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Hi All,

Quick question (or two) about spine.

1: Is the stated spine of an arrow based on the uncut length (which would be tricky as so many different lengths) or standardised at 28"?

2: If the latter, is there a rough guide/formula to how much spine increases (arrow weakens) for draw lengths/arrow lengths over 28"?

Ben

(asking as I apparently have very broad shoulders/long arms, which puts my measured draw length at 32.75 to back of riser)
 

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It's for a 28" arrow and with a 125 grain point. A rule of thumb is to reduce the dynamic spine by 5# for each inch over 28" and add 5# for each inch under 28". This RULE OF THUMB generally works REALLY well for wood, is OK for Aluminum and I have NO clue for carbon. In your 32.75" situation, round up to 33" and go from there. It isn't exact.

Arne
 

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It's for a 28" arrow and with a 125 grain point. A rule of thumb is to reduce the dynamic spine by 5# for each inch over 28" and add 5# for each inch under 28". This RULE OF THUMB generally works REALLY well for wood, is OK for Aluminum and I have NO clue for carbon. In your 32.75" situation, round up to 33" and go from there. It isn't exact.

Arne
Hey Arne,

Thanks much for the info - I think it helps a little if I can get my head round it!

Trying to translate into spine charts

My bow is 35# @ 28", so that roughly equates to 45# with my draw (can't find a scale in this damn country to measure it properly!)

With what you say above, 33" arrows would mean I'd be looking at shafts rated for a bow (5" x 5#) 25# heavier? Would that be heavier than the 35#@28", or #45#@33"?)

That seems a lot? It may be spot on, but like I said - trying to get my head round it.

Aaarggh!


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b -

What bow are you using?

Viper1 out.
 

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Viper - don't eave me hanging (I know, I know... you probably have one of those 'real life' things I hear about! ;) )

Your opinion/feedback/input highly valued this side of the table!


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In answer to your first question, an arrow shaft's stated "spine" is actually determined independent of the shaft's length final finished arrow length.

Spine is determined by measuring the shaft's "deflection" by one of 2 methods.

There is the old AMO method. The shaft is placed on 2 supports, 26" apart. A 2# weight is hung from the middle of the shaft. How far the shaft bends, as measured in thousandths of an inch is the shafts deflection. The AMO method is now basically used only for measuring and classifying wood shafts.

Then there is the ASTM method. Same idea, but the shaft is placed on supports 28" apart and a 1.94# weight is hung from the middle of the shaft. deflection is again measured in thousandths of an inch.

So you can see, that physical property of the shaft, its deflection, would remain the same regardless of how long the finished arrow is.

The using an arrow selection chart, or even an arrow selection calculator, one would find a recommended deflection to use as a starting point, and then proceed with the arrow build by following a tuning procedure.

The "Rule of Thumb" that Moebow mentioned works pretty well, but there are LOT of other variables such as degree of center shot that a particular bow may have, and elements of a shooter's form and execution that will mess with it.
 

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There are a couple of methods I know of to account for length variances. One is called the spine strength method, the other, and the one I use, is the ratio of cubes.

In the latter approach you adjust the static spine of the shaft using the formula:

Spine at length = Spine at 28 in. * (length) ^3 / 28^3 where ^3 means to the cube power, for example, 28 x 28 x 28.

One thing to bear in mind is that the dynamic spine will change based on draw length. I have a 32 inch draw and will put far more energy into a 33 inch shaft than someone with a 28 inch draw that likes to shoot long arrows.

As an example, my Easton ACE 430 give me a static spine in the 670 to 680 range when left full length.
 

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Discussion Starter #9
There are a couple of methods I know of to account for length variances. One is called the spine strength method, the other, and the one I use, is the ratio of cubes.

In the latter approach you adjust the static spine of the shaft using the formula:

Spine at length = Spine at 28 in. * (length) ^3 / 28^3 where ^3 means to the cube power, for example, 28 x 28 x 28.

One thing to bear in mind is that the dynamic spine will change based on draw length. I have a 32 inch draw and will put far more energy into a 33 inch shaft than someone with a 28 inch draw that likes to shoot long arrows.

As an example, my Easton ACE 430 give me a static spine in the 670 to 680 range when left full length.
Dammit, you're all giving me math!! :)

But thanks for the reply, Hank, and the detail in it. It's what I was looking for. Gotta try get my head round this one too.

Unfortunately, I don't have the luxury of buying a few shafts of differing spine to play with (island life - 50% duty, on top of (cost + shipping), plus 7.5% tax on top of all that means its gets pricey real quick). I gotta try and get as close to what I need in a one time purpose and tune out from there (various point weights way cheaper than various arrows)

Based on your own experience as a 'long person', where would you recommend a starting point for a 32.75" draw on a 35#@28 bow?




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Spine at length = Spine at 28 in. * (length) ^3 / 28^3 where ^3 means to the cube power, for example, 28 x 28 x 28.
This brings me to about .620 for my CE Heritages which is workable (if it applies to carbons) .

Time to buy some varying points and start playing around with them, methinks.
 

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For 35 pounds I have been shooting VAP 500. They only have 30.5 inch shafts. I am currently tuning a bow that is 34.5 at my 32 inch draw and so far, 32 inch Black Eagle XImpact arrows are doing great.

I used full length 32 5/8 inch ACE 430 when shooting 40 to 44 pounds with my FITA barebow. I have also shot 470 with my lighter bows. So that seems to be my range, anywhere between 430 and 500 depending on the setup.

I just ordered some 600 to try, just to make sure that the 500 I am shooting are best. Using my formula I figured that I would have to cut them back to 30.5 to equal the 500. I bought a few singles just to make sure that I had the right arrow.
 

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For 35 pounds I have been shooting VAP 500. They only have 30.5 inch shafts. I am currently tuning a bow that is 34.5 at my 32 inch draw and so far, 32 inch Black Eagle XImpact arrows are doing great.

I used full length 32 5/8 inch ACE 430 when shooting 40 to 44 pounds with my FITA barebow. I have also shot 470 with my lighter bows. So that seems to be my range, anywhere between 430 and 500 depending on the setup.

I just ordered some 600 to try, just to make sure that the 500 I am shooting are best. Using my formula I figured that I would have to cut them back to 30.5 to equal the 500. I bought a few singles just to make sure that I had the right arrow.
Hank,

Thanks again!

Quick one - if you have a 32" draw, how were you shooting the VAPs at 30.5"?


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