FYI
An arrow's spine deflection is the arrow's measured resistance to bending. Basically ... an arrow's spine rating tells you how stiff the arrow is. As you may know, arrows should neither be too limber or too stiff. For proper safety and best performance, the arrow spine must be matched to the output of the bow. Arrows which are too stiff or too limber will not fly well and will degrade the accuracy of your bow. Arrows which are dramatically underspined (way too limber) can even present a failure hazard. So selecting the proper arrow spine is very important.
According to the modern standards (ASTM F2031-05) an arrow's official spine deflection is measured by hanging a 1.94 lb. weight in the center of a 28" suspended section of the arrow shaft (not to be confused with the old AMO standard of 2 lb. and 26"). The actual distance the 1.94 lb. weight causes the shaft to sag down is the arrow's actual spine deflection. For example, if a 1.94 lb. weight causes the center of a 28" arrow to sag down 1/2 inch (.500"). Then the arrow's spine deflection would be .500". Stiffer arrows will, of course, sag less. More limber arrows will sag more. So the stiffer the arrow is, the LOWER its spine deflection measurement will be. The more limber an arrow is, the HIGHER its spine deflection measurement will be.
