Bow efficiency?
11-08-2002, 07:10 AM
#11
Nontypical Buck
Join Date: Feb 2003
Location: CT, USA
Posts: 3,058
RE: Bow efficiency?
<BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>
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.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
Oh yes, yes...should have been more clear...as arrows approach the 5 grains per LB threshold. I thought I had stated that in my initial post, but perhaps I forgot. <img src=icon_smile_big.gif border=0 align=middle>
<BLOCKQUOTE id=quote><font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade 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....<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
As would I! I suspect Kevin has hit upon some combination of various elements that no-one else has, OR more likely has been able to "smooth out" enough to where people would buy them. I do know the original BK cams on the Oregon BK also showed similar results.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
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.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
Oh yes, yes...should have been more clear...as arrows approach the 5 grains per LB threshold. I thought I had stated that in my initial post, but perhaps I forgot. <img src=icon_smile_big.gif border=0 align=middle>
<BLOCKQUOTE id=quote><font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade 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....<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
As would I! I suspect Kevin has hit upon some combination of various elements that no-one else has, OR more likely has been able to "smooth out" enough to where people would buy them. I do know the original BK cams on the Oregon BK also showed similar results.
JeffB <img src=icon_smile_big.gif border=0 align=middle>
11-08-2002, 07:38 AM
#12
Fork Horn
Join Date: Feb 2003
Location: Kenosha, Wi USA
Posts: 499
RE: Bow efficiency?
Now that you mention it- you did say "as approaching 5gr/#"... Duh. my mistake, very late evenings and fuzzy mornings lately. <img src=icon_smile_wink.gif border=0 align=middle>
I'm not so sure it is just the cams that are optimized for lighter arrows, but what about the whole system (entire bow) as a unit? Is there something different about the limbs and the way they store the energy? Has anyone found this same scenario with a lower-poundage BKII? What was Matt/PA's poundage bow? It'd be cool if someone with a 55# setting on a BKII would report in...
But now that I think about it- who's going to buy the ultimate speed bow and set it for 55#?!?
Edited by - Black Frog on 11/08/2002 10:46:27
I'm not so sure it is just the cams that are optimized for lighter arrows, but what about the whole system (entire bow) as a unit? Is there something different about the limbs and the way they store the energy? Has anyone found this same scenario with a lower-poundage BKII? What was Matt/PA's poundage bow? It'd be cool if someone with a 55# setting on a BKII would report in...
But now that I think about it- who's going to buy the ultimate speed bow and set it for 55#?!?
Edited by - Black Frog on 11/08/2002 10:46:27
11-08-2002, 08:25 AM
#13
Nontypical Buck
Join Date: Feb 2003
Location: CT, USA
Posts: 3,058
RE: Bow efficiency?
Just to give a few examples of what I was getting w/ the BKII
61 Lbs peak weight 28" modules. with the grip off the brace hovered at 6.75" and the AMO draw length was 28.75" with the grip off.tru-peep, leeches and a loop installed.
With a 313 grain arrow I was getting 314 FPS: Initial KE:68.5 FLbs.
with a 325 grain arrow I was getting 308 FPS: Initial KE: 68.47 Flbs
With a 400 grain arrow I was getting 277 FPS: Initial KE: 68.1 FLbs
With a 475 grain arrow I was getting 254 FPS: Initial KE: 68.06 Flbs
Granted we are splitting hairs here. It's just interesting to note how the BKII cams were as efficient or better as arrow weight decreased. They really pick up some speed once you get in the 5.25 to 5 grains per LB range.
I did not test as thoroughly with the 29" modules. With the grip on (In order for me to shoot it comfortably)The brace height was 6.3" and the AMO draw ran at 29.5" IIRC. I was getting 327 FPS with the 313 grain arrow: Initial KE of: 74.3. Not too shabby for a 61 Lb bow.
I did do some tweaking of the harness to get the cam as straight as possible, It's possible I over-rotated the cam timing a bit resulting in a few more FPS than "normal". Regardless, the speed is incredible (and more importantly,very shootable) I'm looking forward to testing the BKII cam's replacement.
J<img src=icon_smile_big.gif border=0 align=middle>
61 Lbs peak weight 28" modules. with the grip off the brace hovered at 6.75" and the AMO draw length was 28.75" with the grip off.tru-peep, leeches and a loop installed.
With a 313 grain arrow I was getting 314 FPS: Initial KE:68.5 FLbs.
with a 325 grain arrow I was getting 308 FPS: Initial KE: 68.47 Flbs
With a 400 grain arrow I was getting 277 FPS: Initial KE: 68.1 FLbs
With a 475 grain arrow I was getting 254 FPS: Initial KE: 68.06 Flbs
Granted we are splitting hairs here. It's just interesting to note how the BKII cams were as efficient or better as arrow weight decreased. They really pick up some speed once you get in the 5.25 to 5 grains per LB range.
I did not test as thoroughly with the 29" modules. With the grip on (In order for me to shoot it comfortably)The brace height was 6.3" and the AMO draw ran at 29.5" IIRC. I was getting 327 FPS with the 313 grain arrow: Initial KE of: 74.3. Not too shabby for a 61 Lb bow.
I did do some tweaking of the harness to get the cam as straight as possible, It's possible I over-rotated the cam timing a bit resulting in a few more FPS than "normal". Regardless, the speed is incredible (and more importantly,very shootable) I'm looking forward to testing the BKII cam's replacement.
J<img src=icon_smile_big.gif border=0 align=middle>
11-08-2002, 09:59 AM
#15
Thread Starter
Join Date: Feb 2003
Location:
Posts: 2,413
RE: Bow efficiency?
<BLOCKQUOTE id=quote<font size=1 face='Verdana, Arial, Helvetica' id=quote>quote:<hr height=1 noshade id=quote>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.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
Not quite PAB,
I wanted to know if one of the cam styles was more efficient. If they both produce the same kinetic energy, was the same amount of energy put into each draw to acheive it?
I think that is what you are asking.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face='Verdana, Arial, Helvetica' size=2 id=quote>
Not quite PAB,
I wanted to know if one of the cam styles was more efficient. If they both produce the same kinetic energy, was the same amount of energy put into each draw to acheive it?
11-08-2002, 10:20 AM
#16
Join Date: Feb 2003
Location: Grand Forks ND
Posts: 258
RE: Bow efficiency?
Jeff,
Very interesting.
Energy not transferred the arrow ends up going back into the bow causing vibration and sound, right?
So higher efficiency means less noise and vibration.
Meaning that a lighter arrow in your BKII will be quiter and cause less vibration than a heavier one. I guess a sensitive db meter would be needed because of the very little difference in KE. But nonetheless, that is what would happen, correct? Seems to go against everything we've been taught! Cool.
Very interesting.
Energy not transferred the arrow ends up going back into the bow causing vibration and sound, right?
So higher efficiency means less noise and vibration.
Meaning that a lighter arrow in your BKII will be quiter and cause less vibration than a heavier one. I guess a sensitive db meter would be needed because of the very little difference in KE. But nonetheless, that is what would happen, correct? Seems to go against everything we've been taught! Cool.
11-08-2002, 10:42 AM
#17
Boone & Crockett
Join Date: Feb 2003
Location: Lehigh County PA USA
Posts: 12,157
RE: Bow efficiency?
Straightarrow,
I think that would depend on the very specific shape of each of the respective force draw curves.
You could have a 60 lb bow with a huge plateau at peak weight that might have the same KE as a 70 lb bow with more of a true bell-shaped curve draw/force curve. The total amount of draw weight pulled over the entire draw cycle would have to be a figure that needs to be considered.
This is something that I have been thinking about recently. A new technical term that would summarize the total amount of weight that needs to be drawn during the draw cycle for each respective bow. An example of which would be....
At 1 inch..... 5 lbs
At 2 inches... 8 lbs
At 3 inches... 8.5 lbs
...
...
At 10 inches....27 lbs
...
At 20 inches....59 lbs
...
At 29 inches...22 lbs
...etc..
Total weight drawn: 800 lbs
Now that figure seems rather large and in and of itself means nothing but it could be used as a useful tool in comparing the draw force curve from bow to bow.
I think that would depend on the very specific shape of each of the respective force draw curves.
You could have a 60 lb bow with a huge plateau at peak weight that might have the same KE as a 70 lb bow with more of a true bell-shaped curve draw/force curve. The total amount of draw weight pulled over the entire draw cycle would have to be a figure that needs to be considered.
This is something that I have been thinking about recently. A new technical term that would summarize the total amount of weight that needs to be drawn during the draw cycle for each respective bow. An example of which would be....
At 1 inch..... 5 lbs
At 2 inches... 8 lbs
At 3 inches... 8.5 lbs
...
...
At 10 inches....27 lbs
...
At 20 inches....59 lbs
...
At 29 inches...22 lbs
...etc..
Total weight drawn: 800 lbs
Now that figure seems rather large and in and of itself means nothing but it could be used as a useful tool in comparing the draw force curve from bow to bow.
11-08-2002, 10:55 AM
#18
Fork Horn
Join Date: Feb 2003
Location: Kenosha, Wi USA
Posts: 499
RE: Bow efficiency?
Frank-
Norb does this in his reports. It is the "Stored Energy" value. It is expressed in ft-lbs and is determined by measuring the force necessary to draw the bow. Norb does it at 1" increments from brace height to full draw. The summation of the force readings in 1" increments gives the stored energy in inch-pounds. Dividing that value by 12 converts the stored energy to ft-lbs.
Norb does this in his reports. It is the "Stored Energy" value. It is expressed in ft-lbs and is determined by measuring the force necessary to draw the bow. Norb does it at 1" increments from brace height to full draw. The summation of the force readings in 1" increments gives the stored energy in inch-pounds. Dividing that value by 12 converts the stored energy to ft-lbs.
11-08-2002, 11:46 AM
#19
Thread Starter
Join Date: Feb 2003
Location:
Posts: 2,413
RE: Bow efficiency?
Frank,
Yes, that's what I want to compared. The stored energy value (as Black Frog called it) to the kinetic energy produced for a given cam style. I'm wondering which cam gives us the most efficient use of our efforts when comparing to different cam styles that put out the same kinetic energy. For this to happen, obviously the less agressive cam would have to have higer peak weight limbs.
Yes, that's what I want to compared. The stored energy value (as Black Frog called it) to the kinetic energy produced for a given cam style. I'm wondering which cam gives us the most efficient use of our efforts when comparing to different cam styles that put out the same kinetic energy. For this to happen, obviously the less agressive cam would have to have higer peak weight limbs.
