Primitive Archer
Main Discussion Area => Bows => Topic started by: Hrothgar on May 11, 2016, 06:28:32 am
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Pat, if this isn't the proper place to ask this question, please feel free to move it.
I've heard that anytime a string gets a knot in it the strength decreases by half. I was wondering how accurate this statement is. I have an old bow stringer which has accumulated several knots over the years and wonder if it is still safe to use, also a 'long string' for tillering purposes in a similar condition.
Any engineers, physicists, or enlightened individuals who could explain the dynamics involved here please chime in.
Thanks
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Never had a bow stringer that didn't get knots after it has been used for a while, don't know what but mine always do, never had a problem with one breaking. :)
Pappy
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I wouldn't worry about a bow stringer with a few knots in it. A long string usually isn't under much stress so I wouldn't worry about a few knots in the long string either but I would not use a bow string with a knot in it.
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The first knot decreases the strength by half - any further knots don't have an effect. Don't ask me to explain that statement, it is what I was taught in my working at height course.
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The first knot decreases the strength by half - any further knots don't have an effect. Don't ask me to explain that statement, it is what I was taught in my working at height course.
Correct. It's because the knots are of equal strength/weakness. It's like a weak link in a chain. If you have two equally weak links, the chain will break at the strength limit of one of them.
But bow stringers never have as much strain on them as a bow string.
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The stringer is only pulling the bow to - or just slightly beyond - brace point. It is a very odd and rare bow holds anything like full draw weight tension at brace. Your stringer should be fine in any case.
I would not mind if someone would be inclined to share why a knotted string is only half the strength. My limited little pea brain is not able to contrive a reason that such would be the case. I can imagineer a few excuses for the string to perhaps be a bit less strong, but why would it be 'half'? Also, what is a knot? There are any number of knots possible. Do they all subtract an arbitrary half of the strength of any given string?
OneBow
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Might make sense with single strand small diameter cordage that are able to cut themselves within the knot. I doubt the number is that high with a 10 strand reverse twist FF string, I would wager strength loss to be more like 5-10% in that scenario.
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A continuous loop string allows some slippage of each strand to even themselves out in tension during use...I think when a knot is introduced it can cause uneven tension on each strand which can cause problems...You can't put a percentage on the strength, to many variables...May break on the first shot or it may never break...I just couldn't continue shooting if I knew a knot was there...JMO
Don
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I would be willing to bet that the "half" is an engineers rule of thumb. Kind of a worst case scenario to cover their butts ;)
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The most powerful bows made had knotted strings.
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Isn't a bow under the most stress while at brace?
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Lots of interesting thoughts here, but the OP was considering a bow stringer, not a bow string.
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The most powerful bows made had knotted strings.
Hornbows or English bows? Do you have any details? Apparently some heavier Tatar bows had essentially flemish ramie strings with double loops.
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Only thoughts I have had were knots tied in terminal tackle for fishing. Different knots have different strength and provide different action for lures or better weedless qualities or other stuff. I guess it is how the knot distributes the pull of the parts of the knot that makes some knots better than others.
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Asharrow,
He mentioned a string, a bow stringer and a tillering string, so we are all on topic sir...
Don
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Asharrow,
He mentioned a string, a bow stringer and a tillering string, so we are all on topic sir...
Don
Right you are. I guess it seemed to me that he wanted most to know if his stringer was safe to use.
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Isn't a bow under the most stress while at brace?
NO, I'm pretty certain that there is considerably greater stress on a bow while at FULL DRAW.
OneBow
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The string is under the most stress at brace while the limbs are at the most stress at full draw.
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The string is under the most stress at brace while the limbs are at the most stress at full draw.
HELP! I don't get the physics of that idea. A bow's limbs and the string attaching them seem like a closed system to me. If there is increased stress on the limbs, that stress has to be equivalent on the high tension string connecting them. ...am I missing something fundamental here? ...are you being funny maybe and I'm too dull to get the joke? I'm getting more coffee! ??? ??? ???
OneBow
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I don't understand it clearly but I have measured it. On a 40# bow the string is under about 43# at brace and then drops off steadily to 40# at full draw. It's really bizarre to watch the scale drop as you draw the bow. The lower the brace height the higher the string tension is. At least until the scale ran into the bow, that's when I stopped.
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The string is under the most stress at brace while the limbs are at the most stress at full draw.
That is what I meant. >:D
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HELP! I don't get the physics of that idea.
OneBow
Smartphones are not ideal to explain this, but IMO it comes down to string^tip angle increasing during the draw, and the resultant force on the string decreasing because of that. When a string breaks, it happens at the end of the return to draw when an arrow has been shot, most of the time.