Spine tuning should not be used to adjust point of impact?

[doughalysh]

Hello all, not new to reading tradgang, but I am new to posting.

I wanted to see if others agree with a notion that I have come to which is:

changing dynamic spine  of a properly tuned arrow to try to move arrow point of impact left or right is not ideal because you are sacrificing ideal arrow flight to do so.

If an arrow is well tuned to the bow and shooter ideal flight would have the nock in line with and following with the tip of the point along the arc of the arrow going to the target (except for the normal oscillations from being shot)

If you intentionally weaken or stiffen dynamic spine to adjust point of impact, the nock will not be in line with the point anymore and will result in reduced performance of the arrow

So assuming that an ideally tuned arrow is not hitting where I want it to, my only options are to change my anchor point or to "hold-off" to hit where I want to? 

[McDave]

I agree with what you said about not purposely using an arrow that is not tuned to the bow to move the POI.

However, there are several options other than changing your anchor point or “holding off.”

In target archery, it is very common to use a cushion plunger to move the POI without changing the tune of the arrow.  Traditional archers can achieve somewhat the same thing by changing the material used for the strike plate.  Changing to a softer material for the strike plate is similar to reducing the spring tension on a cushion plunger.  This will move the POI to the right for a RH shooter.  An example of this would be changing the strike plate material from the loop side of Velcro to the soft side of Velcro, or for an even greater effect, changing to Martin rug rest material.  An example going the other direction would be changing the strike plate from Velcro to a hard piece of leather to move the POI to the left.

I have noticed when teaching that few people are really maximizing their back tension, grip, alignment, and releases.  Our typical side of the face anchoring position often results in the dominant eye not being over the arrow, which can be corrected without changing the anchor by tilting the head over the arrow.  All of these often result in left misses for a RH shooter., and can and should be corrected without changing the spine of the arrow.

[doughalysh]

McDave, thanks for reply. For the cushion plunger, I still have trouble wrapping my head around how that works.

I know shelf build out affects how much the arrow needs to flex to clear, but the "softness" of the shelf/plunger, does it just increase or decrease the amount the tip end of the arrow "bounces/rebounds" off the shelf at the very start of arrow oscillation/flex, within the first few inches of arrow forward movement,  thus moving the point of impact?

Also I agree with leaning your head, it effectively does the same thing as moving your anchor point(changing where the nock is), but probably is a better way of doing it, depending on the person.

[McDave]

I know shelf build out affects how much the arrow needs to flex to clear, but the "softness" of the shelf/plunger, does it just increase or decrease the amount the tip end of the arrow "bounces/rebounds" off the shelf at the very start of arrow oscillation/flex, within the first few inches of arrow forward movement,  thus moving the point of impact?

That is correct, and is the same thing that happens when a cushion plunger is adjusted.

[doughalysh]

Alright, and this bounce/rebound amount does not significantly affect the amount of arrow flex then? I assume not because otherwise it would change the dynamic spine of the arrow? The "bounce" is a new concept to me, I only ever thought about tuning as a  function of the arrow flex/arrow dynamic spine.

[McDave]

The arrow flex doesn’t change. That is determined by the force of the string on the nock and the dynamic spine of the arrow. The arrow will bounce off the strike plate whether you change the strike plate material (or adjust the spring tension) or not. However, the amount of bounce can be controlled by changing the strike plate material or adjusting the spring tension of the cushion plunger.

[doughalysh]

Alright, sounds good.

Here is a good unique video of two arrows being shot together in slow motion, the channel has a bunch of good ones, that shows why using spine to adjust POI is not a good idea for best flight and penetration.



One arrow flies pretty well, nock mostly behind the point(ignoring shaft flex oscillations). The other arrow, not so much, it yaws heavily and the tip hits to the right.

The big key to me is both arrows mostly travel the same straight flight path to the target, the out of tune arrow just ends up with a bunch of yaw, pointing the tip off to the side and then the tip hits to the side, while the "arrow's central momentum"  is still mostly on that original path to the target. This yaw is what leads to lower efficiency of flight and reduced penetration.

I think, the people who advocate using spine to adjust POI think that the arrow nicely "curves" its trajectory to one side or the other with the nock staying in line with the tip. (I used to think that)It doesn't happen. 

Also just in case you think it is just the arrows messing with each other in flight, here is this video, which shows  one arrow having the "straight" line of momentum, but with yaw. It even shows that the tip hitting left or right is not even a reliable way of determining correct spine, nor a reliable way to adjust your POI. Because with a fletched arrow, the fletchings "pull" the nock back and start a yaw oscillation and depending on the distance to target the tip could hit left, center, or right.