1.) There are two industry standards in measuring the deflection of an arrow shaft.
For wood arrows (=old AMO spine)= measure center deflection of a shaft on supports 26" on centers while putting a weight of 2# in the center of the shaft. Grain must runs perpendicular with the direction of the deflection.
2.) For modern (uniform)material (aluminum and carbon arrows): measure the center deflection of a shaft on supports 28" on centers with a weight of 1.94# in the center
These are the two deflection measurement methods. Now we have to determine, what deflection shoots well out of a particular bow. This is the static deflection to dynamic spine (bow poundage) correlation. The correlation was done empirical.
When it was developed, the predominant arrow material was wood and most bows were between 30# and 55# draw weight.
In this spectrum of draw weights, this spine weight correlation has the greatest accuracy.
The correlation formula for wood arrows is 26"/ deflection in inches equals the poundage of a straight limbed longbow where this static measured deflection results in a well flying arrow. To get a spine weight for new arrows, you need to make adjustments based on the kind of bow/ length of arrow and point weight. This will yield a well shooting arrow for most shooters.
The modern method allows to test stiffer shafts with better accuracy.
On my personal spine tester for wood, I increase the weight by 50% for spines over 60# which will give greater deflection reading accuracy together with an adjusted scale.
For arrows under 30# it is better to reduce the weight by 50%, because you don't want to deform your shaft permanent by measuring it. Everything must happen in the elastic range (= deformations will be go back 100% to straight= this is an ideal)
One can easily convert modern material deflection values into wood spine bow weight.
Formula: (26"/ deflection of modern material in inches)*1.2114 = bow draw weight.
This correlation is generally true for shafts down to 5/16" diameter. If it gets more slender, then this will start to take effect.
Generally, if you use standard carbons/aluminums and get to a required wood spine weight using the adjustments which come from experience and do the conversion, then these arrows will be close, too.
When it comes to buying arrow shafts unseen, you will need generally some help with this.
Stu Millers dynamic spine calculator is a good starting point, if you know how to feed it the correct information.
There is still one factor, and this is your form. This can make the difference between one shaft or the other and that may change over time if your release gets better.
Topic: Spine...not trying to beat a dead horse, but..... (Read 756 times)
