Arrow spine (compound)

Arrows from compound bows work differently to those shot from a recurve/longbow. The arrow still vibrates (flex) although the flex is up and down which is largely related to roll over action of the cams (cam timing) and associated nock point travel. When setting up a compound bow, as you draw the bow you want the nocking point to travel back in as straight a line as possible or in an upward curve. The nock should never travel in a downward line. Both cams must reach full draw at the same time, so they exert even pressure on the arrow when released. Ideally it is best to have a slight upward nocking travel back to full draw, this will assist in creating the initial downward flex of the arrow upon release. (many archers will advance the top cam slightly to achieve this) An arrow shot from a compound bow will vibrate (flex) up and down not side to side like an arrow shot from a recurve bow. Depending upon the type of arrow rest used you may also want the arrow to flex so the tail of the arrow does not make contact with the arrow rest as it passes. (i.e. not too stiff a spine).
The side to side effect associated with finger release does not happen when using a release aid, also the arrow is in motion when the maximum bow weight is exerted on the arrow. Ideally the bow should be set up so the initial forces upon release are pushing down on the rest causing the initial arrow flex to be downward. This downward force continues about half of the arrow travel from full draw to brace height. About half draw the arrow starts to flex up, this upward flexing action continues allowing the tail of the arrow to pass over the arrow rest and out from the bow. The arrow then continues this up and down flexing action for a while, until as the song says, it straightens up and flies right. Various factors affect how far it travels before true flight i.e. composition of the shaft, wood, aluminium, carbon all recover differently with carbon recovering faster. State of tune, spine, and of course the archers form all have an effect. It is important to note the arrow leaves the bow string at brace height, although there are of course factors that affect this, whether there is a string stop fitted or not, fit of nock to string (very important) being two such factors. This flexing is the same although to a lesser extent as happens with recurve/longbow but is in the vertical plane rather than horizontal, and can be tuned to achieve clearance of the rest just as in recurve archery.
There should be no side to side motion on the arrow upon release. If there is this is related to either;
1:  Equipment setup i.e. centreshot, (arrow rest) or cam lean, which can be caused by incorrect setting of cable slide rod or simply poor setup of split cable (yoke)
2:  Shooting technique i.e. bow hand torque, clearance associated with face or chest contact. Due to the let off associated with compound bows, shooting technique and bow set up are critical factors to good arrow flight.
With the light weight let off it does not take much to move the arrow out of line, face contact with the string (even a small amount) or contact with the chest will have a major effect on the arrow flight and accuracy.

Copyright Archery Australia
June 2007
Written by Jim Larven
Technical information and Research by James Park
Reference material Archery Australia Shooting Technique


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