Dyneema and Not Dying



01 Introduction

02 What Is Dyneema (or UHMWPE or HMPE or Spectra)?

03 Before You Read More: Where BARE Amsteel Sucks

04 The CoF Conundrum

05 That Heat Issue

06 More on Munters and Carabiners

07 The Heat Conductivity Advantage

08 Knots, Not-Knots, High-Friction Sleeves, Heat Again

09 Shrinkage and Heat Again!

10 Construction Stretch and Maximum Force

11 Samthane and Melting

12 Dyneema  “Friction” Knots

13 Appearances Can Be Deceiving

14 Finally

15 References


01 Introduction

This document* summarizes my experiences using bare (uncovered) and polyester-covered Dyneema for short rappels and ascents. Initially, I wanted to test thin Amsteel for emergency raps down a snowfield, using minimal equipment. However, I was stunned by the number of negative reponses I received, all based purely on the low melting point of dyneema (145 °C (293 °F)) and some odd views on energy transfer. I had done many tests rappelling on bare dyneema, and I had never found the rope to heat significantly. So I thought I would dig into the matter more. I’ll discuss some of the issues limiting use. I've been stuck at home with kidney stones for about 2 months, and had lots of time for testing.

To summarize some of the most important points:

1) BARE Dyneema has a very low surface coefficient of friction (CoF) [REF 01], probably 0.2-0.3x that of polyester or nylon. The low CoF makes it less likely to heat up in a rappel “device,” but creates serious issues when tying knots. However, there are partial solutions for knotting, and the low CoF isn’t as serious a rap issue as you might think. As we’ll see, some rappel “devices” depend little on the traditional surface  CoF for braking power, and more on energy lost to deformation of the rope. Such energy loss mainly stays in the rope, and probably contributes less to device heating.

2) Dyneema has a radial thermal conductivity (perpendicular to the fibers in the rope) just slightly higher than nylon or polyester, so it takes a long time for heat to diffuse “into” or “out of” a rope. HOWEVER, the axial conductivity (parallel to the fibers and rope) is very high, 15-50x polyester or nylon [REF 02]. That means that heat that does manage to diffuse into the rope spreads out and diffuses away (axially) much more quickly, so Dyneema is unlikely to reach melting in many scenarios.

Before you give me a lecture about the low melting point of Dyneema, check the video linked below. Yes, the video progresses through the word slides slowly, but I’m hoping you will either fast forward, or actually read. For equivalent test conditions and rope diameters, in a friction heating test, the polyester fails 4x faster than the Dyneema.  The output from this test is mainly for a thermal model, but it does give a visceral understanding.

I’ve repeated this test with 8.5mm  polyester rope in a 1” nylon sling; that failed 2 times as fast as the Dyneema, but the geometry is not identical and not as useful for model testing. I've actually done 8 of these fail tests; they are pretty consistent.

I test new techniques and materials many, many times before I use them in serious situations. The ceiling of my stairwell is reinforced, with several clip-in points; I can do 10’ of hanging rap, or ascent, at home. I test strength of ropes and knots using: 2 trees in my backyard; an industrial pull scale; a come-along ratchet; and 97% efficiency pulleys for mechanical advantage. In some cases I can test the “strength” of high modulus materials with practice falls.  When I feel fairly confident, I go to a test area (below, 5 miles from my house), and try the methods and materials there on 100’ of rap. Such tests are not guarantees everything I’ve tried is safe, especially for someone who doesn’t like to heed the cautions below. Use this information at your own risk.



The reinforced ceiling above my stairwell allows me to test carabiner/cordage combinations in relatively safe situations, and also allows me to measure effective friction coefficients and brake factors. I can directly instruments webbing/rope with thermocouples. Below I am using double-strand supermunter to rap 10' hanging on 2.8 mm cord in my stairwell. You can see the rap is rather smooth and can be controlled with the tips of the fingers of one hand.

Still, it is hard to do a real 100', double-strand hanging rap, where all one's body weight is trasmitted through the rope, in a safe testing environment. As described below, I have devised a way to do a simulated rap, to measure the highest possible temperature rise on the "rappel device."

02 What Is Dyneema (or UHMWPE or HMPE or Spectra)?

Dyneema is a trademark for ultra-high molecular weight polyethylene (UHMWPE); that particular trademark is owned by DSM Dyneema corporation. Spectra is another type of UHMWPE, trademarked by Honeywell. UHMWPE is also referred to as high modulus polyethylene (HMPE), for the very large tensile modulus, compared to nylon and polyester fibers. Chemically, UHMWPE is much like the polyethylene used in plastic bags and bottles; but instead of having short, unoriented molecular chains, it has very long individual molecules, arranged in parallel. UHMWPE is also characterized by a very low surface coefficient of friction (CoF), high axial heat conductivity, and low melting point (~148 C) compared to nylon and polyester. UHMWPE loses strength above 70 C,  more so than polyester and dry nylon. However, the mechanical/thermal properties of UHMWPE typically mean it  may not reach the melting or failure point any faster than other synthetic fibers in rappel and ascent. The video at the beginning of this document demonstrates this point.

This document mainly discusses one variant of Dyneema: Amsteel BLUE, by Samson Rope. My experience is with BARE Amsteel in diameters of  7/64" and 1/8” (2.8 and 3.2 mm, intended for emergency raps or pull line), 3/16” and 1/4” (6mm). (The quoted diameters of Amsteel may be slightly misleading, as the rope narrows considerably under slight tension.)  I have also tested the 1/4” (6mm) polyester-sheathed Amsteel, as well as an 8mm Bluewater Canyon Pro, a  polyester-sheathed rope with Dyneema core that is NOT Amsteel Blue. I have tested New England Ropes Spyder line (polyester-covered Dyneema) in 3.8mm (1900 lb) and 4.8mm (2500 lb) for prusik loops.  The thinnest Amsteel is an 8-strand, single braid cord; all others are 12-strand single-braid. The BARE ropes are soft and highly flexible (picture below). This softness is due to a very low lay angle in the braid, meant to maximize the strength per weight. Those properties make BARE Amsteel rope less suited for situations where it may snag on rough rock.

Dyneema braided rope, similar to Amsteel,  IS used for rappels in some European rescue groups [REF 03], with special cautions. The longest recorded rap on BARE Dyneema rope is 760 METERS. ALL RAPS are done with Munters and supermunters; devices specifically for rappels, such as ATCs, are not allowed, because of the heating issue (Munters get part of braking force from rope-rope contact). In rope rescue systems, the low CoF means much less rope drag; a carabiner can get over 60% efficiency with Dyneema, vs ~53% for nylon or polyester.  Since rescues may require hundreds of meters of rope, the low weight of Dyneema (per unit strength) is a significant advantage.  However, the high modulus means that short falls can generate large forces, so energy dissipaters are used in-line.  The material doesn’t absorb water, and is very resistant to damage from UV light, which is advantageous when material must be left out for days.

The EFC (French) have experimented with a different kind of Dyneema, made by Beal, for canyoneering [REF 04].  They use 5 mm cord and have found ways to use devices such as figure 8s and Piranhas with extra wraps, and have even used ascenders on this thin cord. The Beal cord is 100% Dyneema, with a tightly-woven sheath.  Such a construction will not have the snagging and abrasion problems of Amsteel. EFC are well aware of the low melting point, and report an average device temperature of 40C (104F), with an observed maximum of 72C (158F) on a 100 METER rap. These are far above the temperatures I have seen with BARE Amsteel, but then again, they do longer raps, and use “real” rappel devices (not munters). I will not discuss this use further.

03 Before You Read More: Where BARE Amsteel  Sucks

First, Dyneema is static (though Amsteel is, in practice, nowhere as stiff as the reported modulus would imply; see “construction stretch” below), and is definitely not for belaying. Because of the very high moduli, one can generate large forces in a short fall.   Amsteel  can be used for a haul rope, or a pull line (many canyoneers use it to pull fiddlesticks).  But if you really want a light pull line, consider arborist throw lines. Like all thin cords, 1/8” Amsteel is tangly, and the very-loosely-woven braid tends to catch on rough rocks when not tensioned. When it catches, it abrades faster than you might imagine.  It makes a very poor handline, because it is slick and very thin for its strength. BARE Amsteel  is not good for canyoneers, as a general rope; because:  1) of the abrasion and catching issue;  2) it is hard to knot securely; and 3)  it cannot be used with "normal" rap devices without extra wraps and a chance of over-heating. Unsheathed Dyneema would be mainly for escape situations; 200’ of 1/8”Amsteel Blue weighs one pound and has a test strength (static) of 2500 lbs. I find people are more likely to take emergency gear if it is light.

I have rapped a lot on this stuff using Munters (“German” position) and supermunters;  however, the speed of a rap on a Munter is harder to control , than a rap with (e.g.) a Piranha on a polyester-, technora-, or nylon-sheathed rope. Ironically, the large, round-cross-section HMS carabiner is least suitable for a munter on thin Dyneema. A small D-shaped biner, such as Grivel’s Plume dual wire-gate, is far more suitable for thin bare Dyneema; the tight turns provide much more control and provide more “internal friction.”  With a large HMS, your speed is either slow and perhaps bouncy (supermunter) or near-catastrophic (regular Munter on thin cord).  You MUST test with the type of carabiner you will use. With a  dual wire-gate, I can quickly put in an extra wrap, before I plan to go down an overhanging section; but that option is tough with screwgates. If you feel good with the supermunter on you carabiner, I highly recommend you use that knot; it does NOT twist the rope, and provides a smoother, safer experience, and generally you won't have to worry about adding extra twists.

Secondly,  I weigh 148 lbs in my clothes.  Don’t do anything on the thin stuff until you have tested it with your weight.  If you weigh 250 lbs, try 9/64” Amsteel  (at least) instead of 1/8”, and go for the supermunter instead of the Munter  on thin cord. The longest I have rapped with BARE Dyneema is 100’double-strand, pushing as hard as I could. Typically the “device” (Munter on biner) is < 25F above ambient at the very end of the rap, right beneath the Munter. The most extreme delta T I have witnessed on a real double-strand rap was 41 F with 7/64"; the actual rap was about 90 feet, and was accomplished with a single Munter on a plume carabiner in 20 seconds. The T was measured with a NIST-traceable, very thin thermocouple placed on the aluminum right below the munter as soon as I finished (the dyneema itself remained fairly cool). The very highest delta I have seen was 54F, for a simulated rap on 100' of a double-strand supermunter with bare 7/64" Amsteel on an Attache carabiner. For the simulation, the carabiner was attached to a tree, my wife acted as the brake hand, and the Amsteel was attached to the back of my harness. I tried as hard as I could to "run" down the steet, pulling the amsteel through the supermunter on the biner, and pulled 100' (200' in double strand) in about 30 seconds. From tests pulling against an industrial scale, I know that I can generate > 100 lbs of force when leaning 45 degrees into the "run," on normal pavement. However, this is about twice the speed I can normally rap on a supermunter, so the friction energy/time simulates a 200 lb person taking a minute to rap 100' double-strand. So this was actually a fair test. However, if your ambient is 100F, that would put you just below the maximum recommended working T for Amsteel; but the measured T is dominated by the carabiner T, which is significantly above the Amsteel T at the end of the test.  In short, if you are thinking of trying this stuff for a rap, I recommend first doing a hanging rap from a tree over a relatively short distance.  The supermunter may surprise you with how slow it is on this stuff.

04 The CoF Conundrum

The surface CoF of Dyneema and other synthetic rope fibers, is defined by the familiar Coulomb Law. The surface Cof is the  proportionality coefficient µ in

F = µN

Where F is the force of friction opposing the motion, for a surface sliding over another, and N is the force normal to the surface.  This is a reasonable approximation when N is small. Ideally, for synthetic fibers, µ is obtained in experiments with small forces and small fibers or bundles of fibers to avoid deformation, and one distinguishes between static and kinetic friction. This is the friction right on the interface, most likely (e.g.) to heat a rappel device as rope moves over it.  In reality, µ is often measured for thick ropes, with substantial forces on a capstan, and terms proportional to rope dimensions add to F. The available literature has widely varied values for µ; values reported by rope manufactures generally come from the supplier of the fibers, and do not reflect the effects of incorporating the fiber into a soft braid versus a tightly-woven mantle. The CoF is also supposed to be dependent on temperature [REF 05], but my tests for nylon up to 150 C have failed to show this. Ideally, the reported values would describe the other surface in contact with the synthetic fibers – e.g. more synthetic fibers, steel, or aluminum; often that information is lacking. Dyneema Corp., Samson ropes, and many other sources give fiber-on-fiber ranges of 0.05-0.07 for Dyneema, and 0.15-0.25 for nylon and polyester. For braided, thick ropes on rough metallic surfaces, the static CoF may be 2x the kinetic CoF, and the values for polyester (Dacron) and nylon ropes vary widely, from similar low values (0.15) up to ~0.7 [REF 06].

Below is the CoF for Dyneema for yarns (fiber bundles) ON METAL, compared with other polymers. The low value (0.04) is less than 1/4th the CoF of polyester  (1/5th that of nylon), according to DSM, the manufacturer. These are possibly the most relevant CoF, for (e.g.) movement over a carabiner. However, we will see that application to real life is not so straight-forward.

CoF polymers


05 That Heat Issue

Here’s what I hear a lot: “Yeah, well I’ve used rap devices that got really hot! They would melt through Dyneema!”  Well, your device got hot because the sheath on your rope – which overwhelmingly controls the heat production when rappelling on a ATC , figure 8 or Piranha – was made of material with a much higher surface CoF.  Nylon, polyester, or Technora have surface CoF roughly 3x-6x that of Dyneema.  It is irrelevant that the core of your rope may have been made of Dyneema.  I have rapped 100’ with the same biner, same hour, same place, on:  1) bare 6mm Dyneema (8300 lb static test);  and 2) 6mm polyester-covered Dyneema (4900 lb static test). With the same knot, the rap was faster for (1), and the biner was about 20-25F above ambient at the end (tested by NIST-traceable thermocouple).  For (2), the biner gave me 2nd degree burns, suggesting the biner was ~100F above the ambient.

Dyneema does lose strength as temperature rises (REF 12). By 75 C, the tensile strength has dropped to 1/2 the 25 C value; by 100 C, it is 1/3 the 25 C value.  That's actually a bit better than nylon in humid air, but such nylon will continue with low strength (perhaps 10% of the 25 C value) up to just below the melting point 250 C, whereas Dyneema melts at just 148 C.  But remember,  Dyneema is much less likely to reach higher temperatures.

06 More on Munters and Carabiners

I've done hundreds of raps and many pull tests on Munters, so this advice is not given lightly.  It is something of a hobby to test every carabiner I have with unlikely cordage (I've even rapped off a quicklink).

The choice of carabiner is extremely important for Munters on BARE Amsteel.  Below is the brake factor (ratio of force on anchor side, to hand force needed for steady descent on brake side).  The test used  3/16” Amsteel on a “German” position Munter (brake hand UP for extra friction). I used 3 types of carabiners, 4 trials each and the uncertainty in the brake factor is about +/- 1. The results mean that much more brake force must be supplied by the hand of the rappeller, for a traditional round-cross-section HMS carabiner, than for the other two. Most folks learn that the big, round-cross-section  HMS biner is best for a Munter; and that may be true with polyester or nylon ropes, but it's dead wrong with thin Amsteel. Folks also tend to learn the Austrian hand position as well (probably because instructors like to ramble on about unwinding screwgates).

Note that if you were to calculate the brake factor from the capstan equation, using the known CoF for Dyneema, you would get a value of ~2, which is obviously in discordance with measured values of 8-15. While CoF strongly affects the heating of the device, it doesn’t limit how the rope dissipates energy by other means.  There are  other mechanism of energy dissipation that do not involve surface friction, yet still slow down the transit of rope; these are ambiguously described in the literature as “internal friction” or “viscous friction” [REF 07].  Bending and distortion dissipate energy, and in modern rap device,  outweigh the energy loss to surface friction  [REF 08], and these results show the same is true for Munters, especially with Dyneema, which has a low CoF.  Since this dissipation is generated within the rope, and the radial heat conductivity of the rope is very low, a rappel device won’t absorb much of this energy before the rope moves off.  The surface CoF is often measured with as light a load as possible, on single fibers or small fiber bundles, to avoid deforming the fibers.  In reality, most Dyneema ropes are very deformable; they tend to flatten out when pulled over a carabiner. In my own tests, it takes about 16 lbs of force to pull a 10  lb weight 180° over an aluminum carabiner, using 3/16” Amsteel; it takes 20-24 lbs to pull with an 8.5 mm polyester rope. Clearly the Dyneema is less frictional, but the formal surface CoF suggest much lower force than does the experiment. (The capstan equation is clearly inappropriate here, as the rope diameter is not small with respect to the carabiner thickness; nonetheless, the capstan equation does get used for analysis of Munters [REF 07].) Note that for 180 degree turns over the rounded portions, the three biners above give the same efficiency; it's only with larger turns that the irregular cross section makes a difference.

For 1/8" cord, a single Munter WITH EXTRA WRAP does well with a small carabiner. An extra wind around the biner spine helps to control the descent, and use the hand in UP position to maximize force.  My experience with “twisting” of double strand:  it isn’t a BIG issue with 100’ rap with this STATIC braided rope, even with Munter. It does happen, and one must be careful to let the end of the rope untwist before bagging it. Supemunter doesn’t twist rope. DO NOT use a simple Munter with Dyneema and a large HMS-style carabiner with oval cross-section; you will simply go too fast.  Use a supermunter on large burly biners. Yes it’s slow, and bouncy, but on the plus side, you won’t die. I've tested the 7/64" (2.8mm, 1600 lb test) only with a double-strand supermunter on the Attache carabiner, as well as a tiny (3") mettolius wiregate, and only in my in-house set-up on drops of 10'; that arrangement was surprisingly smooth, and I could control a hanging rap with a few fingers... but again, I weigh 148 lbs. The is a clear advantage in using a larger carabiner: there is more metal and surface area to dissipate and adsorb any heat buildup.

07 The Heat Conductivity Advantage

The plot below contrasts the temperature rise for Dyneema and polyester ropes, when insulated bundles of the two rope types were placed against an insulated can containing water at ~200F. (Oriented so the rope fibers were parallel to the diameter of the can.) The temperature was measured ~3.5 cm away from the can, inside the rope. In this type of geometry, the time required for the T “front” to reach a particular point x, goes as x2 . If an increment of heat is added to a Dyneema rope, within 5-10 seconds, it has blurred out (axially) about 5 mm on each side of the initial point of addition, greatly diluting the temperature rise. In the polyester rope, the equivalent blurring would be about 1mm, so a lesser volume receives heat per unit time.  While polyester rope is about 15% denser than Dyneema rope, it has just 2/3 the heat capacity per unit mass, so the same amount of added heat leads to a greater temperature increase per unit volume in polyester, below the glass point. However, above the glass point of polyester, the heat capacities are comparable.

[Sophisticated in-kitchen experiment, results plotted above. Boiling water was placed in insulated can, insulated rope bundles were attached to can with conductive tape.]


08 Knots, Not-Knots, High Friction Sleeves, and Heat Again

If you are just using BARE Amsteel  for an emergency 2-strand rap, you don’t need any “knot” except the Munter or supermunter.  But there are times when you need to provide a clippable end, as when using the cord to haul.

it is pretty common to have “trustworthy” knots fail at 35% of the rated strength, when Dyneema is tied to Dyneema  [Ref 09].  It is very useful to witness a Dyneema knot failure in real time, because on-line videos rarely show the force of breaking, and may be slowed down. In my two pull tests on Dyneema-Dyneema unions, the knot failed by having a tail pull through. The failure, once started, was very fast; if you think you are going to tie longer tails to give you some more time, think again.  Be wary of videos that show tails pulling through slowly; the machine that provides the pulling may not be keeping up with the sudden rope surplus, thus the tension is suddenly at a plateau; this gives the illusion that longer tails will save the day. In reality you might get just a few more seconds. Lastly, here is another good reason not to tie Amsteel to Amsteel: once the knots are weighted with just 150 lbs, they are extremely hard to untie.

Thin Dyneema, when paired with thicker nylon ropes, performs pretty well in some knots.  Here is a figure eight on a bight with 1/8” Dyneema paired with 8 mm nylon; the application is a pull cord for 8mm rope, when used with a carabiner block. I took the pair up to 1400 lbs, with no sign of failure. On left I pulled both ropes; on right, just Dyneema was pulled.  I’m sure I could have gone farther, but I am really not interested in using more than body weight on a pull cord.

Many  climbing sites state that one should never use an EDK  to pair ropes with different diameters, but  these sites are typically thin on explanation or guidelines.  I paired 1/8” (3.2 mm) Amsteel to 8 mm nylon, with two EDKs (the same short piece of Amsteel as in the previous test). This knot failed rather spectacularly (pictured below), when I brought the knots to 1100** lbs tension the 4th time. In the 2nd EDK (on the right), I had allowed Amsteel to cross Amsteel at nearly a right angle; effectively, one piece of Amsteel sawed through the other one. I see now that it is easy to put lop-sided (higher) stress on the smaller cord in an EDK. But 1100 lbs is not too bad for a rap, where the thinner rope would mainly be used as a pull cord under body weight. This is clearly a case where more (the 2nd EDK) was not "better.".  Since the nylon rope was not stressed in the right-side EDK, this may have been an unrealistically harsh test. As tied and weighted, these were essentially water knots. It is notable that in DMM's tests on Dyneema slings gave a water not failing at less than 24% of the full strength, considerably worse than the results of this test (failure at 44% of 1/8" Amsteel rated strength).


Another solution to the “knotability” issue is to sheathe the Dyneema in a short section of nylon.  For the example below, I sheathed  3/16” Amsteel Blue in 9/16” thin tubular webbing (available from Fish Products). Then I tied an overhand on a bight directly on the sheathed portion.

There are other ways of making permanent connections in bare Dyneema ropes. Splices are used by many mariners; and there are many youtube videos on splicing Amsteel.  In the Tatras SAR, they combine splices with the metal connectors used for cable.

WARNING WARNING WARNING The following discussion describes sewing of weight-bearing materials. This activity can be very dangerous, especially if you are inexperienced. If you have the least doubt about your sewing ability with strong threads, immediately stop reading.

 You can also sew a loop termination with many stitches of 300 lb-test fishing line. I first sew two strands of 1/8” Amsteel together, near the loop, with 15-20 penetrations of each. Then I slip a short piece of ˝” tubular webbing sheath (ClimbSpec, 1000 lb test)  over the two cords below the loop, and sew about 15-20 more penetrations through each cord, bringing the nylon tube into the stitching.  Then I dip the whole end in Seam Grip urethane, which has been thinned with Cotol 240. If you wanted an end that could be used for a girth hitch, you wouldn’t urethane coat the loop; and you would make a larger loop, and use a shorter sewn/tubing-covered section.

I have pulled this type of termination, on 1/8" Amsteel, to 1400 lbs without a sign of failure. However, I've had an odd "sort-of" failure, when I was using this type of termination to connect to a pull-scale, in order to test the knot in the image 3 steps above (edk test). The upstream knot (edk) gradually had one strand fail at a time, till just one strand was left, suddenly tensioned at 1100 lbs.  That single strand should have just broken (200 lbs breaking strength!), but it pulled out from the nylon sleeve several inches away, leaving the sewn loop termination looking pretty much intact. I don't really consider this a failure of the termination, since the failure of the tested knot was the actual cause.

If you must connect the ends of two strands terminated as above (e.g. for a long save-you-butt rap), the first option is to hitch the two eyes, with a girth-hitch (what sailors call a cow hitch), but that might get tiresome with 100’ of rope. You might consider tying an EDK just below the terminations (LEFT side in photo above). You will reduce the strength of the double-strand to ~1700 lbs, and the knot will be hard to untie after loading. EDKs are supposedly prone to inversion; I if I try to invert the EDK by hand, it hits the wide terminations and stops. That would probably be the point of failure in a pull tests, where the rope would simply break. My preference would be to connect the loops with a screwgate or dual-gate biner.

And of course, you can buy sheathed Dyneema; some canyoneering ropes have polyester or polyester/Technora sheath (e.g. old Canyon Pro and Canyon Pro DS). The 2011 test in Heaps Canyon, on the heating of ATCs while rappelling, actually used Canyon Pro DS. I’ve seen posts on canyoneering blogs where it is claimed that the Technora is for heat resistance; the manufacturer (Bluewater) claims the Technora is for cut resistance.  Apparently there was concern "on the street" that heat from the rap device could melt the Dyneema core. However, the radial conductivity of polyester is so low, and the cooling of a rap device by radiation and convection is so fast, that it is unlikely the Dyneema core will ever see significant T from a hot rap device.

To illustrate this point(video linked below), I pre-heated an ATC to ~96C (204F), placed Canyon Pro rope into the device, and measured the temperature just inside the sheath. Just 1 mm in, T never exceeded 36C (96F).  Frankly, I'd worry more about a polyester sheath being above the the glass point, where it might weaken, contract, and then pull apart, exposing the core. At least one manufacturer has recommended that ATCs not be used on polyester-sheathed Dyneema, because of core slippage. Core slippage has occurred on Polyester-sheathed, Dyneema (non-Amsteel) core ropes, but the cause is not really clear. Apart from that caution, these 8mm+ canyoneering ropes are meant to be used with “normal” rap devices.

There is also Samson-manufactured cord with a polyester sheath on Amsteel. The Ľ” (about 6mm OD) sheathed material is 4900 lb breaking strength and weighs 1.6 lbs for 100’. I’ve actually prusiked up this stuff, and it will hold a light person with some  versions of real ascenders; again, I weigh 148 lbs in my clothes, so adjust accordingly.  I’ve tested this stuff in the field, right after testing Ľ” bare Dyneema; it is MUCH more frictional, and requires an entirely different rap device. The Mammut Nano Eight is more suitable for this 6mm polyester-sheathed Dyneema. But beware, you will *almost certainly* need to wrap the 6mm cord once over the sidearm to be in control of the rap.

09 Shrinkage and Heat Again

There is one caution about polyester-sheathed ropes with non-polyester cores. Polyester can shrink a LOT when heated above the glass point, and that shrinkage may enhance core separation. The figure below shows the sheath removed from an 8.5 mm all-polyester rope. At top is the sheath after I tried to stretch it by hand;I couldn't get more than a few % stretch, and it immediately relaxed back. The center of the figure shows the sheath immediately after ironing (yes, with a hand iron on wool setting); it never got above 250 F (121 C). The bottom of the figure shows the ironed sheath after stetching by hand; it immediately relaxed back.  The shrinkage is about 12 +/1%. Most polymers shrink when heated above the glass point, as the increase in entropy is taken up by increased disorder in the form of shortened, more tightly wound molecular chains.  Note this significant change happens well below the melting point, and is probably not noticed.  This is a fairly good reason NOT to showboat and rap fast on polyester ropes, especially if the cores are of a different material. Even with all-polyester ropes, the core rarely gets as hot as the sheath. Supposedly, Dyneema shrinks less than 1% at 100C, but mariners report shrinkage of 3-4%, possibly from UV exposure.

10 Construction Stretch and Maximum Force

I always test knots and other rope terminations by simulated falls.  The ceiling of my stairwell has been reinforced a LOT, so I suspend about 10-12’ of rope down, clip my harness in, and take “falls” of at least two steps (16”). This definitely hurts on a high-modulus rope, but I do it about ten times to make sure I trust the termination.

It is relatively straight-forward to  calculate the maximum force in such a short fall, given the moduli or the static elongation for a given rope, the amount of rope played out, and the length of the fall. There are many derivations on the web under “rope physics,” and one can do the energy balance with high-school physics. There is about a 20% down correction for the relative "softness" of the human body, compared to the inelastic weights assumed in the simple equations [13].

I repeated this test with 3/16” amsteel, and didn’t notice undue discomfort.  When I calculated the supposed max force, it was around 2000 lbs.  I assure you I didn’t feel anything like that, compared to other tests I have done.

The answer to this conundrum is in two parts, First, if we look at the figure below, from [REF 10]. The we see 11 tension cycles, with 9 to half-breaking-strength, and the 11th to actual breaking. The "final" modulus is achieved only after the rope has been tensioned a few times; even then it tends to spring back and requires some light tensioning just to reach the "linear" range. Perhaps more interesting, is that this test was done on just 10' of dyneema, and gives a naive modulus just about 10% of the quoted modulus. The authors atribute that difference to construction stretch in addition the the splices used to hold the test section. (The hysteresis stretch is in addition to the quoted elastic modulus, and takes several hours to relax back.) There is a soft "construction stretch" in many braided dyneema ropes, added when the separate fibers are woven into yarns, and when the yarns are woven into a rope. Before ropes are tested to determine elastic moduli and hysteresis stretch, they may be cycled 50 times, and can gain several % in length.

Why is there so much construction stretch in Amsteel?  The main uses for Amsteel are in yachting, ocean sailing, logging, and other heavy industries where ropes are tensioned to a significant fraction of breaking as a matter of course. Slack is taken up by capstans, knots, and electric motors.  In contrast,  climbing ropes rarely see more than body weight. 

The second part of the answer is a little more subtle. In sailing, much of the construction stretch never comes back, because the ropes are nearly always in tension. Loosely-braided ropes, with low lay angle, will gain much of the construction stretch back, when they are simply bagged or coiled. It's also rare that we use a rope without one or two knots in the system; tightening the knots lowers the effective modulus of the rope during a fall.

How much does this extra stretch moderate the "static" nature of dyneema? Consider a practical example. I have 100' of Endura 6mm rope, with an SK-78 Dyneema core, and a polyester sheath. I have taken many 800lb falls on the rope to test terminations, and I have used it under my body weight for raps; but I always bag (as a canyoneer might do) it bewteen uses. New England Ropes claims this rope has 0.65% stretch at 900 lbs (20% breaking strength), for a linear modulus of ~1.38e5 lbs. I put two knots as terminations on an 18' length, which was doubled back through a 97% efficiency pulley, and brought the rope to 100 lbs for a minute. Then I placed a Sharpie mark across both strands, and over several minutes brought it to 300 lbs, using a come-along and industrial scale. When the force plateaued I measured the stretch. The corresponding linear modulus was 1.48e4 lbs, more than 9x lower than the manufacturers specifications.

Key to this behavior is the 12-strand, low-lay angle braid of Amsteel and many other dyneema ropes; they recover a softer effective modulus when bagged or even coiled. In practice, the linear modulus of this 6mm rope is very similar to that of 11.5 mm PIT/Maxwear nylon caving rope. The knots also inherently reduce the effective modulus, unless they are pretensioned by a hard pull. Note this soft braid is very different from the tight, high-lay angle braid used in Dyneema slings.

11 Samthane and Melting

Amsteel is coated with a proprietary urethane coating, “Samthane,” which contains the coloring and supposedly increases abrasion resistance.  The coating leaves the individual fibers distinct; the coating thoroughly saturates the thinner ropes, but is on the outside fibers for thicker ropes. Some of the coating wears off after repeated friction, such as passage through a Munter.

The most obvious effect of the urethane treatment, is that it makes the Amsteel almost impossible to melt with a flame. At best, the Amsteel chars in a flame, leaving little burnt-rounded terminations on groups of fibers.  This is quite unlike the other Dyneema and Spectra  cordage I have, and accounts for part of the odd results in the first video linked in this document.  I melted the ends of two polyester-sheathed , Dyneema core ropes.  The first was 8mm Canyon Pro; the core was NOT urethane treated. The sheath and core melted simultaneously with a flame, quite easily.  In contrast, below is a picture of Samson “WarpSpeed II,” which has an Amsteel (urethane-treated) core; the polyester melted away easily, leaving an unmelted Dyneema core.  To seal the ends of Amsteel, I use Seam Grip urethane glue, and work it into the fibers, then shape the ends as the urethane begins to cure.

What I don’t like about this coating, is the uncertainty of what it does to the CoF.  Does the CoF change as the coating wears off?

12 Dyneema  “Friction” Knots

The thin 6mm, polyester-covered dyneema (4900 lb test) actually works with Camp Solo2 ascenders, but the wear on the rope is noticeable. In addition, if one is going through the trouble of reducing weight with thin rope, all mechanical ascenders seem heavy. I have tested prusik loops (and pusik knots) made with 3.8 mm Spyderline (1900 lb BS, polyester-coated dyneema), and they work reasonably well. However, if one has mastered sewing dyneema (SEE CAUTIONS ABOVE), it is possible to make stronger, lighter "hybrid" prusiks that will work on the 6 mm polyester-covered cord, and even on 9mm double ropes. The video below shows the concept; the pair of hybrid prusik loops weighs 1.8 oz, for a BS > 2500 lbs. I recommend true prusik knots, NOT klemheist knots, on this thin, flexible 6mm rope. Klemheists have a very nasty tendency to pull the main 6mm "rope" into the friction knot, completely locking, unless one is very fussy with dressing the knots.

And for irony, the video below shows an “experimental” use of Amsteel. The setup uses prusik loops made from  1/8” Amsteel, tied with 5-turn klemheist knots, to 6mm polyester-sheathed Amsteel.  I tried this test 5 times, and the knots were a bit tight by the time I shot the video.  The analogous situation, with nylon prusiks on thin rope, is the one place I have melted nylon.  On thin cord, klemheist knots can get very tight; despite being called a “friction” knot, much of the strength of klemheists comes from the deformation of the rope when the knot is loaded. With nylon, the relatively high CoF can actually melt the prusik knots when they are moved.  For a time I had aramid prusik cords, because I was so afraid of melting the knots.  I’ve learned how to loosen the knots, so the friction issue is less extreme, over the years.  But in these tests, the prusik knots didn’t melt at all.  I examined them in detail after the 5 tests; not a single melt spot.


NOTE these are NOT Dyneema WEBBING SLINGS. I have tried Dyneema slings, and they are too stiff and slick. The key to the success of the Amsteel is that it deforms, and is so thin that it is pretty easy to tie a 5-turn (rather than 3-turn) klemheist.

The Amsteel cord is about 3.2 mm (2500 lb test); I used a knot to make a loop, then slid a 4000 lb test piece of tubular webbing over the knot, and sewed the 4 pieces of rope coming out of the knot to the webbing with 100 loops of 300 lb test spectra fishing line.  Then I soaked the tubular webbing in thinned Seam-Grip urethane, so individual stitches were unlikely to pull through if abraded, and the webbing was stiff enough to form a step.

These results are somewhat consistent with [REF 11], though they were testing the tightly woven Dyneema slings.

Does this mean I recommend using thin Dyneema prusiks for ascending 6 mm ropes?  Of course not, it’s a PITA.  But it is nowhere as unsafe as some predict. People with lower upper body strength may have a really hard time with thin prusik loops on 6mm cord, no matter what they are made of; the cord just deforms too much.

13 Appearances Can Be Deceiving

Sometimes the Amsteel rope deforms quite a bit, especially after untying a knot;  the deformation may be disturbing to folks who are used to the kernmantle ropes that seem almost indestructible.  Usually the deformation goes away simply by holding the rope on each side and compressing it. Perhaps this will ease your mind about the significance of the appearance ([REF 10], in a discussion of logging):

“The current project did not plan to test abrasion or degradation in synthetic ropes, but we did obtain a sample of AMSTEEL 815 synthetic rope used three years as a guyline for tail trees and intermediate support trees. The 9/16 inch rope was markedly increased in diameter (more than double) and had dirt and debris among the fibers and strands. Yet the residual strength of the test sections was more than 65% of the original specifications.”


14 Finally

I ask again: are you big-boned?  The weight that can be handled by a rope roughly goes as the diameter *squared*.  So if an 8mm rope is “safe” for someone who weighs 150 lbs, and you weigh 250 pounds, you need a 10.3 mm rope for a similar safety margin.  The same rule applies for thin cords, except the strength rating may drop off faster with diameter. In  kernmantle ropes, the sheath may be constant in thickness, as the core size increases; and thin, purely braided ropes have other issues with strength when the braids are loose. But if you are just using Amsteel for a pull cord, 1/8 “ is probably adequate.  If you want an emergency rap, go for the 9/64 with a 4000 lb rating and supermunter.


* I wrote this guide in honor of Zadig (by Voltaire).  Zadig, when asked about the laws against eating griffins (which were thought to be imaginary), tried to find a neutral course; but he still offended those who had never seen griffins, but harbored strong opinions.

**Why just 1100 lbs? I had an operation 2 days ago, and the doc put me on lifting restrictions, so I didn't bother setting up the mechanical advantage pulley system. Alas, the scale tops out at 1100 lbs.


15 References

01) Low UHMWPE CoF.





02) Conductivity of Polymers.

Rantala, J. (2001) The Anisotropic Thermal Conductivity of Plastics. Design, Materials, Compounds, Adhesives, Substrates, Number 3, Technical Data, Test & Measurement, Volume 7.

Kawabata,S. and Rengasamy, R.S. (2002) Thermal conductivity of unidirectional fibre composites made from yarns and computation of thermal conductivity of yarns. Indian J. Fibre & Textile Res. 27, 217-223.



03) Use of Dyneema in Europe. http://www.alpine-rescue.org/ikar-cisa/documents/2016/ikar20160404002117.pdf


04) Dyneema in French Canyoneering: http://cnc-ffcam.fr/wp-content/uploads/2015/01/cahiers-efc2.pdf


05) Temperature Effect on CoF.

Mens, J.W.M. and de Gee, A.W.J. (1991) Friction and wear behaviour of 18 polymers in contact

with steel in environments of air and water . Wear, 149,  255-268.


06)  Friction Coefficients of Synthetic Ropes. Available on web as

http://www.dtic.mil/get-tr-doc/pdf?AD=ADA036718  or ADA036718 .pdf

Kane, B.C. (2007) Friction Coefficients for Arborist Ropes Passing through Cambium Saver Rings. Arboriculture & Urban Forestry, 33(1), 31-42.


07) Viscous Friction in Brake Devices.

Fuss, F.K. and Niegl, G. (2010) Understanding the mechanics of dynamic rope brakes. 8th Conference of the International Sports Engineering Association (ISEA).


08) Rappel Devices and Rope Deformation. (http://www.bolt-products.com/Glue-inBoltDesign.htm ; https://link.springer.com/article/10.1007/s12283-013-0147-6




09) Strength of Dyneema Knots. Joel Hartter thesis. https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/9800/Hartter,%20Joel%20%20MS.pdf?sequence=1


10) Construction Stretch.



11) Comparative Testing of High Strength Cord [Spectra Friction Knots]


12) Temperature and tensile strength UHMWPE.

van der Werff, H.  and Pennings, A.J. (1991) Tensile deformation of high strength and high modulus polyethylene fibers, Colloid & Polymer Science Colloid Polym Sci 269:747-763

13 Holden, T; May, W.; and Farnham, R. (2009) On the Utility of Rescue Randy Mannequins in Rescue Systems Drop Testing. Available on the web as Rescue_Randy_Mannequins.pdf