spine----again

[jeffp51]

I keep forgetting, and TBB just isn't very clear to me.  Let me see if I have this even close--standard spine assumes 125 grain tip and a 28 inch arrow. 

1. For every inch of arrow, subtract 5# of spine (assume 5# weaker spine).
2. For every 5 grains of tip weight subtract 1# of spine (heavier tips cause weaker spine/more flex)

Question:  With Arrow length, is it physical length or draw length--if I have a 32" arrow but only draw it 28 inches, do I still assume 4" of weaker spine?

Or am I completely wrong on everything?
And this time, I am going to write it all down in my arrow-making notebook.

[make-n-break]

It's the length of the arrow, not the draw. If you have a bow that draws 45@24".. But you're shooting a 32" arrow, you're going to exert the same inertia forces as someone shooting a 45@30" bow. Although the flex point would be different because the shorter bow would have the paradox at 24", The energy has to travel through the same length of shaft.

I hope I'm explaining it right.

Here's an exaggerated experiment you can do that proves this point. Imagine your hands as the sources of force. One hand is the string exerting forward inertia forces; and the other hand is the point, exerting rearward inertia forces. You can consider your "draw weight" as your maximum arm/chest/back strength used to squeeze your hands together as hard as you can, regardless of what that number might be.

Now take a 3/8 dowel that's 48" long. Put it between your hands like this |=======|   Get it bending a little and squeeze your hands together until it snaps (there will be a lot of flex and it wont take much effort). Then take a 3/8 dowel that's 26" long and perform the same experiment. It will take much more force to get it bending and will be difficult to break. If you wanna go real extreme, do the same experiment with a 3/8 dowel that's 10" long. You won't be able to get it to flex and it will be extremely difficult to break with only forward and rearward force.

Your arms, chest, and back strength stayed the same throughout the experiment, but the dowels became increasingly difficult to bend and break.

In physics, the term used is torque. This is force applied multiplied by distance.

So, using a nice round number, say 100# is your maximum squeezing power...

48" x 100# = you are exerting 4,800 units of force on that shaft
26" x 100# = 2,600 units of force
10" x 100# = 1,000 units of force

All controls stayed the same. The "draw weight" stayed the same. The diameter of the shaft stayed the same. The only variable was the shaft length.

[Pat B]

For each inch over 28"(arrow not draw) you subtract 5# of spine weight. For each inch under 28" add 5# of spine weight.
 For every 5gr of tip weight added over 125 subtract 1# of spine weight and vice versa.

[jeffp51]

Thanks--that physics explanation was what I was thinking too--but well explained.  In TBB the article simply says draw length, and that is what confused me.
Pat, that was exactly what I needed--of course that is just a starting point, because non-center shooting bows will affect spine, as well as a number of other factors.

I feel like I want to make a set of very closely matching spine and weight arrows so that I can separate what is bad shooting technique from what has been bad arrow building technique. 

Anyone bare shaft test arrows with self nocks?

[Danzn Bar]

Anyone bare shaft test arrows with self nocks?

Heck yea all the time.......

[Pat B]

I haven't shot a center shot bow since my compound days back in the 1980.
 Cane and hardwood shoot arrows are easier because the natural taper makes them more weight tollerant

[jeffp51]

When bare shafting do you start stiff and sand down, or do you start light and cut off ends until stiff enough?  Is it different with cane/boo than with shoot shafts? (My wife agreed to go cut dogwood shoots with me for our date tonight- I have it good)

[Danzn Bar]

When I'm using cane or boo I generally don't have a choice, I just deal with the shaft I have, some are stiff some are weak.  But most of the time I try to start out with a long (weak) shaft a cut it about 1/2" at a time to suit.  I don't sand or reduce diameter of river cane but will do it some with Tonkin cane (boo).  I don't have any experience with shoot shafts, but I have some arrow vibirum drying and would think reducing the diameter would do the trick with shoot shafts...I'll find out in a few months.
DBar

[DC]

When bare shafting do you start stiff and sand down, or do you start light and cut off ends until stiff enough?  Is it different with cane/boo than with shoot shafts? (My wife agreed to go cut dogwood shoots with me for our date tonight- I have it good)

I start stiff and sand them down until I've gone too far then I start cutting the ends off until it's too stiff and then I increase the tip weight til it's right. With luck

[jeffp51]

something isn't adding up.  I shoot a 32" arrow.  by the formula given above, that would mean subtracting 20# of spine--or having a measured spine of 75#  for a 57# bow.  But my best shooting arrows measure 53# and 59#  with tips that are about 130 grain.  The bow isn't center shot, which means the arrow needs a lighter spine--say 5#?

So the bow needs (in theory)  maybe a 52# spine at 28" arrow.  But my best shooter is 59# - 20 (for length) -1(for tip weight)= like 38# spine.  That seems a long way off.  Am I missing something or just overthinking it?

[DC]

I'm no expert by any means but I've found the one of the biggest factors is the thickness of the handle(arrow pass). You can have two virtually identical bows except for handle thickness(width) and the fat handle will take a drastically lighter arrow. String alignment can mimic this. The fibreglass recurve guys have an adjustable button to change this and it takes very little to dial in the spine.

[Pat B]

Generally with selfbows you will want to start with arrows spined 10# lighter than the bows draw weight. With center shot bows the spine can be a few pounds stiffer spine than the bow's draw weight.  My Treadway Longbow(FG), Mike recommended 65# spined arrows for my 55# draw weight and they shoot great.

[make-n-break]

It gets more complicated when you're matching arrows. In the beginning you were asking what happens to spine when the shaft is longer or shorter, but now you've gotta factor in arrow pass width, etc.

I really think the point of paradox varies greatly between each bow tune/arrow match. I had the same thing Pat B described : my 68# self bow with a 1.5" wide handle actually needed 75# spine to get good flight. I don't think it's a golden rule that the spine must be lighter than the draw weight for other-than-centershot. I don't know the science but I'm guessing the 60-65# shafts just weren't getting the paradox at the right spot and were bouncing off the arrow pass.

You're gonna have to test arrows that are spined below your draw weight as well as above your draw weight.

[JoJoDapyro]

Have you built a spine tester? I think mine cost about $17 all said and done. A digital caliper from Harbor Freight, a 3/8 wood dowel, a chunk of 2x4 a clamp, and a weight.

[DC]

I made this one from scraps from the shop and a picture from the internet. Total cost nada. grain scale optional at extra cost