Someone please explain ...
I can't tell how many times I've heard that bows , typically over 65 lbs, become less efficient than lighter bows .
But for the life of me I don't understand how that's possible . Nor am I understanding , apparently , what " efficiency " means in this context .
Thoughts ?
I do not know how to explain that, did they give you the reason?
I do believe that traditional bows are more efficient with heavy arrows (to a limit). Te reason is that not all the fibers of the bow release their energy at the same time, therefore a heavy arrow, which takes longer to leave the bow, allows additional fibers to discharge their energy on the projectile. That is why a large increase of arrow weight leads to a relatively small loss of speed, resulting in greater momentum and kinetic energy. To a limit, of course: if you use such heavy arrow that all the fibers have given their energy, you are not going to gain anything more, will only loose speed, and have a more curved trajectory.
That is how I understand it, please correct me if you think I am wrong. I cannot explain why a heavier bow would behave differently.
Just my two cents :)
It all depends on the design and build of the bow. Some bows like ASLs are less effected by increase in arrow weights, the faster bow will be faster with any arrow weight, but the difference between heavy arrows to light arrows may be greater. Now for the original post. Again it all depends on what one calls efficiency. I have used ASL bows from 38 pounds all the way to 96 pounds from the same maker. I have never once been able to go over spine on the very heavy bows, I was always been under spined. Example with one 90 pound bow at my draw and arrow length, best cedar arrow spine was 65 pounds with a 125 grain shaft cut to 27.5". Anything stiffer flew loggy. 70 pound spines at 27" bop with heavy heads were the correct spine for me. On the flip side I have found arrows over spine to always be the correct spine for the light bows. Out of the 38@26 bow 45 pound spines 27" with with 125 grain heads to be. I tried a variety of lighter spines and always end up back to the 45 pound spine arrows. Striking force, the heaviest bows always win, because of the cast to weight of arrow ratio. However, when playing with a chronograph, it was a bit surprising to find that the 38 pound bow shot 11 grains per pound at the same speed as the heavier bow shot 8 grains per pound. Again it will be different with every bow design.
It really depends on what Efficiency you're talking about....
All things being equal a heavier bow will always be more powerful....
A 100 pound bow with a 1000 grain arrow will always be more efficient than a 50 pound bow with a 500 grain arrow..... At killing water Buffalo and most extra big game.
Again...all things being equal.
"Striking force, the heaviest bows always win, because of the cast to weight of arrow ratio."
Quote from: Aaron Proffitt 2 on March 13, 2019, 05:24:53 PM
Someone please explain ...
I can't tell how many times I've heard that bows , typically over 65 lbs, become less efficient than lighter bows .
It could be because heavier bows tend to have heavier limbs, which per se sucks up some of the speed/energy
Quote from: Aaron Proffitt 2 on March 13, 2019, 05:24:53 PM
Nor am I understanding , apparently , what " efficiency " means in this context .
Thoughts ?
You are right, we should first define what we mean by efficiency.
The way I intended it, efficiency is the proportion of stored energy that the bow manages to give back to the arrow. Let me explain:
You pull the string, and you get at full draw; right now the bow has potential energy, which is waiting to be released; the moment you loose the string, the fibers of the limbs go back to where they were; that potential energy, stored by the deformed limbs (deformed as you are at full drawn) will be transmitted to the arrow. Not all of it will end up in the arrow though; there will always be losses (e.g. the attrition with the air). If the arrow is really light, it will leave the bow before all its fibers have released their energy, therefore the "surplus" will end up on the bow itself (and that's why if the arrow is too light you can damage the bow, similarly to a dry fire).
So, the efficiency of a bow is determined by the proportion of potential energy that is actually transmitted to the arrow. I completely agree with Pavan, one of the variables determining the efficiency is the design of the bow.
Hope this makes sense. Are there any physicists among us, by any chance? :tongue: That would help
Quote from: madmaxthc on March 13, 2019, 09:21:39 PM
You are right, we should first define what we mean by efficiency.
The way I intended it, efficiency is the proportion of stored energy that the bow manages to give back to the arrow. Let me explain:
Or, to be more precise, I think we should talk of efficiency of a bow/arrow combination.
If I weren't single right now, I probably wouldn't be so pedantic :tongue:
I am sorry for such long posts
Quote from: pavan on March 13, 2019, 08:45:48 PM
I have used ASL bows from 38 pounds all the way to 96 pounds from the same maker. I have never once been able to go over spine on the very heavy bows, I was always been under spined. Example with one 90 pound bow at my draw and arrow length, best cedar arrow spine was 65 pounds with a 125 grain shaft cut to 27.5". Anything stiffer flew loggy. 70 pound spines at 27" bop with heavy heads were the correct spine for me. On the flip side I have found arrows over spine to always be the correct spine for the light bows. Out of the 38@26 bow 45 pound spines 27" with with 125 grain heads to be. I tried a variety of lighter spines and always end up back to the 45 pound spine arrows.
That is interesting. I wonder if this is a common occurrence, and what other archers' experience is.
Damn I'm glad deer aren't mathematicians and physicists or they would never fall prey to my inefficient efforts. Seriously, If I am understanding these discussions, it would seem that efficiency of a given bow is a matter of what percentage of its stored energy it actually imparts to the arrow upon release. If that is correct, it would seem that a heavy bow can be less "efficient" than a lighter bow but still transfer more total energy to the arrow than the lighter one. Therefore, it seems it could be less mechanically efficient, yet be a more efficient killer. How far off base am I?
Quote from: Sam McMichael on March 13, 2019, 09:49:48 PM
Damn I'm glad deer aren't mathematicians and physicists or they would never fall prey to my inefficient efforts. Seriously, If I am understanding these discussions, it would seem that efficiency of a given bow is a matter of what percentage of its stored energy it actually imparts to the arrow upon release. If that is correct, it would seem that a heavy bow can be less "efficient" than a lighter bow but still transfer more total energy to the arrow than the lighter one. Therefore, it seems it could be less mechanically efficient, yet be a more efficient killer. How far off base am I?
That could be true. But I am still not sure that a heavier bow would be per-se less efficient.
Quote from: Sam McMichael on March 13, 2019, 09:49:48 PM
Damn I'm glad deer aren't mathematicians and physicists or they would never fall prey to my inefficient efforts.
:biglaugh:
C'mon, pragmatism and physics are not mutually exclusive ;)
In the end, if your arrow/bow combination work, there's no need to fix them. :laugh:
I think what generally happens is that there is a diminishing return of increased momentum after 60#. In other words, if the momentum increases by 10 (arbitrary number) going from 55-60#, then it will increase by 9 going from 60-65#. There is always an increase in momentum as poundage increases, but the rate of increase is less with each increase in bow weight.
Good stuff , guys ! Great insight !
Quote from: McDave on March 13, 2019, 10:10:22 PM
I think what generally happens is that there is a diminishing return of increased momentum after 60#. In other words, if the momentum increases by 10 (arbitrary number) going from 55-60#, then it will increase by 9 going from 60-65#. There is always an increase in momentum as poundage increases, but the rate of increase is less with each increase in bow weight.
That could be. Do you have any direct observation? Or hypothesis on why that might happen?
Edit: I re-read what I just wrote, hope I did not come out as rude, it is a genuine question.
Many many moons ago I had a 64" very skinny tipped 42 pound Black Widow, The difference in speed from target aluminum arrows to 1816s with 145 screw in points was remarkable. Those 1816s seemed like lead balloons compared to the target arrows. I had a longer draw and shot that bow with sights. I was also amazed that I hardly needed any adjustment at the bow sights at 20 yards. At a Sioux City shoot I compared it to a 55 pound longbow at the chronograph. The 55 pound longbow did not like the 1816s, but the Black Widow shot the 510 grain cedars pretty good. So I tested the bows with the cedars. The BW was still 3 fps faster for an average, but when shooting, the longbow seemed faster. In those days I would not have considered hunting deer with any 42 pound bow. I shot an arrow through a deer that year with an 85 pound longbow, because I considered the 55 to be dog after I did the comparative chronograph test. The guy I gave the 55 to, has shot 4 deer with it, before he bought a 47 pound Hill longbow. Perspectives can be a misleading thing.
In my understanding, lighter bows do have a better efficiency #, but physics dictates the heavier poundage you shoot the better your momentum #'s you will see pound for pound... Dependent on bow design... Your typical "D" bow won't compare to a R/D bow or a Recurve... with actual #'s, but still better then a lighter "D" bow pound for pound...
And if you are a hunter, penetration is what you are looking for period! Not efficiency of your setup... Thus momentum #'s are more important then the efficiency #'s!!! Given accuracy...
Which again tells to the adage, shoot the most poundage you can shoot accurately! Especially if you are after what can be deemed as "dangerous" game...
Quote from: madmaxthc on March 13, 2019, 10:30:07 PM
Quote from: McDave on March 13, 2019, 10:10:22 PM
I think what generally happens is that there is a diminishing return of increased momentum after 60#. In other words, if the momentum increases by 10 (arbitrary number) going from 55-60#, then it will increase by 9 going from 60-65#. There is always an increase in momentum as poundage increases, but the rate of increase is less with each increase in bow weight.
That could be. Do you have any direct observation? Or hypothesis on why that might happen?
Edit: I re-read what I just wrote, hope I did not come out as rude, it is a genuine question.
There is a good discussion of this in The Traditional Bowyer's Bible, vol. 1, "Bow Design and Performance" by Tim Baker. While his article relates to self bows, the general principles should apply to other types of traditional bows as well.
Clearly, my statement is only applicable to bows made the same, with only changes in poundage. Changes in materials or technology will result in bows of the same poundage that have different efficiency, so there will be bows of higher poundage that have higher efficiency than other bows of different design have at a lower poundage.
I'm not aware of any study that has tested bows for efficiency that were built the same except for poundage. I equate efficiency with momentum, since that seems to be the best guide for hunting efficiency, per Dr. Ashby's tests. However, in any design there has to be a point at which maximum efficiency is reached, with diminishing returns on either side. This seems to occur at around 60#. That is not to say that you shouldn't shoot a 100# bow if you can handle it, because it will shoot the arrows that are best tuned for it at a higher momentum than a 50# bow with equivalent arrows that are best tuned for the 50# bow. Since everything I have read indicates that maximum efficiency is reached around 60#, it simply means you pay for the extra momentum you get from shooting a 100# bow with more than a proportionate amount of extra effort.
McDave ,
Did we read that in Ashby's report about the 60-65 lb threshold ? Been wracking my brain . Just know I've read that quite frequently.
Quote from: McDave on March 13, 2019, 10:10:22 PM
I think what generally happens is that there is a diminishing return of increased momentum after 60#. In other words, if the momentum increases by 10 (arbitrary number) going from 55-60#, then it will increase by 9 going from 60-65#. There is always an increase in momentum as poundage increases, but the rate of increase is less with each increase in bow weight.
I have always found this to be true in pretty much every test I've seen. I'm not sure what the formula is to prove it though. It's been a long while since I took those classes. :biglaugh:
Sam got it right. :clapper: It is the principal of diminishing return. The relationship between draw weight and energy imparted to an arrow is not linear. You eventually reach a point where increasing bow weight will lead to more total energy imparted to the arrow, but the efficiency of doing so is less than with a lower draw weight bow.
QuoteIt could be because heavier bows tend to have heavier limbs, which per se sucks up some of the speed/energy
I'm not sure if this is relevant to overall efficiency, but two bows of a given and duplicate design except draw weight will have the weight of the actual working limbs lighter in the heavier draw weight bow...... I think. Just my opinion. :)
Monterey, I believe the opposite is true. It takes less material, i.e. less thickness of core and or glass, to make a 40 pound bow versus a 60 pound bow. The heavier bow will have more material in the limbs, thus making is ever so slightly less efficient in regards to dynamic efficiency compared to stored energy. It may only go from a dynamic efficiency of 86% to 85%, but some of the stored energy is required to move the additional mass of the limbs.
I think that has to be true. A bowyer isn't going to make limbs that are any more massive than are needed to get the job done, and if lighter weight limbs would get the job done on a 65# bow, then they would be no reason to use heavier weight limbs on a lighter weight bow. Of course, limbs can be made lighter by using different material or design, but then in order to be comparable, the lighter weight bow would also have to be made out of the same material and design.
Such a moot point. There are no good studies to quantify this, and there is no point where the diminishing returns outweigh the significant performance gains of shooting a higher poundage bow of the same design and gpp arrow. Now if you want to argue about what is necessary to kill something, that is a different story.
Nope
I look at it this way.
A 60# bow will usually toss a 600gr arrow a bit faster than a 45# bow.
A 60# bow is 33% heavier than a 45# bow but the arrow usually is not going 33% faster.
That being said, not all bows of same weight shoot the same. Differences can be more or less or none.........
I will still pick heavy bows to shoot heavy arrows........ YMMV
I'll start off by saying that I don't know what Pavan meant by his "nope." That said, enough of us were curious about the issue to want to discuss it. Curiosity about the way the world works is a basic human characteristic; sometimes it results in useful information, sometimes it doesn't. If you're not curious about what we're discussing, you're welcome to browse elsewhere.
McDave, I would actually be curious if some meaningful data on the subject were to be made available that could point someone in a direction for how heavy of a given bow to shoot game with. I just don't see how that can be, especially with no good trials, just hypothesis that go against what I have observed . I am all ears for someone to show the data, and how they used that to decide what bow they would shoot.
If I offended someone by saying it is a moot point, I didn't mean that it wasn't something that can't be discussed. Just if it isn't a useful tool for deciding which equipment to use, it loses it's value to me.
I'll certainly be curious to know about something that , on a basic physics level, doesn't make sense . If a given bow delivers so many pounds of thrust , ie. draw weight , it should deliver 'x'. Now comes the variables such as bow design , string material , etc..
And I find it pretty interesting the diversity of thought that comes through in discussions like these. You hear from guys who are bow builders and others who've studied this stuff to a degree that I dont. I'm a " tune it and shoot it " kinda guy. So I love hearing the 'why's ' from guys who've studied it.
I shoot 55-65 lbs and am happy with them. I know I could easily go to a 70 or 75 , and I wouldn't hesitate if I saw a used one in that weight in a model I'd love to have. Why ? Cause I'm an American .
Don't know if this will help but had a bowyer explain it to me that a 50-55 pound bow will give you the best speed with a 10 Grain per pound arrow. When you get above this speed doesn't increase proportionately with added arrow weight. Although the benefit comes shooting a heavier arrow, just not much faster. He had a graph that plotted this out.
PDK, you said: "Now if you want to argue about what is necessary to kill something, ...."
and pavan replied: "Nope"
If you are talking bow efficiency as in stored energy versus momentum or KE, then you are correct that after a point most bows loose efficiency when you get beyond 60 pounds. The reason is that the amount of material in the limb no longer gains in a linear equation. Instead of adding say .001" core thickness per pound you end up with .0015 or .002 additional core per pound in the heavier weights.
This adds mass weight to the limb and thus uses more stored energy to move the limb and has a negative effect on efficiency.
This makes it difficult to build the same limb design from lightweight (30) to heavy weight (60+).
This is my experience with building a few thousand sets of limbs.
Mike
I find this an interesting discussion but also a bit academic. What I mean is, it doesn't matter much for practical applications.
If the definition of "efficiency" above is correct ( and I like it) than heaver bows are almost certainly less efficient. Does of course depend on bow design, arrow weight, correct spine...
BUT, 100# war bow at 60% efficiency (let's assume it is roughly a poundforce of 60) will still be impacting more than a 50# bow at 90% efficiency (pound force 45). Not very efficient for the heavy bow, but definitely more punch.
I think what was said above defines efficiency at the point were the arrow leaves the string.
That doesn't cleanly translate into what happens at the target: arrow design, correct spine and distance come into play.
Also, depends on kinetic energy and momentum, and those are two different things that shouldn't be confused.
Practically, it isn't that important and bow choice is more of a case of what you need ( and what you like shooting).
A 50# bow with good efficiency will go through a deer at 20yard. If you try to penetrate chain mail at 50 or 100 you probably need something a bit heavier, "efficiency" be damned.
Quote from: wingnut on March 15, 2019, 12:53:00 PM
If you are talking bow efficiency as in stored energy versus momentum or KE, then you are correct that after a point most bows loose efficiency when you get beyond 60 pounds. The reason is that the amount of material in the limb no longer gains in a linear equation. Instead of adding say .001" core thickness per pound you end up with .0015 or .002 additional core per pound in the heavier weights.
This adds mass weight to the limb and thus uses more stored energy to move the limb and has a negative effect on efficiency.
This makes it difficult to build the same limb design from lightweight (30) to heavy weight (60+).
This is my experience with building a few thousand sets of limbs.
Mike
Ahhh. I gotcha.
Every design has its own point of of diminishing returns. In my own testing the fast light limbed bows show the most difference in cast when arrow weight is increased, but they will remain faster than heavy limbed slower bows. A heavy limb longbow will not show as much of a percentage of speed loss with arrow weight gain, if there is a point where it can pass up a light limbed bow that is faster at 10 gpp, it would be at such a heavy weight arrow that such an arrow would be impractical with either bow. Now for the big one, some bows that are very fast show more differences with draw length variations and release energy variations, while some bows that are slower do not show as much variations in the energy of the shot. It all depends what one's own variations are as to which bow will be better for them. I know that I sometimes I have a half inch variation in the point where the string leaves my fingers, sometimes a quarter inch shortener sometimes a quarter inch longer. I have seen bows that really come to their own at a specific draw length. My wife's short and very fast hybrid is good at her standard 26" draw, but at 26.25" the tips have a bit more motion and up that bow really comes on strong, at 25.5" it is no faster on the chronograph than her Hill longbow that is 1 pound lighter with the same arrow pulled to 25.5" draw. Then the question is how much will that loss of cast affect the arrows cast and which bow can she better adjust to complement the weaker shot. So far I have never met a human Hooter Shooter, it is a person by person thing, and impossible to declare precise outcomes.
Wow,
been away a couple of days, found now a lot of interesting posts. Thank you all for sharing your insight and experience.
Quote from: wingnut on March 15, 2019, 12:53:00 PM
If you are talking bow efficiency as in stored energy versus momentum or KE, then you are correct that after a point most bows loose efficiency when you get beyond 60 pounds. The reason is that the amount of material in the limb no longer gains in a linear equation. Instead of adding say .001" core thickness per pound you end up with .0015 or .002 additional core per pound in the heavier weights.
This adds mass weight to the limb and thus uses more stored energy to move the limb and has a negative effect on efficiency.
This makes it difficult to build the same limb design from lightweight (30) to heavy weight (60+).
This is my experience with building a few thousand sets of limbs.
Mike
Thanks for this answer, Mike, it is very informative :thumbsup:
Seems like saying more horsepower = less efficiency. Sure, the most efficient engine will be the one which is just powerful enough to move the vehicle. A 50 HP motor running at maximum power will use less fuel than a 300 HP motor driving the vehicle the same speed, and you will not get 6 times more speed out of the 300HP motor, but it will be faster by quite a bit.
Quote from: Bladepeek on March 17, 2019, 12:38:10 PM
Seems like saying more horsepower = less efficiency. Sure, the most efficient engine will be the one which is just powerful enough to move the vehicle. A 50 HP motor running at maximum power will use less fuel than a 300 HP motor driving the vehicle the same speed, and you will not get 6 times more speed out of the 300HP motor, but it will be faster by quite a bit.
Interesting anology, but I think WE are engine, while the bow and arrow is the transmission, driveshaft, differential, wheels, etc.