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Looking at both GT and Three Rivers charts the recommended spine is much much stiffer than I use for full length arrows. Am I not seeing something?
 

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Bart Harmeling
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3,441 Posts
I've always found the spine calculator to be better as it allows for more specific variable data inputs. However their spine chart matches my set up pretty well. Good enough that it would get me into the right spine choice and from there I'd trim to tune or adjust point weight.
 

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straight arrow
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612 Posts
The 3Rivers lookup arrow chart says for a 50#@28 bow drawing ~58#@31" I need 32" 300 spine arrows with 175g tip. I actually shoot 340 spine.
For my 40# bow (~47#) it says I need 340 spine. I shoot 400 spine.

I've played mostly with the Stu Miller calculator, although the 3Rivers calculator works just as well. In the calculator they have a 'Personal Form' fudge factor entry that, once I figured it out, made my setups match their calculations more closely. Without that I was always an arrow spine level weaker compared to the bow calculations.

Also I didn't assume the 'default' programmed bow dimensions. On my old Sage 50# hunting bow I measured the actual cut-past-center dimension - it was actually .060" less than cut-past-center default of -.1875" in the 3Rivers calculator. You can't change that default so I need to change the strike plate field entry to make up for that. I make the 1/8" strike plate I actually have to be 3/16" (.1875").

This setup works for me, good to 40yds:
50#@28 bow, 31" draw = 60.5# dynamic
7595 GT Hunter carbon (340) 32" 175g tip = 61.1# dynamic
Fudge factor -8 (if I leave it set to '0', my bow dynamic spine is 68#)
 

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My take on spine charts, whether they be plugged into a calculator, or printed out and pinned up on the wall, is that they are rough starting points. There only purpose is to get you in the ball park, from there on it's up to you to work out the fine points based on your personal shooting style.

Way too many guys think they can use them to buy the perfect shaft and then can't understand why things aren't working out. Modern age I guess.
 

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I suspect all those spine charts are written by 'modern' archers who are used to mechanical releases and a perfect clean release. The calculator I've played with (Stu's) has a 'form factor' which I suspect is a nice way of saying 'your release may need work' ;-)
 
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