Primitive Archer
Main Discussion Area => Bows => Topic started by: Jjpso on October 12, 2014, 01:01:17 pm
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Hello there.
If all the rest is the equal, wich bow has more stored energy: a 50 pounds at 26 inches or a 50 pounds at 28 inches?
And between a 50 pounds at 26 inches and a 44 pounds at 28 inches?
Thanks in advance.
Joćo Pedro
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The longer draw will store more energy but the 50#@26 will store more than the 44# even at the longer draw.
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My physics is a little rusty but for gravitational potential energy: E = mass x force (gravity) x height
I think the same principle would apply to a drawn bow.
50 @ 28 would have more stored energy.
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Mitch it is similar to the gravity but with gravity the force is constant. When measuring a bow you simply add up the force applied at intervals usually 1". A 50# weight dropped 24" would have about 100# of stored energy where a 50# bow drawn 24" ( plus a 6") brace height would likely store a little over 50# energy depending on the force draw curve.
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wow
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Badger - you are correct, the force on the arrow could (would) vary throughout the "stroke" of the string being released.
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Graph it! Draw weight at each inch. Draw weight on x axis, draw length on y-axis. You get a force-draw curve. A computer, regular addition, (or exhaustive calculus) can tell you the amount of area under the comparative curves, which will equal the stored energy. Longer draws are usually extra inches of near-maximum energy storage, depending on the shape of the curve (More energy!). For instance, a 25" draw 40# compound bow will store more energy than a 28" draw 50# longbow, usually, because of the relative curve shapes and area under their curves. Efficiency is how well stored energy becomes moving-arrow energy. You can detect inefficient bows because they are loud. Increase efficiency by increasing arrow weight --- you'll notice that bows get quieter with heavier arrows.
Although I've studied it, these ideas are not mine- all are explained in TBB volumes.
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Potential Energy (PE) = draw x final poundage /2 (approximately)
however, the draw length is the effective draw length = draw length - brace height
a 28" draw with an 8" brace has the same effective draw length as a 26" draw with a 6" brace height.
Ken
I say approximately because the actual Potential Energy available is the area under the force-draw curve on the return, not the draw. But we can never truly know this curve.
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I say approximately because the actual Potential Energy available is the area under the force-draw curve on the return, not the draw. But we can never truly know this curve.
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Ken, there actually is a method you can use to track this curve, by establishing the virtual mass of a bow. If no hysterisis is present the virtual mass will stay consistent regardless of arrow weight sshot form the bow. The way wood reacts when shot hysterisis will become more pronounced as the limb speed increases with lighter arrows. I started a thread on this the other day with regards to set and how it affects the shot. The concept is simple but I am not good at trying to explain it. Part of this concept you can see with the naked eye. Say you unstring a bow, if you could neasure the set like within 1/10 of a second you might have 2" set. 1 second later it may be 1 3/4", 10 seconds later 1 1/2" 1 min later 1 1/4" and 10 min later 1". You see a pattern develop that shows the closer you get to unbraced profile the slower the limbs return, with a super high speed camera you might be able to see this same phenomina continuing as the bow is shot possibly stretching into the bow 6" before brace height. It is happening fast but not as fast as the arrow leaves the bow so we are loosing power.
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Thanks for the answers guys.
Joćo Pedro
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I'm with ya Brad. WOW. ;) :) got some very smart guys on this board. :) Don't know anything about the numbers but I know from experience that a 50@28 will shoot quite a bit harder and sink the same arrow in the target much deeper than a 50@26 will.So I would have guessed it stored more energy. :) :)
Pappy
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I'm with ya Brad. WOW. ;) :) got some very smart guys on this board. :) Don't know anything about the numbers but I know from experience that a 50@28 will shoot quite a bit harder and sink the same arrow in target much deeper than a 50@26 will.So I would have guessed it stored more energy. :) :)
Pappy
I expected the same, but not from my own experience...
The only problem is that my draw length is only 24 inches... :D
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Mine was only 25 , I worked on getting it a little longer just for that reason,now I am at about 26,every little bit helps in that respect.
Pappy
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Ok, so I just read all this... My head hurts. I just make sticks that bend. I don't get all that mumbo jumbo. Physics class was in the morning, I was in a fraternity, u see where I'm going...
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Blafair, there really is no need to worry about all that stuff unless you are flight shooting anyway. Its fun to try and figure out why one bow is faster than another but as long as you are using good wood and good demensions and doing a good job tillering past that you are just splitting hairs. You don't need any math to know that if a bow took too much set it was because it was either wet, underbuilt or poorly tillered. No need to know much more than that unless you just enjoy crunching numbers.
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Blafair
your head ach reminds me why I don't do math
and as long as Steves numbers keep proving me right I don't have to get a head ach either
so keep up the good work Steve !
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Maybe I'm not understanding what is being asked but, wouldn't you also have to assume or know an arrow speed and weight then just calculate the foot pounds the arrow would impart upon a target. I guess the performance of the bow material and design would have to be super uniform.
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Hey everyone. Just started making bows, but I've been a nerd for a while.
If you're trying to discuss potential energy as it applies to projectiles, you technically have to integrate the force applied from release to the arrow leaving the string and apply that over time to get impulse as force translated to the arrow, then divide by mass to get flight speed or skip a few steps and assume negligible wind resistance and finish with impact force. But if you don't want to go about it the long way, Trad Gang answered this question already for us.
http://www.tradgang.com/ashby/Momentum%20Kinetic%20Energy%20and%20Arrow%20Penetration.htm (http://www.tradgang.com/ashby/Momentum%20Kinetic%20Energy%20and%20Arrow%20Penetration.htm)
But here's the cliff's notes: A bow held at the same force, but a longer draw will then apply force for a longer time, leading to higher impulse, which translates to more force. Or, bigger numbers will always result in bigger numbers. But why bother? Shouldn't you just make the bow that is right for you?
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Actually, the original question was which scenario would have the most stored energy. Stored energy= potential energy. What you mentioned above would actually be related to realized (Kinetic) energy, which includes efficiency. Put simply KE = PE x Eff. Or put in full mathematical form (WxV^2)/(gc x 2) = (D x P)/2 x eff
W = weight
V = velocity
gc = gravitational constant ( only for ft-lb systems) = 32.14 ft/s^2
D = effective draw = draw length - brace height
P = poundage at full draw
If you change P to grains/pound you will find that at 100% efficiency, your max possible velocity is about 214 ft/s for 9grains/pound arrows and a 22in effective draw (28-6), though this is not actually possible. At 200 ft/s your efficiency has to be pretty close to 92%. This does assume a perfectly triangular force-draw curve which may not be realistic for some types of bow designs. This is just the "static" math that helps explain what is happening.
Ken
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Pinhead, to figure an arrows potential energy you don't use time you use force and distance. The force changes over the length of the stroke being the highest at full draw. This is a well established accepted archery method for measuring a bows stored energy. It has no relation to how efficient the bow is. Truth is with a wooden bow you will never get an accurate measurement of force applied to the arrow by trying to measure it because of hysterisis in the wood. But measuring the force draw curve and stored energy will get you pretty close. The chrono will tell you what actually went into the arrow.
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Oh, I was just answering the question with physics, I didn't know there were already thumb rules in archery.
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A few tips for getting more stored energy:
1) High early draw weight (and weight throughout draw) - So avoid string follow, and shoot what # you can handle.
2) Avoid stacking, as this gives most of your draw weight & energy in only the last few inches. Many short bows have this prob.
3) Lower brace height a bit. This lengthens the effective draw, BUT slightly decreases draw weight.
4) As mentioned above, pull it back further.
5) Release quickly upon reaching full draw --- holding it back there will decrease stored energy because of limb stresses.
And then don't waste your energy with a sloppy release or with arrows that are too light to take what the bow has stored.
It is both simpler and more complicated than we make it sound, based on your perception and which details concern you.
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Did you know this guy's tests?
http://goldenhordebows.blogspot.com/2013/07/hysteresis-tests-part-2-wooden-and.html?q=draw+return
he did some videos too(bit chaotic):
http://www.youtube.com/watch?v=HcSJTObuMV0
With his tests in mind I would conclude time has a much more impact on hysteresis than I thought.
Michael