Dynamic spine & FOC? for you physicists

[Red Dwarf]

I have been playing with 2 different arrow set-ups and have the following question:

Does %FOC alter the dymanic spine?

Both arrows are unfletched carbon, the same length and the same static spine, with the same weight up front.
One shaft has thicker walls, is therefore heavier with less FOC.
It seems to me that the arrow with less FOC shoots "stiffer". Am I correct or is this just a matter of poor form on my part?


Red Dwarf

[Javi]

Did you measure the spine or take the manufacturers word for it. I've seen several shafts from different manufacturers test very different than advertised.

In fact I just had a dozen that were susposed to be .500 actually measure .430... and a set of .500 measure .480..  big difference between the two..

[Rob DiStefano]

it's not just the foc, nor the dynamic spine, it's also everything about the shooter - the usual forgotten part of the arrow spine formula; the form, the aiming methodology.  each of us is an 'experiment of one' when it comes to getting great arrow flight with a bow-safe arrow mass weight - and make NO mistake, THAT is what we WANT, not foc.

[Red Dwarf]

Javi

Both shafts measure the same on my tester.

Rob

I have shot both arrows many, many times; the one with greater FOC has great flight but the arrow with less FOC flys nock right. (I shoot righthanded)


Red Dwarf

[Javi]

Rather than type out a long explanation of beam theory I'll just post a link to a good article on this phenomenon….  


 http://www.africanarcher.com/dynamicSPINE.html

[Rob DiStefano]

Originally posted by Red Dwarf:
...

Rob

I have shot both arrows many, many times; the one with greater FOC has great flight but the arrow with less FOC flys nock right. (I shoot righthanded)

Red Dwarf

more than likely both will fly just fine for me, using your bow.

you do need to supply more, actually all the details about the arrow and the bow.

[Red Dwarf]

Javi

Great article by Harry Marx...thanks.

What he is saying (if I am reading things correctly) is if you increase the weight of the shaft, but keep everything else as is (FOC will obviously change), the shaft will flex less upon release; just like a shaft with a stiffer static spine, which I would expect to fly nock right for me.
I think that this explains what is happening in my scenario.

Rob

Not sure what additional info is required and why?

Thanks

Red Dwarf

[EL Mejor]

GOOD LINK,,,,

[Javi]

Originally posted by Red Dwarf:
Javi

Great article by Harry Marx...thanks.

What he is saying (if I am reading things correctly) is if you increase the weight of the shaft, but keep everything else as is (FOC will obviously change), the shaft will flex less upon release; just like a shaft with a stiffer static spine, which I would expect to fly nock right for me.
I think that this explains what is happening in my scenario.


Thanks

Red Dwarf

Yes that is what he is saying..

I believe Rob is referring to your form and release.. the cleaner the release the weaker the spine you can shoot..

[Red Dwarf]

Javi

The "weaker" arrow is consistently flying better for me, so I think it is an arrow issue not a release thing, as per the Harry Marx article.

Red Dwarf

[Javi]

Originally posted by Red Dwarf:
Javi

The "weaker" arrow is consistently flying better for me, so I think it is an arrow issue not a release thing, as per the Harry Marx article.

Red Dwarf

Sounds like it to me... throw some weight on the stiff one and see what it does..

[Friend]

I feel Rob's advice is in on the mark. Neither one may be correct. You could be so mismatched that you are getting false results. Also, you can only tune as well as you can shoot. Bare shafting will magnify form errors.

I only use knock position at 5 yards for arrow   set-ups that I am initally concerned. Bare shaft tune in 5 yard increments back to 20 yards and fine tune at 25 yards, then re-valuate my tuning at 5 yards.

[Red Dwarf]

Friend

I have shot the lighter arrow set-up for the past two years, they are well tuned to both my bow and shooting form.(Bareshaft planing method as per Mr. Adcock's recommendations)
I bought shafts of the same s-spine but from a different manufacturer, set them up identical to the first set but found a difference in flight characteristics.
It appears, from the aforementioned article, that this variance in flight is due to the increased shaft weight, although the change in FOC may also be playing a part.

Red Dwarf

[CoilSpring]

Red, Different total-arrow weights and different FOCs will both affect Dynamic Spine.  

You implied in your original post, and confirmed later, that your thicker-walled shaft was heavier (ie. total arrow shaft wt is heavier, since pnt-wt & length were the same). The heavier arrow (all other things being equal) will always fly stiffer than a lighter wt arrow (of same spine) because it is moving SLOWER than the lighter wt arrow. Your bow (or any stickbow with fixed limbs) at a fixed draw length has only a certain amount of stored energy to apply to the arrow (ie. the area under the force-draw curve - see integration in calculus). It simply takes more energy to move a heavier (mass) arrow (at the same speed) as a lighter arrow. The main power source (engine) for your arrow is your bow, and it puts out only a fixed amount of energy at your draw length. With that fixed amount of energy to apply to each (2 different weighted) arrows, the heavier arrow moves slower (because is takes more energy (Horsepower, Ft-lbs, etc) to move it; therefore, it has effectively higher dynamic spine, even though its static spine is equal to the lighter wt arrow.

A lower %FOC stiffens (increases) dynamic spine because more of the arrow's weight is CLOSER to the nock end of the shaft.  Having the center of mass weight closer to the nock (as is your lower FOC arrow) effectively makes the shaft act stiffer.

PS. Rob may have been refering to more info about your arrow/bow because it does matter.  Nock tightness, string type/diameter, arrow outside diameters and bow riser-cut all affect Dynamic Spine. In your case (same bow & same string) riser-cut and string are irrelevant to your question; however, nock tightness and arrow outside diameter,OD, are.  If nock tightness and OD on both arrows are the same, disregard the following... If all other things on your 2 arrows are equal, the tighter nock will cause an effectively stiffer dynamic spine, because it takes energy away from the arrow, slowing it down. Also, the larger OD shaft, will effectively cause a stiffer Dynamic spine, because a LARGER OD shaft's center-line is pointed at a greater angle away from the target-to-bowstring centerline, not allowing the arrow to bend enough around the riser (archers paradox) to get to the target. This is why you can shoot a stiffer STATIC spined arrow out of a more center-shot or cut-past-center bow riser, than out of a riser not cut or cut-before-center...it isn't required to bend (in paradox) as much to get to its intended target.

[Flying Dutchman]

As a matter of fact: I make my own arrows for years now. FOC is my starting point. I want to have a FOC around the 8 to 9%.
This because I am a 3D archer who also shoots at great distances.
On my 11/32 Stitka Spruce shafts a 100 grains fieldpoint does the job, with this I am on a 8.5% FOC

Further I make sure that the dynamic spine is on the right value for my bow and my way of shooting and I iake cere I am in the proper weight range. In my case 9,5 to 10 gpp.

[JJB2]

Great explanation coil spring.