# What is gpi?



## weasle414 (Dec 31, 2006)

I see it on the sides of just about every arrow I see. I have no idea what the gpi numbers are suposed to mean! Could someone explain it for me?
Thanks,
Alex


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## Bob Kellam (Apr 8, 2004)

Grains Per Inch

Arrow Spine & Weight

What is arrow spine?

There are two different types of spine - STATIC spine and DYNAMIC spine.

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. The number of inches the arrow deflects or bends due to the weight, is the spine size or measurement of an arrow.

DYNAMIC spine describes the way an arrow reacts from the stored energy of a bow as it is shot. Several factors determine the way an arrow is going to react when shot out of the bow, including method of release (fingers or mechanical release), amount of energy applied by the bow, the bow's cam system (single, round wheel, hard or soft), weight of the arrow, spine of the arrow, length of the arrow, point weight, nock weight and fletching weight. Even nock set material (traditional brass nock or serving nock), along with string and serving material can influence dynamic spine. Because of the nearly unlimited variables in determining dynamic spine, arrows are usually measured using static spine.

What does the numbering system on aluminum arrows mean? (ex. 2213)

The four-digit number refers to the outside diameter and wall thickness of the shaft. The first two numbers are the outside diameter in 64ths of an inch. The second two numbers are the wall thickness in thousands of an inch. For example, a 2514 shaft would be 25/64th of an inch in diameter and .014 of an inch wall thickness. OD and wall thickness are the two variables in controlling spine for aluminum arrows.

Why is the spine of an arrow important?

Spine is very important when it comes to tuning, shooting and grouping your arrows. If you do not have the correct arrow spine for your bow set up, you are going to get erratic arrow flight and poor shooting groups. Having the proper arrow spine is key to optimizing the grouping of your arrows and for the best possible accuracy. Use the Easton Shaft Selector software (download free here) or reference the Shaft Selection Chart in the back of the Easton Arrow Guide to make sure you are shooting the correct arrow spine for your set-up.

How does Easton measure arrow weight?

The industry standard measurement for weight is grains per inch (GPI). There are many factors that make up GPI including: arrow diameter, wall thickness, and shaft material. The GPI weight of listed arrows does not include the weight of the point, nock, insert or fletchings.

I found this on the Easton Site

Bob


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## weasle414 (Dec 31, 2006)

Lol, about 2 minutes after you posted this I was on the exact same page. I hadn't seen this yet but it pretty much answered my questions.

I've got a couple more questions, though. What gpi would be a good one from a bow set at 68 lbs? Should I go lighter and faster or heavier and slower? How would each end of the spectrum fly? Sorry for all the questions, I just want to get the right stuff the first (or maybe 2nd) time so I don't burn a huge hole in my wallet finding out the expensive way and have a crap-ton of rarely-used extra arrows.


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## bowtechin (Apr 8, 2007)

What brand arrows and what is your draw length?


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## weasle414 (Dec 31, 2006)

I'll be getting carbon Eastons and my draw length is just shy of 30".


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## bowtechin (Apr 8, 2007)

http://www.basspro.com/webapp/wcs/store ... ARROWCHART

Can read through here. Might help, might just confuse ya too.


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## weasle414 (Dec 31, 2006)

It did a good job at confusing me, but that's alright. I'm easily confused on a good day and today I'm sick as a dog. Maybe I'll understand a bit when I feel better.


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## bowtechin (Apr 8, 2007)

When you do you can go through a part on there where you input your draw,lbs. such and it gives you which arrow you need. Otherwise, I'm terrible at explaining but will do my best.


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## weasle414 (Dec 31, 2006)

Alrighty then, thanks bowtechin.


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