# Arrow length affect on spine?



## PB26 (Dec 31, 2006)

Yes, very much so. The longer the arrow, the weaker the spine of the arrow becomes.


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## Capital_Ken (Mar 13, 2012)

But when using their arrow chart, they have a column for draw length. Wouldn't that correspond to an arrow length, or are they assuming the arrow is being used at full length, which wouldn't make sense.


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## bigbadwoolfe (Jan 1, 2013)

I could try to explain, but read this and it should be clear... the "arrow spine and weight" question...

http://www.eastonarchery.com/frequently-asked-questions

And if not, post again and you'll get more explanations.

Sent from a smartphone. Ignore typos and carry on.


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## dwagoner (Sep 27, 2007)

Capital_Ken said:


> But when using their arrow chart, they have a column for draw length. Wouldn't that correspond to an arrow length, or are they assuming the arrow is being used at full length, which wouldn't make sense.


their charts are by arrow length and NOT draw length.... always arrow length


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## Capital_Ken (Mar 13, 2012)

This is interesting for us short draw shooters....

STATIC spine is measured by the amount of flex in the arrow when an 880-gram (1.94 lbs.) weight is suspended from the center of the arrow. The arrow must be 29" in length and supported by two points, which are 28" apart.


So when they measure spine weight, they are doing it at points that don't even exist on an arrow that is 26" long. It would make sense to me that a shorter arrow is going to have an increase spine, but I'm no engineer.


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## bigbadwoolfe (Jan 1, 2013)

Static spine is uniform, applies to all length arrows. Details on how it's calculated are in that write up. It's like a unit of measurement... people have agreed on a standard and everyone is using it to classify arrows coming out of factories.

Now, static spine number for an arrow means nothing to you (it's just a starting point when picking arrows) as it's just a number obtained in a controlled environment (a lab with a specific arrow length and precise weight hung at a specific point on that arrow). The minute you change that environment (by changing arrow length, adding weight front and back to that arrow, bow weight, bow "explosivness"...), that spine changes for the new environment, and that's your dynamic spine - the one you actually care about... so depending on your specific environment (bow setup), you get arrows with dynamic spine that match it...

And, shorter arrow has less spine, not more. Try bending a short and a long arrow. Which one bends easier?


Sent from a smartphone. Ignore typos and carry on.


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## Capital_Ken (Mar 13, 2012)

I get what you're saying. I misspoke.


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## Strodav (Apr 25, 2012)

I often try to understand how things work by looking at the extremes. Take, say, a 30" 500 spine arrow shaft and cut off a 4" piece. How much force does it take to flex that 4" piece with your hands. Now take the larger 26" piece in your hands. How much force does it take to flex it. Even though both pieces come from the same arrow and have the exact same properties, the longer one acts weaker and the shorter one acts stronger. What's just a little less intuitive is that for a given arrow length adding weight makes it act weaker and subtracting weight makes it act stronger. When tuning, I start with 100 gr point, but try 85 and 125 to see if there's any difference.


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## RunsUpRiver (Jul 22, 2009)

Strodav said it pretty well.

Can you break a brand new full length pencil with your hands? How about when it is 4 inches? How about 2"?

The longer the pencil, the less force needed to break it because of leverage. 

The spine is listed by AMO at 28 inches, just for simplicity. Many of the charts will show you what spine you are actually getting at your arrow's length.

You can also do your own measurements. Just make two points whatever distance apart that is the length of the arrow you want, and suspend the required weight from the middle of the shaft and measure the bend in inches.

When they list an arrow as having a 500 spine, it is a .5 inch deflection at 28 inches. That is a standard. At 26 inches it might be a 400 (.4 inch deflection). At 20 inches it might be a 100 (.1 inch deflection)

The higher the number, the more it flexes under weight. The spine number 400 corresponds to a deflection of .4 of an inch.


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