Could spine affect dynamic arrow weight?

[mmgrode]

Hey Gang,
    I just got to thinking:
        - Let's say you have two arrows that weigh say, 520 grains each.  One of these arrows has a weaker spine and one a stronger spine.  Now, with the same 65# bow tuned to each of these(with a built up strike plate to tune the weaker spined shaft), would not the weaker spined arrow seem to resist less to the bowstring than the stiffer one due to the larger paradox?  So, for a weaker spined arrow would'nt a guy need to actually increase the weight of the arrow in order to maintain the general 8 grains per pound safety margin? Thoughts? Comments?  Matt

[Molson]

No.  The bow is pushing the weight of the arrow, not the spine. 520 grains is 520 grains.

[mmgrode]

Aye Molson, but is it really? The bow is pushing the weight of the arrow, but the resistance to the bowstring has to involve the spine by my reasoning.  
Think of it this way; take a stiff nonbending rod and put a weight on the end. Put your hand on the other end and throw the rod by pushing on that end(like throwing a spear from the very end)...all of the weight is there to resist that throwing action.  Now take the same setup, but use a flimsy rod instead.  When this stick is pushed the flimsy stick will bend and gyrate and you won't be able to get the same amount of force into it.  The hypothetical bowstring in this case being the force you exert thus would not have the same resistance throughout the power stroke in both of these cases(less in the flimsy one).  The arrow with stiffer spine will have a more inline resistance to the force of the bowstring, therefore also more velocity in the long run.  But for safety in this case I believe this also to be true.  

Perhaps another example:
     Take these same two rods without the weight on the ends. Now, stand them vertical on the ground and push down on each only on the ends. When the stiffer one is pushed on it will penetrate deep into the soil. The flimsy one will again bend and possibly even break. The man doing the pushing will not be able to push this flimsy rod to the same depth as the stiffer one.  

So, to my reasoning, I hypothesize that the dynamic weight of the arrow(the resistance that it causes to the released string) will be different for these different spines.  I suppose a proper test of this would be to try the two spines tuned on the same bow with a chronograph and shooting machine. I would be willing to bet the stiffer one would be not only faster, but safer for the bow.  I'd also bet the noise would be less for the bow with the stiffer arrow.  

Thoughts?   Comments?  Alternate theories?  

Cheers, Matt Grode

[O.L. Adcock]

Molson is correct...the arrow bends first toward the riser and then away about half way through the power stroke. It doesn't just colapse as you are suggesting. The only time you will see a lose of velocity is if the spine is so far off you get riser/shelf contact....O.L.

[mmgrode]

Thanks for the reply O.L.
 - How could you not get riser/shelf contact?
 
Perhaps I should have been more specific. Let's say we are shooting a longbow bow cut to center for the stiff spine and built out to say 3/16" to accomodate the weaker spine. The arrow paradoxes partly because of the stiffness(or more specifically lack of stiffness), but also partly because of the built out strike plate, no?
This also brings up another point: the weaker spined arrow will not be as much in line with the power stroke of the string. To my estimation then, the string could not possibly exert as much force on the weaker arrow as the stiffer one. It would be likened to pushing partially sideways on the weaker arrow in the second example above.  Wasted energy through paradox.  Where does that energy go?...straight into the limbs.  Compromised safety margin. The more bend the more energy lost/less there to accelerate in that straight line. Remember, same arrow on both setups(point, shaft and the rest), just different spine. Thoughts? Comments? Alternate theories?  Cheers, Matt

[O.L. Adcock]

Matt, it's not "lost"...It's contained in the elasticity of the arrow cause it's going to flex the opposite way before it leaves the bow and several cycles after. It would have to leave something behind to lose anything!    This ain't theory, you can test it yourself. Even without proper tuning, you'll see no velocity difference caused by spine at 3'. You'll darned sure see it down range as the fishtailing arrow bleeds off energy flying sideways and trying to correct. Do a search on tuning for the threads where folks hear a "clack" when they shoot.....Thats riser contact, usually caused by overly stiff spine, they don't bend enough and therefore hit the riser. It darned sure robs a lot of energy. Even properly tuned off the shelf compared to an elevated rest robs a lot but that's more so down range....O.L.

[O.L. Adcock]

OOPS, misread you statement on riser contact...If properly tuned, that arrow only touches the shelf for maybe 1". Arrows paradox vertical as well as sideways....O.L.

[mmgrode]

Thanks for the replies O.L.
"It would have to leave something behind to lose anything!"
That makes sense. My thinking is that because the arrow does bend there is not the same amount of resistance to the released string throughout the entire power stroke.  Instead of pushing the full arrow weight directly down the shaft(as in the stiffer spine) wouldn't a weaker spined arrow lose some of that resistance to the bowstring in inducing the bend in the shaft?  

Yet(as you mentioned), at the same time, that energy which is drained into the weaker arrow goes directly into causing that bend.  Therefore, that extra energy that the weaker arrow lacked in resistance is made up in the bend of the arrow during paradox and subsequent fishtailing flight. O.K. that makes sense to me.      

So O.L., if there is energy put into the arrow to cause it to bend(in addition to moving it forward) and not solely into pushing the shaft forward as in the stiffer shaft, how can there not be a decrease in velocity with the weaker shaft? The energy has to go somewhere right? Wouldn't that equate to velocity loss in the weaker arrow? That just doesn't seem to make sense to me. Trying to learn.  Thoughts?  Comments?  Cheers, Matt

[Jeff Strubberg]

You didn't ask if the weaker arrow had less velocity, you asked if the weaker arrow provided less resistance to the bowstring during the shot.  Two very different things.  The energy output from your bow is constant, assuming a stable draw length and good form.  The energy absorbed by different arrows can be quite different.

Whether the shaft bends or not, you still have to accelerate it.  Same weight, same mass, same amount of energy to get it moving to an identical speed.  As O.L. said, the only way this changes is if your spine is so radically wrong that you pick up additional drag through excessive shelf contact.  There are only two variables on the power side of the equation, stored energy and efficiency.  There are only two variables on the projectile side of the equation.  Mass and drag.

Now, things are a little different on the other end of the equation. When the shaft STOPS, less energy will be expended in the direction we want it to go (in the case of the lighter-spined shaft) precisely because of the bend.

The weaker spined arrow will have less velocity downrange because it is wasting more energy in paradox. BUT, if both are tuned to the bow, as you stated in your first post, the difference is going to be so small that you are going to have to get quite a ways downrange to measure it.

[O.L. Adcock]

Matt, the string and therefore the arrow is accelerating at a rate governed by it's mass...It doesn't matter if that mass is a arrow, feathers, or a rubber ball....All the string sees is the mass and it's end speed will depend totally on that. Any resistance it sees like friction on the riser it will see but the arrow wiggling around doesn't cause any resistance to slow the string down. Last time I checked the front end is going just as fast as the back end!    I see what you are saying but if it does lose something, it's too small to measure.....O.L.

[stickflngr]

WOW,you guys actually made my hair hurt trying to keep up with you...... But keep up the good work, I think?

[mmgrode]

Jeff- The very fact that the resistance to the bowstring would be less by the induction of the bend pertains directly to the idea of arrow velocity variability. More energy into bend= less energy into arrow acceleration.

Perhaps I am being a bit too picky here(as if that wasn't obvious;).  My assumptions are that the bow has a given amount of energy at full draw and that energy will have to go somewhere upon release(law of conservation of energy). With this said the bow is pushing that same mass in the weaker arrow as the stiffer one...O.K. So part of that energy would have to be expended in causing the arrow to bend right? Even if it is small, it is still there right? I suppose this along with the better penetrating qualities of a stiffer shaft give cut past center bows a bit more of an advantage, even if it slight. So, in conclusion, as long as the energy is drained from the bow, whether through arrow acceleration or bend induction, the safety margin should be consistent with these two arrows of different spine.


O.K. lads, thanks for the input, expertise, and thoughts. I enjoyed this.  Any other thoughts?  Cheers, Matt