Arrow Spine Deflection

[bigcountry]

I see folks refer to arrow spine two different ways.  Usually cedar, wood arrows are referred to 45-50, 50-55, etc.  Where carbons and aluminums are in inches of deflection.

How do they correlate?  In other words, is a POC arrow with 45-50 spine .5" deflection or what?

[Bob Walker]

Try this....

 http://www.rosecityarchery.com/AMOspine.html

[bigcountry]

Wow, thats a wealth of information. Thanks Bob.

[Cupcake]

Bigcountry,

There is a simple correlation once you understand that wood is measured differently than aluminum and carbon.

Woodies are measured with the ATA method of applying a 2.0 pound weight in the center of a shaft that is supported at at a span of 26 inches.  The wood pounds spine is defined as 26"/deflection in inches.

Aluminum and carbon are measured with a 1.94 pound weight applied in the center of a 28 inch span and the deflection is used.

To convert aluminum or carbon to the wood deflection simply multiply the 1.94lb., 28", deflection by .825.  You can then divide this number into 26" to get the equivalent wood spine.  A more direct method for aluminum and carbon is to  divide the 1.94lb., 28", deflection into 31.5".

For example: a 2016 aluminum has a deflection of .531", so 31.5/.531=59.3lb. spine.

I derived this conversion using beam deflection theory when I designed my Spine-O-Meter spine tester.  The scales on it have all three shaft types on them so you can convert spines at a glance.

I have a write-up on the derivation on can email to anyone interested.

Kevin

[bigcountry]

Ok, so I am little confused.  The AMO charts show a 2016 alum to have a 49-50# spine.  But your saying its a 59.3lb spine.  

So with all that said, if I have a bow that shoots a 30" 1916 alum arrow well, but want to order some sitka spuce tapered shafts, what do I ask for from the arrow builder like Rose City, or "Arrows by Kelly", or Old South Archery?

Do I take your formula of 31.5/.623= 50.56lb and ask for a 50# spine arrow, or do I take the AMO chart and pick a 41-42# arrow.  Or should I ask the arrow maker first?

[SCATTERSHOT]

Go to Foxfire Archery website   http://www.arrowsbykelly.com/,  formerly Arrows by Kelly. There is a spine chart there that will give you the spine of aluminum shafts, using a spine tester like for wood shafts.

[Cupcake]

I would definitely talk with the arrow maker about the correct spine.

I think you are using the AMO chart incorrectly.  The aluminum shafts are measured differently than the woodies so you can not use the Easton published deflection to calculate AMO spine.

Using the AMO test method the deflection will be .825 times the Easton deflection, or .531"x.825=.438".  The shortcut method is to use 31.5/Easton deflection, or 31.5"/.531"=59.3lb.  That is because 26"/.825=31.5".

A 1916 shaft is 31.5"/.623"=50.6lb.