Spine tuning is a dark art. and hence accompanied by much misinformation! The aim is to ensure the arrow clears the bow during launch, that the arrow leaves the bow with minimal rotation, and ideally that the arrow moves along the bow's center-line. This is the same as most other tuning goals! Present research indicates a perfect solution may not always be possible, but arrow spine tuning is likely to provide the best compromise.
Spine tuning is desirable for most bow types, although some more than others. This is especially true for all traditional bows and other non-center-fire bows. It is also necessary for all finger release situations where lateral nock movement at release is unavoidable.
Older compound bows were susceptible to moving the nock in a vertical
plane, particularly in the valley, but generally can accept a wider range
of spine - so wide that the manufacturer's spine recommendation is
probably fine. For modern compounds, a good stiff arrow is nearly always
fine.
The following diagram is should give a feeling for the spine versus
grouping by bow type:
The chart shows likely spine versus grouping for
various bow types.
So, the process is much the same, just in different planes and different adjustments. Ultimately the objective is tight groups on center-line.
A bow applies great force to the nock of an arrow. This force accelerates the various distributed masses along the length of the arrow. Ideally this force would be perfectly aligned to the arrow’s axis, but in reality this never happens. Indeed, when the rest is off center as with most traditional bows, proper alignment is impossible. When misalignment occurs, the misaligned component of the force will tend the bend the arrow in that direction. The much larger aligned component of the force places the arrow under great compressional load and it amplifies the bending with Euler column buckling effect.
The bending can be very complex and tends to cause a resonant oscillation, dominated by the fundamental harmonic of the arrow resonant frequency. One reason for the complexity is the change of constrains during the progress of a launch. For example, when the arrow breaks contact with the string just beyond the brace height, its vibration mode changes from an end constrained mode to a free mode. Another example is the constrain provided by the plunger and rest that can apply a force that moves along the shaft during launch.
Spine tuning involves matching the bending characteristics of an arrow to the bow's dynamic draw curve. Spine tuning is particularly important for traditional bow's where a well matched setup will ensure the arrow not only clears the bow but also travels the center-line despite the fact the arrow is initially pointing well off-center. The fact that this is even possible is the Archer's Paradox.
Simplified lateral arrow movement during and after launch from a recurve bow. Red line is the nock path and the blue line point path. The vertical axis is exaggerated by factor of ten.
With all modern center-line bows, the arrow no longer needs to move around the riser, it simply needs to avoid any unintended contact.
An arrow has just three adjustment parameters: static spine, length and various masses. This gives ample scope, provided arrow mass can be kept within an acceptable range for the archer's goals.
The two most important tuning issues are riser clearance and zero rotation.
Other requirement are secondary.
The bow can be tuned also but it is preferred to reserve this for the fine
tuning process where hopefully only small changes are required to control
rotation.
It is assumed the archer has chosen the arrow static spine to the manufacturer's recommendation. This hopefully ensures the dynamic match to the bow is within a sensible range. The implication of a miss-match is that the arrow can strike the bow. An aim of the tuning process is to get a feel for the clearance margin and to ensure it is large enough to ensure ALL looses clear.
For the recurve bow the arrow is likely to strike the plunger or other vertical part of the riser. For compound bows the risk is more likely strike on the rest. Such strikes are always undesirable as they lead to variable arrow flight.
Check the arrows for wear along the back third of the shaft, especially on the plunger and rest sides. Check the fletching for signs of contact. On the bow in the rest and plunger areas for signs of wear or plastic deposits from fletches. Sometimes arrows can strike the sight when set for long ranges on low draw weight bows.
If a strike is suspected, apply some sort of marker to the bow shelf area or to the arrow's fletching. Talc, flower, spray on deodorant and lipstick are all candidate materials.
Even if nothing is found, the author likes to add 3-5 mm high strips or beads of Blu-Tack© to likely strike areas to confirm a margin and to get an idea of the effect of tuning adjustments. The great advantage of this method is its 3D - giving a better picture of exactly what is happening. The only problematic area is with drop rests - placing Blu-Tack on the fast moving parts on the rest is likely to impact its timing, so is to be avoided. Leave the Blu-Tack in place while spine tuning, but remove for final tune confirmation shots.
Should strikes be confirmed, in order, try:
rotating the nock so the fletching is more likely to clear,
loosen over tight nocking points by reducing serving thickness,
complete basic tune,
confirm arrows are of same static spine, weight and length,
adjusting the arrow's dynamic spine,
trim fletches a few millimeters.
Should none of the above work, you need help!