Explaining the Spine Calculator

[VA Elite]

Here is what I got when i punch these numbers in for one of my experiment arrows. Can someone explain if this a good starting point for when I do finally get started with this process?


Bow Results   Arrow Results   Arrow Specifications
Dynamic Spine
47.4   Dynamic Spine
49.5   Calculations below are based on the
arrow information entered BELOW.
These two numbers should be nearly equal (within 2#).
 
Fine tuning can then be done by adjusting the bow's brace height.
If the arrow is slightly weak (lower dynamic spine) for what the bow needs,
then lower the brace height.
If the arrow is slightly stiff (higher dynamic spine) for what the bow needs,
then raise the brace height.
Arrow Total Weight (grains): 454   
Arrow Specific Weight (GPP): 9.7   
Front Of Center (FOC) %: 17.1   
Arrow Speed (feet per second): 182   
Arrow Energy (ft-lbs):33.7

[Orion]

So, you're bow is about 47#@28 inches.  Is that your draw length?  What kind of bow is it?  How is the shelf cut?  What kind of string are you using? I expect you plugged that information into some kind of calculator to arrive at the numbers you got. Can't really answer your question without answers to the previous questions.

BTW, you buy arrows by stipulating static spine, not dynamic spine.  You can't change the static spine, but you can change the dynamic spine a lot by changing arrow length and/or point weight, and changing bow brace height and side plate thickness.

[VA Elite]

Bow is 50# at 28. I pull to 27. Yeh I plugged in the right specs according to my bow and my arrows. I don't know what dynMic or static spine is. These were the numbers it gave me. I'm just getting an idea of what would be a good starting point. I'm a  long way off from getting tuned arrows.

[Orion]

OK. I just divided the gpp into total arrow weight to arrive at 47#.  Just assumed it was at a 28-inch draw.  

I don't use the calculator, but can get pretty close to the same results if I know something about the bow you're shooting and how you shoot it.  A high performance string requires more spine than a a dacron string.  A bow cut past center requires more spine than a bow cut to or proud of center.  A long draw requires more spine than a shorter draw.  A heavier point requires more spine than a lighter point.  Etc.

You know the numbers you plugged in, but we don't.  Without them, we can't be of much help.

[VA Elite]

samick sage 50# @ 28
I draw it to 27" so Im around 47#
arrow is cut 30" with 50 gr brass insert and 125 gr head
three 5" feathers
nock 11.5 gr
Gold Tip 3555 .500 spine
strike plate 1/8
cut on center shelf
fast flight 18 strand

[Orion]

GT 35-55 is about right.  It's static spine is 60# plus, measured with a 2# weight at 26 inches.

The 50 grain brass inserts softens the (dynamic) spine about 5-10#. Subtract about 5# of spine for every every inch of arrow length beyond 28 inches -- 10#.  If the strike plate is cut past center 1/8 inch, that would require another 5-10# of spine.  If it's cut to center, about 5# of additional spine would be needed.  Likewise a fast flite string usually requires about 5# more spine than dacron.

So, adding and subtracting all of this -- 60# - 20# + 10# = 50#.  That's the dynamic spine you end up with, which is what the calculator predicted.  I'm doing the same thing that Stu's Calculator does, though the Calculator is more refined and now based on thousands of measurements.

Based on your initial post, I thought you were asking what is the spine you should select for your bow.  Didn't know you already had selected the shafts you were to use.  Regardless, you selected the correct spine, IMO.  

Could also go up another 25 grains on the front end without any ill effect. Could add even more weight up front if you shorten the arrow an inch or two.  Shortening the arrow stiffens it, but adding point weight softens it.  Can plug those figures into the calculator as well.  Or, just do it by trial and error. Good luck.

[VA Elite]

orion, thanks for that. I get it now. I did actually plug another 25 gr up front and it still had me within the 2# margin. I went out and shot a 145 gr head with a 125 gr head, and you are right, no difference at all.