels where the weight of the hanging rope on the other side of the rappel ring would prevent the rope from easily sliding through the rappel ring when you pull on the bag side.
- Another common mistake when rigging a figure 8 block is using the length of rope between the figure 8 and the anchor to secure the rigging. This mistake prevents conversion of the block to a lowering system while under weight without additional techniques.
- The ability to release rope can be vital in some situations (because someone is rappelling into extreme flow or because length of the rope is uncertain and no means to ascend the rope exist). In these cases, it is better to NOT lock the figure 8 with the second bight (rig the figure 8 with only one loop) and block the rope going to the bag by wrapping 3 times around the waist or leg of the person manning the station and to sit on the loose end. This way the rope is ready to be released but at the same time there is no risk for the person manning the station to get distracted and let go.
- Another mistake is to clip the safety carabiner to the anchor or to clip it to the rappelling rope, or to loop the rope around the eight in such way that the rigging is blocking the pull down of the rope. This can easily be prevented by testing the pull down before the last man rappels down, just a foot or so will be enough to prove the rope is going to pull fine. Additionally, clipping the safety carabiner to the anchor or a dogbone can prevent fatal consequences for the first bullet, though the last person will have to remember to unclip this from the anchor. Caution should be taken to ensure clipping in to the anchor does not load the 8 direction, preventing release.
Dangers
- While extremely uncommon in practice, some of the old figure 8 block variations can fail to provide sufficient friction to prevent the bag-side rope from sliding through the block if the figure 8 is prevented from squeezing the quick link against the rope. This can be solved by having the person manning the station hold the rope that goes to the bag when the first person gets on rappel, if the rope starts to slide firmly holding on the rope will be enough to stop the slide. Besides the first person down, this safety practice should also be applied with heavy loads, like two people rappelling together. This issue may have been the cause of a fatal accident.
- Some figure 8 block variations can lock under load preventing easy conversion to a lowering system. This can usually be addressed by attaching a carabiner to the hole of the 8 and to one harness and then weighing on the 8 with one weight; in most cases this is enough to free the rope from the pinch and be able to unlock the 8.
- In a dry canyon, it is safer for LAMAR to rappel DRT than SRT without the figure 8's safety carabiner attached.
Euro 8
Constructing
- Press a bight from the pull side through the small eye of the 8.
- Use the bight to start a braid.
- Make your way through the braid with 2 bights. Note that a larger braid with more bights will increase the chance of the block getting stuck between rocks and cracks during pull-down. (Rich Carlson tested this block using different numbers of bight repetitions and found 2 is sufficient).
- Lock the last bight with a carabiner to the bag side strand, or extend it into a larger bight and hold it to be ready for lowering, with a second pair of hands, or Prusik to your harness as backup. At this point the block is set for rappelling. Also, you don't need to lock the carabiner; you want to be able to quickly remove it if you need to lower, and there is no force on it requiring it to be locked.
Releasing
- Remove the safety carabiner (or remove your hand from the bight).
- Grab the bag side strand and pull firmly and briskly to release the daisy chain. Do not let go of the bag side strand and firmly hold onto it, as that is now what's preventing the rappeler from falling.
Old Variations
Critical steps
Figure 8 block variations differ at only three points in the construction process:
- Direction the bag-side rope goes directly after the first bight passes through the big hole of the figure 8. There are two possibilities:
Possibilities for direction the bag side rope can go after passing through the figure 8's big hole the first time
Left: Opposite side; the rope crosses the figure 8 from bottom to top before going around the outside edge
Right: Same side; the rope goes around the closest outside edge of the figure 8 immediately- Same-side. The bag-side rope wraps directly back around the same side of the big hole of the figure 8 that it passed through on the first rigging
- Opposite-side. The bag-side rope crosses over the figure 8 before wrapping around the opposite side of the big hole of the figure 8 that it passed through on the first rigging
- How many wraps around the stem of the figure 8 before passing the second bight through the big hole of the figure 8. "Zero" wraps means that the second bight is passed through the large hole without any unnecessary wrapping around the stem. A half-wrap causes the second bight to pass through the big hole of the figure 8 in the opposite direction as zero wraps.
- Second bight twist. Both the amount and direction of twist can be varied.
Compact secure
Same-side, half wrap, no twist The compact secure variant is currently the most widely accepted non Euro version of the 8 block. It takes less rope and time to construct and is less likely to get caught during the pull than the Euro 8. This variation is constructed with a same-side direction, a half stem-wrap, and no second-bight twist. A distinguishing characteristic is that the second bight enters the big hole of the figure 8 from the same direction as the first bight. This variant is the easiest to teach and inspect of all the variants, with the 2 bights going the same direction, and no rope crossing over the middle of the large hole in the figure 8. It is more resistant (though not immune, like the Tom Jones variation) to low-friction conditions when the figure 8 and quick link are forcefully separated. The construction of this variation is pictured in the Constructing section as well as in this video.
Short
Same-side, no wraps, no twist
This variation uses the smallest amount of rope, though that is simply a distinguishing characteristic and not an advantage of this variation. It can fail catastrophically if the figure 8 is prevented from squeezing the rope against the quick link or rappel ring. It is constructed with a same-side direction, no stem wraps, and no second-bight twist.
This variation is used in:
- Bogley post by Brian in SLC (maybe)
- Non-locking figure 8 block in Tom Jones' Bogley post
- Video showing the weaknesses certain figure 8 variations
Crossover
Same-side, no wraps, non-capturing half twist
This variation is very similar to the short variation, but it adds a twist to the final bight just before capturing the small side of the figure 8 the second time. It is constructed with a same-side direction, no stem wraps, and a half-twist of the second bight that does not capture the bag-side end of the rope.
This variation is used in:
- Cerberus Canyons (canyonquest.com)
- Bogley post by Brian in SLC (maybe)
- Canyoneering.net post by Rich Carlson
- Bogley post "Figure 8 block with lower"
Tom Jones
Same-side, no wraps, capturing half or full twist
This variation is the one described by Tom Jones on the Bogley forum. It is the only variation described on this page where a high-tension section of rope passes over a lower-tension strand of rope, causing the system to lock down on itself regardless of rope size (similar to a clove hitch). It is very similar to the crossover variation, but the twist of the final bight is in the opposite direction as the crossover variation. It is constructed with a same-side direction, no stem wraps, and a half or full twist of the second bight that captures the bag-side end of the rope. This variation can be difficult to release when weighted; see this video. However, it has the advantage that, after dressing, it will not slip regardless of load.
This variation is used in:
- Locking figure 8 block in Tom Jones' Bogley post
- Failure to release after loading in pull testing video at 5:15
- Release difficulty test video
Petzl
Opposite-side, no wraps, no twist
Petzl secure
Opposite-side, half wrap, no twist This variation is constructed with an opposite-side direction, an additional half stem-wrap, and no second-bight twist.
<needed: picture of this variation>
Discussions
The figure 8 block has been the subject of many discussions. Some of them are listed here.
- Pros and cons of the Tom Jones locking variation (December 2015)
- Releasability of the Tom Jones variation (July 2015)
