Modern strings

[avcase]

Since the FF string won't stretch like the dacron, the limbs are going to have to move a bit more to reach the same draw length.

The opposite is actually true. The tension in the string at full draw is less than for the braced bow. As the tension on a string is reduced, it contracts or shortens. More tension on the string, it stretches or lengthens. For a “stretchy” material like Dacron, change in string tension has a bigger effect on the string length than a material like FF.  So what happens when the bow is at full draw and the tension on the string is less than at full draw?  The string contracts or shortens at full draw, but it contracts more for the Dacron string than it does for the FF string. If the Dacron string contracts more at full draw, then this requires the limbs with a Dacron string to bend a little farther than to make up for it compared to a FF string.

Alan

[IrishJay]

I've heard the idea that string tension is less at full draw before, but I can't see how that would work from a physics standpoint. In physics a general rule of thumb for everything from springs, to fluids in a pipe to electricity is everything moves from high to low. Ergo at full draw the limbs are at a higher potential than at brace. When we remove outside influence (your string hand holding the system at the higher potential) the system will move back toward the lower potential,  in this case brace. Because brace has the lowest mechanical potential energy possible in the system, tension at this point would also have to be the lowest at this point.

Another more practical way to look at this is, brace a bow, put it on your tillering rig. There is a certain amount of tension in the string to hold the limbs at brace. Now with your scale hooked up, slowly pull until the limb tips start moving (the string will move some before the tips due to stretch in the string). Once you find the force at which the tips just start to move, you've found, approximately, your brace tension. And this will of course be considerably less than full draw weight.

[maitus]

+1 ....! Ununderstandabl how can something i pull, lose the tension

[bradsmith2010]

I am so confused,,

[DC]

It has to do with the mechanical advantage of the bow and string. There is a name for it, something to do with beams. At low brace the bow has the advantage over the string. A forty pound bow can put up to 60-70# tension on the string, I've measured it. As you draw the bow the string loses its MA and string tension drops. I put a scale in the string and as you draw it the tension goes down.

[PatM]

Pope mentioned this in his book.

[IrishJay]

If you had a low enough string to limb angle at brace it could happen. The example numbers I crunched were a 45 degree string limb angle at brace (mechanical advantage to the limb) verses a 90 angle ( string in line with direction of limb movement at draw, no mechanical advantage) limb tension at brace would have to be 70.7% of full draw tension for tension to stay constant. So for a 50lb bow, to have 50lbs string tension at brace, the limbs would have to already have 35.35lbs of tension in them.

[DC]

We discussed this here
https://www.primitivearcher.com/smf/index.php/topic,64869.30.html

My 40# bow actually has around 100# of string tension at brace. Remember we are talking string tension, not bow weight.

[IrishJay]

DC I'm not doubting that you measured more then draw weight at brace, I'm just saying that the tension will go up at full draw. String tension at draw is always going to be higher than draw weight. The only way draw weight will equal string tension is if you are pulling directly along the length of the string. Of course even at full draw there is some angle between the string and the direction you're pulling on it. Tension will be draw weight divided by the sin of that angle.

[DC]

I can't remember what the string tension was at full draw. I do remember that it dropped dramatically. You don't forget when things go so opposite to what you expected

[IrishJay]

Did a quick test.  I'm not sure what the draw weight was doing during any of this because my only scale was hooked up inline with the string. But at brace string tension was 5.32lbs at about 26" draw it was just over 31lbs.

[DC]

What is the draw weight normally?

[IrishJay]

34# at 28" but the test string with the scale tied into isn't the same length as the normal string for this bow.

[DC]

I don't know. Unless your bow has an awful lot of set. Can we see it unbraced? I'm not the only one that has done this test. Yours is the first one I've seen with that low a reading at brace.

[IrishJay]

That now does have alot of string follow, I'll try it on another bow at some point, probably won't be until later tonight though.