Spine Formula

[riarcher]

Definately.
I've a set of carbons I shoot in 3 bows. Just changing the tip accordingly.

Personally, I feel that gathering a set of known test shafts (once in a lifetime thing) is far superior to any formula. But, that's just me.

[Mechslasher]

thanx for working out the details for me eddie.  been busy for most of the day.  only about half of this formula pertains to shooting parallel wooden shafts.  unfortunately, not much at all to carbons since carbons are designed to be shot through center cut bows.

dana, as to limb shape, the more the tips are in front of the grip, the more force/pound they will impart to the arrow.  to see this, look at a force/draw curve.  to compensate for this, you need to raise the spine.  as to head weight, there is no escaping the physics.  base spine is calculated at 28" shafts with 125gr heads.  the more weight up front, the more force is applied to the shaft to move it.  this has to be balanced to take advantage of archer's paradox.  too much weight up front and not enough spine will make the arrow fly right while recovering.  not enough weight up front and the arrow flies left.

jack, the taper has to be eyeballed.  say if the point rivercane is 3/8" and tapers to 1/4" nock, then subtract 8-10#.  tonkin tapers very little so maybe 3-5#.

i know a lot of people say all this takes some of the fun/mystery out of "primitive archery."  i have enough mystery as to why my arrows only hit were i aim half the time.  i don't need to be wondering if its my arrows or me.  i've also heard that the indians didn't do all of this when they made arrows, but i guarantee they did, just not with numbers.  if you are depending on making a living with arrows, you wouldn't want any mystery involved.  i hope all this help clear up spine calculating.  if ya'll have any more questions, fire away.

[Badger]

Click here: http://heilakka.com/stumiller/Dynamic%20Spine%20Calculator%20Rev%2011-22-09.xls

Here is a link to one of the best spine calculators published so far; I didn't know how to hyper link it so you need to copy and paste. Steve

[Mechslasher]

i thought mine was complicated.

[Badger]

     It does look complicated on the surface but pretty easy once you try it. I was surprised how close it comes out. Steve

[Justin Snyder]

I can see where the confusion is. When you add a heavy point you decrease the effective spine of the arrow. By saying "head weight (add 5# for every 25gr. over 125gr" he is saying that you have to have add 5 pounds to the actual spine of the shaft so the effective spine will come in the same. (I want a spine of 60# but use a heavy point so I must add 5#, using a 65# spine to get the arrow where it needs to be.)

[El Destructo]

                       Gee....I thought I had already read that somewhere else in this post........... ...........

[NTD]

"+/- length (add 5# for every inch over 28" or subtract 5# for every inch under 28") "

Wouldn't it be +/- 26" since we spine test based on 26 inches??

[Justin Snyder]

                       Gee....I thought I had already read that somewhere else in this post........... ...........

That is funny, its not what I read.

[CraigMBeckett]

NTD,

The AMO spine charts are for 28 inch arrows supported at 26 inches, so no it is add or subtract 5lb for every inch different to 28 inches.

Craig.

[Barrage]

Chris, when you're measuring spine on a tapered shaft, do you measure closer to the nock end?  Point end?  Middle?  (does that make sense?)  Or does it not affect the measured spine much?

[DanaM]

Chris, when you're measuring spine on a tapered shaft, do you measure closer to the nock end?  Point end?  Middle?  (does that make sense?)  Or does it not affect the measured spine much?

Good question?

So the consensus is that for every 25g over 125g the point is you add 5lbs to the spine in the formula that is?

I understand that a heavier point reduces spine

[Mechslasher]

B, measure in the middle, like a parallel shaft.  the reason for subtracting is because of the taper when the arrow leaves the bow, it's closer to center than at full draw.  when this was realized this, people started tapering wood shafts to simulate the taper found in cane shafts.

you got it dana.  5# for every 25gr.  my formula is just a good starting point.  i've lost count of how many cane arrows i've made using my formula and so far, all have flew like darts out of the bow they were made for.  of course, they have to be oriented correctly with the stiff side against the strike plate.

[NTD]

Thanks Craig!  And thanks Chris for sharing your formula.  Mods, is there anyway this thread can be moved to "How To's and Build-A-Longs" so we have it for easy future reference?

[DanaM]

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