spine vs. deflection

[skidmark]

Recently I have been playing with arrows and spines looking for the perfect hunting arrow. In some reading I found deflection the topic instead of spine. Can some one enlighten me on the difference, if any? Is it measured the same way? An example would be a 2117 with .325 deflection and .400 spine. Thanks.....Dave

[TexMex]

I think deflection is used to figure out the spine. Spine is calculated by dividing 26 by deflection.

[soopernate]

And If I rememeber correctly the spine of a 2117 is around the 81# for the XX75 with a slight increase to like 83 or 84 for the gamegetter shafts.  The deflection of the 81 pound xx75 should be in the neighborhood of .320 inches if my math is correct.

[yamapup]

Tex, that is interesting. The 26 is the distance apart that the spine is measured on the shaft. So, why in my 08 Easten arrow chart does it give  spine at "28" span?" I also thought that spine referred to woodies and deflection was the actual amount of bend in fractions (decimal) of an inch. Can anybody clarify this? If woodie makers would give the average spine numerically, it would be easier to match woodies and alums or carbons.

[DesertDude]

If my mind is working right today, wood shafts are measured 26" and carbon and aluminium are measured at 28". A friend of mine (CupCake) A sponser here makes and sells a spine tester. His  indicator gives the spine reading for all three on one indicator. So when you test a wood shaft you can see where in relation to alu & carbon it measures. Very easy to use, so if your measuring a carbon shaft, you can get the woodand the alu shaft equal. Remember that this is a static spine.
 Mark

[Steertalker]

Mark...you are correct.  Woods(commercially sold) are spined at 26 inches whereas carbons and aluminums are spined at 28 inches.

TexMex is also correct except that you divide 28 by the deflection to calculate spine for the carbons and aluminums.

Also....carbons, but I'm not sure about aluminums, have a strong direction as well as a weak direction.  This is called "spline".  In case anyone is interested.

Brett

[JRY309]

Spine=the measurement of delfection in inches.The correct spine for 2117 is .407 measured at 28" span.The deflection in inches is the spine of an arrow they are the same.

[Cupcake]

Skidmark, See if this helps.

Wood arrow deflection are measured at 26" span and 2.0lb. weight applied.  The spine, in pounds, is defined as 26"/deflection.

Aluminum and wood shafts are measured at 28" span and 1.94lb. weight applied.  There is no spine defined for this system, just the deflection.

But, in developing the Spine-O-Meter I derived a conversion factor between these two systems using beam deflection theory.  So if you know the deflection of an aluminum or carbon shaft multiply by .825 to get the deflection in the wood method.  Then you can divide 26" by this number to get the pounds spine.  A shortcut to get the wood equivalent spine directly from the aluminum/carbon system is simply: 31.5"/deflection.

For example: a 2016 aluminum shaft deflection is .531", to convert to wood .531"*.825=.438, then; 26/.438=59.3lbs. spine.

Or directly, 31.5/.531=59.3lbs. spine.

Kevin

[snag]

So, if I got this right...GT 5575s are spined at .400 divided by 31.5 = a little less than 80lbs spine....?

[JBiorn]

Ok, so what about a tapered shaft such as the Grizzly Stik? Typically they spine very heavy, yet they shoot fine from the lower poundage bows. Does the tapered design affect the deflection that much?

[Mr.Chuck]

Grab a hand full of shafts, and shoot'em.  The best flying one "is the one".

[Cupcake]

Snag, that is correct; 31.5/.400=78.8.  I have verified this by measurement.

The tapered shafts are different dynamically and the spine measurement is static.  I have yet to delve into tapered shafts much so I don't have any inight yet.
Kevin

[skidmark]

Thanks all, Kevin...that was a big help. Nice to have a little more info for selection. Dave

[wapiti]

After talking with some in the archery manufacturing industry deflections charts in 1/000 of an inch were used for both wood and aluminum to determine spine /poundage. Everything was done on the same 26" centers according to AMO standards using a 2 lb weight placed at 13"(center). The 28" span came about from a combination of the newer high energy compounds and the use/ popularity of carbon shafting. The newer bows require stiffer shafting. To make pundage labeling understandable and consistent the 28" was developed giving,for example, a .400 deflection on a particular shaft. On a 26" span it may only read .325. A reading of .325 correlates to 80lbs. but aan archer with a compound may only be shooting 70lbs but needs a shaft of .325.

[snag]

This is probably a really dumb question...but what is "deflection"? Is that have much it flexes when put under a load? Like determining spine of wood shafts?

[Cupcake]

Snag,
The deflectionn is the amount the arrow shaft flexes on the spine tester.  Your 400 shaft arrow deflects .400 inches when 1.94lbs is applied in the center of the 28 inch span.

If you measured it using the wood system, 2.0lb. and 26 inch span it would deflect .330 inches and the spine would be 26/.330=78.8lbs.

[snag]

Thanks for verifying that for me. I am trying to figure out carbons.