Primitive Archer
Main Discussion Area => Bows => Topic started by: Selfbowman on February 08, 2022, 12:45:08 am
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Question: string angle is often talked about on here and my related math holds me back on understanding. Does an arrow take on more energy from a longbow or a short bow? The string is longer on a longbow and the angle is lower but does it stay on the string longer creating more energy? Even though the draw is the same? I’ll listen or try to. Thanks in advance. Arvin
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The idea is that the string pushes harder for longer.
Think of it this way, how does a #50 @28" longbow feel at half draw? What about a 50# @28" short bow?
Because the longbow pushes harder over more of the stroke it accelerates the arrow more.
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The idea is that the string pushes harder for longer.
Think of it this way, how does a #50 @28" longbow feel at half draw? What about a 50# @28" short bow?
Because the longbow pushes harder over more of the stroke it accelerates the arrow more.
I need to work up a fdc on a 60” vs 66” bow all things else being equal. I do not think that the weight or tension at half draw will be higher on the longer bow. I think where you get into a performance loss with short bows is when the string angle gets to the point that you start to stack. If you have a fluid build to full draw without stacking, I cannot see how or why the shorter bow would not out perform the longer one. Shorter bow limbs has less mass, and if the fdc is equal or near, then performance should go to the shorter limbed bow. One doesn’t push the arrow longer than the other, unless one is drawn further. At least that’s the way my thinking works.. looking forward to following this thread.
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I think that as long as the string angle doesn't get past 90 degrees you are not wasing energy.
As it gets past 90 you are starting to pull slightly along the length of the limb, trying to stretch it rather than bend it.
E.G. Imagine a skinny limb growing straight out from a tree at shoulder height.
Tie a yard or string to the end and pull down (string angle 90 )... it bends the limb. Now try pulling directly outwards in line with the limb (string angle 180), it won't bend at all! This shows that "feel" can be deceptive.
Del
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It isn't that when it hits 90 degs you lose all advantage. Every inch pulled from brace loses the advantage over the limb. The longer bow stores more energy as drawn because the string angle is proportionately lower through the whole draw. People talk about the stack when a bow string hits 90degs because you can't ignore it then! If you get to the point your bow is actually hitting 90 degs it is likely a bit short for the draw anyway.
Force draw curves are plotted from brace for a reason ;)
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Arvin: when it comes to the string angle topic my thinking never was focussed on the the arrow side but the drawing side: how much of your drawforce actually gets stored in the bow and how much of it is just lost in space? the higher the angle of attack of the string, the less energy finally is stored in the bow; vice versa: the lower the angle the more efficient the energy you put into the bow really gets stored.
to understand this its best to visualize with the force vectors operating at the tips. maybe somebody has a pic of it... similar to Del's explanation.
this also greatly helps to understand the term of "stacking"
cheers
I'm not claiming full understanding in this topic. But other than Del I think its relevant even beyond a 90 degree angle. Like bownarra said its a gradual loss of efficiency with increasing draw and angle.
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It isn't that when it hits 90 degs you lose all advantage. Every inch pulled from brace loses the advantage over the limb. The longer bow stores more energy as drawn because the string angle is proportionately lower through the whole draw. People talk about the stack when a bow string hits 90degs because you can't ignore it then! If you get to the point your bow is actually hitting 90 degs it is likely a bit short for the draw anyway.
Force draw curves are plotted from brace for a reason ;)
How is it storing more energy unless the force it takes to draw it is greater? Why does string angle alone play into how much energy is stored?
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Not sure it's just a string angle thing here.Talking only about performance wise.
The weight of the arrows makes a difference between short to long bows.Shorter bows shoot lighter arrows more efficiently.
Longer bows shoot heavier arrows more efficiently.That's a limb mass difference thing.
All bows shoot heavier arrows more efficiently really.
Accuracy wise at least for me a smoother draw/lower string angle helps.
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Ed that’s what I’m seeing happen in flight bows. Sleek made impressive shots in flight arrows with his short bows. I seem to be shooting arrows farther than him in broadhead class with a longer bow. I would like for everyone to think about this some. My force draw curves usually come up in the first third of the draw and then a short flat spot and tail off at twenty eight inches or so. Arvin
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Morgan what does the average 60 inch bow weigh vs say a 67” bow. My 50 pound bows come in at 21-24 oz usually. Depends on width at fades. That’s with a 9-10” handle weighing close to 7 oz.
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String angle has always confused me to Arvin. I never read the Bible. Seems like I need things explained in simple ways.
I don’t know if it’s right, or if it makes since to anyone else.
The way I understand it now. As the string angle increases a certain angle. It starts to cause the bow limb to push out more than ahead. I can imagine how that could be less efficient.
Bjrogg
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Morgan what does the average 60 inch bow weigh vs say a 67” bow. My 50 pound bows come in at 21-24 oz usually. Depends on width at fades. That’s with a 9-10” handle weighing close to 7 oz.
Arvin, I just weighed one that is 59” long and 55 lb draw @ 27”. Closest one I have at the house to what you have. Minimal handle section that has just a hair of bend only @ full draw I can’t see the bend but can feel it. It is 15.4 oz overall, no idea what the handle weighs.
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String angle has always confused me to Arvin. I never read the Bible. Seems like I need things explained in simple ways.
I don’t know if it’s right, or if it makes since to anyone else.
The way I understand it now. As the string angle increases a certain angle. It starts to cause the bow limb to push out more than ahead. I can imagine how that could be less efficient.
Bjrogg
I like the way you explained that. I can see where that could be a factor.
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Morgan that draw weight and that mass weight sounds like you ended up with not having to much extra unneeded mass and good distribution. Seems like most bend in the handle bows come in around 15-16 oz. if I remember right.
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Been thinking about this. So if you bend a short bow to 28” the limb tips have to move further than if you bend a long bow the same. Does that come into play in the performance? If the stroke is carrying the weight of the limbs further it would kill performance. Or am I off base? Most all of y’all posting are far more knowledgeable than myself, I’m just trying to wrap my head around performance factors.
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my understanding with my physics background is to separate the string tension from the pulling weight.
The most efficient pulling angle is always 90 degrees which gives the longest effective moment arm. Above and below that are less effective but since string tension is so low when the bow is initially drawn the inefficiency is not noticed. Until the 90 degree angle is achieved as string tension increases the moment becomes more efficient mitigating the increased force created in the string. It is only as the angle passes 90 degrees that it becomes less and less efficient and of course the string tension is also increasing so the lack of mechanical advantage becomes more apparent. Think of leverage - hold a 30lb weight straight out is much harder than holding is straight down or straight up but the force is the same. In this example the weight is the string tension, the position of your arm is the string angle, and the effort is the pulling weight.
As explained above stacking occurs when the combination of high string tension and large angle(smaller moment arm) create an exponential increase in force to draw the string back.
So a recurve that measures 35lbs pulling weight might have the same string tension as a straight bow that measures 50lbs at the same draw length. Or put another way if both bows measure the same pulling weight, the string tension and energy stored will be greater in the recurve due to a better mechanical advantage. That is why those short horn bows worked so well but no wood could handle that level of string tension.
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That is why those short horn bows worked so well but no wood could handle that level of string tension.
Not sure what you mean by this. Wooden bows aren't limited by string tension.
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This is from TBB4 but it doesn't explain the why. It also looks like a pretty negligible gain if two bows are close in length.
"One of the cruel realities of bow design is that shorter straight bows can’t be as fast per pound as longer straight bows, even at equal draw length. Between 35” and 60” possible performance rises roughly 1 fps per inch of bow length. Cast rises slowly from there to around 68”, then only minor improvement from there to 80”, and only then if given more elliptical tiller."
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I like it when the smart guys share.🤠
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What I meant was that longer bows have fatter FD curves, which is the same as saying that they store more energy all else equal.
Stack is the opposite of a fat F/D curve, so a bow that stacks hard at full draw will store as much energy as a longbow that draws the same weight at the same distance.
This also depends on brace height of course, as it cuts into the total 'draw length'.
Stack happens when the string angle gets large because of changes in leverage, as indicated above.
How this translates to arrow speeds depends on several other variables.
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This is from TBB4 but it doesn't explain the why. It also looks like a pretty negligible gain if two bows are close in length.
"One of the cruel realities of bow design is that shorter straight bows can’t be as fast per pound as longer straight bows, even at equal draw length. Between 35” and 60” possible performance rises roughly 1 fps per inch of bow length. Cast rises slowly from there to around 68”, then only minor improvement from there to 80”, and only then if given more elliptical tiller."
The last part of the quote from TBB goes parallel with what Bj was saying earlier about pushing forward rather than outward. There has been much discussion on proper tiller shape for a given front profile. I’m not certain if that is for longevity or performance or both but if the forward not out movement is what lends to better performance, wouldn’t an elliptical tiller always be desirable from a performance stand point?
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Ok this pic is not at 90 degrees cause I can’t my bow some for better sight on bringing the broadhead to full draw . What’s the string angle?
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Probably had 1-11/2 “ to get to full draw.
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Probably between 60 and 70 degrees between the arrow and string.
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A good ways from 90.
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my understanding with my physics background is to separate the string tension from the pulling weight.
The most efficient pulling angle is always 90 degrees which gives the longest effective moment arm. Above and below that are less effective but since string tension is so low when the bow is initially drawn the inefficiency is not noticed. Until the 90 degree angle is achieved as string tension increases
Thank you chasonhayes for your thoughts! I think its a good idea to string tension into play. It maybe helps understanding what really happens. Altough I must say your basic assumption is very wrong: String tension definitly is highest at brace and then goes down when drawing the bow.
(-P
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my understanding with my physics background is to separate the string tension from the pulling weight.
The most efficient pulling angle is always 90 degrees which gives the longest effective moment arm. Above and below that are less effective but since string tension is so low when the bow is initially drawn the inefficiency is not noticed. Until the 90 degree angle is achieved as string tension increases the moment becomes more efficient mitigating the increased force created in the string. It is only as the angle passes 90 degrees that it becomes less and less efficient and of course the string tension is also increasing so the lack of mechanical advantage becomes more apparent. Think of leverage - hold a 30lb weight straight out is much harder than holding is straight down or straight up but the force is the same. In this example the weight is the string tension, the position of your arm is the string angle, and the effort is the pulling weight.
As explained above stacking occurs when the combination of high string tension and large angle(smaller moment arm) create an exponential increase in force to draw the string back.
So a recurve that measures 35lbs pulling weight might have the same string tension as a straight bow that measures 50lbs at the same draw length. Or put another way if both bows measure the same pulling weight, the string tension and energy stored will be greater in the recurve due to a better mechanical advantage. That is why those short horn bows worked so well but no wood could handle that level of string tension.
I'm sorry but the basis of your deduction is incorrect. String tension decreases as a bow is drawn. It's at its highest at brace.
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Ok this pic is not at 90 degrees cause I can’t my bow some for better sight on bringing the broadhead to full draw . What’s the string angle?
Bravo!A very good question, (which I've asked before)... is it the angle with the last 2" of limb? The last 10"? Half the limb? The point nearest the tip which flexes.
this illustrates nicely how complex the physics and geometry. Even the most extensive analysis relies on simplification.
Del.
PS. My instinct would be to draw a line from the very tip to a point about 4" or so, down from the tip and use that to measure the angle.
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Yes at the bows' tip is the angle that's important.
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sometimes simplificated pics help.
I made 3 drawings for you:
Situation A: String angle less than 90
Situation B: String angle 90
Situation C: String angle more than 90
As you can see, the drawforce induced from the string (vector) at 90 degrees goes 100% into the bend of the limb.
In Situation A and C its different.... this force at the tip is split into two vectors:
In Situation A its being split into one part that goes into bend and one part working somehow like a compression force to the limb (GREEN)
In Situation C its split into one part that goes into bend and another part that goes into stretch of the limb (RED)
According to my primitive understanding GREEN = GOOD and RED = BAD.
I can also understand why RED = BAD, but not fully why GREEN = GOOD :)
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Good analogy.As good of a drawing to understand this as I've seen.
I agree with your final conclusion also.
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"String Angle and Stack The angle made by the string and arrow is the important angle. The near-tip string angle is less significant. If limb and string are nearly parallel for a high percentage of limb length then braced string tension is greater, increasing early draw weight."
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"String Angle and Stack The angle made by the string and arrow is the important angle. The near-tip string angle is less significant. "
wrong. only the angles at the tip are relevant when we think about putting energy from string into the limbs.
I would even say, THE GREENER THE BETTER :) regarding energy storage the first inches of draw are most efficient; the further the draw, the less efficient your draw becomes. Also that's why you are doing tricks with recurves etc. to keep good energy storage even on further draw.
this also (partly) could explain why a low braceheight makes a faster arrow.
but what do I know :)
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regarding energy storage the first inches of draw are most efficient; the further the draw, the less efficient your draw becomes. Also that's why you are doing tricks with recurves etc. to keep good energy storage even on further draw.
this also (partly) could explain why a low braceheight makes a faster arrow.
but what do I know :)
Awhile back I lowered the brace height of my 60" recurve and found it did appear to increase velocity of the arrows. It also reduced a tendency for the arrows to buck to the left and they entered the target in a straighter line.
I overdid it then walked it back to what appears to be just right for my bow. Just a bit lower than recommended factory specs.
Its hard to compare the effects of limb length when discussing self bows and horn bows. According to authorities on the long bow quoted by Saxton Pope the average English war bow if drawn past 30" was "7/8ths broken".
The limit of safe draw length, to avoid limb damage and future failure, varied greatly according to length of the bow. The longer bows could be drawn further.
The long bows were intended for heavy hard hitting arrows rather than maximum range. The longer limbs helped accelerate the heavier arrows.
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Maybe a bit off topic from Arvin’s original question but I always reference this discussion when it comes to stacking. https://www.tapatalk.com/groups/paleoplanet69529/stack-and-string-angle-t6218.html
I think the relationship to arrow speed is more complicated and relates to limb mass as well as others have stated.
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So if the tips are flipped as in my pic . The flipped tip is adding some string tension at full draw? Would we figure the angle at full draw as if it was a straight limbed bow? I think I agree with Del. Ryan Interesting info from days gone by thanks . That’s going to take some studying for me.
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Nice tiller, Arvin.
My understanding is that as 90 degrees is approached for string angle, efficiency decreases.
String angle deceases for recurves and reflexed bows.
As limb length increases for straight limbed bows string angle decreases but there arises another host of problems.
Then there is the issue of finger pinch on shorter bows which need to be reflexed.
Anyway, I don't really make myself short bows nor do I make myself recurves.
Great discussion!
Jawge
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So if the tips are flipped as in my pic . The flipped tip is adding some string tension at full draw? Would we figure the angle at full draw as if it was a straight limbed bow? I think I agree with Del. Interesting info from days gone by thanks . That’s going to take some studying for me.
Arvin: With your flipped tips you are only cheating towards MORE GREEN and LESS RED. That's why you are doing them :)
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I think string angle is not very important for the energy transformed to the arrow....
Stacking begins at high 80° angle and at 90° it is bad stacking....
The area below the force draw curve represented the stored energy...
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The bow above shot the broadhead arrow 260 yds. 50 pound bow. The recurve in the tips did work some toward full draw. I do think the pyramid does cut down on any unneeded mass in the limb.
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Question: If my bow in the pic is at say 60 degrees and I trap the back 25-30 percent decreasing mass and allowing the tension and compression to be the same at full draw is this a win win??? Just thinking here guys.🤠
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The way I understand this is that the energy imparted to the arrow is the area under the F/D curve. I think some knowledge of integrals would be needed to really calculate this but here is a way to show it. After all a given weight at full draw is the same regardless of string angle. If a bow stacks curve is steeper than say a longer bow with a linear force curve. Mass and internal friction being equal the longer bow that stacks less would have more energy. Hope this helps and a lot of this is explained in TBB 1.
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"The way I understand this is that the energy imparted to the arrow is the area under the F/D curve."
Yes.
Flipped tips? Oh y'all mean recurved tips. :)
Jawge
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The limbs on a shorter bow has to travel further at a given draw length; does that come in to play? Or is that a false statement and an optical illusion? I’ve never done a comparative measurement, but it sure looks like they do.
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With respect to draw length I think they move the same distance. With regard to actual tip travel, they may travel further due to bending in a tighter radius curve.
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That fact shows where the energy is coming from on the bows' limb.Longer bow versus shorter bow.
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Ok Jim Hamm at one point thought that adding weight to the tips would make a bow faster. Then the slowest bow in the world was built to prove a point. But is there truth in what he was thinking if you distribute that mass thru the whole limb. Meaning The longer bow shoots a heavier arrow farther because of the mass going forward. Train vs Volkswagen . The extra mass with a light arrow does not store the energy the same. You have less drag with a lighter arrow but the bow is now not as efficient. So short bow light arrows And longbow heavier arrows.
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I have a question and assumption to throw out here. First, take 3 identical length ttt, same mass bows. One straight, one deflex/reflex, and one reflex/deflex at tips. All have tips in line with handle like the straight bow. At brace the string does not touch the limb on the deflex/reflex. All bows are drawn to equal draw length and angle from string to tip and straight line to fulcrum point at hand never exceeds 90 degrees. Are the F/D curves different and which shoots the arrow faster? If so, why?
The only thing that I can reasonably think that would cause the deflex handle to reflex limb to excel is because of the lever effect of having a longer arm during most of the draw cycle. Essentially saying that the bow will start with high draw weight due to the arc of the limb and the unraveling effect (limb becomes more straight as opposed to arc which makes the distance from string tip to fulcrum longer). Much like pushing a door in the center vs the handle. The same force may be exerted but the door with move farther with the gain in leverage. The straight bow will have a gradual increase but no peaks so to say and the deflex tip bow will have a flatter F/D than the other 2 bows because at brace the bow will act as though it were a shorter straight bow because of the arc but it can unravel and lengthen giving more leverage effect. It will increase in weight but not gain advantage doing so.
As far as different length bows, I think this also explains it, but different length bows are apples to oranges. We have additional variables. Mass being a big one so there's always a sweet spot.
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there is some nice info on string angle in Vol 4 by Dan Perry,, he talks about string angle and mass etc,, energy storage of string angle and also then has some specs on the energy storage is not as important as one would think on how far an arrow will shoot, I found it very informative, he tells how far the different bows are shooting,,which is a great reference,,
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there is some nice info on string angle in Vol 4 by Dan Perry,, he talks about string angle and mass etc,, energy storage of string angle and also then has some specs on the energy storage is not as important as one would think on how far an arrow will shoot, I found it very informative, he tells how far the different bows are shooting,,which is a great reference,,
I guess that’s good reason for me to get vol.4
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With respect to draw length I think they move the same distance. With regard to actual tip travel, they may travel further due to bending in a tighter radius curve.
That fact shows where the energy is coming from on the bows' limb.Longer bow versus shorter bow.
More energy is gotten from the inner limbs with lighter tips than the outer limbs of shorter bows.This might not be the total analogy but I feel it has something to do with it.
I'll explain my observations and reasons for stating this.When tillering removal of wood the same amount of thickness from the inner limbs reduces poundage more than removal of material from the outer limbs.So outer limbs carry less poundage when shooting and inner limbs carry more.
Everyone knows a whip tillered bow is not very efficient shooting heavy arrows.
It correlates with the TBB 4 book on page 117 showing stress break down along a limbs length.Although it is for a D bow but see it hold true for stiff handled bows too.
Whether string angle has a say in this is another thing,but obviously a whip tillered bow would have a higher degree of string angle also.
Making a long bow to work mostly on the outer limbs is about like shooting a short bow.
I've tested the two types myself inner versus outer limb working bows[same poundage bows] and can tell you the whip tillered does not shoot the heavier weight arrow as fast as the inner working limbed bow and it takes it's advantage away from it not having more limb working to store more efficient energy.
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That is why those short horn bows worked so well but no wood could handle that level of string tension.
Not sure what you mean by this. Wooden bows aren't limited by string tension.
No short hornbows 'work so 'well' because they are physically light, horn/sinew can store a lot more energy than wood alone and they stored the energy because they bend 'close' to the handle and don't stack until the end of the draw. Wood can't handle the bend radius or energy storage.
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You could theoretically make a hornbow profile bow out of wood with the same bend radius and energy storage. But it would be extremely thin and wide meaning the limbs would be massive, not to mention the issue with air resistance at what would be huge widths.
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Most bow designs are optimized for the materials they are using. Fiberglass, wood and horn bows are not that far apart when it comes to speed using 10 grain per pound arrows. The horn bows really seem to excel with lighter arrows primarily because they can be made shorter and are more efficient. Most of our energy losses are due to vibration. Energy storage is just one factor and when it comes to wood bows hysteresis is a major cause of energy losses because of over stressing the wood. High energy storing designs are more prone to over stressing and that's why you will often see much lower energy storing designs able to beat them out.