Question please..

[akila]

Thank you for all your trouble to answer my question...i have to admit, taht I't never came to my mind to try such a simple experiment with the fishierman's scale, altough, I'm using one to tiller my bows..so  as a conclusion we could say, that the tips of a specific bow, dosent deal with more then half of the final draw force of that specific bow .

[bubbles]

I also remember that from TBB's. Can't remember where. Its definitely the opposite of what seems logical.  But, when bracing a highly reflexed bow to a very low brace height, there's a lot of tension on that puppy.

[DC]

TBB 4 Page 143. I've had a night to think about this and although it's an interesting tidbit I don't think it affects the arrow much, if at all. When you pull the bow the weight starts at zero and climbs to max draw weight. That's what the arrow feels (in reverse) when you release it. The tensions within the string stay in the string.

[Slackbunny]

DC, I'd like to see the pictures of your complete experiment. That would be cool if I was wrong. I always like it when things defy my intuition. It opens up new doors of understanding.

Where was the fishing scale positioned in the string? Was it in the middle where you were pulling? Or was it in between where you gripped the string and the nock?

Now you've gone and peaked my curiosity. I won't be able to get anything done at work now 

[DC]

The scale was right above the nocking point. I might try it with my tillering scale, the long spring in the fish scale is what made it so tough to string. I had to bend the bow and stretch the string(scale) at the same time. I finally got it to low brace and decided that was good enough. Later I tried to tighten the string a bit by winding it on a pair of pliers. The string broke and fired the scale across the room. Fortunately nothing was broken. A video showing the scale and draw at the same time would be the answer but I don't know how to do video.

[Badger]

  Another easy way to test it is to just take a string that is a little too long and tie a loop into it. Brace the bow and pull straight down on the loop with your scal and how much tension it takes to put a little slack in the string below the loop. Braced is the most tension on your string.

[Slackbunny]

Yeah, I'm sure that I was flat out wrong now. I've sat down and done some calculations. I used the initial string tension and bow draw weight figures that DC supplied from his bow to make sure I was using real data. Then I assumed some angles and did some force balancing and trignometry.

Sure enough, the tension on the string does actually go down as you draw the bow. I assumed that at full draw the angle between the string protruding from either side of your hand was 90 degrees for ease of calculation. But if you put the data into a spreadsheet and vary that angle, you still have reduced string tension. If you increase that angle (which would correspond to letting off the draw a bit), and lessen the draw weight accordingly, the string tension will increase until it reaches the original max tension at brace.

This is totally counter-intuitive at first glance, but the math confirms it. What we have going on here is a pulley-like effect where your fingers are the pivot point. The tension on the limbs is no longer being held by 1 string, but by 2, so the tension on either length of the string will be smaller.

I would be curious to see if there was a point at which stacking (the exponential growth of draw weight near the end of the draw) would be able to force the string into a state of higher tension than at brace, or if the string angle would continue to decrease enough to outpace string tension until failure.

[scp]

Well done, DC.

Now we need to figure out a way to measure the shear force at any point in the working limb.

[Springbuck]

I also remember that from TBB's. Can't remember where. Its definitely the opposite of what seems logical.  But, when bracing a highly reflexed bow to a very low brace height, there's a lot of tension on that puppy.

True, and sometimes at low braces, they seem impossibly strong.  I once strung a R/D bow with a string too long during tiller, so the string contacted most of the limb, and was only braced at maybe 2" by the handle.  Pulling that thing the first few inches was astoundingly hard, but once the string lifted from the limbs you could feel it toggle over and let off.

[scp]

Now we need to figure out a way to measure the shear force at any point in the working limb.

Actually in practice, we don't need to do this. All wood bend quite a bit before they break, especially at the thickness we see in bows. Therefore we can safely narrow the tips until they start to bend at full draw. I guess, to be safe, we better stop as soon as we feel or see any movement there.