Heavy weight equals less efficiency ?

[Aaron Proffitt 2]

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 ?

[madmaxthc]

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

[pavan]

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.

[Terry Green]

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.

[pavan]

"Striking force, the heaviest bows always win, because of the cast to weight of arrow ratio."

[madmaxthc]

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

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?  That would help

[madmaxthc]

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 
I am sorry for such long posts

[madmaxthc]

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.

[Sam McMichael]

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?

[madmaxthc]

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. 

Damn I'm glad deer aren't mathematicians and physicists or they would never fall prey to my inefficient efforts.

 

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. 

[McDave]

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.

[Aaron Proffitt 2]

Good stuff , guys ! Great insight !

[madmaxthc]

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.

[pavan]

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.

[Bowhunter4life]

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...

[McDave]

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.

[Aaron Proffitt 2]

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.

[Todd Cook]

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. 

[JohnV]

Sam got it right.     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. 

[monterey]

It 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.