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Author Topic: Formula for increase in draw weight  (Read 2762 times)

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Offline WillS

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Formula for increase in draw weight
« on: April 17, 2013, 04:16:01 pm »
Quickie question for anybody who knows - if you know the draw weight of a bow at a certain draw length, is there a formula / calculation to work out what the weight will be at a different given length?  For example, if a bow is say 50# at 18", is it possible to work out what the draw weight will be at 20", 22" and so on?
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Offline PEARL DRUMS

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Re: Formula for increase in draw weight
« Reply #1 on: April 17, 2013, 04:17:52 pm »
I use 2.5# per inch and it will get you withing a few pounds.
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Offline WillS

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Re: Formula for increase in draw weight
« Reply #2 on: April 17, 2013, 04:20:23 pm »
Cheers Pearl.
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Offline PEARL DRUMS

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Re: Formula for increase in draw weight
« Reply #3 on: April 17, 2013, 04:33:53 pm »
Will if you scale your bow at 18" and it reads 42#, you can basically expect it to remain close to 2.33# per inch as you pull farther until stacking occurs past 90 degree's.
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Only when the last tree has died and the last river has been poisoned and the last fish has been caught will we realize we cannot eat money.

Offline adb

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Re: Formula for increase in draw weight
« Reply #4 on: April 17, 2013, 04:39:11 pm »
I find my bows are more 3-5# per inch. Some of my warbows above 90# have been as high as 7# per inch.
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Offline WillS

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Re: Formula for increase in draw weight
« Reply #5 on: April 17, 2013, 04:52:22 pm »
How much does the brace height affect this?  I'm asking as I'm currently tillering a yew longbow, and it's at a low (2 inch) brace height so basically just a v. tight long string at this stage and it's dead on 50#@18".  Does the formula still remain the same, or does a higher brace height change things?  If we average out Pearl and Adb's answers and say about 3#/inch, does that mean that at 28" I should be expecting 80#, or will that go up/down once I'm at full brace?

Normally I don't consider this stuff, I just tiller out to my goal draw weight and so on, but it's too dark outside to use the tiller so I'm in "think" mode instead!
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Offline PEARL DRUMS

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Re: Formula for increase in draw weight
« Reply #6 on: April 17, 2013, 04:57:27 pm »
Your math sounds right to me Will. Brace height doesnt affect draw weight.
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Only when the last tree has died and the last river has been poisoned and the last fish has been caught will we realize we cannot eat money.

Offline adb

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Re: Formula for increase in draw weight
« Reply #7 on: April 17, 2013, 05:44:19 pm »
I don't know for sure, cuz I've only ever measured this at full brace. I wouldn't think it would change much, if at all.
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Offline PEARL DRUMS

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Re: Formula for increase in draw weight
« Reply #8 on: April 17, 2013, 05:48:38 pm »
The limbs have to bend the same distance whether it starts at a 3" brace or a 6" brace, therefore the draw weight doesnt increase with a higher brace. The string will be tighter at a full brace, but the end draw weight will remain.
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Only when the last tree has died and the last river has been poisoned and the last fish has been caught will we realize we cannot eat money.

Offline WillS

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Re: Formula for increase in draw weight
« Reply #9 on: April 17, 2013, 05:54:58 pm »
Great, cheers for the replies guys!
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