B55 stretching at full draw?

[lefty4]

OK. Can someone explain why there is more tension on the string at resting brace than at full draw?

By the way, I am not a rocket surgeon, so keep it simple.

[katman]

From Chad "Technically speaking, the string has the most load on it at rest (2 points of contact). At full draw (3 points of contact), supposedly that's when the string acts like two separate strings that share the load. At least that's been the explanation I've gotten...think O.L. Adcock was the first one I heard tell it like that."

Makes sense since each half of the string is pulling weight of one limb. The resistance of both limbs gives final draw weight.

[lefty4]

But they can't be two separate strings if they are still connected, ie still one string and will share the load equally.

I understood what Chad said, just do not understand what it means or how that works. Still scratching.

[DanielB89]

I agree with lefty4.  I see what you're saying, but theres not secret that when drawn, the bow gains poundage, so wouldn't it make the weight heavier at full draw?


either way, this is interesting.  I have another question i'm going to post just to see what people think about it.

[Fletcher]

Originally posted by DanielB89:
I agree with lefty4.  I see what you're saying, but theres not secret that when drawn, the bow gains poundage, so wouldn't it make the weight heavier at full draw?


either way, this is interesting.  I have another question i'm going to post just to see what people think about it.

Clearly, the bow's draw weight increases.  String tension is different and is greatly influenced by string angle at the tips.  I have access to a tensiometer, I'll see if I can measure the actual tension.  Should be able to at least get relative comparisons.

[moebow]

In a March 1931 publication of "Sylvan Archer" (a magazine of the period) C. N. Hickman writes that (this is paraphrased folks) the tension of a bow string at brace height is at it's highest tension.  then as you draw, the string tension decreases as you approach HALF draw the increases again until full draw is reached. AND, the tension in the string APPROACHES the tension of the string at brace height but doesn't quite reach it.

Now, in the article, he (Hickman) refers to mathematical results that predict this. And then goes on to show testing that confirms the math.  In a quick review, I don't see that he addresses the WHY , just the results.  For those of you that have an aversion to mathematics and physics this article and most all Hickman's writing are NOT for you. There are formulas and many graphs showing these results.

As a matter of interest, he also talks about the effect of brace height on the weight of the full draw.  Basically, A bow's brace height (WITHIN REASON) does not affect the full draw weight of the bow significantly, if at all; but can greatly affect the stress in the limbs.  This all was written in a period of time where all wood bows were the norm and linen strings were used.  So things MAY have changed but again, maybe not.

A large set of articles written for archery magazines in the 1930s and 1940 are compiled in a book called "Archery _ The Technical Side" and has articles by Hickman, Klopsteg, Nagler and others.  For those of you with a math and physics interest, MUCH can be learned from these writers.

There are many examples of things that would APPEAR to make sense that don't stand up to scientific study; I believe this is one of those.

Arne

[moebow]

PS.  IF the above is true -- that the tension on the string is ABOUT the same at brace height as it is at full draw, you probably won't FEEL any "stretch".  Maybe you are experiencing something else?

Arne

[McDave]

I'm assuming the tension in the string is different from the force required to pull the string, i.e., the force draw curve that we are familiar with.  The force draw curve for most bows increases for each inch the bow is drawn within its usable range, so is at a maximum at maximum draw.  The direction of the force vector is always at a right angle to the handle of the bow; i.e., the direction we would pull the string when we shoot it.

Tension in the string would be the force required to hold the bow at a given bend by pulling the string along its length.  For example at brace height, if you took half a string with a loop in the end and pulled it toward the other nock, the tension would be the force required to pull the bow into its braced position.  The direction of the force vector would be toward the other nock at brace height.  As the bow is drawn, the force vector remains an extension of the string, and so changes direction as the bow is drawn.  As the tension at any point could be measured by putting the bow in a vice and pulling with half a string in the angle the string would normally be at that point in the draw, the string does act like two strings for tension measurement purposes.  Intuitively, it would appear that there should be some mechanical advantage in pulling the string at an increasing angle as the bow is drawn, which, unlike the normal force draw curve, might be enough to offset the increased force required to draw the bow as it is bent.

These are just my thoughts as I try to understand the physics of what's going here; please correct me if I'm wrong.

[lefty4]

Thanks Arne for the references.

McDave, I agree with your assessment of the force vectors and angles and was thinking therein lies the answer. But I just can't see it in my head. I have been away from physics classes for a long long time.

I am anxious to see what Fletcher discovers.

Good discussion and thought provoer.

[LBR]

I'm not going to pretend I understand the physics behind it.  I like to keep it simple.  

Put the bow on a scale with two different strings of the same length (giving the same brace height).  Measure the draw weight at a given draw length (or several) with both strings.  Record the difference--if there is one, it will be minimal.

Immediately after letting the bow go back to brace (off a scale or shooting), measure the brace.  I'm sure Daniel or anyone else who has put a string on a stretcher can verify, they contract very slowly.  If the string stretched at full draw, you would be able to get a noticeable difference in the brace height.

I've seen some weird things blamed on strings, and it's come from people who had a reputation for knowing their stuff.  

For instance...a claim that a string (high performance material) contracted so much at full draw that the bow gained 4# of draw weight; that a Flemish string caused the limbs to twist on a 72# longbow;  that Flemish strings are less accurate and could be dangerous...

Not doubting the honesty of these people (not all of them anyway)...but however they came to these conclusions, there was a hiccup somewhere along the way during testing.

[Orion]

Your explanation appears sound to me McDave.  

We know there are differences between dacron and modern low stretch materials, and we can see and have measured some of those differences.  The disagreement seems to lie in how large those differences are.

As they say in scientific circles, more research is needed.

[DanielB89]

Originally posted by LBR:
I'm not going to pretend I understand the physics behind it.  I like to keep it simple.  

Put the bow on a scale with two different strings of the same length (giving the same brace height).  Measure the draw weight at a given draw length (or several) with both strings.  Record the difference--if there is one, it will be minimal.

Immediately after letting the bow go back to brace (off a scale or shooting), measure the brace.  I'm sure Daniel or anyone else who has put a string on a stretcher can verify, they contract very slowly.  If the string stretched at full draw, you would be able to get a noticeable difference in the brace height.

I've seen some weird things blamed on strings, and it's come from people who had a reputation for knowing their stuff.  

For instance...a claim that a string (high performance material) contracted so much at full draw that the bow gained 4# of draw weight; that a Flemish string caused the limbs to twist on a 72# longbow;  that Flemish strings are less accurate and could be dangerous...

Not doubting the honesty of these people (not all of them anyway)...but however they came to these conclusions, there was a hiccup somewhere along the way during testing.

by "hiccup", you much mean bud light..

[LBR]

If I had to guess, something a good bit stronger...

I put a fresh, unstretched B50 on a 96 pound longbow and tested the weight.  20 minutes later my brace height was down to three inches from 6" nad the  bow pulled less pounds.  Once that string was in use for a couple of weeks, it was stable and bow remained at 96 pounds.   I may be OCD, but that stability and nock fit for my 5/16 mercury nocks is a major deal, regardless of the lack of hand shock, faster  arrow speed, and greater spine tolerance with the BCYs.  I need to order a couple BCYs with a turn or two less in the loops than my standard narrow loops for a couple of tiny tip bows.  Is there a minimum number of turns?

[DanielB89]

Pavan,

what do you mean by minimum turns?  Are you referring to the number of twists in the loops?  if so, no.  That is how you adjust the size of the loop.  A few less twists would make the loops smaller(assuming that is what you're wanting)

When I made my B50 strings, I would twist them much harder than normal, resulting in an apparent thinner string.   When I made strings for heavy bows with 18 or 21 strand strings, it took a lot of pressure in the twist to keep the loop small enough for bows like Schulz longbows.   I was kidding about the number of twists, but I do need a couple of strings that have a couple less twists, I need to count the urns on the strings I have on my current bows.   Some longbows do not kick all that hard, but when I put a Chad string on them, I can still see what I was missing with the B50s.  Chad seems to put a graduated twist towards the loops  in the main string, I imagine it does more than just look cool.

[DanielB89]

I am not sure what you mean by "graduated twists", excuse my ignorance. I have seen several of Chad's strings and he definitely knows what he is doing!


I can see what you're talking about about needing the loops smaller, but also needing a high count of strands.

On my Chad strings about five inches or so from the area where the loop strands begin, the twist is about twice as tight as the middle section.  i tried duplicating this on a B50 string for an old bow and as soon the bow is strung they start going away and are gone with the first shot.   I have been shooting the Chad strings for many thousands of shots and that tight twist prior to the loop doubling remains.  I don't know he does that.

[Fletcher]

Here we go!  I borrowed a tensiometer from work.  This is an accurate and calibrated tool that is used to measure the tension of aircraft flight control cables.  Test bow is a Great Northern Field Bow, 41 lb at 28".  I chose it for its lower draw weight, making the test easier to perform and its nice round string which leads to consistent readings with the tensiometer.  With the bow at brace height, the meter indicates a 34, which translates to about 55 lb actual string tension.  At 28" draw the meter shows a 24, which translates to about 36 lb actual string tension.  Watching the meter as I drew the bow, the tension dropped steadily to about 1/3 draw the held steady for another 1/3 then slowy started to climb approaching 28" draw.  This  matches Hickman's findings mentioned in Arne's post above and pretty much settles it for me.  Interesting and fun.

Tensiometer at brace height:
   

Tensiometer at 28" draw:
   

[lefty4]

Thanks Fletcher for doing the "test". This is not what I would have guessed at the beginning of this thread. Glad you could show some "proof" that concurs with Hickman's findings.