Is Arrow Length and spine Important .
To get the most accuracy and consistency it is absolutely critical to have a correctly spined arrow. The arrow spine is really summed up as the stiffness of the shaft . This is determined by
- bow draw weight
- head weight
- Brace Height
- Bow cam type
- Arrow length
- and even the shaft weight.
Any changes in any of the above will result in a change in how the arrow bends as the string pushes it out of the bow .So what is the effect of an arrow which does not have the correct spine.
- Too soft a spine ( a soft or week spine will allow the arrow to bend to much ) will result in erratic and inconstant arrow flight A soft arrow will shoot to the right for a right handed shooter. If it is a lot to soft it can snap and go through the shooters arm or hand for example a 700 spine arrow being shot out of a 70 lbs bow.
- A spine which is to stiff will not allow the arrow to bend enough causing it to shoot to the left on a right handed shooter .The arrow needs to bend a small amount to stay on target .If it doesn't bend enough it will be pushed off target.
When you buy your arrow shafts you will notice numbers written on the shaft . These numbers will tell you the spine and on most shafts the grains per inch . The grains per inch is the weight of the shaft .
ie on a 27 inch shaft at 7.1 grains per inch (gpi) the shaft will weigh 7.1 gpi x 27 inch = 191 grains total shaft weight without the insert ,head ,nock or fletching.
A heavy shaft has more momentum than a light shaft and therefore it will resist moving more than a lighter shaft. When the bow is fired the back end is being forced forward while the front end is momentarily resisting the moving. This results in the shaft bending away from the bow.The more force and speed on the back of the arrow the more it will bend . To stop it bending to far you need to stiffen the arrow shaft with a lower spine ( you can also cut the shaft down or reduce the head weight ) . A lighter arrow shaft will move more easily and therefore a lighter spine ( higher number ) will need to be used .
Basically the higher poundage bows shoot faster than lower poundage bows and need a stiffer ( or lower number ) spine .
The spine is measured buy suspending the arrow shaft over 2 points 28 inches apart and hanging a 2 lbs weight in the middle . How much it bends (in hundredths of an inch ) is the spine .ie if it bends only 300 ths of an inch it is a 300 spine a softer shaft at say 500 spine will bend 500 ths of an inch.This is the static spine rating.
A higher number means a softer spine and a lower number means a stiffer spine.
Many people mix up the spine of an arrow with the shaft weight which is obviously incorrect. you can have a 30 inch shaft at 300 spine that weighs 400 grains and you can have a different 300 spine shaft with a weight of 280 grains at the same length.
To increase the stiffness of the arrow you can cut it to a shorter length.Which brings us to the arrow length part of this article. So what is the correct arrow length and is it necessary to cut the arrow down.. If an arrow is at full factory length ( normally between 29 and 32 inches ) as long as it is in the correct spine for your set up than the only reasons to cut it down are
- to lighten it
- or to give less wind effect from crosswinds
So when ordering arrows from the internet if you need them cut always take your measurement from inside of the nock groove to where you require the cut .The inside of the nock groove is where the string applies it force and so this is where your spine is calculated from.
STANDARD OF MEASUREMENT for arrow length .`
The standard AMO Method of measuring an arrow is the distance between the bottom of the groove of the nock (where the string rests in the nock) to the end of the arrow, not including the tip or insert. We measure and trim all arrows to length using this standard AMO (now the ATA) method.
Remember full length arrows can be used as long as the spine is correct .An arrow that bends to much has a week spine and will shoot inconsistently and can be very dangerous if the spine is too soft ( if it breaks it can cause serious injury ) .