Carbons- non linear effects on dynamic spine; shaft length vs point weight?

[ozy clint]

So I've been doing some tuning of some unorthodox arrows for a water Buffalo. Looks like I've settled on an axis 400 with 300gr tuffhead, custom adapter 340gr. (From memory), long 2020 footing about 5". Total-1007gr. Shoots nicely. About 35%FOC. 70# border recurve.

Now before you say how is a 400 spine ever going to work, consider the long footing. It reduces the flexible portion of the shaft to 24". This stiffens it considerably!

This leads me to my anecdotal findings. That carbons stiffen in a non linear relationship with point weight. The shorter you go the more and more point weight needed to maintain dynamic spine per unit of length trimmed. I've found 50gr upfront equates to about 1" of shaft length between full length and 30" but as you going shorter more point weight is needed per inch trimmed. Anyone else found this?

For making a UEFOC arrow the long footing is a blessing. It enables the use of a lighter GPI shaft which makes it easier to keep the back end lighter. Plus the footing adds weight and strength to the front and gives you another option for tuning since you can vary the length of it in place of trimming the shaft. It also allows you to get the balance point within the footing which I hope should eliminate breakage behind the footing since the center of mass isn't acting upon the back of the footing/ insert as in conventional designs.

Thoughts?

[McDave]

Makes sense to me. I conceptualize these things by going to extremes. The indigenous people of the Amazon use very long arrows and no point weight, just a sharpened tip. Imagine what a small amount of point weight would do to a 6’ arrow that was balanced without any point weight. OTOH, imagine a 6” long arrow, and how much point weight it would take to cause the shaft to flex.

[Don Stokes]

The same relationship holds true with unfooted wood arrows. Maybe I'm missing something... I can shoot a weaker spined arrow if I either decrease point weight significantly or shorten the shaft, and vice versa. The relationship between spine and draw weight isn't linear. I did extensive testing to confirm this. The curve fits a 5th order polynomial equation. That's why the graduations on an Adams-type spine tester get closer together as spine increases.

[Steve O]

Ozy, I’ve found that to be very true. Seems to me cutting length of forbtuning carbons does not matter much until you get to a specific point (different for every bow/arrow/point weight combo), them a little goes a long way. I’ve had much better results leaving my standard, non buffalo type arrows full length and tuning with point weight. I think you are onto something.

[Roger Norris]

Ozy, I’ve found that to be very true. Seems to me cutting length of forbtuning carbons does not matter much until you get to a specific point (different for every bow/arrow/point weight combo), them a little goes a long way. I’ve had much better results leaving my standard, non buffalo type arrows full length and tuning with point weight. I think you are onto something.

You guys are waaaaay flippin smarter than me. But I have always left my shafts as long as possible, trying to get things tuned with weight up front (Or otherwise, I have gone so far as to fix a length of 20 awg copper wire inside of carbon shafts) rather than altering spine with length.

[Chain2]

I’m with you Roger. Those guys make me wish I’d have stayed in school. But I’m the opposite, I want my arrows as short as I can. Which seems to be 32” BOP. They are tough in a quiver on horseback.

[Don Stokes]

Clint, I think you've invented the overdraw, traditional style. Having a 5" tip that isn't flexing with the rest of the shaft is like having a point that's 5" long. At some point in your testing you might want to be careful about overstressing the part of the shaft that does flex. There will be a concentration of stress at the point where the two sizes meet... interesting stuff.

[streamguy]

While I haven't pushed things out as far as you have, I have noticed that when I foot my wood arrows, and shorten the length of shaft that flexes, I see a noticeable shift in spine/arrow flight.  I'm typically on footing about 3 or 4 inches below the point, and I've done it internally with steel rod and externally with aluminum tubing/shafts.  While I definitely can see the stiffer spine due to less shaft length flexing, I've been curious about the physics of the flexing shaft, then a short non-flexing section with the weight of the point (200-300 grains) out in front.  it makes the math a little complicated.  I've been able to get them tuned in and I think the stiffer spine is important to recovery with that heavy point suspended out in front of the flexing part of the shaft.  I've only fooled with it to get my arrows shooting well, I've haven't experimented more broadly to see what happens.

Matt

[acedoc]

Nowhere close to your example but carbon is non linear and your draw length and release play a big role.
I used full length Beman ICS and used to think they were weak spine at my 32 inch draw. A post by Big Jim set me right and I put on heavier field points. The 340 spine full length arrow tunes well with 250 grains up front and a 2 inch section of footing is used . The longer 300 spine black eagles tune well with the same point weight and even with 50 grains less.
This is like a journey down the rabbit hole !

[TomMcDonald]

My experience with the longer footings is that the shafts are more likely to snap as they get further up the shaft. I find 1" footings to be ideal for me. Have you shot anything hard yet?

[ozy clint]

Don- yes, I did think the same exact thing. It's basically an overdraw without the point coming back behind the riser.

Tom- my experience is the opposite. The shorter the footing the more prone to breakage since the center of mass has a longer lever with which to work against the back of the footing or insert. That why for this setup the balance point lays within the footing. Not sure what inserts you use but unless the back of the footing is further back than the back of the insert then the only main benefit from the footing will be mushrooming resistance and a little gain in FOC.

[ozy clint]

At some point in your testing you might want to be careful about overstressing the part of the shaft that does flex. There will be a concentration of stress at the point where the two sizes meet... interesting stuff.

But as you shorten the flexible portion it becomes stiffer and can handle more stress.

[Cari-bow]

Ozy
Very interesting that your footing is beyond  the balance point. I had stopped putting footings on because other then adding point weight  did not see a benefit.

Now the question for you is how do you hold the long footing on while testing?
How tight a fit between the shafts?

I don't shoot 70#'s but I will be trying this to see if I can achieve this with a weight and combo that works for me.

I shoot 340's and have a total of 700grs. with an FOC of just over 34% without a footing. I also don't hunt Water Buffalo.

[ozy clint]

i use hot melt so that i can disassemble and modify during tuning. final hunting arrows will be epoxy.

not sure what shafts your using but 2020 aluminium fits snugly on an axis 340. fits almost as good on 400.

with the longer footing you'll likely have to drop back to 400.

[Cari-bow]

I'm using Victory 340's at 6.4 grs. per inch.
I'm thinking I might need to go down to 500
with your length of footing or longer. Nice thing is the change if it works won't be for this year.
Thanks

[ALDO]

Great stuff here guys. I use a 500 Axis with a 1" footing and 325 grains with the BH, adapter and arrow insert.  28% foc and seems to work well.  Once I get a chance I may try a longer footing as I believe I am a the most I can put up front without sacrificing spine. 

[Cari-bow]

Well I have been playing with Ozy's theory of having a footing run past the balance point. I have been trying to get an arrow closer to the 650grs and still maintaining a UEFOC of over 30%.
My first attempts yield arrows 740 then 720 and my last ones were 700grs with an FOC over 34%.

Now to Ozy's suggestion of reducing spine of the arrow shaft but then adding a long footing to bring the dynamic spine back up.
Well I have achieved an arrow just over the 650 threshold at 658grs with an FOC of 33.6%.
The arrow measures 29.375" from insert to nock depth.
I have SS insert and a 5.8125 aluminum 1814.
The point weight is 350grs.
I have 1.5" of footing past the balance point.
Goal achieved now to the testing of the arrow's durability.

I arrow flies great and the trajectory is certainly much less then with 700grs.
I have been testing it around branches and trees with good results.
I will say I'm not one that will just shoot an arrow against a rock or concrete block.
That test may have merit for some but neither one tastes that great to me.
Are there others that have tried this and tested arrows in a more realistic or repeatable manner.
Ozy's theory made sense to me, now I'm wondering how to prove or test the theory.

Side note I have hunted now for many years using arrow that are over the 700gr mark and like the results.
I'm almost regretting having achieved the 650 threshold as this seems a little light right now.
The trajectory is the big plus. With the long footing one has more things to play with the over all spine which may help or confuse.

[Joeabowhunter]

Abe did you drop from a 340 to a 500 spine?  What is your draw and poundage?

Joe

[Tedd]

Also agree. Recently using Black Eagle Vintage w 250 gr points 50 grain inserts. While working them shorter for tuning I hit a point where they were tuned, but I could then swap the 50 grain insert for a 100 grain insert and get the exact same tune. (the 100 grain insert being slightly longer helps with that). I can go back and forth between 50 and 100 grain inserts if I want.
Tedd

[Cari-bow]

Joe
I went from 340 spine to 400 spine.
I shoot a longbow 57#@28" but don't draw 28"
The longer footing changes or adds to the running.