Floating anchors: But what if the anchor blows?

Introduction

Floating anchor: but what if the anchor blows?
Isn't it going to rip the other anchor out?

I hear this as a common statement quoted back to me when talking about a two-point floating anchor (aka self-equalising, magic X, etc.).

I can only think it’s based on the assumption you have:

  1. Built or chosen really poor anchors
  2. Rigged the anchors together in a way that does not share the load between the two, and
  3. As one anchor fails and dynamically loads the rigging and ropes, there is nothing to absorb any force.

We wanted to dig a bit deeper and find out more about this type of anchor. However, the deeper I got down the rabbit hole, the more questions we had, resulting in more testing. We finally finished up having undertaken over 270 tests with 800 data points recorded.

While there is always more to do, we have got to a point where I understand a great deal more about this type of anchor and have some recommendations at the end about what to use, the best configurations and what not to use, as well as a different way to think about anchor rigging.

As a bonus at the end of the post, we have included a link to a 200+ page summary report with more detail on the testing.

Anchor types

Usually, we use two anchors because we have judged that one anchor by itself is not enough for the context and purpose of use. You need two (or more) marginal anchor points joined together to make a bombproof.

Marginal anchor point

An assessment that a single anchor point is not able to hold the entire load. Several marginal anchors are joined together to form a suitable anchor with sufficient margin.

Bombproof anchor point

An assessment that a single anchor point can hold the entire load with sufficient margin. You do not need to have a second anchor point or back up this type of anchor.

Load sharing

Where multiple anchor points are rigged in a way that shares the load as equally as possible.

Focal point

Where the rigging from multiple anchors comes together, and carabiners are attached. 

Fixed focal point

Rigging for a multi-point anchor where the focal point cannot adjust. The load sharing capability has a limited range.

Floating focal point (aka self-equalizing, magic X) 

Rigging for a multi-point anchor where the focal point can adjust and move left and right under load. The load sharing capability has more range.

Context and purpose

A floating anchor, self-equalizing rigging, magic X (or whatever you choose to call it) has long been used in rescue to automatically attempt to share the load between a two-point anchor without any manual input. That is, as you are busy lowering, as an example, you don’t have to monitor and adjust the force on each anchor. The rigging method is fast to tie and uncomplicated to set up in your rescue environment.

2 point floating anchor carabiner pulleys

For backcountry rescue applications, we have used this configuration for a slope lower where the fall line can change (the load moves side to side) and where the anchors are close together or inline, such as 2 bolts. In these cases, it is difficult to predict where the focal point of the 2-point anchor will be, or any movement off the centre line will result in one anchor being loaded (negating the point of having a second anchor).

Testing overview

What we wanted to know?

Some key things we wanted answered:

  • Does the floating focus anchor work and automatically share load between anchors?
  • What configuration is most suitable for sharing the load between two anchors?
  • If one anchor fails in a floating focal will it rip out the other anchor in comparison to a fixed focal point?
  • Does the rigging material, length and type of the 2-point anchor effect the drop test result?
  • What is the suitability of each configuration during slow pull testing both in two point configuration and after it has failed onto one anchor?

Materials used

Software

Edelrid X Tube 25mm tubular webbing

Aspiring 16mm tubular webbing

PMI 8mm accessory cord

CT Dynamic Rope Sling (sewn)

PMI 7mm accessory cord

Nautilus 6mm dyneema cord

Hardware

ISC Eiger Pulley 36kN

ISC Eiger Pulley 36kN
ISC Eiger Pulley 36kN

Petzl Partner 15kN

Petzl PARTNER 15kN
Petzl PARTNER 15kN

SMC Micro Double PMP 22kN

SMC Micro double PMP
SMC Micro double PMP

Petzl Rollclip Z Triact Lock 20kN

Petzl ROLLCLIP-Z-TL 20kN
Petzl ROLLCLIP-Z-TL 20kN

Rock Exotica Mini PMP 30kN

Rock Exotica Mini PMP
Rock Exotica Mini PMP

Configurations

2 point fixed anchor

  • 90cm loop end to end.
  • Loop tied with a double fisherman’s bend, tape bend or sewn.
  • Focal point tied with an overhand knot.
2 point anchor fixed focus
2 point anchor fixed focus

2 point floating anchor

  • 90cm loop end to end (for most variations),120cm end to end (for the cords only) and 180cm for the quad.
  • Loop tied with a tape bend, double fisherman’s bend or sewn.
  • Limiting overhands tied 20cm apart – 10cm either side of centre. 
  • NOTE: 20cm has been chosen as a distance that limits fall distance while still allowing movement side to side.
  • The drop-tested length was around 65cm tied with limiting overhands with the knots being around 18-20cm from either end.
2 point anchor floating focus 1 strand 1 biner
2 point anchor floating focus 1 strand 1 biner
2 point anchor floating focus 2 strand 2 biner
2 point anchor floating focus 2 strand 2 biner
2 point anchor floating quad 2 strand 1 biner
2 point anchor floating quad 2 strand 1 biner
2 point anchor floating 1 strand 1 pulley
2 point anchor floating 2 strand 2 pulley
2 point anchor floating 2 strand 2 pulley
2 point anchor floating 2 strand double pulley
2 point anchor floating 2 strand double pulley

Side to side tests - does it work?

Test setup

  • The two-point anchor setup was pre-loaded with a 100kg load (100kg bag).
  • The two-point anchor were ring hangers through-bolted into a 150×50 wooden beam 60cm apart.
  • Force was measured using Rock Exotica Enforcer load cells into each anchor set on slow mode (2Hz).
  • Pre-movement centred force (kN) was recorded on both anchors.
  • The load was moved sideways by 40cm (in both directions) with a jigger attached to the bag’s base.
  • The force was recorded on each side at the anchors.
  • The angle moved off centre was approximately 12.5 degrees.
  • The base of the bag was approximately 1.7m from the focal point.
  • The anchor angle at the start was approximately 50 degrees.
START CENTRE Side to side load sharing test
START CENTRE Side to side load sharing test
PULL RIGHT side to side load sharing test
PULL RIGHT side to side load sharing test
PULL LEFT side to side load sharing test
PULL LEFT side to side load sharing test

Materials used

Materials used 16mm webbing 25mm webbing 8mm cord 8.2mm dynamic 7mm cord 6mm Dyneema
2pt fixed, 2 strands, 1 carabiner
2pt floating, 1-2 strands, 1-2 carabiners
2pt floating, 2 strands (one with a twist), 1 carabiner
2pt floating, quad, 2 strands, 1 carabiner
2pt floating, 1- 2 strands, 1-2 pulleys

Pulleys used

  • ISC Eiger,
  • Petzl Partner,
  • SMC Micro Double PMP,
  • Petzl Rollclip Z Triact Lock, and
  • Rock Exotica Mini PMP.

Test results

Side to side load share tests, 40cm, 100kg load

# tests LHS anchor kN RHS anchor kN LHS anchor kN RHS anchor kN LHS anchor kN RHS anchor kN
Rigging START CENTRE PULL RIGHT PULL LEFT
2pt fixed, 2 strands, 1 carabiner 6 0.52 O.58 O.79 0.32 0.38 0.78
2pt floating, 1 strand, 1 carabiners 6 0.51 0.46 0.65 0.38 0.40 0.64
2pt floating, 2 strands, 2 carabiners 6 0.47 0.50 0.64 0.32 0.32 0.65
2pt floating, 2 strands (one with a twist), 1 carabiner 6 0.56 0.52 0.82 0.34 0.36 0.79
2pt floating, quad, 2 strands, 1 carabiner 3 0.50 0.53 0.73 0.43 0.43 0.72
2pt floating, 1- 2 strands, 1-2 pulleys 6 0.53 0.52 0.62 0.50 0.51 0.62

Analysis

Fixed focal:

  • The fixed focal point for anchors close together does not do a great job (80/35) at sharing the load between anchors if you move away from the fall line.
  • When the load is on the fall line or the direction of loading is managed (e.g. with a redirect pulley) this anchor system can perform well.
  • The material used in the anchor (16 and 25mm webbing, 6mm Dyneema cord, 7 and 8mm cord and 8.2mm dynamic rope) did not make a major difference to the result.

Floating focal with carabiner(s):

  • Using a floating focal between two limiting overhands with one or two carabiners in either one or two strands improves the system’s ability to automatically share the load between two anchors (65/30-40).

  • Using a floating focal quad between two limiting overhands with one carabiner with two strands gives a small improvement in the systems’ ability to automatically share the load between two anchors (70/40).
  • The material used in the anchor (16 and 25mm webbing, 6mm Dyneema cord, 7 and 8mm cord and 8.2mm dynamic rope) did not make a difference to the result.

Floating focal with a twist/magic X:

  • Using a single carabiner into both strands (one with a twist – or known as magic X in webbing) did not work at all well (80/35). Of all the testing undertaken it performed the worst. As both strands were evenly loaded, these things were happening in addition to the friction on the carabiner; 

    With cord:

    1. The strand with a twist wrapped around the carabiner much more (up to 50%) than just clipping to one strand thus increasing friction.
    2. When the carabiner moves, the strands are moving in opposite directions, increasing friction.
    3. Where the carabiner is narrow, the two cords roll together, further increasing friction.

    With webbing:

    1. One piece of webbing is on top of the other, thus increasing friction.
    2. The focal point did not move; this was the same as tying a knot at the focal point.
    3. Both the narrow end and the wide end of an HMS alloy carabiner was tested with no difference in the result.

Floating focal with pulley(s):

  • Using a floating focal between two limiting overhands with a pulley(s) gives a significant improvement in the systems’ ability to share the load between two anchors (60/50) automatically. It’s as close to equal sharing as you can get.
  • The force hardly changed as it did not have to overcome the friction of the carabiner. Using a single (small) pulley in one strand, two pulleys into two strands or a double pulley into two strands all work equally as well.
  • We only tested with 8mm cord as previously it was found the material did not affect results.

Drop Tests

Test setup

  • One side of a two-point anchor was failed by cutting a 3mm cord with a knife.
  • The two-point anchor setup was pre-loaded with a 200kg mass (2x 100kg bags) which applies initially (approximately) 2kN of force (or more exactly 1.96kN).
  • The two-point anchor; ring hangers bolts coach screwed (10mm x 100mm) into a large tree 20cm apart.
  • The anchor angle was approximately 15 degrees.
  • Force was measured using Rock Exotica Enforcer load cells (500Hz) attached on each anchor set on fast mode.
  • Predrop force (kN) was recorded on both anchors and post drop MAX on the side released onto.
  • Pre-drop and post-drop length (cm) was measured the side released onto.
  • Both sides were tested:
    • the bend and the non-bend side when tied or
    • the sewn and non-sewn side of the dynamic sling.
    • Note: the sewn side was grouped with the side without the bend as it had less stretch.
  • NOTE: the testing is the worst case with no ropes and limited length rigging to absorb the fall’s energy.

Fixed focal, 1 carabiner, 2 strands clipped, 90cm sling

2 point fixed 0cm drop testing
2 point fixed 0cm drop testing

Test results

2-point fixed, 1 carabiner, 2 strands clipped, 0cm drop, on the side WITH the bend, 90cm sling

Material Avg. (3) kN Comments
Edelrid 25mm webbing 2.57 Dropped on side with bend
Aspiring 16mm webbing 2.51 Dropped on side with bend
PMI 8mm cord 2.59 Dropped on side with bend
CT 8.2mm dynamic rope 2.45 Dropped on side without stitching
PMI 7mm cord 2.51 Dropped on side with bend
Nautilus 6mm Dyneema/Polyester cord 2.99 Dropped on side with bend

2-point fixed, 1 carabiner, 2 strands clipped, 0cm drop, on the side WITHOUT the bend, 90cm sling

Material Avg. (3) kN Comments
Edelrid 25mm webbing 2.60 Dropped on side without bend
Aspiring 16mm webbing 2.55 Dropped on side without bend
PMI 8mm cord 2.79 Dropped on side without bend
CT 8.2mm dynamic rope 2.39 Dropped on side with stitching
PMI 7mm cord 2.77 Dropped on side without bend
Nautilus 6mm Dyneema/Polyester cord 2.71 Dropped on side without bend

 Analysis

  • Drop test 0cm, 200kg mass.
  • Dropped on the bend side:
    • Most anchor materials were around the same value (2.4-2.6kN) – all are suitable for fixed rigging.
    • The 6mm Dyneema was the only tests that increased due to the low stretch (3kN).
  • Dropped on the side without the bend:
    • Anchor materials were around the same value (2.5-2.8kN) – all are suitable for fixed rigging.
  • If one anchor fails then, as long as you have built a good anchor, the other one should not fail no matter what the anchor rigging material used.

Floating focal, 1 carabiner, 1 strand clipped, 90cm sling

2 point floating 10cm drop testing
2 point floating 10cm drop testing

Test results

2-point floating, 1 carabiner, 1 strand clipped, 10cm drop, on the side WITH the bend, 90cm sling

Material Avg. (3) kN Comments
Edelrid 25mm webbing 5.18 Dropped on side with bend
Aspiring 16mm webbing 5.00 Dropped on side with bend
PMI 8mm cord 4.96 Dropped on side with bend
CT 8.2mm dynamic rope 3.90 Dropped on side without stitching
PMI 7mm cord 3.90 Dropped on side with bend
Nautilus 6mm Dyneema/Polyester cord 5.71 Dropped on side with bend

2-point floating, 1 carabiner, 1 strand clipped, 10cm drop, on the side WITHOUT the bend, 90cm sling

Material Avg. (3) kN Comments
Edelrid 25mm webbing 6.32 Dropped on side without bend
Aspiring 16mm webbing 5.26 Dropped on side without bend
PMI 8mm cord 5.45 Dropped on side without bend
CT 8.2mm dynamic rope 4.19 Dropped on side with stitching
PMI 7mm cord 5.53 Dropped on side without bend
Nautilus 6mm Dyneema/Polyester cord 8.23 Dropped on side without bend

 Analysis

  • Drop test 10cm, 200kg mass.
  • Dropped on the side with the bend:
    • 7mm cord and 8.2mm dynamic rope was around 4kN;
    • 8mm cord, 16 and 25mm webbing was around 5kN and
    • 6mm Dyneema was around 6kN.
  • Dropped on the side without the bend:
    • 8.2mm dynamic rope was around 4kN;
    • 7/8mm cord, 16 webbing was around 5kN,
    • 25kN webbing was around 6kN and
    • 6mm Dyneema was around 8kN.
  • The dynamic rope provides the best result of all the testing and is recommended.
  • 7/8mm cord provides good all-round performance (can be used with a pulley) and is recommended.
  • 16mm webbing could be used with a carabiner.
  • 25mm webbing is not recommended for this type of anchor system as test results are over 6kN.
  • 6mm Dyneema should not be used for this type of anchor system as the max force was over 8kN.

Floating focal, 1 carabiner, 1 strand clipped, 120cm sling

Test results

2-point floating, 1 carabiner, 1 strand clipped, 10cm drop, on the side WITHOUT the bend, 120cm sling

Material Avg. (3) kN Comments
PMI 8mm cord 5.07 Dropped on side without bend
CT 8.2mm dynamic rope 3.67 Dropped on side with stitching
PMI 7mm cord 5.09 Dropped on side without bend

Analysis

  • Dropped 10cm. 200kg mass.
  • Dropped on the side without the bend/sewn side:
    1. 8.2mm dynamic rope was around 4kN
    2. 7/8mm cord was around 5kN
  • Going from 90-120cm length sling did not significantly affect the max force at failure.

Floating focal, quad, 1 carabiner, two-strands clipped, 180cm sling

Quad drop test start
Quad drop test start
Quad drop test finish
Quad drop test finish

Test results

Quad, 1 carabiner, 2 strands clipped, 10cm drop, side WITH the bend, 180cm sling

Material Avg. (3) kN Comments
PMI 8mm cord 6.30 Dropped on side with bend
PMI 7mm cord 5.43 Dropped on side with bend
Nautilus 6mm Dyneema/Polyester cord 9.20 Dropped on side with bend

Quad, 1 carabiner, 2 strands clipped, 10cm drop, side WITHOUT the bend, 180cm sling

Material Avg. (3) kN Comments
PMI 8mm cord 5.03 Dropped on side without bend
PMI 7mm cord 4.79 Dropped on side without bend
Nautilus 6mm Dyneema/Polyester cord 7.07 Dropped on side without bend

Analysis

  • Dropped 10cm. 200kg mass.
  • Dropped on the side with the bend:
    • 7mm cord was around 5kN, 
    • 8mm cord was around 6kN and
    • 6mm Dyneema was around 9kN.
  • Dropped on the side without the bend:
    • 7/8mm cord was around 5kN, and
    • 6mm Dyneema was around 7kN.
  • 7mm cord provides good all-round performance (can be used with pulleys) and is recommended.
  • 8mm cord is not recommended for this type of anchor system as max force was over 6kN.
  • 6mm Dyneema cord should not be used as the max force was over 9kN.

Floating focal with 1 or 2 pulley(s), 1-2 strands clipped, 90cm sling

2 point floating 1 pulley 1 biner drop test start
2 point floating 1 pulley 1 biner drop test start
2 point floating 1 pulley 1 biner drop test finish
2 point floating 1 pulley 1 biner drop test finish

Test results

2-point floating, 1 -2 pulleys, 1-2 strands clipped, 10cm drop, 90cm sling

Material Avg. (3) kN Comments
PMI 8mm cord 5.04 Dropped on side with bend. 1 strand. 1 pulley. 1 carabiner.
PMI 8mm cord 4.45 Dropped on side without bend. 1 strand. 1 pulley. 1 carabiner.
PMI 8mm cord 5.07 Dropped on side without bend. 2 strand. 2 pulley. 2 carabiner.

 Analysis

  • Testing with 8mm cord found that there was no more or less force compared to carabiners and with no damage to the cord by 1 or 2 pulleys.

Slow pull tests

Testing procedure

  • A 100kN vertical testbed (60Hz) was used at Aspiring Safety, 1/6 Burdale Street, Riccarton, Christchurch https://www.aspiring.co.nz.
  • New cord, rope or webbing was used for the testing.
  • All tests were completed between 12mm pins, 10mm mallion rapides or 12mm steel carabiners.
  • Anchors were attached to the outside holes of a 5 hole rigging plate as shown below.
  • 5cm tails were used as the standard.
  • All knots and bends had hand tension. All strands were pulled to the get bend/knot tight and compact.
  • Three tests were undertaken on each variation.
  • Testing speed was set at 100mm/minute.

Fixed focal, 1 carabiner, 2 strands clipped

2 point fixed anchor testing slow pull

Test results

2 point fixed, 1 carabiner, 2 strands clipped

Material Avg. (3) kN Comments
Edelrid 25mm webbing 36.23 Broke at fixed overhand, 1 strand, leg without bend
Aspiring 16mm webbing 22.32 Broke at fixed overhand, top side, 1 strand, leg without bend
PMI 8mm cord 24.47 Broke at fixed overhand, top side, 1 strand, leg without bend
CT 8.2mm dynamic rope 30.24 Broke at fixed overhand, both strands, bottom leg
PMI 7mm cord 19.77 Broke at fixed overhand, top side, 1 strand, leg without bend
Nautilus 6mm Dyneema/Polyester cord 28.03 Broke at anchor rapide, leg without bend

 Analysis

  • Slow Pull test 100mm/min
  • All the materials tested (25mm webbing, 8mm dynamic, 8mm, 7mm, 6mm cord) were well over 20kN except for 7mm cord which was just under (19.77kN).
  • All the materials are suitable for 2-person loads in this configuration.

Floating focal, 1 carabiner, 1 strand clipped

Test results

2 point floating, 1 strand, 1 carabiner clipped

Material Avg. (3) kN Comments
Edelrid 25mm webbing 25.09 Broke at limiting overhand, 1 strand, leg without bend
Aspiring 16mm webbing 17.22 Broke at limiting overhand, 1 strand, leg without bend
PMI 8mm cord 18.36 Broke at limiting overhand, bottom side, leg without bend
CT 8.2mm dynamic rope 18.07 Broke at limiting overhand, 1 strand, on sewn leg
PMI 7mm cord 15.60 Broke at limiting overhand, 1 strand, leg without bend
Nautilus 6mm Dyneema/Polyester cord 16.53 Broke at limiting overhand, 1 strand, leg without bend

Analysis

  • Slow Pull test 100mm/min.
  • Only 25mm webbing was well over 20kN and suitable for 2-person rescue loads.
  • 8mm cord and 8.2 dynamic rope were over 18kN and suitable for 2-person rescue loads when using a 10mm rope.
  • The 7mm cord, 6mm cord, 16mm webbing is between 15-18kN and suitable for single person loads.

Floating focal, 2 carabiner, 2 strands clipped

2 point floating 2 strand 2 carabiner

Test results

2 point floating, 2 strands, 2 carabiners clipped

Material Avg. (3) kN Comments
Edelrid 25mm webbing 36.64 Broke at limiting overhand, both strands, overhand disintegrated, leg without bend
Aspiring 16mm webbing 27.42 Broke at limiting overhand, both strands, overhand disintegrated, leg without bend
PMI 8mm cord 29.61 Broke at limiting overhand, top and bottom, leg without bend
CT 8.2mm dynamic rope 31.53 Broke at limiting overhand, both strands, on sewn leg
PMI 7mm cord 26.39 Broke at limiting overhand, both strands, leg without bend
Nautilus 6mm Dyneema/Polyester cord 29.93 Broke at limiting overhand, 1 strand, top side, leg without bend

Analysis

  • Slow Pull test 100mm/min.
  • All the materials tested were over 20kN.
  • All the materials are suitable for 2-person loads in this configuration.

Floating focal, quad, 1 carabiner, 2 strands clipped

2 point quad anchor testing

Results

2 point floating, quad, 2 strands, 1 carabiner clipped

Material Avg. (3) kN Comments
PMI 8mm cord 36.71 Broke at limiting overhands, both strands, bottom side, leg without bend
PMI 7mm cord 28.66 Broke at limiting overhands, both strands, bottom side, leg without bend
Nautilus 6mm Dyneema/Polyester cord 32.12 Broke at limiting overhands, leg with bend, stripped sheath second strand

Analysis

  • Slow Pull test 100mm/min.
  • All the materials tested were well over 20kN.
  • All the materials are suitable for 2-person loads in this configuration.

Floating focal, 1 carabiner, 2 strands clipped (1 with a twist)

2 point floating with a twist
2 point floating with a twist

Results

2 point floating, 2 strands, 1 twist, 1 carabiner clipped

Material Avg. (3) kN Comments
PMI 8mm cord 29.01 Broke at limiting overhand, 1 strand, top side, leg without bend
Aspiring 16mm webbing 24.35 Broke at limiting overhand, 1 strand, top side, leg without bend
Edelrid 25mm webbing 33.68 Broke at limiting overhand, 1 strand, top side, leg without bend

 Analysis

  • Slow Pull test 100mm/min
  • All the materials tested were well over 20kN.
  • All the materials are suitable for 2-person loads in this configuration.

Floating focal, 1-2 strands, 1-2 pulleys clipped

2 point floating 2 strand 2 pulleys

Results

2 point floating, 1-2 strands, 1-2 pulleys, 1-2 carabiners

Rigging Avg. (3) kN Comments
2pt floating 1 strand 1 pulley 1 biner 18.44 Single overhand, broke overhand limiting, 1 strand, side without bend, ISC Eiger 35kN pulley
2pt floating 1 strand 1 pulley 1 biner 17.83 Single overhand, broke pulley, Petzl partner 15kN pulley
2pt floating 2 strand 2 pulley 2 biner 31.25 Single overhand, broke overhand limiting, 1 strand, side without bend, ISC Eiger 35kN pulley
2pt floating 2 strand 2 pulley 2 biner 32.46 Single overhand, broke overhand limiting, 1 strand, side without bend, broke 1 pulley, Petzl partner 15kN pulley

Analysis

  • Slow Pull test 100mm/min
  • Single Petzl Partner pulley is recommended for single person loads.
  • Single ISC pulley is recommended for 2-person loads.
  • All the 2-strands, 2-pulleys are suitable for 2-person loads in this configuration.

Single leg, post drop loop configuration, between the limiting overhands

Post drop loop slow pull test setup
Post drop loop slow pull 1 strand 1 pulley 1 biner
Post Drop Loop 2 strand 2 pulleys 2 biners
Post drop slow pull loop 2 strand 2 pulleys 2 biners

Results

2 point floating, 1-2 strands, 1-2 pulleys, 1-2 carabiners

Rigging Avg. (3) kN Comments
Post drop loop 1 strand 1 biner 15.08 Single overhand, broke overhand limiting at carabiner end, 1 strand
Post drop loop 2 strand 2 biner 15.37 Single overhand, broke overhand limiting at carabiner end, 1 strand
Post drop loop 1 strand 1 pulley 1 biner 17.04 ISC Eiger 35kN pulley, single overhand, broke overhand limiting at pulley end, 1 strand
Post drop loop 2 strand 2 pulleys 2 biners 12.64 ISC Eiger 35kN pulley, single overhand, broke overhand limiting at pulley end, both strands
Post drop loop 2 strand 2 pulleys 2 biners 15.42 ISC Eiger 35kN pulley, doubled overhand, broke overhand limiting at pulley end, 1 strand
Post drop loop 2 strand 2 pulleys 2 biners 11.30 Petzl partner 15kN pulley, single overhand, broke overhand limiting at pulley end, 1 strand
Post drop loop 2 strand 2 pulleys 2 biners 16.31 Petzl partner 15kN pulley, doubled overhand, broke overhand limiting at pulley end, 1 strand
Post drop loop 2 strand 1 double pulley 1 biner 9.90 SMC micro double 22kN pulley, single overhand, broke overhand limiting at pulley end, 1 strand
Post drop loop 2 strand 1 double pulley 1 biner 9.98 SMC micro double 22kN pulley, doubled overhand, broke overhand limiting at pulley end, 1 strand

Analysis

  • Slow Pull test 100mm/min
  • Single carabiner and double carabiners are all over 15kN and suitable.
  • Two small single pulleys (rigged with two carabiners) failed well under 15kN with a single overhand limiting knots. When rigged with a doubled overhand limiting knots failed over 15kN and is recommended.
  • A small double pulley is under 10kN when tested with any variation and should not be used.

Conclusions

1. Build quality anchors

  • Make sure you choose and build anchors with sufficient margin with the expected worst-case loading – ideally 2x 10kN for rescue.
  • Prevent a failure from occurring in the first place.

2. Share the load as equally as possible between the anchor points.

2.1 Use fixed focal anchor rigging for fall line loads

  • If you are heading down the fall line and your direction of loading is unlikely to change, tie a fixed focal anchor.
  • A failure of one anchor puts relatively little additional force on the second anchor.
2_point fixed overhand 2
2 point fixed overhand closeup
2_point fixed overhand 1
2 point fixed overhand overview

2.2. Use floating focal anchor rigging with off-centre loads

  • Use a floating focal with limiting overhands when you have a situation that warrants it. Where it is difficult to keep the force centred between the two anchors.
PULLEYS
  • Where you have pulley carabiners (20kN+), clip them into the strands, between the limiting overhands of the floating focal, to allow close to even loading between the two anchors.
2_point floating carabiner pulley 2
2 point floating carabiner pulley closeup
2_point floating carabiner pulley 1
2 point floating carabiner pulley overview
  •  When using two small pulleys (20kN+) tie doubled limiting overhands – two wraps (rather than one)  – to get the strength of a post drop single anchor leg well above 10kN. 
2_point floating small pulley 1
2 point floating small pulleys closeup
2_point floating small pulley 2
2 point floating small pulley overview
  • Not all small pulleys are suitable. Single pulleys with thicker side plates and rounded edges are preferred.
  • Don’t use a small double pulley as the centre plate has sharp edges and the residual strength of a post drop single anchor leg can be below 10kN. Although we only tested one brand, all the small double pulleys we own have the same narrow and sharp centre plate.
CARABINERS
  • Clipping into one or two strands with a one or two carabiners is better than fixed focus and is recommended.
2_point floating carabiners 1
2 point floating carabiners closeup
2_point floating carabiners 2
2 point floating carabiners overview
QUAD
  • Clipping into two strands with a one carabiners gives a small improvement over fixed focus.
  • If you need the additional strength and redundancy for some specialised rigging jobs, then a quad may be appropriate if used with pulleys and 7mm cord.
  • Considering you use double the amount of cord to other styles tested, there were no particular gains in performance in any testing carried out when comparing 4 to 2 strand rigging.
PUTTING IN A TWIST/MAGIC X
  • We don’t recommend a twist (or magic X in webbing) in the second strand into one carabiner as it performs poorly by increasing friction and negates the point of having a floating focal.
  • There is no need to have a second strand twist. When clipped into the middle of the limiting overhands loop, the load can only drop as far as the limiting overhands. 
  • Note: in the distant past we used to twist when there was no limiting overhands and failure of one leg would mean you would slide off the end. 
  • If you need two strands clip in two carabiners.

3. The materials you use does matter if one anchor fails

  • Using dynamic rope sling anchor legs significantly decreases the single anchors’ load (around 4kN) should one anchor fail.
  • 7mm and 8mm nylon cord provides a balance between using a non-specialist general rigging and keeping forces to a reasonable level (5kN) while allowing you to use pulleys for better load sharing.
  • 16mm webbing is around 5kN and can be used.
  • 25mm webbing is over 6kN and is not recommended.
  • Low stretch anchor rigging material in floating anchors, such as 6mm Dyneema, is not recommended as they provide little shock absorption should an anchor failure occur (over 9kN).

Final thoughts

Question:

Floating anchor: but what if one anchor ‘BLOWS’ – isn’t it going to rip the other anchor out?

Answer:

The statement above is back to front thinking for anchors. Our priorities should be the other way round. If we build quality anchors with sufficient margin and share the load as equally as possible between them, they are unlikely to fail.

Therefore the hierarchy of thinking for 2-point anchors should be:

  1. Choose and build quality anchors with sufficient margin for the expected worst-case loading.
  2. Make sure you share the load equally between the two anchors using the most appropriate rigging method for the situation – fixed or floating.
  3. If you fail on #1 and #2 and put a shock load onto one marginal anchor, dynamic rigging material will significantly reduce force for a 2-point floating focus. 

However, let’s do a better job, and #1 and #2 and not get to #3.

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