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Thread: How much will lose off of IBO when at 60 lbs.

  1. #1
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    How much will lose off of IBO when at 60 lbs.

    Because most bow manufacturers base IBO ratings at 70# I wondered if a 60# bow have a lower IBO even though your still shooting an arrow at 5 grains per pound?

    I though I read somewhere here on AT that a 60# bow would IBO a little less than a 70# rating because of the energy difference?



    Just curious and wanting to learn, lol. Thanks
    2012 Pearson Stealth II, Hogg-It sight, Limbdriver rest, Bernies 10" XROD & QD, Easton Flatline 500's, Vanetec Swift 2.25, Stan Black Jack release


  2. #2
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    As a rule of thumb you lose 5 to 7 fps when going from 70 to 60 lb limbs.
    Quote Originally Posted by Kevin Strother
    I'm adding you to my slander, defamation of character lawsuit, I'm really tired of all the BS you and L-Train spread so you both will be added to the proceedings and named as a defendant.

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  3. #3
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    Well i know when I changed out the limbs on my old Marquis, at 72# 360gr it was shooting 328, at 61# 305gr it shot 331.
    I drank the Koolaid - Triumph and really bad aim

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    Quote Originally Posted by DeepFried View Post
    As a rule of thumb you lose 5 to 7 fps when going from 70 to 60 lb limbs.
    That's what I've always heard too
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  5. #5
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    IBO is 5 grains per pound. if you maintain the 5gpp ratio between the limbs, you theoretically shouldnt lose anything. reality doesnt always play by the same rules.


    manufacturers use the 30"/70lb parameters because it looks better in numbers......and numbers sell bows.
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    Quote Originally Posted by rock monkey View Post
    IBO is 5 grains per pound. if you maintain the 5gpp ratio between the limbs, you theoretically shouldnt lose anything. reality doesnt always play by the same rules.


    manufacturers use the 30"/70lb parameters because it looks better in numbers......and numbers sell bows.
    well put

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    Quote Originally Posted by DeepFried View Post
    As a rule of thumb you lose 5 to 7 fps when going from 70 to 60 lb limbs.
    DeepFried: Do you mean you lose 5 to 7 FPS per pound of draw weight? Or, do you mean you lose only 5 to 7 FPS total going from 70lbs to 60lbs? Thanks.
    *PSE Matt Forest Green "Dominator Pro 3D Max" for Outdoor 3D. (AKA: BOOGER) *PSE Matt Forest Green "REVO" for hunting. (AKA: YOGURT) ...........................May Da Schwartz Be With You!!!

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    My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.

    I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.

    Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.

    I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.

    After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...

  9. #9
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    Hmm interesting. It does make since though that it takes less force to move a lighter object the same speed. I need to find me some 275gr TW arrows for 3D
    Quote Originally Posted by FSUBIGMAC
    Well i know when I changed out the limbs on my old Marquis, at 72# 360gr it was shooting 328, at 61# 305gr it shot 331.
    BoCoMo FAT Slings

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    Quote Originally Posted by spiker_01 View Post
    My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.

    I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.

    Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.

    I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.

    After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...
    When you decrease poundage by backing the limb bolts out, you will increase draw length and also INCREASE brace height. They will both increase because the limb tips move back as you back the bolts out which also moves the string back. If your brace height decreased your draw length would shorten. I would think the increase in brace and draw would cancel out any speed you'd gain but alot of it has to do with cam timing and rotation. You can sometimes over or under rotate cams for increased speed.

    To the OP, if you shoot the same arrow at 70 and 60 lbs, the 60 lb draw will be about 20 fps slower. I got the following from Backcountry Bowhunting:

    For every inch of draw length you go down you will lose 10 FPS

    For every 5# draw weight you go down you will lose 10 FPS

    For every 3 grains of weight your arrow is over IBO ( 5 grains per pound) you will lose 1 FPS

    For every 3 grains of weight you add to your string you will lose 1 FPS.
    Speed is fine. Accuracy is final.

  11. #11
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    there is going to be some drop in efficiency,your moving the same parts on the bow with less draw weight so something gets lost there,however due to the increase in dl and brace you loose less of it.if dl and brace stayed constant like say going from 70lb max limbs to 60lb max limbs you should loose more fps due to the brace and dl staying the same.i think kevin strother told me 6-10 fps if i remember correctly.

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    Quote Originally Posted by BRUKSHOT View Post
    DeepFried: Do you mean you lose 5 to 7 FPS per pound of draw weight? Or, do you mean you lose only 5 to 7 FPS total going from 70lbs to 60lbs? Thanks.
    The last part. If you take a bow that is 70lbs, 30 inch draw and a 350 arrow then have the same bow but with 60 lb limbs and a 300 gr arrow and 30 inch draw the 70lber would be around 5 fps faster.
    Quote Originally Posted by Kevin Strother
    I'm adding you to my slander, defamation of character lawsuit, I'm really tired of all the BS you and L-Train spread so you both will be added to the proceedings and named as a defendant.

    Have a great one.

  13. #13
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    Quote Originally Posted by spiker_01 View Post
    My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.

    I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.

    Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.

    I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.

    After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...
    Your brace height will actually increase as you decrease poundage... just think about the geometry. Or, better yet, do you have a string stopper on your bow? If the string is flush with the stopper at 70 lbs as you back it down to 60 lbs it will pull away from the stopper (AKA, longer brace height)

  14. #14
    I put my Guardian through OT2 and here's what I came up with.

    At 70#, shooting a correctly spined arrow, OT2 said the Guardian should be shooting 299fps.

    At 60#, shooting a correctly spined arrow, OT2 said the Guardian should be shooting 286fps.

    This was plotted changing nothing but the draw weight and matching it to the correct spine using the CX 3D Select shafts for both draw weights.

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  15. #15
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    Quote Originally Posted by DeepFried View Post
    The last part. If you take a bow that is 70lbs, 30 inch draw and a 350 arrow then have the same bow but with 60 lb limbs and a 300 gr arrow and 30 inch draw the 70lber would be around 5 fps faster.
    Thanks DeepFried.
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  16. #16
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    Quote Originally Posted by klemsontigers7 View Post
    Your brace height will actually increase as you decrease poundage... just think about the geometry. Or, better yet, do you have a string stopper on your bow? If the string is flush with the stopper at 70 lbs as you back it down to 60 lbs it will pull away from the stopper (AKA, longer brace height)
    Oops, brain fart...sorry, you are correct about the brace.

  17. #17
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    Quote Originally Posted by automan26 View Post
    I put my Guardian through OT2 and here's what I came up with.

    At 70#, shooting a correctly spined arrow, OT2 said the Guardian should be shooting 299fps.

    At 60#, shooting a correctly spined arrow, OT2 said the Guardian should be shooting 286fps.

    This was plotted changing nothing but the draw weight and matching it to the correct spine using the CX 3D Select shafts for both draw weights.

    Automan
    Wow? That's a difference of 13 fps.
    2012 Pearson Stealth II, Hogg-It sight, Limbdriver rest, Bernies 10" XROD & QD, Easton Flatline 500's, Vanetec Swift 2.25, Stan Black Jack release

  18. #18
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    Quote Originally Posted by Bowhuntertim View Post
    When you decrease poundage by backing the limb bolts out, you will increase draw length and also INCREASE brace height. They will both increase because the limb tips move back as you back the bolts out which also moves the string back. If your brace height decreased your draw length would shorten. I would think the increase in brace and draw would cancel out any speed you'd gain but alot of it has to do with cam timing and rotation. You can sometimes over or under rotate cams for increased speed.

    To the OP, if you shoot the same arrow at 70 and 60 lbs, the 60 lb draw will be about 20 fps slower. I got the following from Backcountry Bowhunting:

    For every inch of draw length you go down you will lose 10 FPS

    For every 5# draw weight you go down you will lose 10 FPS

    For every 3 grains of weight your arrow is over IBO ( 5 grains per pound) you will lose 1 FPS

    For every 3 grains of weight you add to your string you will lose 1 FPS.
    this numbers are much more in line with what i have seen in real life,,,

    10 pounds of weight loss will not give the same speed as 70 even if you maintain 5gpi,, if this were true a 40# bow at 5gpi would be as fast as a 70# bow.... its till being lanched at lower poundage.. the small difference in brace height and draw length is not worth trying to figure out,,as far as speed is concerned

    i have found the speed studs from g5 to actually work and when placed right gain up to 8 fps.. in most bows. if placed wrong they will rob speed so that weight added to the string does not always cost you speed,,

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