I am an old time aluminum arrow guy, but the world has changed. I am trying to figure out the spines of carbon arrows so I can intelligently get some new arrows for my daughter. What is the algorithm or some sense of logic they use to figure out what to use per draw weight? With her length and draw weight (29" arrows and about 30 pound draw) the spine is heavier than the same length with a 50 pound draw, why is that? Just don't want to explode an arrow through her hand like those pics I have seen out there. Need to get her a kevlar glove too, yes I am a little protective.
Thanks guys,
Indy

Spine is a measure of deflection for a given weight at a given length.  Stiffer arrows have a smaller spine number (less deflection under weight), and the shorter the arrow, the stiffer it will be.  A longer arrow will bend more than a shorter arrow made from the same stiffness of material.    

Are you sure a 29" arrow is the right length for your daughter?  I'm 6'1", my bow is set for a 29" draw, and I shoot 28" arrows.

Arrow School

Easiest method is to pick a brand, and look at their spine charts online.  You'll need arrow length, and draw weight.  Some may require the weigh of the tip as well.

In general, lower number = stiffer for carbon arrows.

340 means the arrow deflected .340" in the middle when the standard weight was applied.   400 means the arrow bends .400" in the middle.  Etc.

Like said, the best way is to refer to the manufactures selection chart.  Here is Easton's shaft selector computer based on their selection charts.

http://www.eastonarchery.com/store/shaft_selector/

At that weight she could probably get away with 500 spine arrows (very light).

It seems most women shoot 500's