Bow efficiency?
11-07-2002, 09:12 AM
#1
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Bow efficiency?
I know I've seen the answer to this on here before, but I can't remember and it was too long ago for the search.
Are the bows, designed for more speed, that much more efficient than the slower models, or are you really only get a similar percentage of the energy you put into the draw, back out on the shot?
If one model shoots a 500 grain arrow at 230 fps at 60lbs and another bow shoots a 500 grain arrow at 230 fps at 70lbs, then isn't the energy going into the draw, about the same? Without a doubt, some cams seem easier to pull than others, regardless if it's more peak weight.
Another question. If the energy going in, is about the same, then wouldn't a similarly spined arrow be required for both, even though one is 10 lbs more peak weight?
Are the bows, designed for more speed, that much more efficient than the slower models, or are you really only get a similar percentage of the energy you put into the draw, back out on the shot?
If one model shoots a 500 grain arrow at 230 fps at 60lbs and another bow shoots a 500 grain arrow at 230 fps at 70lbs, then isn't the energy going into the draw, about the same? Without a doubt, some cams seem easier to pull than others, regardless if it's more peak weight.
Another question. If the energy going in, is about the same, then wouldn't a similarly spined arrow be required for both, even though one is 10 lbs more peak weight?
11-07-2002, 09:48 AM
#2
Fork Horn
Join Date: Feb 2003
Location: Kenosha, Wi USA
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RE: Bow efficiency?
SA-
It's not only about peak weight, but the area under the draw force curve. Bows designed for speed take advantage of more stored energy with harsher draw curves (harder cams).
If you have a 500gr, 230fps coming out of both a 60# and 70# bow, obviously the 60# bow will have a harsher draw force curve than the 70# bow. The 60# (with hard cams) will have more severe curves to store the same energy at full draw as the 70# bow (with round wheels). The area under the curve is proportional to the stored energy when the bow is drawn.
Edited by - Black Frog on 11/07/2002 10:49:37
It's not only about peak weight, but the area under the draw force curve. Bows designed for speed take advantage of more stored energy with harsher draw curves (harder cams).
If you have a 500gr, 230fps coming out of both a 60# and 70# bow, obviously the 60# bow will have a harsher draw force curve than the 70# bow. The 60# (with hard cams) will have more severe curves to store the same energy at full draw as the 70# bow (with round wheels). The area under the curve is proportional to the stored energy when the bow is drawn.
Edited by - Black Frog on 11/07/2002 10:49:37
11-07-2002, 10:34 AM
#3
Nontypical Buck
Join Date: Feb 2003
Location: CT, USA
Posts: 3,058
RE: Bow efficiency?
BF is pretty right on...I'd like to make two comments...the first is a personal thing, the second, just food for thought.
1)because the 60 lb is at a much lower peak weight,individually it may or may not feel harsher than the 70 lb bow...for me it does not..I prefer less peak weight that may be stout throughout the entire draw rather than a cam that ramps up to 70 pounds super quick and dumps just as quick..it feels "sloppy" to me, and annoys me to no end.
2) look at the majority of Norb Mullaney's Bow reports...here we see bows that should "AMO" in in the 230-235 range with the specs they have, yet the efficiency is so poor many are hitting in the mid or low 220's...The Clearwater Storm model done fairly recently is a perfect example...6.5" brace height and it won't hit 230 FPS AMO. The PSE Quantum is another example.. 7.25" brace and it was clocking along at 225..and these are not easy cams to draw either. I remember a Pearson hard solocam with brace hovering around 6" that clocked 229 AMO. That's poor. I'd rather take the more efficient design any day. Why put out the effort for a "loss"? For example: bows Norm Reported on like the Mathews Zmax, and 2K Bowtech Extreme Solo were clocking 13-15 FPS faster with the same appx. brace heights as the Pearson model The cams are all similar in feel (I've shot and/or owned them all). So many times the same amount of effort doesn't produce the results because of inefficiency in the particular cam/bow system.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
1)because the 60 lb is at a much lower peak weight,individually it may or may not feel harsher than the 70 lb bow...for me it does not..I prefer less peak weight that may be stout throughout the entire draw rather than a cam that ramps up to 70 pounds super quick and dumps just as quick..it feels "sloppy" to me, and annoys me to no end.
2) look at the majority of Norb Mullaney's Bow reports...here we see bows that should "AMO" in in the 230-235 range with the specs they have, yet the efficiency is so poor many are hitting in the mid or low 220's...The Clearwater Storm model done fairly recently is a perfect example...6.5" brace height and it won't hit 230 FPS AMO. The PSE Quantum is another example.. 7.25" brace and it was clocking along at 225..and these are not easy cams to draw either. I remember a Pearson hard solocam with brace hovering around 6" that clocked 229 AMO. That's poor. I'd rather take the more efficient design any day. Why put out the effort for a "loss"? For example: bows Norm Reported on like the Mathews Zmax, and 2K Bowtech Extreme Solo were clocking 13-15 FPS faster with the same appx. brace heights as the Pearson model The cams are all similar in feel (I've shot and/or owned them all). So many times the same amount of effort doesn't produce the results because of inefficiency in the particular cam/bow system.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
11-07-2002, 11:01 AM
#4
Fork Horn
Join Date: Feb 2003
Location: Kenosha, Wi USA
Posts: 499
RE: Bow efficiency?
SA-
And to answer your question about the spine of the arrow- you are correct. The arrow only "sees" the amount of energy stored under the draw force curve, not just peak weight. That's why many shaft charts have different columns for round wheel, soft cam, hard cam, etc... The charts are trying to compensate for the draw force curves in combination with peak weight.
And I think SA and JeffB may be talking about two different "efficiency" terms. There's the efficiency having a 70# peak weight, but a draw force curve that doesn't take advantage of much of the peak weight. I think that is what SA is referring to when talking about speed bows being more efficient. The use more of the peak weight to store more energy. For total "efficiency" of the draw force curve, you would have to compare the draw force curve of the bow in question to a absolute draw force curve. In that I mean that the absolute bow starts out at the PEAK weight, and stays at that peak weight throughout the entire draw. In other words, a draw force curve that is a straight line across the entire draw cycle.
There is also the efficiency of the bow as it is, as a system, regardless of the drawforce curve. Norb expresses these values as VIRTUAL MASS and DYNAMIC EFFICIENCY. The virtual mass takes into account the bow and arrow combination, because as the arrow gets lighter, the effiency goes down. Virtual mass is the weight in grains of an imaginary mass shot fro the bow at the same time and the same speed as the measured arrow. VM represents all the inefficiencies in the bow and arrow combo. The sum of the KE of the arrow and the KE of the imaginary mass equals the stored energy of the bow (area under the draw force curve). Norb's idea of VM is not constant (but close), and will change with different arrows on a given bow, but allows good representation of what's going on in the system.
Dynamic Efficiency is the ratio of the KE of the arrow at launch to the stored energy of the bow- expressed as a percentage.
Edited by - Black Frog on 11/07/2002 12:05:24
And to answer your question about the spine of the arrow- you are correct. The arrow only "sees" the amount of energy stored under the draw force curve, not just peak weight. That's why many shaft charts have different columns for round wheel, soft cam, hard cam, etc... The charts are trying to compensate for the draw force curves in combination with peak weight.
And I think SA and JeffB may be talking about two different "efficiency" terms. There's the efficiency having a 70# peak weight, but a draw force curve that doesn't take advantage of much of the peak weight. I think that is what SA is referring to when talking about speed bows being more efficient. The use more of the peak weight to store more energy. For total "efficiency" of the draw force curve, you would have to compare the draw force curve of the bow in question to a absolute draw force curve. In that I mean that the absolute bow starts out at the PEAK weight, and stays at that peak weight throughout the entire draw. In other words, a draw force curve that is a straight line across the entire draw cycle.
There is also the efficiency of the bow as it is, as a system, regardless of the drawforce curve. Norb expresses these values as VIRTUAL MASS and DYNAMIC EFFICIENCY. The virtual mass takes into account the bow and arrow combination, because as the arrow gets lighter, the effiency goes down. Virtual mass is the weight in grains of an imaginary mass shot fro the bow at the same time and the same speed as the measured arrow. VM represents all the inefficiencies in the bow and arrow combo. The sum of the KE of the arrow and the KE of the imaginary mass equals the stored energy of the bow (area under the draw force curve). Norb's idea of VM is not constant (but close), and will change with different arrows on a given bow, but allows good representation of what's going on in the system.
Dynamic Efficiency is the ratio of the KE of the arrow at launch to the stored energy of the bow- expressed as a percentage.
Edited by - Black Frog on 11/07/2002 12:05:24
11-07-2002, 11:46 AM
#5
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RE: Bow efficiency?
This one is tough to articulate. I'll try again.
The draw force curve represents a certain amount of stored energy. One that comes to peak weight quickly but at lower weight has a more rectangular shaped curve. One that gradually comes to peak weight, but goes to a higher weight has a different shaped curve. Let's pretend that both represent the same amount of store energy. Will the kinetic energy produced, be greater from one or the other, because of the efficiency of the design of the cam that puts forth that particular draw force curve?
I'll try it another way. Let's assume the cams were put on two identical bows, only difference being the draw weight of the limbs. The heavier limbed bow will have the cam with the higher peak weight, but the more gradual buildup. Lets assume both bows store approximately the same amount of energy, will there be a diffence in kinetic engergy produced by either bow?
I realize some bows will have inferior limbs or cheap bushings or other things that take away from efficiency. That's not what I'm wondering about. I'm curious if a particular cam design is more efficient at converting the stored energy into kinetic engergy.
The draw force curve represents a certain amount of stored energy. One that comes to peak weight quickly but at lower weight has a more rectangular shaped curve. One that gradually comes to peak weight, but goes to a higher weight has a different shaped curve. Let's pretend that both represent the same amount of store energy. Will the kinetic energy produced, be greater from one or the other, because of the efficiency of the design of the cam that puts forth that particular draw force curve?
I'll try it another way. Let's assume the cams were put on two identical bows, only difference being the draw weight of the limbs. The heavier limbed bow will have the cam with the higher peak weight, but the more gradual buildup. Lets assume both bows store approximately the same amount of energy, will there be a diffence in kinetic engergy produced by either bow?
I realize some bows will have inferior limbs or cheap bushings or other things that take away from efficiency. That's not what I'm wondering about. I'm curious if a particular cam design is more efficient at converting the stored energy into kinetic engergy.
11-07-2002, 12:09 PM
#6
Nontypical Buck
Join Date: Feb 2003
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RE: Bow efficiency?
SA,
In your second example..If both bows store the exact same amount of energy regardless of how it is done, they will only produce the same speed and KE if the percentage of efficiency (as BF posted about) is the same between the two.
Also a note regarding arrow weight adn efficency as BF posted.
Yes in general, the lighter the arrow the less efficient the system becomes (and KE is lower). However that is not always the case: the BKII cams in particular are an exception: At the various arrow weights I have tested it, they produce the same or HIGHER KE as arrow weight is dropped. The cams are optimized for lighter arrows. Once a certain threshhold is reached when lowering arrow weight, the bow gains roughly 1 FPS for every 2 grains of arrow weight dropped..as the arrow approaches 5 grains per LB it is producing slightly more KE initially than it does with heavier arrows.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
In your second example..If both bows store the exact same amount of energy regardless of how it is done, they will only produce the same speed and KE if the percentage of efficiency (as BF posted about) is the same between the two.
Also a note regarding arrow weight adn efficency as BF posted.
Yes in general, the lighter the arrow the less efficient the system becomes (and KE is lower). However that is not always the case: the BKII cams in particular are an exception: At the various arrow weights I have tested it, they produce the same or HIGHER KE as arrow weight is dropped. The cams are optimized for lighter arrows. Once a certain threshhold is reached when lowering arrow weight, the bow gains roughly 1 FPS for every 2 grains of arrow weight dropped..as the arrow approaches 5 grains per LB it is producing slightly more KE initially than it does with heavier arrows.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
11-07-2002, 09:08 PM
#7
Fork Horn
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RE: Bow efficiency?
JeffB-
How do you think the BKII's cams are optimized for lighter arrows? I know you found that in your setup that lighter arrows worked a little better, but do you think that can be atributed to finding the "magic spine" for your setup?
They would use the same cams on a bow set at 55#'s as they would on a 70# bow. How would the cams be designed for lighter arrows?
Edited by - Black Frog on 11/07/2002 22:11:05
How do you think the BKII's cams are optimized for lighter arrows? I know you found that in your setup that lighter arrows worked a little better, but do you think that can be atributed to finding the "magic spine" for your setup?
They would use the same cams on a bow set at 55#'s as they would on a 70# bow. How would the cams be designed for lighter arrows?
Edited by - Black Frog on 11/07/2002 22:11:05
11-08-2002, 06:09 AM
#8
Boone & Crockett
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RE: Bow efficiency?
Darn, it looks like I missed out on a great technical discussion....again...<img src=icon_smile_sad.gif border=0 align=middle>
I guess that is what I get for having to go to a seminar yesterday...<img src=icon_smile_wink.gif border=0 align=middle>
Both Black Frog and Jeff have this pretty much nailed down but in order to make it simpler for less technically oriented folks...
...When all else is equal, the draw force curve has no effect on the Kinetic Energy level delivered between two bows producing the same amount of speed with the same weight arrow.
To put into even more practical terms.....a 60 lb Bowtech Patriot spitting out a 400 grain arrow at 290 fps isn't generating anymore kinetic energy than a 70 lb Mathews Legacy shooting a 400 grain arrow at 290 fps though the Bowtech might have a more plateaued draw force curve.
I think that is what you are asking.
I guess that is what I get for having to go to a seminar yesterday...<img src=icon_smile_wink.gif border=0 align=middle>
Both Black Frog and Jeff have this pretty much nailed down but in order to make it simpler for less technically oriented folks...
...When all else is equal, the draw force curve has no effect on the Kinetic Energy level delivered between two bows producing the same amount of speed with the same weight arrow.
To put into even more practical terms.....a 60 lb Bowtech Patriot spitting out a 400 grain arrow at 290 fps isn't generating anymore kinetic energy than a 70 lb Mathews Legacy shooting a 400 grain arrow at 290 fps though the Bowtech might have a more plateaued draw force curve.
I think that is what you are asking.
11-08-2002, 06:15 AM
#9
Nontypical Buck
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RE: Bow efficiency?
<BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>
JeffB-
How do you think the BKII's cams are optimized for lighter arrows? I know you found that in your setup that lighter arrows worked a little better, but do you think that can be atributed to finding the "magic spine" for your setup?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I don't think so, as the greater speed and energy was the case with various spined arrows in the same weight range out of my bow: various Carbon express shafts of different sppine values, , as well as two different spines of A/C/C's. In addition, I know of two other owners of the BT Pro-38DC (which uses the same general cam) who have run into the same scenario. Also, IIRC Matt/PA's results were similar to mine. Perhaps he will jump on here and comment.
<BLOCKQUOTE id=quote><font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>They would use the same cams on a bow set at 55#'s as they would on a 70# bow. How would the cams be designed for lighter arrows?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I'm not sure what you mean here regarding the different draw weights. Draw weight did not factor into my discussion re: the increased efficency of the BKII cams with lighter arrows. I did dicuss it regarding overall bow efficiency however.
Now, HOW are they optimized? Good question... I am no engineer.It is my OPINION it's done with where the cam stores more energy (in a relative sense) than most other cams: at the last few inches of draw weight. I believe this gives a lighter arrow (especially) a bigger initial "push" than most other cams as they travel back to brace.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
JeffB-
How do you think the BKII's cams are optimized for lighter arrows? I know you found that in your setup that lighter arrows worked a little better, but do you think that can be atributed to finding the "magic spine" for your setup?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I don't think so, as the greater speed and energy was the case with various spined arrows in the same weight range out of my bow: various Carbon express shafts of different sppine values, , as well as two different spines of A/C/C's. In addition, I know of two other owners of the BT Pro-38DC (which uses the same general cam) who have run into the same scenario. Also, IIRC Matt/PA's results were similar to mine. Perhaps he will jump on here and comment.
<BLOCKQUOTE id=quote><font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>They would use the same cams on a bow set at 55#'s as they would on a 70# bow. How would the cams be designed for lighter arrows?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I'm not sure what you mean here regarding the different draw weights. Draw weight did not factor into my discussion re: the increased efficency of the BKII cams with lighter arrows. I did dicuss it regarding overall bow efficiency however.
Now, HOW are they optimized? Good question... I am no engineer.It is my OPINION it's done with where the cam stores more energy (in a relative sense) than most other cams: at the last few inches of draw weight. I believe this gives a lighter arrow (especially) a bigger initial "push" than most other cams as they travel back to brace.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
11-08-2002, 07:01 AM
#10
Fork Horn
Join Date: Feb 2003
Location: Kenosha, Wi USA
Posts: 499
RE: Bow efficiency?
JeffB-
I mention draw weights because the cams are used on bows with a variety of draw weights with different limb deflections. When you say lighter arrows, I'm wondering if the better terminology would be "lighter grains/pound"? Otherwise, what constitues "lighter" for the different poundage bows? You'll certainly be using a much lighter IBO arrow on a 55# bow as you would on a 70# bow.
I know Norb has stated many times: <BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>"The dynamic efficiency of the bow and arrow combination is affected by the weight of the arrow. Lighter weight arrows are launched with lower efficiency than heavey arrows. This is an inescapable fact of physics."<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I would agree with Norb in a physics sense, but I don't doubt your findings on the BKII- that's why this is very interesting to me! I want to know more....
I mention draw weights because the cams are used on bows with a variety of draw weights with different limb deflections. When you say lighter arrows, I'm wondering if the better terminology would be "lighter grains/pound"? Otherwise, what constitues "lighter" for the different poundage bows? You'll certainly be using a much lighter IBO arrow on a 55# bow as you would on a 70# bow.
I know Norb has stated many times: <BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>"The dynamic efficiency of the bow and arrow combination is affected by the weight of the arrow. Lighter weight arrows are launched with lower efficiency than heavey arrows. This is an inescapable fact of physics."<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
I would agree with Norb in a physics sense, but I don't doubt your findings on the BKII- that's why this is very interesting to me! I want to know more....