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Discussion Starter #1
I know that IBO speed is based on 70#, 30in draw, and a 350 grain (5 grains per #) arrow.

It's my understanding when caculating IBO speed you deduct 1 fps for every 3 grains above IBO arrow weight.

Is the deduction based on total arrow weight or grains per #?

Example at 65# with 455 grain (7 grains per #) arrow:
Total arrow weight: 105/3 = Deduct 35 fps
Grains per #: 7-5 = Decuct 1 fps

Thanks
 
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Garuki said:
I know that IBO speed is based on 70#, 30in draw, and a 350 grain (5 grains per #) arrow.

It's my understanding when caculating IBO speed you deduct 1 fps for every 3 grains above IBO arrow weight.

Is the deduction based on total arrow weight or grains per #?

Example at 65# with 455 grain (7 grains per #) arrow:
Total arrow weight: 105/3 = Deduct 35 fps
Grains per #: 7-5 = Decuct 1 fps

Thanks
If your arrow weighs 455 gr and IBO spec's says 350 gr, that's 105 gr difference. 105 divided by 3 = about 35 fps less.
 

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I may be wrong about this, but you are saying that you are pulling 65#. Therefore, an IBO arrow would weigh 325g, not 350g. So, you aren't over weight by 105g, but 130g. 130g/3= a deduction of 43 f/s. I think.
 

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Discussion Starter #5
I pull 65# have a 29" draw, 455 grain arrow and my string has a loop, kisser and peep.

My bow's (60-70# bowtech Liberty) IBO speed is 296-305. When I make all the deductions (10 draw weight, 10 draw length, 35 arrow weight and 15 string) the caculated IBO speed is 226-235.

I don't believe it!!!!
 

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Garuki said:
I pull 65# have a 29" draw, 455 grain arrow and my string has a loop, kisser and peep.

My bow's (60-70# bowtech Liberty) IBO speed is 296-305. When I make all the deductions (10 draw weight, 10 draw length, 35 arrow weight and 15 string) the caculated IBO speed is 226-235.

I don't believe it!!!!
Mathematical formulas are great, if you're a mathematician. :) But IMHO, I think it depends on your bow, what you have on the string (peeps, silencers, etc...etc). Limb Savers on the limbs slow down your arrow speed. There are a lot of factors involved.

IBO is measured as you said at about 70 pounds with a 5 gr. per pound of dw arrow....but most importantly...a stripped down bow. No sights, nothing on the limbs, exact brace height....a totally bare bow. Anything you add can affect your speed.

The only true way to calculate your speed, is to set your bow up how you want to shoot it...then go to a pro-shop that has a chronograph and check it.

:shade:
 
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