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#### spike camp

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how do you figure out spine?

say you take a 400 spine arrow and cut 4 inches off full length....how do you figure out what the finished spine will be?

does that make sence?

#### grandteton

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Well, considering spine is calculated on I think a 28" arrow with a certain weight applied to the shaft it is different depending on overall length, weight pushing it, and head weight. This is where the OT2 type programs come in handy to put it all together for you.

Because there is no standard I don't know what your "full length" is, and which brand/model you are talking about. There may be a way to enter the values into OT2 to come up with what you are looking for, but then I'm not sure why it is you are looking for this data because I can't see the value in it. Can you explain more?

#### Roskoes

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It is still a .400 spine arrow. Only now it is relatively stiffer. Shorter shafts and lighter points will make an arrow stiffer, and vise versa. Between those elements, and adjustment on the draw weight of the bow, we can tune the elements to get perfect arrow flight. If it weren't for these variables, they would have to sell arrows in a lot more spine increments than just .300, .340, .400. and .500.

#### spike camp

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i understand that there are many factors that contribute to a finished arrow in regards to spine.

lets use my arrow as an example...goldtip 55/75 is a 400 spine at factory length.
when i cut them to 27.5,it changes the 400 spine to somewhere in the 300's.

how do i know/figure exactly where that arrow will spine out at that length?

i asking because it was suggested to me to get a 340 spine arrow cut at 28.5 for my set up, and that my 55/75's were way underspined.

when i asked a world renowned tuner,he said that my GT's were not even close to underspined.

so how do i know that a 400 spine arrow cut at 27.5" doesnt equal a 340 spine...of course figuring tip weight and DW into the mix.

or am i just not understanding the concept of spine?

#### Widgeon

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I think you're missing the point just a tad. The "spine" of the arrow is the rating that it is given by testing it as a full length shaft on supports 28" apart and using a 1.94 lb weight. This never changes. By cutting the arrow and adding components, you now have changed the overall stiffness of the arrow, but not the spine rating.

When you look at manufacturer's charts or use the different software packages, they use the spine rating of the arrow and take into account how you setup the arrow by cutting, adding point weight, etc. and suggest which spine rating you should start with.

#### ebonarcher

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What arrows are you shooting now , out of what bow set at what poundage and what components?
The arrows spine never physically changes.

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I think you're missing the point just a tad. The "spine" of the arrow is the rating that it is given by testing it as a full length shaft on supports 28" apart and using a 1.94 lb weight. This never changes. By cutting the arrow and adding components, you now have changed the overall stiffness of the arrow, but not the spine rating.
This measures STATIC spine. It doesn't change no matter how short you cut the arrow or how much weight you put on the point.

What changes and what matters is DYNAMIC spine. The charts and software predict how the arrow will act with various combinations of draw weight, cam type, point weight, etc. The charts are mostly based on experience than any formula.

Finding optimum spine for each individual set up begins with the charts or programs, but to get the perfect spine requires trial and error . John Dudley describes how to do this in one of his articles on his website. It's basically group tuning and adjusting draw weight to achieve the best groups.

John has a lot of other good articles there.

#### spike camp

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i get it now...thanks for the replies!

im working on getting mt SR-71 set up and an getting inconsistent tears.

my bow is a 29/70 and my arrows are 55/75's cut at 27.5 c to c and using 100 gr. FP's.

i may need different arrows to get better results but deffinatly needed to understand the spine concept first!

thanks again!

#### AZBowhunt

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not underspined

I would agree with a previus post that you are not even close to being underspined. I will try to run it through TAP and see what I get.

#### AZBowhunt

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eat my words

gonna have to eat my words. according to tap, using goldtip 5575's (not sure your exact arrow) it shows it underspined. It shows 7595's to be a better fit.

For what it's worth.

#### TN.Frank

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This is something that I've been having a fun time trying to figure out. Using the online Easton Shaft Selector my 29.5" ST Epics 400's are borderline with a 100gr tip on my bow set at 64# but if I drop back to a 75gr tip then I'm fine. I wish I could find a more accurate spine calculator or even a downloadable program that'd run on my iMac to calculate all the factors so I can arrive at wether or not the 400's are correct or not. Anyone know of such a web site or program?

#### ebonarcher

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That's correct. Using a lighter point or shortening the shaft will make the arrow act stiffer.

#### TN.Frank

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That's correct. Using a lighter point or shortening the shaft will make the arrow act stiffer.
And that's why I've gone form a 100gr point to an 85gr point. At 64# with a 100gr point I was marginally spined with a 29.5", 400 but with an 85gr point I'm perfectly spined. :teeth:

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