spine calculators..does this sound right?

[olddogrib]

Just because a mind is a terrible thing, FMI, I plugged the numbers into Stu's calculator to determine what it would take to step up/down one spine level from my current arrow. I had to plug in my bow info, but it's irrelevant for this exercise. I'm shooting 3R Easton Trad only cut 30" BOP, with 175 gr. points, 3-5" fletching.(this arrow is about 25 lb. shy of the dynamic spine the charts says I need, but they fly perfectly) I just wanted to see what it would take to reproduce the dynamic spine of this arrow with 400's and 600's. I was expecting to have to alter length and tip weight, but unless I screwed something up the chart claims it can be done easily with point weight alone. If I leave the make, length and fletching the same, the charts says I could shoot 600's with a drop to 125 gr. point and 400's with an increase to 250 gr. Does this jive with your real world experience?  I thought it would have taken that much plus length adjustments of an inch or more. What am I missing? (disclaimer..these numbers are approx.,i.e. nocks for 600's may be a few grains lighter, but I didn't have that info so left it the same.)

[Hermon]

Dynamic spine may be the same, but the grains per pound of draw weight (gpp) is going to be drastically different.

Don't know the answer to your question, but it does sound reasonable. I have tried going from .400 spine to .500 spine by only changing length (I use a broadhead I do not want to change) and the length difference was less than 1.5" with everything else the same.

Also, if you use Stu Millers calculator, you can input your arrow that you know works good, then set the PFF (personal form factor) to make that arrow match your bow. Once I did that, all future calculations come out really close.

Bisch

[dbd870]

If I use Stu's numbers I should be running 100gr points with my KMag. When I do they show stiff. Switching to 145gr points they hit dead on so nothing beats actually going out and testing. His numbers work for the Grizzly.

[damascusdave]

My experience is that carbon arrows do not respond much to change in point weight whereas they respond rather quickly to changes in length...my other experience is that you will never know until you try it and it sounds to me like you may be trying to fix something that is not broken...I am not so sure your bow information really is irrelevant for this exercise...in general the centreshot of a bow is a huge factor in how well it will shoot a variety of spines...my dual shelf bows are pretty finicky when it comes to dynamic spine...the bow I shoot that is least picky about spine is my Hoyt Desperado with a flipper/plunger rest set up...there is a reason FITA shooters use very adjustable arrow rests

DDave

[Bobaru]

DDave, you write: "in general the centreshot of a bow is a huge factor in how well it will shoot a variety of spines."

I don't fully understand what you are saying there.  Could you explain that a little more?

I'm very interested in this thread because I have 4 bows where Stu's calculator is almost spot on.  The 5th bow, my Blacktail Elite, insists on shooting a far, far weaker arrow than Stu suggests.  That bow is cut 3/16" past center, as are a couple other of my bows.

I simply don't understand what is so different about my blacktail bow.  (All my bows are recurves between 62" and 66").

[olddogrib]

Bobaru, DD is pointing out that center shot is probably the single most overlooked/misunderstood factor in determining spine preference and he is correct. Simply stated, the further a bow's riser is cut past center the less paradox(flex)the arrow must undergo to wrap around the riser and continue in a straight path to the target. My riser is also 3/16ths past center, as far as I know this is fairly typical of recurves.  I think most would agree that long bows, due to less structural mass in the riser often are not cut that far. You hear all the time about "XYZ Brand" bows being "so powerful" they require stiffer arrows.  All that tells you if the risers are cut past center is the owner's an ignorant dunce! DD, what I meant by the bow information being irrelevant is that the specs. input would remain the same in all three cases, because my objective is to theoretically get three differently spined arrows to fly well out of the same bow.  For the curious, the chart calculates that the bow should require dynamic spine in the low 70's.  It is what the chart refers to as a generic "performance recurve".  The actual make/model wasn't listed, but I'm drawing about 48# @ 28.5 ". I think the .500's that I mentioned are about 8-9 gpp and shoot 185 fps., respectable but not "earth shattering", so I don't think I'm exaggerating. The chart seems to give more credit than I would for the 6 strand string and the centershot. The arrow that the chart would recommend that I shoot would be a .400 cut 29" w/175 gr. points.  I've tried them, they're stiff, not terribly and I can shoot them if I modify my anchor, but I'm too old to start changing stuff.  And I know the chart has a "form (fudge) factor" field.  I'm not asking why my bow likes a weaker spine than the chart says, there are countless individual form variables that can affect that. I just want to know how many have been able to move up or down a whole spine class with just a 50-75 grain tip weight change?  I'm not sure why the chart limits entry of sideplate info if certain models are selected.  My has a simply calf hair side plate. I measured it with vernier calipers, it's approx. .05" thick, so I entered it...maybe I shouldn't have?

[Bobaru]

Thanks for your input, Richard.   I must say, I still don't fully understand some of the results - as it looks like you also.  I can say that, going from a 500 spine carbon arrow to a 400 spine carbon arrow, my head weight increases approximately 75 grains for the arrow to tune.  So, in that regard, as you write: a whole spine class affects my tip weight by about 75 grains.  At least, that's roughly what I see with my Blacktail.

[9 Shocks]

The spine calculator claimed I could shoot a heritage 150 cut to 28.5 with a 225 grain tip to match my bow. That would be insanely stiff. Bare shafting has got me to a 30 5/8th inch arrow with a 225 grain tip.  I know release plays into it but I feel my arrow is always weaker than what the calculator says it should be.

Originally posted by 9 Shocks:
The spine calculator claimed I could shoot a heritage 150 cut to 28.5 with a 225 grain tip to match my bow. That would be insanely stiff. Bare shafting has got me to a 30 5/8th inch arrow with a 225 grain tip.  I know release plays into it but I feel my arrow is always weaker than what the calculator says it should be.

That's where the Personal Form Factor in Stu's calculator comes into play!

Bisch

[9 Shocks]

So using the personal form factor would just indicate I need a weaker arrow?  I guess I dont really understand the personal form calculator.

[olddogrib]

9, Stu built it in because there are those that have some nuance in their form that the chart doesn't work.  It might be that your unique facial geometry causes you to slightly pull the string outside the center-line of string travel with the limbs with a mouth-corner anchor. The factor is just a tool.  If you know your form is repeatable and your particular arrow flies perfectly for you regardless that the dynamic spine #'s don't match up, you can plug in a +/- form factor to equalize the numbers and it becomes a unique constant for you each time.  It does the same thing as remembering the calculated number you got that wasn't right for you.

[VA Elite]

my arrows come within that 2# mark. with both 125 and 145 heads. I shot both and they both flew the same. But I don't even know what dynamic spine is or static spine, take me with a grain of salt.