More String Geekiness

[LBR]

Here's a bit more verifiable information about strings/string materials, primarily the claim that certain string materials are causing damage to certain bows, and the "information" used to try and back that claim. This information wasn't published by any string maker or string material company. I can't take credit for finding it--it was shared with me by someone who looked it up (that person can chime in if they choose to be identified). Either way, it's independent and factual info. that shows the simple difference between static and dynamic elongation...what we commonly refer to as "stretch".  It's based on rope, but we use the same materials in bow strings.

http://ropewiki.com/Rope_comparison

Static vs Dynamic elongation

Static ropes are subjected to a static elongation test. That is, suspend weight for some period of time, carefully, on the rope and measure its length again. This test almost certainly does not describe how the material will react in an accidental dynamic fall, or bouncing on ascent/rappel. As an example, there are accounts of the Sterling C-IV (polypropylene core) feeling particularly subject to dynamic behavior not reflected by its static elongation testing figure. Be cautious when relying on the static elongation figure alone, however polyester and high modulus fibers seem reliably static for canyoneering use.


Key phrases here: 

"This test almost certainly does not describe how the material will react in an accidental dynamic fall, or bouncing on ascent/rappel."  Or as it applies to bow strings, at the shot when the bow returns to brace.

"...behavior not reflected by its static elongation testing figure."  Or, simply put, how the string acts at brace is going to be different than how it acts at the shot.  You can't use one to state how it's going to act during the other.

"Be cautious when relying on the static elongation figure alone,..."  See above.  If someone is trying to tell you that how a string stretches under a consistent and steady load (like at brace) will tell you what happens under a dynamic or shock load (like at the shot)...well, they can't.  The two aren't directly related. 

I use good 'ol silly putty as an example.  Pull it slowly (static load), it stretches like bubble gum.  Pull it apart quickly (dynamic load), it snaps.  Practical application as it applies to bow strings...even though you may get some slow elongation (creep) over time with the bow strung, it's still not elastic in the sense that it will stretch/snap back in the split second it takes for the bow to return to brace at the shot.

Keep in mind this won't change any warranties, and may not change any minds.  I just thought it was interesting.

[McDave]

I used to be a rock climber.  Static ropes are used mainly for hauling things and rappelling.  That is, activities where no shock load will be applied to a climber.  Dynamic ropes are used in climbing, to catch falls.  The rule of thumb was that the shock of a 6’ fall on a static rope was enough to break bones.  The length of fall that can be taken on a dynamic rope without breaking bones is almost unlimited, because the longer the fall, the more rope is let out, and as more rope is let out, the dynamic stretch increases.  In fact, if we felt that a falling climber would be caught after only falling 6 feet, we would try to let out a little more rope to cushion the fall, even on a dynamic rope, unless it looked like the climber was about to hit something.

In my mind, this relates to bowstrings, because if the shock of a six foot fall on a static rope can break bones, the shock of a static bowstring could break a bow, unless the bow was designed to handle it.

[McDave]

Sorry double post

[LBR]

Makes sense, except there's not any bowstring material that has any appreciable degree of elasticity at the shot.  Even Dacron isn't a rubber band, and "primitive" materials like linen and silk aren't either.

Supposedly the most load is on the string at brace.  If it were going to stretch, this is when it would happen.

[SteveB]

Bow strings are subjected to 2 very different forces. That's why I went looking for the info Chad posted.
   -1st is the static force that is applied when drawing the bow to anchor. Easy to measure the elasticity if any present at this force.

   -2nd is the force applied at the conclusion of the shot. Same dynamic force McDave describes as a climber falling with a harness or falling from a tree stand while harnessed up. Takes a far more sophisticated setup up to measure this force and even more so to measure the elasticity present if any. The article shows that the easily performed static measurement is not  a good indicator of what the harder dynamic test will show. Therefore it makes no sense to use it when deciding what string materials are best/safest for a bow.

I would think a bowyer who is concerned enough about string materials to limit some of the modern FF ones would want to do so based on actual test numbers. And nor rely on non pertinent testing or small samples of anecdotal reports which easily leave out most of the other possible contributing factors.

Yep - we are geeks 

[DanielB89]

ok, i'm trying to understand exactly what you two are trying to prove here.  Are you saying that neither B55 or and FF material will stretch when shot? 

I tend to believe along the lines of what McDave is stating.  There are definitely different types of ropes out there that have very specific applications.

I'm also assuming these posts are based off of Border Archery's not accepting some BCY materials such as BCY X.   Yes, I have lost a few string sells to a few customers over the years, but I haven't given it too much thought.  If a guy builds a $700+ set of limbs and I purchased them, I would definitely play by his rules. 


Now, my question is, What type of test has been done to prove the elasticity or the material on the shot?  My head hurts even trying to figure that one out. 

[Orion]

I'm out of my element here, but I do prefer "low-stretch" strings over dacron.  Given that bow limbs themselves are very flexible, it seems their vibration at the conclusion of the shot becomes a long term repetitive stress factor. Thus, reducing the amount of vibration with a lower stretch string should prolong limb life.  Don't know if that makes any sense, and I don't know of how one would test for it.

Regarding "low-stretch" materials contributing to limb failure.  All of the examples I've seem seem to be of a different type of stress.  That caused by very skinny, hard material strings that cut into the nocks. The lower stretch may exacerbate this problem, but it seems that padding the loops would pretty much overcome it.  At least that's been my experience for the past half-dozen years or so since I switched to low-stretch strings.

[SteveB]

ok, i'm trying to understand exactly what you two are trying to prove here.  Are you saying that neither B55 or and FF material will stretch when shot? 
Not trying to prove a thing. Saying there is no testing to show the elasticity - if any - of a typical string on a typical shot.   .

I tend to believe along the lines of what McDave is stating.  There are definitely different types of ropes out there that have very specific applications.
Yes there are. And all have extensive testing done that shows their difference in the conditions being used. No dynamic stats are derived from static testing


I'm also assuming these posts are based off of Border Archery's not accepting some BCY materials such as BCY X.   Yes, I have lost a few string sells to a few customers over the years, but I haven't given it too much thought.  If a guy builds a $700+ set of limbs and I purchased them, I would definitely play by his rules.
Haven't lost a single order as I offer D97 and it's on the list. I make  anyone ordering a Border string aware of their policy and allow the customer to make their own decision.


Now, my question is, What type of test has been done to prove the elasticity or the material on the shot?  My head hurts even trying to figure that one out.
 None that we are aware of. That's the point - it is impossible to make an informed decision without it. The engineer from the company who supplies BCY has said there is likely no detectable elasticity with at the typical shot with any string made of any HMPE material. Did not offer test results but I tend to go with the guy's opinion who designs the stuff. I really don't care what anyone chooses to shoot or recomend for a bow they build. Just want to see that they are not using unproven claims to make their decisions .

 

[LBR]

Steve B. covered Daniel's inquiries better than I could.  I agree 100% on all points.  It's not that we are trying to bash anyone's bows, it's the simple fact that we string makers (and also string material manufacturers) have been bashed over it (I've seen it, been on the receiving end of it), and the "information" being used may look/sound scientific but it's not.  You simply cannot use static creep to determine dynamic stretch.  They might appear to be related, but they are not, and using one to try and prove the other is misleading at best.  It's like saying "look, pull on silly putty slowly and it stretches, so it's only reasonable to say that if you pull faster, it will stretch faster".  Sounds logical, but it's false, even when the person stating it claims to be an engineer.

Orion, you bring up some points I've thought about before, but I never thought about the elasticity of the limbs themselves.  I don't think you are in over your head at all!

...reducing the amount of vibration with a lower stretch string should prolong limb life.

I've had the same thoughts for years, but I thought about it like this:  since "low stretch" materials can greatly reduce hand shock, wouldn't that mean they are better for the bow?  I may be missing something there, and I have no idea how to test it, but it makes sense to me.

All of the examples I've seen seem to be of a different type of stress.  That caused by very skinny, hard material strings that cut into the nocks.

Same here.  I've seen lots of failures/delaminations on lots of different bows.  Pretty much without exception if the bow has a "fast flight" string on it, the string gets the blame...but if it has Dacron, then it's a mystery what caused it.  I've known of at least two bows that delaminated the first time they were drawn to anchor.  I've seen strings get blamed for a twisted limb (on a longbow!).  Drives me nuts...the string seems to be the red-headed step-child when it comes to bow failures.  Surely at least some of them couldn't be due to faulty bow design or a mistake made in the build?   

[LBR]

More thoughts....

As Steve noted, it would be a complicated test (I don't know if there's a machine to do such tests) to measure the amount of elasticity, if any, at the shot.  And if that's not complicated enough...

There are countless variables.  Simply put, there's no possible way to honestly state "this many strands of this material is fine, but if you use more, or if you use a slightly different material, you will damage the bow".  There are different grades of HMPE.  Different strand counts in the string.  Variations in how one string makers works vs. the next.  Different string lengths.  Different bow designs, even variations  from one bow to the next in the exact same design.  Different bow lengths.  Different draw weights.  Different draw lengths.  Difference in one person's release to the next.  Different arrow weights.  Even the spine of the arrow can matter if you get right down to it.  And on and on and on.
 

[DanielB89]

ok, i'm trying to understand exactly what you two are trying to prove here.  Are you saying that neither B55 or and FF material will stretch when shot? 
Not trying to prove a thing. Saying there is no testing to show the elasticity - if any - of a typical string on a typical shot.   .

I tend to believe along the lines of what McDave is stating.  There are definitely different types of ropes out there that have very specific applications.
Yes there are. And all have extensive testing done that shows their difference in the conditions being used. No dynamic stats are derived from static testing


I'm also assuming these posts are based off of Border Archery's not accepting some BCY materials such as BCY X.   Yes, I have lost a few string sells to a few customers over the years, but I haven't given it too much thought.  If a guy builds a $700+ set of limbs and I purchased them, I would definitely play by his rules.
Haven't lost a single order as I offer D97 and it's on the list. I make  anyone ordering a Border string aware of their policy and allow the customer to make their own decision.


Now, my question is, What type of test has been done to prove the elasticity or the material on the shot?  My head hurts even trying to figure that one out.
 None that we are aware of. That's the point - it is impossible to make an informed decision without it. The engineer from the company who supplies BCY has said there is likely no detectable elasticity with at the typical shot with any string made of any HMPE material. Did not offer test results but I tend to go with the guy's opinion who designs the stuff. I really don't care what anyone chooses to shoot or recomend for a bow they build. Just want to see that they are not using unproven claims to make their decisions .

I would definitely agree that you can't just a materials dynamic qualities based off of a static measurement.  That is the exact same situation with a carbon arrows spine deflection.  I can't recall how many times i've read where you need to remember the measurement given on them is a static measurement, not a dynamic.

I have seen some of his claims and I am wondering where he gets his numbers from.  Are they all based off of a static resting stretch? 

[LBR]

I have seen some of his claims and I am wondering where he gets his numbers from.  Are they all based off of a static resting stretch? 

As far as I know, yes--all based on static numbers.  I don't know that there's a machine to even measure the miniscule amount of stretch at the shot, plus you would have to run countless tests to cover the practically infinite number of variables.

He also has a load limit of 1,000# (based on what he thinks is the breaking strength of the "approved" materials, which doesn't coincide with manufacturer specs).  I asked where he came up with that number, and if it applied to a 30# bow the same as a 70# bow.  Did it apply to a 24" draw the same as a 32" draw.  How did he come up with that number, and how could it be equally applicable to every bow regardless of draw weight or draw length.

He called me "uninformed" and blocked me.   

[LBR]

The best analogy I can come up with, which the engineer I conversed with at DSM agreed (DSM is the manufacturer of Dyneema), is this.

Technically speaking, two hairs will weigh twice as much as one hair, but on the back of an elephant that weight is totally insignificant. 

That's what it comes down to when you get into how much difference there is in elasticity at the shot between different HMPE and HMPE blends.

I'll say again...nothing said here is going to change a warranty.  ALWAYS stick to what your bowyer or manufacturer says to avoid nullifying your warranty.  Deciding whether or not it makes sense is something you do before deciding to make a purchase, not after.  I've seen warranties cut in half based on the type string you use...even when the string that cut the warranty was often shipped with the bow.  I've seen warranties voided if you used a Flemish string, or a Dacron string of any type.  Stranger things have happened.

[McDave]

You could design your own test for this if you wanted to.  Similar to climbing ropes, I’m sure that the longer the test strand, the easier it would be to measure stretch, since stretch increases with length.  So you could get 100’ pieces of bowstring, and toss them off a building a little higher than 100’, like Galileo did with an Apple and a canon ball off the leaning tower of Pizza, with a weight attached that is a little too light to break the strand.  You could have someone stand at the bottom and measure the stretch (if any).  If there is no stretch after falling 100’, then we can assume they are pretty much static.  To recoup the costs, you could make bowstrings out of the strands and sell them for a little more than your usual price, as “pre dynamically stretched.”

[Roy from Pa]

Mr. McReynolds

LMAO

 

[LBR]

Dave, believe it or not it did cross my mind to try something like that, just not quite that extreme (I don't think there are any 100' buildings less than a couple of hours from me, and I ain't about to climb 100' up a tree).  I was thinking about attaching a string to something secure...say a beam...attaching a weight then adjusting it so it's just barely above the floor.  Pick it up, drop it, and see if it touches the floor.  Then I thought about it some more...what material?  How many strands?  Which type string?  How long a string?  How heavy a weight?  How far do I suspend it over the floor?  How far do I drop it?  Where am I going to find the time or motivation to repeat this process with at least 6 different types of string material, in different strand counts, in different lengths, in different styles of strings, with different weights attached, dropped from different heights...just to show one guy who hates my guts and wouldn't pay the least bit of attention anyway that he is mistaken?   

I'm with Steve.  I'll just use one of the "approved" materials when a customer needs a string for one of those bows. 

Meanwhile, I'll keep on testing new materials on my el cheapo Samick Journeys...one of them is going on...7 years I think, always had one of those dastardly bow-breaking materials on it (BCY-X), even had endless on it at times (another no-no).

[bamboo]

Quote
...reducing the amount of vibration with a lower stretch string should prolong limb life.


----my way of thinking is when a low/no stretch material smooths the shot feel it is transferring the energy to the arrow that was wasted [left in bow!]by polyester/dacron materials -

---i've never put a no stretch string on a bow and not seen an improvement in shot feel-noise or performance.

[LBR]

I've thought the same thing bamboo.  Hand shock is wasted energy resonating from the limbs to the riser to your hand.  A string won't change the draw weight, so that energy has to be going somewhere.  The puzzling part is the chronograph doesn't normally show much if any speed difference in equal weight strings.  Energy going into the arrow paradox?  Danged if I know.  I just know hand shock is usually reduced.

[McDave]

I've thought the same thing bamboo.  Hand shock is wasted energy resonating from the limbs to the riser to your hand.  A string won't change the draw weight, so that energy has to be going somewhere.  The puzzling part is the chronograph doesn't normally show much if any speed difference in equal weight strings.  Energy going into the arrow paradox?  Danged if I know.

Perhaps because the arrow has already been launched before the limb vibration starts?

[LBR]

Perhaps because the arrow has already been launched before the limb vibration starts?

I don't think so, because the arrow is the only thing that could absorb the energy other than the bow. 

This kind of stuff intrigues me.  The more I learn, the more I realize how much I don't know.