Arrow Spine Selection
The arrow spine is a number that measures it's stiffness. This shows how much the arrow flexes when is released from your bow. When the arrow leaves the string, it travels through air like a spiralling snake, bending and spinning like mad (there are some very interesting slo-mo videos on the internet capturing this). This bending is the reason why the Archer's Paradox occurs.
If an arrow is too weak for the bow you are shooting, it will bend too much. If the arrow is too stiff, it will not bend enough. In both cases, your shooting results will not be great because your grouping will not be consistent.
So, how do we select the right spine for our arrows? Simply put, the stronger your bow is, the stiffer your arrow should be. But, as always in archery, things are far from being simple or straight forward. There are some other factors and measurements that will influence the spine selection:
- Your bow weight.
For beginner and intermediate archers, reading the numbers written on the limbs will suffice, as this will be, approximately, the poundage that you draw. For example, 24#.
For the advanced archers, that want to achieve maximum accuracy, we can measure their exact draw weight using a bow scale. This is the exact poundage that they draw at full length with their bow. For example, somebody with a #36 bow can only draw #35. We can calculate this figure fairly accurately if we know your draw length.
The heavier the bow poundage, the more the arrow will flex during its flight.
- Your arrow length.
Your arrow length is influenced by your draw length. Normally, the length of your arrow should be 1" longer than your draw length, even 1 1/2" for beginner archers or 2" for compound archers.
It's important to know that a longer arrow will flex more in its trajectory to the target and a shorter arrow will flex less.
- Your point weight.
That is the exact grain of your points - for instance 100g or 120g. To have an idea why this is important, you should know that a heavier point will make the arrow flex more while travelling towards the target.
All arrow shaft manufacturers have arrow selection charts (have a look at Easton's target shaft selector). Unfortunately, they all have different marking systems, and may use different systems for different arrow materials, so the numbers you read on your arrow shafts from one manufacturer will not match the ones from a different manufacturer. Nevertheless, it's quite easy to find the recommended shafts in the cell at the intersection of your bow weight and arrow length. Keep in mind that, if the chart does not specify a point weight, it means it uses a standard 100g point.