# Question about Kevlar strings



## cerelestecerele (Aug 5, 2019)

I've been looking unsuccessfully for some more information about Kevlar recurve strings as I'm curious about their relatively brief heyday. There's plenty of stories about their longevity and relative lack of creep, but I haven't found a direct comparison of their other properties - their weight or the resulting arrow speed or bow noise when compared to Dacron or the original dynaflight/fastflight that replaced it. As it's pre-internet and pre-Archery Focus, and the few books I have from around then don't go into detail, I've been struggling to find more information.

Does anyone have any old notes or access to articles or advertisements from the "transition period" when polyethylene strings had already been introduced, but Kevlar was still being used? 

Kevlar as a material is slightly denser than Dacron, and has a modulus slightly lower than UHMWPE though still much higher than that of Dacron. So based on some back of the envelope calculations, I would guess that a Kevlar string was slightly heavier than a Dacron one of the same diameter (though lighter if fewer strands and a thicker serving were used), and about halfway between Dacron and UHMWPE in speed. I don't know whether that's close to the reality of it though.

Is there another sport that still uses a similar Kevlar string material for some other application? I've only found it in braided form for kite strings and sewing heat resistant clothing, neither of which would be suitable for making a test string.


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## Grasshopperglock (Sep 9, 2020)

They still use it in fishing. The other material used the same as Kevlar is Spectra. Its the main competitor to Kevlar. You can find Spectra at any fishing store.


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## cerelestecerele (Aug 5, 2019)

Grasshopperglock said:


> They still use it in fishing. The other material used the same as Kevlar is Spectra. Its the main competitor to Kevlar. You can find Spectra at any fishing store.


Spectra isn't the same as Kevlar. It's a brand name for UHMWPE, just like Dyneema. 
I've only been able to find non braided Kevlar as a fly tying thread in 200 denier (3/0) size, which is about 10 times thinner than regular string materials. Is there a different (thicker) Kevlar non braided thread used in fishing that you meant?


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## Stepping wolf (May 5, 2019)

Check with Jerry from South Shore Archery. I think he is using Kevlar for his crossbow servings, weather he makes strings from it I don't know.
Jerry used to go by southshorerat and Tappnation here on AT but I don't know how active he is now, he was looking to retire.


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## Stash (Jun 1, 2002)

I still have most of a spool of Kevlar string material. If you’re seriously interested, I can make up a string and weigh it compared to a modern synthetic or dacron string material. My current strings are Astroflight, discontinued. Don’t have any actual branded Fastflight material. I could give you a subjective opinion on the noise level and speed. My Chrony crapped out a few months ago so I can’t do an accurate speed test.

I made tons and tons of Kevlar strings for myself and friends/customers back in the day. One thing we discovered was that Flemish kevlar strings significantly outlasted endless loop ones, and that’s what I used until Fastflight hit the market.


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## cerelestecerele (Aug 5, 2019)

Stash said:


> I still have most of a spool of Kevlar string material. If you’re seriously interested, I can make up a string and weigh it compared to a modern synthetic or dacron string material. My current strings are Astroflight, discontinued. Don’t have any actual branded Fastflight material. I could give you a subjective opinion on the noise level and speed. My Chrony crapped out a few months ago so I can’t do an accurate speed test.
> 
> I made tons and tons of Kevlar strings for myself and friends/customers back in the day. One thing we discovered was that Flemish kevlar strings significantly outlasted endless loop ones, and that’s what I used until Fastflight hit the market.


That would be great if you've got the time! Also more than happy to buy a string worth of material off you to make and chronograph+record and compare it against other materials I have. 
Does it look like the colour/strength of the spool have changed significantly with age? When left outdoors in the sun for a year it degrades significantly after a few months, but I don't know how the combination of a few decades of storage would work out.


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## Stash (Jun 1, 2002)

I’ll make up a kevlar string the same length/thickness as one of my Astroflights and weigh them. 

And I’ll be happy to mail you some material, but we have a Covid lockdown here, so I think I’ll wait a bit - don’t want to make a non-essential trip to the Post Office. PM me your mailing address. 

The spool I have looks like the wax of outer layer has darkened. I’ll peel off a few yards to expose the fresh stuff and see if I can detect any loss of strength.


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## Stash (Jun 1, 2002)

I seem to be running low on serving material, so rather than waste it making a Kevlar string I’m not going to use, here’s what I did. Hope it answers some of your questions.

Same nock fit using #4 Brownell Nylon serving:
B55 dacron uses 12 strands
Astroflight uses 16 strands
Kevlar uses 18 strands

I wrapped the appropriate # of strands between the posts of my string jig (11” apart), then weighed the material. A 66” string (approximately suitable for a 68-70” bow) would be 5 times that length. Weights in grains, not grams, BTW.

B55 12 str x 11” weight 19.5g, so 66” = 117.0g
Astro 16 str x 11” weight 15.2g, so 66” = 91.2g
Kevlar 18 str x 11” weight 17.3g, so 66” = 104.8g

Now that’s for the string material only. Kevlar (at the same thickness) seems to be about halfway in weight between dacron and spectra type materials.

I had a 16 strand 55” Astro string, total weight 107g. According to the above numbers, the material only for a 55” should be 76g, so the weight of the serving would be 31g (6” ends, 7” center).

Therefore, without actually weighing them, total weight for a string for a 68-70” target recurve would be approximately
B55 = 150g
Astro = 120g
Kevlar = 135g

I wrapped a strand of each of the materials around a couple of screwdriver handles and tried breaking it by pulling across my chest. B55, I could definitely feel it stretch, and it broke fairly easily. I couldn’t break either the Astro or the Kevlar, but I’m old not as strong as I used to be. 😄 So I’m confident the Kevlar is still sound after all these years.

Incidentally, if it hasn’t been pointed out, Kevlar strings didn’t fail because they weren’t strong enough under tension. They always failed at a bending point - the nock loops or nocking point - where they were subject lateral stress, not tension.

So if you still want to try the stuff, let me know and I’ll send you enough material for a couple of strings.


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## cerelestecerele (Aug 5, 2019)

Thank you so much for doing that - that's really informative. I wasn't expecting it to be so light compared to Dacron based on the rather generic density information about each fiber. Messaged you


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## cerelestecerele (Aug 5, 2019)

Thanks to Stash, I've now made and tested two kevlar strings. One is about 1cm longer than the other so that I could compare the same brace height but different twists. They're endless loop and have halo servings, which isn't strictly traditional but it offers a very direct comparison with more modern strings. Here are the results and comparison:

Building: as expected, far less irrecoverable stretch of the kevlar string when the bow was left strung overnight for the first time. I'd expect a ~5 mm decrease in brace height with dynaflight 97 (SK75 dyneema) and got 1 mm for each kevlar string. The kevlar is really a lot less forgiving of imperfect initial strand tension, while dynaflight is easy to "even out" any loose or tight strands when wrapping it around the strin jig, kevlar seemed to stick to itself and not allow any relative movement of the strands.

Finished strings
Kevlar string 1 (brace 22.5cm with 0 twists): 115 grains
Kevlar string 2 (brace ~20 with 0 twists, 2 strands less): 107 grains
Dynaflight 97 (brace ~21 with 0 twists) : 104 grains.
All endless loop, halo ~0.017" on the servings, about the same nock fit. That's not the optimised setting for kevlar, but it means I can compare it more directly.

Speed comparison - I am confident in the relative speeds here from my choronograph though the absolute value could be a few fps off. Speeds in m/s, for fps just multiply by 3.28. Average from at least 6 shots per setting. 68" bow, 40.6lbs, 267 grain arrows. Recommended brace height range 21.5-23.5 so I tested it at the ends and halfway in between. Standard deviation for each setting between 0.3 and 0.5 m/s after at least 6 shots.
Dynaflight 97: 62.7, 62.1, 61.8 twisted to brace heights of 21.5, 22.5, and 23.5 cm
Kevlar 2: 61.1, 61.0, 60.4 at the same brace heights.
Kevlar 1: 61.4, 60.8 at 22.5 and 23.5

So the kevlar was consistently about 1.2 m/s (1.9%) slower than a dynaflight 97 string with about the same weight and nock fit. The heavier shorter kevlar string looks slightly faster than the slower one, which could be a fluke related to my build quality or some other variable. With this setup and my regular string, a 1.2 m/s speed reduction is equivalent to dropping my draw weight by 4lbs, which is quite substantial. I imagine that, back when kevlar was commonly used, the prospect of reducing draw weight by 4lbs while keeping the same arrow speed would have been pretty compelling, even if the new material still had kevlar's breakage issues.

Noise/vibration comparison - this was measured with my phone about 1m diagonally behind me. The bow had a full set of long+side rods with dampers and weights. I put the recording of each shot into audacity and took the A-weighted RMS noise level of the first 50 milliseconds. This is pretty sensitive to phone position, nearby furniture/echoes etc but values can be compared if they were all shot in the same session and the archer/phone positions are constant. Larger numbers are louder.
Dynaflight 97: 896, 876, 721 for brace heights 21.5, 22.5, 23.5 cm. standard deviation about 30
Kevlar 2: 1149, 957, 864
Kevlar 1: -, 799, 707
So they were all pretty similar at the middle and high brace height, but the longer kevlar string was quite a lot louder at the low brace. I repeated this process on a different day and got pretty similar values. So It looks like between the dynaflight and kevlar strings with about the same length and weight, the kevlar was reliably both slower and louder, and the loudness was more dependent on the brace height.

Hoping to get a more generalisable result, I repeated the comparision with a pair of 23lb limbs that give a speed around 48 m/s with the same arrow. Unfortunately my chronograph went rather unreliable, but the speeds looked about the same between the two materials for the same brace height to within my experimental error that day (about 0.5 m/s). The noise tests were also rather inconclusive but I might revisit them later.


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