# Formula for arrow spine?



## Flyboy718 (May 20, 2008)

Is there a formula or equation to figure out what a shaft spines at if it is shorter or longer than 28"?


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## Viper1 (Aug 21, 2003)

Flyboy -

See if this helps (and yes, it's a copy/paste from another site, but it's legit, as I wrote it!):

IN GENERAL for 28" ARROWS +/- 1: ( 27 - 29") and 100 - 125gr heads:

#25 - #33 1716
#34 - #42 1816
#43 - #52 1916
#53 - #60 2016
#61 - #70 2117
#71 - #80 2216
#81 - #100 2219+ (Might want to play with 23xx and 24xx, gets a little dicey at those weights)

If you go to a 30" arrow, jump up one spine, 32". then two spine numbers. Ditto for going shorter, 26" one spine down, 24" two spines down. 

This WILL NOT give you the perfect aluminum arrow for a given bow. It will give you a tuneable arrow, and that's all you need for starters. Once the arrow is tuned, you'll know if you're compensating for a stiff or solf arrow by the tuning requirements. Then you can go in to finer arrow choices. 

For example, if you have a #41 @ 28" bow and are using a 29" arrow, you'd pick an 1816, right? And that would work. If you find that you have to move the rest/strike plate out a little more than you like, then you NEXT set of arrow might be 1914s. Same weight as the 1816s, but a little stiffer.

In addition, if I know a particular bow, I might suggest an arrow that's not one of the primary spine numbers. A certain #57 bow might work very well with a 2114, for example. 

Viper1 out.


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## Flyboy718 (May 20, 2008)

Thanks Viper:thumbs_up


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## AKRuss (Jan 10, 2003)

The old wood arrow AMO spine standard is measured on 26" centers with a 2# weight suspended from the center and expressed as a 28" arrow. The equation for determining spine is: 26" (centers) / deflection" = draw weight. So, if you have a .500" spine it's "weight" is determined: 26" / .500" = 52# at 28". The new synthetic ATA spine standard is similar but they use 28" centers and a 1.94# weight. The ATA standard doesn't assign a designated "weight" but it would be easy to figure out. For a .400" spine shaft it would be: 28" / .400" = 70#. The weight numbers are somewhat useless other than they're easier to remember than deflection readings but they do not really correlate directly to draw weight. 

The old rules of thumb are: for every additional inch of shaft weight or 35 grains of point weight, the dynamic spine is decreased about 5#.


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## LBR (Jan 1, 2004)

> Is there a formula or equation to figure out what a shaft spines at if it is shorter or longer than 28"?


The actual spine stays the same, but the effective spine changes pretty much like AK said.

There are charts all over the www, but any of them are ball-park at best. There can be HUGE performance differnces between different styles and set-ups, not to mention bows that are cut shy of center, to center, or past center, etc. etc. etc.

For instance, a 40# Hill style longbow (cut shy of center) with a fat dacron string is going to need a considerably weaker spine than a 40# ACX longbow that's cut to or past center with a tiny 8125 string.

There's also variables in nock fit, your release, etc.

There seems to be two ways (at least) of looking at tuning. One, you can finnagle around with arrow length, point weight, brace height, etc. etc. etc. and try to "make" a certain arrow work; or you can try a few different arrow spines and use the one that works with your bow set up the way you want it. Being a hunter, and having an elbow that is sensitive to hand shock, I prefer the latter. I set my bow up to be quiet and have the least handshock I can get, then use the arrow that works for it.

Chad


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## Warbow (Apr 18, 2006)

LBR said:


> The actual spine stays the same, but the effective spine changes pretty much like AK said.
> 
> There are charts all over the www, but any of them are ball-park at best. There can be HUGE performance differnces between different styles and set-ups, not to mention bows that are cut shy of center, to center, or past center, etc. etc. etc.


And when Chad says "actual spine" he means that the flexibility of the material stays the same. That measurement is called "static spine." There is another quality called "dynamic spine," which is how the arrow behaves when you shoot it, as it is being accelerated and in flight. Adding weight to the tip makes the arrow behave as if it is more flexible. You can visualized this pretty easily. Say you had a six foot length of 1" PVC pipe. If you were to wave it or snap it forward it would have some flexibility. Now add a two pound weight to the far end and wave it around or snap it forward, the weight will make the pipe much more whippy and seem more flexible. The same is true of adding weight to the tip of an arrow, where adding weight to the tip makes the arrow behave as if it was "weaker" in spine, or more flexible.


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