# Lenght to spine conversion table



## ButchD (Nov 11, 2006)

*Saveable version?*

Vittorio, thanks for that, is it possible to obtain a savable version? Thanks, ButchD


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## STUDENT-ARCHER (Jun 26, 2009)

do you know if those measurements are taken with the shafts cut and having the same overhang as the "Listed" spine


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## Joe T (Apr 5, 2003)

Much simpler to calculate as you can use any initial spine (not just 100's) and any length change (not just whole inches).

i.e. new spine = old spine x new length cubed/old length cubed


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## richardfrog (Jan 24, 2009)

*It's great*

Thanks for the table, most helpful it is


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## richardfrog (Jan 24, 2009)

*Thanks a lot*

It is most helpful


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## Vittorio (Jul 17, 2003)

Joe T said:


> Much simpler to calculate as you can use any initial spine (not just 100's) and any length change (not just whole inches).
> 
> i.e. new spine = old spine x new length cubed/old length cubed


Agree, but people likes easy tables similar to the Easton one, as they can be used easily as multi-entry for arrow choice


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## FS560 (May 22, 2002)

Joe T's formula is much easier to use and more convenient, especially for arrow lengths other than even inches.

I may be wrong, but when using the formula, I would think that it should be utilized with arrow lengths one inch less than actual. This would more readily allow for the predictable creep in the ratio.


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## Joe T (Apr 5, 2003)

FS560 said:


> Joe T's formula is much easier to use and more convenient, especially for arrow lengths other than even inches.
> 
> I may be wrong, but when using the formula, I would think that it should be utilized with arrow lengths one inch less than actual. This would more readily allow for the predictable creep in the ratio.


Don't see this. Depending on how you round the result to the nearest whole number you always end up within 1 or half a spine value. In practice the "old spine" is always going to be standard measured Easton one at 28".

The table is good for an overview of how spine varies with length but if you want the spine of a 29.5" arrow with an Easton spine of 640 then calculation is simpler than trying to interpolate the table. Also note this table is only approximate for barrelled arrows as you have a variable diameter and grains per inch.


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## fivespotshooter (May 12, 2009)

Could someone please explain how to use this table and give an ex. or two please.


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## iharangozo94 (Feb 27, 2009)

how do we know what i too light and what is too heavy, and how what about point weights?


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## Jared Les (Jun 22, 2008)

This is very helpful, thanks.


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## FS560 (May 22, 2002)

A 26" arrow with an Easton spine of .450:

(.450)((27)cubed/(29)cubed) = .363

or

(.450)((26)cubed/(28)cubed) = .360


A 31" arrow with an Easton spine of .450:

(.450)((31)cubed/(29)cubed) = .550

or

(.450)((30)cubed/(28)cubed) = .553

This is the creep, although quite small.

If the objective were to be precise, which would be more appropriate?


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## fivespotshooter (May 12, 2009)

FS560 said:


> A 26" arrow with an Easton spine of .450:
> 
> (.450)((27)cubed/(29)cubed) = .363
> 
> ...


 NOW I'M REALLY LOST!!!

i MIGHT BE ASKING ALOT BUT COULD SOMEONE MAKE A SPREED SHEET WERE WE COULD JUST PLUG IN A NUMBER OR 2?

Hay Tony do you think John could make a spreed sheet for this?


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## Joe T (Apr 5, 2003)

FS560 said:


> A 26" arrow with an Easton spine of .450:
> 
> (.450)((27)cubed/(29)cubed) = .363
> 
> ...


Easton spine is measured at 28". You are using the wrong spine values in the calculation i.e. .450 at 29" and .450 at 28" can't be the same arrow. 

For an example some archers trim arrow length to match bow and arrow. The table would tell you for a specific arrow say the spine change by cutting off 0.25" of shaft. How much change depends on the Easton spine and the initial arrow length.


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## Joe T (Apr 5, 2003)

Maybe an example makes this clearer:

Starting point is the Easton spine e.g. 450 (always at 28")

The spine at 27" is given by 450*27 cubed/28 cubed = 403.49

for the spine at 29" using Easton spine value (28")
spine = 450*29 cubed/28 cubed = 499.96

you could also get the 29" spine from the value calculated at 27" i.e.
spine = 403.49*29 cubed/27 cubed = 499.96

Always the calculated spine has to be related to the Easton spine which is the only one that's actually measured.

What the table does is pre-calculate the spines at 1" intervals based on the Easton spine, so to get the spine difference (on a row) between two different lengths you just need to subtract the two spine values.


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## FS560 (May 22, 2002)

I am fully aware that Easton spine (industry standard) is for a 29 inch arrow with the supports at 28 inch spacing. The measurement of actual spine is always for spacing one inch less than the arrow length. That has been the accepted method for a long time. The Easton spine value may be for 28 inch spacing of the supports but Easton lists it for a 29 inch arrow. And, that is the nominal that everyone works from.

The formula is used to calculate an actual static spine of an identical arrow except for length. For example, the actual static spine of a 31 inch arrow, if measured on a spine tester, would be the deflection measured with the supports at 30 inch spacing.

My point is a question of which calculation is more correct, using the length ratio based upon support spacing or on arrow length. I think it is more correct to use the support spacing, which will be one inch less than the length of the new arrow and one inch less than the 29 inch arrow length listed in the manufacturers spine list.

As the length of the arrow changes, the cubed ratios change, although slightly.

The bottom line is that we are saying the same thing.

Some may be too young to remember that Easton used to publish a spine chart with spine deflections for arrows for a full range of lengths.


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## FS560 (May 22, 2002)

Now, what does everyone think about utilizing this formula to adjust for the length of the point shank?

Example, would you not expect an arrow to react stiffer with a 90 grain NIBB point compared to a 90 grain one piece point. The NIBB has a significantly longer shank, which reduces the flexible unsupported shaft length.

The same analysis could be applied to the tungsten vs. stainless break off points where weights overlap.


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## Joe T (Apr 5, 2003)

FS560 said:


> I am fully aware that Easton spine (industry standard) is for a 29 inch arrow with the supports at 28 inch spacing. The measurement of actual spine is always for spacing one inch less than the arrow length. That has been the accepted method for a long time. The Easton spine value may be for 28 inch spacing of the supports but Easton lists it for a 29 inch arrow. And, that is the nominal that everyone works from.


Finally understand where you're are coming from. 

If put a 5 foot arrow on an Easton spine tester then the shaft weight outside the supports (with no centre weight) would result in the arrow forming an arc. You would be starting with a negative spine before adding the centre weight. 

The length of arrow outside the supports affects the spine reading so you have to standardise on this. Easton spine measurement is for a 28" shaft not 29". You are just defining the overlap being small enough not to make any practical difference. Lots of things affect the spine measurement (temperature, air density etc) and these may be defined in the Easton spine standard (don't know - never seen it) but they make no practical difference when it comes to selecting arrows.

Easton spine is really a measurement of shaft stiffness which is essentially a fixed property and doesn't depend on arrow length.


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## bis (Feb 2, 2005)

> Easton spine is really a measurement of shaft stiffness which is essentially a fixed property and doesn't depend on arrow length.


Not always true.
If you take an ACE shaft as it comes from easton (a bit more than 32") and you center it over the 28" supports you will get a quite different spine result, comparing with the same shaft cut to 29".
This is mainly due to the portion of tail not part of the measurement - 2" vs. 1/2".


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## Joe T (Apr 5, 2003)

bis said:


> Not always true.
> If you take an ACE shaft as it comes from easton (a bit more than 32") and you center it over the 28" supports you will get a quite different spine result, comparing with the same shaft cut to 29".
> This is mainly due to the portion of tail not part of the measurement - 2" vs. 1/2".


Sorry, I shouldn't mentally link posts together - already pointed out in an earlier thread post that this spine table idea doesn't work well for barrelled arrows. Don't know how Easton define spine in this case but presumably the rear end position remains constant. Works even less so in terms of "dynamic spine".


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## Ricfranz (Feb 20, 2005)

Excuse me for my poor english.

If you want, you can find the Excel in the link published on Forum CAM
(I cant post the link 'couse i've less than 5 posts) Perhaps Vittorio will.

You can calculate the correct shaft lenght at the desired spin, sobstituting the values that you'll find in the yellows cells.

Lunghezza asta attuale meamns actual shaft lenght
Spine nomiale nova freccia means Nominal spine of the new shaft at 28"

Lunghezza nuova asta means New shaft lenght

I hope you can understand me.


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## Joe T (Apr 5, 2003)

Link to spreadsheet Spine/Length calculator

You can insert/copy rows and columns however you wish

Worth reading the original thread as it contains some good examples of how to use the table. (with the translator of your choice as needed)

Original Thread

Thank you Ricfranz - nice idea


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## Ricfranz (Feb 20, 2005)

A "thank you" from Joe T is a great honour


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## lc12 (Jul 30, 2009)

Joe T said:


> Link to spreadsheet Spine/Length calculator
> 
> You can insert/copy rows and columns however you wish
> 
> ...


Can this be found in ENGLISH somewhere?


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## Duss (May 23, 2006)

*Other variables*

Some users might find variations if they are using that program for barrelled shafts. What is changing in "barrelled" shafts is the spine. Then the variation with length is not uniform. Moreover, with A/C/E shafts, "spine distribution"along the shaft varies according to the actual "manufactured" spine of the shaft. Some are almost uniform and others are much more tapered. The differences might be more evident between A/C/E shafts that have been severely reduced in length and those that have been left more or less uncut. Please notice the maximum cutting lengths recommended by Easton.


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