? Interesting Perspective on Wooden Arrows for Heavy Bows ?

[Benny Nganabbarru]

G'day, and my apologies for a vague topic and question...

In the dusty recesses of the back of my mind, I recall reading something interesting in the way of someone's findings or research into spine requirements for heavy bows. I may be quite wrong in my recollection, and I probably didn't understand it right anyway, but it might've been something to do with a broader range of spines being suitable in terms of wooden arrows the heavier bows get in draw weight.

Again, my apologies if I'm mixed-up, but I just thought I read something along those lines somewhere, and was wondering if anyone knew more of what I'm trying to remember.

Thanks,

Ben

[Al Kidner]

All I can help you with mate is the following link...

http://www.arrowsbykelly.com/Spine_Charts.html

Has worked for me in the past. I've also found that if you bare shaft an alloy, say a 2216 then the charts are a very good starting point.


ak.

[Benny Nganabbarru]

Thanks, Al.

It wasn't a spine chart. It was more someone's hunch / findings / observations about less difference in required spine for, say, 120 and 130 pound bows than for, say, 50 and 60 pound bows. It might've involved a bell curve, too.

Maybe I dreamt it!

Thanks anyway!

[kerrille]

ben your best bet would be to talk to len or gav they have the big poundage bows and make arras for em

chears nev...

[Len]

G'day Ben. What you have probably seen refference to is when you shoot arrows of 3/8 - 1/2 " out of 100lb + bows, spine doesn't matter much. This is more to do with the size of the arrows, if the bow is big enough to shoot them spine isn't so important.If you shoot more normal or modern sized arrows out of big bows then spine would be more of an issue I think.

[Dave_Bell]

First post here, hi all

I think heavy bows will tolerate a wider range of spine, I think the reason is that the difference in deflection when you spine heavy shafts is very small compared to lighter spine shafts. Eg. if you have some 1/2" shafts, some spine at 120 and others spine at 145 the difference in the deflection (when they're put on a spine tester) wont be a lot, compared to something like a 30 spine shaft and 55 spine shaft where the difference in deflection will be pretty significant. I can't remember where I heard this, I believe it was someone talking about high spine bamboo shafts. Hope that helps.

[Benny Nganabbarru]

Thanks, Dave! Welcome to Ozbow. I'm pretty sure that's the thought I was chasing. Thanks again, Ben

[Benny Nganabbarru]

I got this e-mail from Doug Knight at http://www.surewoodshafts.com :

Understanding spine, especially at the upper end of the scale can be a bit of a challange. As you probably know, spine is determined by hanging a 2 lb weight from the center of the shaft while it is being supported 26" apart (for wood; carbon and aluminium use 28"). With this weight hanging from it and bending the shaft, a measurement (in .000"
inch) is taken to see just how much it is bending, otherwise known as deflection. Once the "deflection" value has been acquried the simple math formula of [spine=26"(support)/deflection] calculates it into spine as we know it. ie, a 100 lb spine shaft deflects .26" of an inch with the weight hanging on it. Take a 110 lb shaft and it deflects .24" of an inch, or a difference of .020" for a 10 lb spread. At the lighter end of the scale, ie 20 lb spine has 1.3" deflection while 30 lb spine has .87" deflection or a difference of .43". Both represent a 10 lb spread but the difference of .43" vs .020" is quite a difference.
This simply tells us that spine is not linear. Do the math for each 5 lb spine group and plot them on a graph, then draw a line connecting all the plotted points. You don't end up with a straight line but rather a nice arching curve or parabola with the deflection steadily DECREASING (at some exponetial rate) while the spine steadily INCREASES. Around the 90 lb rate the curve tightens up and heads towards infinity as we continue trying to add more and more poundages of spine into a finite space as the deflection nears zero.
Recall our 20 lb shaft has 1.3" of deflection and our 100 lb shaft is .26". Now think of the fact we only have that .26" left on our scale (till it reaches 0")to crowd all spine poundages clear to infinity. Thus it becomes clear the higher spine (or less deflection) we have in our arrow shaft the closer we approach infinity as to poundage of bow it can be used with. In theory an arrow of 0" deflection could be shot from a bow of infinate poundage, in fact it could be shot from a gun, a cannon or any other means approaching the speed of light(disregarding the fact of course that it would burst into flames and burn up once it reached kindeling temperature due to friction).

As humans we have limitations as to how heavy a bow can be drawn and have devised this method of calculating spine to suit our needs within that range. We have also developed the formulas of adding or subtracting 5 lbs here 5 lbs there for bow types, head weights, string materials etc. etc.
This method of adding and subtracting is linear. It is a great tool, but recall spine is not. Thus it is most applicable and benificial to those in the (average) 45-60 lb bow weight ranges it was developed by and created for. The more we move out of this average upwards towards infinity of bow weight the less rate of effect these issues come into play.

That's what I was trying to recall.

Thanks, Doug!

[longbow steve]

An interesting read Ben . What weight bows are you currently using?
Steve

[Benny Nganabbarru]

Only 80lbs at 28". I draw it a little toward 29".

[longbow steve]

Wow Do you have ambitions to go any higher or are you happy with that weight for your big game needs?
Did you have to custom order the bow?
What game is on your list for this year?
Sorry

[Benny Nganabbarru]

I don't have any ambition to go higher in weight. I'm just learning about shaft selection for my bow, bearing in mind the arrow length and broadhead weight. I'm just trying to make sure I get the right arrows when the new 240 grain glue-on STOS broadhead arrives later this year.

Yep, it was a custom bow; a Schafer Silvertip recurve made by Dave Windauer.

I do hope to get a couple of little sows with it this year! Who knows what the year holds in store?

[longbow steve]

Keep posting your adventures . Steve

[ed]

try a one ended spine test. There was a thread about one on trad gang a couple of years back.

Essentially a wooden bracket attached to a vertical board or wall. The bracket has to have about a 3/4 inch slot for the shaft to go into. Then hang a weight from the other end and measure the deflection over the entire length. The method the tradgangers were using was a 1lb socket spanner.
I have spined all my wood arrows this way ever since and done so for shafts well in excess of 100lbs spine.
Using the full length of the arrow for the deflection gives more bend for the arrows spine that can be measured (twice the deflection and therefore double the accuracy).

It is a simple and easy to make method, just start by putting in some guide marks from arrows of known spine and the rest is extrapolation...

[heavybows]

Hello Ben Yes i made my arrows 7/16 diameter tapered 11/32 and barrelled to 23/64 they spine from 130# to 200#. I shoot them out of my 140# 150# 160# Hill longbows. Marlon

[White Hawk]

Great thread Ben, some really valuable info in here for aspiring big bow users. I will be directing some of our Warbow Australia members to this to have a read. Heavy spined arras is new territory for alot of people and this will certainly help.

good on ya!

Gav