Dynamic Spine calculations for Wooden Arrows

I've recently bought myself an American Flatbow, a Buck Trail Black Hawk and when I bought it the guy in the shop measured me for some arrows which are okay but I'd like to have a go at making my own and getting a more accurate match to my bow.

I've just come across the Stu Miller Dynamic Spine calculator and this is raising a few questions.

My bow is rated at 35lb (I presume at 28" although it doesn't say). The arrows it was suggested I buy, after my draw was measured, measure 30" from the throat of the nock to the end of the shaft/base of the point, the point adds almost another inch to the arrow. Using a draw length measuring arrow, when I'm at full draw it's at 28.5" at the back of the shelf (which is also the deepest part of the grip) and measuring to the very front of the bow it's exactly 30".

If I add these into the calculator (Bow weight 35lb, rated draw length 28", my draw length 28.5 and center cut 1/8th) it suggests that the dynamic spine I need is 34.7lb.

However, when I use a bow scale to measure the actual draw weight of my bow at my draw length, it's measuring 44lb. If I set that as the bow weight it says that the dynamic spine I need is 44.1lb. That's one hell of a difference!

What is then more confusing is that on all the charts I've seen for wooden arrows, it says that for a Flatbow with a 28" draw with a draw weight of 35lb I would need arrows of 40/45lb but if the draw weight is 44lb then I need arrows of 45/50lb. If you said my draw was 29" then it's 45/50 or 50/55lb respectively.

Coming from a recurve background, a difference in spine calculations of 9.5-20lb is making alarm bells ring. Anyone know which figures I should trust?
 

RMH

Member
Finding the right spine for wooden arrows can be interesting to say the least. I have been making and shooting wooden arrows for quite a while now and the best thing I did was to ditch the dynamic calculators and spine charts. Calculating where to start for spine starts with bow weight at your draw length, forget what is stamped on the bow and measure yourself, my sons black hawk is rated at 45lb @ 28inch, however when measured it was actually 39lb @ 28inch! The best way to measure I have found is to draw an arrow with your fingers as if you were taking a shot and when settled at your anchor point have some one mark or pop a peg on the shaft were it meets the front of the bow. Once you have this then you can attach the bow scale and draw the arrow to the mark/peg again and measure. A given arrow spine on charts etc generally assumes 28inch length with 125g pile, do not include the pile in your arrow length calculations. For every inch your draw length is over 28inch you should add 3-5lb spine and for every inch under 28inch you should subtract the same amount as a starting point. If your draw length is 30inch to the mark and your bow weight at this draw length is 44lb as you say I would look at arrows in the region of 55lb spine. You could always nip to your local archery shed and buy 3 shafts at 50-55 and 3 at 55-60, make them up with 125g piles and try them? Leave them all at full length to start with. If one of the spine range flies weak then cut the shaft down by half inch and try again and so on until they fly as desired. I would resist the temptation to stiffen the arrow with lighter pile to start with, it is by far better to get the correct spine of shaft first and then only fine tune with pile weight (lighter stiffens the arrow and heavier weakens it) hope this helps
 
D

Deleted member 7654

Guest
Short answer...
The only thing you should trust is your observations of how the arrows fly.

The weight of the bow is only a rough guide, as is all this stuff... even if you had a perfect dynamic spine calculator it wouldn't tell you how the arrows would fly. It would only give you a starting point.
The only thing that matters is how they actually fly, and shooting them is the only way to find out.
It's about the acceleration applied to the arrow and this depends of the type of bow not just the poundage (and all the other variables).
E.G A 35# carbon fibre faced AFB will accelerate the arrow much harder than a 35# self wood ELB, and of course the ELB will need the arrow to flex more to get round the grip.
There are so many variables too... just choose some of the variables you want to fix, say fletching size and point weight, then try a few shafts to find what works.
Consistency is the most important thing, which is of course more difficult to obtain with wood than some other materials.
Simple things like getting the grain orientation the same on all the arrows will help.
Del
 
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