Spine vs GPI - completely lost

[Bowrich]

I put aside my 45# 58" bow recently to do some comfortable indoor target shooting with a lighter weight 66" target bow. My draw length is 28" to front of riser and this set up has me pulling 30-31#s. My confusion has nothing to do with ever having to push a broadhead with this set up, just off season target. I'm still shooting off the shelf, split finger. My 500s were too stiff so I went to a 600 (30")  spine Carbon Express which happen to weigh 9.8 GPI. With a 120gr tip, they shoot well. I figured I might try and see how this works for 3D as well and picked up some more arrows (anticipating a few rock walls in my future). I stayed with the 600 spine, but went to a different brand which upon arrival I discovered had only a 5.7 GPI! Both arrows, 600 spine, one at 9.8 the other at 5.7 GPI!! What gives? The carbon express are a much wider diameter. I have not fletched, shot or bare shafted the lighter 600s yet as I would like to understand how the same spine can vary so much in weight and still be considered "the same". I fear the lighter arrows will shoot under spined despite fitting the recommended 600. Could anyone shed some light on this.

[Baylee]

Simple really. Spine is a measure of how much the shaft flexes gpi is the grains per inch the shaft weighs. A cedar arrow could have the same spine as an ash arrow with the two weighing 200 grains different. Same with aluminum or carbon. Just because two shafts flex the same (spine) that has nothing to do with how close their weights will be.

[McDave]

I have two arrows in my hand right now.  They are both Gold Tip 600 spine arrows.  The one I normally use for targets is an Ultralight, which is 5.9 gpi.  The other is a Traditional, which has a wood grain finish, and is 7.8 gpi.  I assure you they are both .600 spine (as closely as carbon arrows usually are, anyway, which is a different subject).  The main difference is the weight of the wood grain finish.  It would be important for you to pick a shaft that has the same gpi and stick with it for your target shooting, because the heavier shaft will obviously have a higher trajectory.

On a slightly different subject, don't confuse gpi with gpp.  Gpp is grains per pound, which is calculated by the shooter, and is used for different purposes than gpi.

[creekwood]

The spine of an arrow is determined like this: The arrow must be 29” in length and supported by two points, which are 28” apart and a 880 gram weight is hung from the center of the 28 inch points. The number of inches the arrow deflects or bends due to the weight is the spine size or measurement of an arrow. So, a 500 arrow bends .5-inches when the weight is applied.  The grains/inch of arrow may have little correlation to the stiffness although usually as an arrow weight increases, so does the stiffness. .

[creekwood]

I made that seem clear as mud.   

[Orion]

First, I'm surprised that the 600s work out of that light set up.  700s would be a better fit. The lighter arrow is likely to act dynamically stiffer (i.e., overspined) than the heavier arrow.  Because it's lighter, it takes less force to start it moving forward than does the heavier shaft.  Or, looked at another way, it won't bend as much because it's not offering as much weight resistance to the string. That will make it act stiffer. On the other hand, because it is also a narrower shaft, that effect might be negated/compensated for by the fact that the center of the arrow will be lying closer to the center of the bow. Moving the shaft closer to the center of the bow enables the bow to shoot a stiffer shaft.  So, it might be a wash.

In summary, the spine is the same with both arrows, but the shaft diameter, physical weight (and the distribution of that weight, and length, which we haven't even discussed) can cause similarly spined shafts to behave dynamically differently (i.e., when shot)

The only way you'll know if the newer shafts will work is to try them.  I suggest you try one or two and see how they fly, adjusting shaft length and point weight to further tune the arrows to the bow.  If you can't get them flying well, you'll likely need to go to a 700 spine shaft.

It would be a good idea to try to hook up with someone who understands the ins and outs of arrow tuning to give you a hand.  Good luck.   

[Bowrich]

I really appreciate all your feedback in trying to clear this up. I kinda get it but I guess the bottom is as Orion said, to do some experimenting. I'll do that in next few days and see where it leads me. If all else fails, I know the 45# with 500s and 145s will work fine for 3D. Its what I used last year. Was just trying to take it a little easy in the off season on the 3 anchors in my drawing shoulder.  Thanks again you guys. 

[Bowrich]

Thanks again all, here is a follow up. I did a little experimenting with the lighter (361 gr total) vs the heavier (446 gr total) both 600 spine, both tipped with 125 gr and was initially surprised at how the lighter arrows bare shafted just about perfect, much better than the heavier shafts. Surprised until I discovered the world of FOC! The lighter shafts had a 15.6% FOC compared to 9.7% with the heavier shafts. Now I understand why 2 arrows, same spine (length, etc), but different weight can act the same (or better) depending on tip weight (and FOC). Thank you all for steering me in the right direction. Too bad my skill level does not increase as quickly as my grasp on this topic!!! But at least I won't have arrows to blame!!!!! In a few months, I'll switch back to hunting weights, but for now, good luck to all and thanks for helping me get a grasp on shooting lighter gear.

[Orion]