Primitive Archer
Main Discussion Area => Bows => Topic started by: akila on December 21, 2014, 09:51:23 am
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Hello all, i have a question, and i hope i will manage to explain as clear as possibile what i have on my mind..im curiouse, if anione, had ever tested, how much poundage, the End's of a bow really take...i think that I'v read at a certain point, that the actualy poundage, that the end of a bow needs to stand is much smaller then the entire force of that specific bow..let's say that we have a 40lbs bow..the poundage at the end of the bow, where the overlays are, is the same or smaller?..i think its smaller, but I'm curiouse id there is a formula to calculate this.
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The total draw weight should be divided in half for the amount effort each limb will have to handle, I think. Also the stresses at the fades is higher because of the leverage of the limbs. These are more common since explanations than scientific explanations.
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Someone posted it a while back and I think it's in one of TBBs. Most stress out of the fades, a little less mid limb, and the least stress on the tips. On a properly tillered bow
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At the actual overlay all the poundage is simply tying to crush the overlay, there is no leverage.
A simple test is to lightly clamp a bit of arrow shaft in a vice (wrapa bit of leather round it to stop it getting crushed)
Loop an old string over the shaft right next to where it is gripped and pull as hard as you can... you won't break it.
Now try with 6" of shaft sticking out of the vice and the string near the end, it will easily break.
Del
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No thanks Del ;) ;D O:)
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what Pat said,, I actually tried to break some tips with a scale attached to the string,, tip in a vise..I narrowed them extremely thin and it was very difficult to break one,, really helped my confidence to narrow a tip to reduce the mass,,
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Thanks all of you for your replays..I'm also doing my tips really narrow now, even for 55-60 lbs bows..i just taught that maybe there is some kind of formula to calculate this moore precise..what is the poundage that the last 6" needs to deal with :)...but there they are so many variables , and its probably quite dificult to find a formula for this
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Also be aware that the bowstring is under the most tension at brace, not at full draw. This is the reason that D/R bow typically perform better than straight standing bows. It all has to do with how close the string is to the end of the limb at brace.
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Not really - r/d bows are fast because the reflexed outer limbs keep string angle low and therefore store more energy through the draw than say a standard straight limbed bow. Similar to a recurve but without the potential excess outer limb weight. Of course better string tension at brace helps with early energy storage too.
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If a bow were a lever then for a 40# bow each end would experience a 20# force depending upon where the fulcrum is. But the bow is a spring so I don't think the tips experience half the poundage for a force...much less would be my guesss. Jawge
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Also be aware that the bowstring is under the most tension at brace, not at full draw. This is the reason that D/R bow typically perform better than straight standing bows. It all has to do with how close the string is to the end of the limb at brace.
Unless I'm over-looking something I'm pretty sure that's not right.
At brace the string is only dealing with the tension that the limbs are exerting on it. When you pull on the string, you are adding tension to the system. This will necessarily increase the tension on the string.
D/R often perform better than straight bows, but its got nothing to do with the string tension.
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Also be aware that the bowstring is under the most tension at brace, not at full draw. This is the reason that D/R bow typically perform better than straight standing bows. It all has to do with how close the string is to the end of the limb at brace.
Unless I'm over-looking something I'm pretty sure that's not right.
At brace the string is only dealing with the tension that the limbs are exerting on it. When you pull on the string, you are adding tension to the system. This will necessarily increase the tension on the string.
D/R often perform better than straight bows, but its got nothing to do with the string tension.
You seemed to answer your own question. At brace the string is holding the braced weight of the bow, and at full draw the string is holding the drawn weight of the bow, the entire limb is working to distribute the weight, not just the tips.
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wow that sounds very confusing to me :-[
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Also be aware that the bowstring is under the most tension at brace, not at full draw. This is the reason that D/R bow typically perform better than straight standing bows. It all has to do with how close the string is to the end of the limb at brace.
Unless I'm over-looking something I'm pretty sure that's not right.
At brace the string is only dealing with the tension that the limbs are exerting on it. When you pull on the string, you are adding tension to the system. This will necessarily increase the tension on the string.
D/R often perform better than straight bows, but its got nothing to do with the string tension.
You seemed to answer your own question. At brace the string is holding the braced weight of the bow, and at full draw the string is holding the drawn weight of the bow, the entire limb is working to distribute the weight, not just the tips.
Yeah, that's why I'm wondering why he thought that the maximum string tension takes place at brace and not at full draw.
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Break out the popcorn.
I know I've read somewhere in TBB that Misslemaster is right, that string tension is higher at brace. I looked and can't find it yet. I decided to try it. I made up a string with a fishing scale in it and braced my 35# bow. It was a real PITA to brace. At low brace the scale reads 56# and as I draw it the weight drops off and by half draw or so is down to 35# and stays there til full draw. I'm convinced. I took some pictures but all I got was pictures of the scale so they don't prove anything. My photographer is out so anything more convincing is out. If you don't believe it, try it yourself, it took 15 min.
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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 :).
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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.
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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.
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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 ;)
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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.
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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.
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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.
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Well done, DC.
Now we need to figure out a way to measure the shear force at any point in the working limb.
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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.
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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.