Building the wood arrow...Part 5...Dynamic spine

       The next step of our building a wood arrow is knowing where to start for spine for your bow. Dynamic spine is relatively easy to understand if certain principles are adhered to and this is basically how is works.  Again, static spine is the spine of the arrow tested on the spine testing machine.  Dynamic spine is how the arrow flexes around the bow and is influence by several outside factors including point weight, release, bow centershot, etc.

      For these upcoming posts, I'll be using my personal bows and arrows as examples.  Please add or subtract from these poundages and spines relative to your own bows and you'll get close enough with your initial attempts to get some really good arrows and with time, and some tweaking, you'll be a good arrow maker, building quality wood arrows that shoot very well for you and your bow.

     My daily shooter bows are around 57# @ 27".  Because I shoot a dynamic fluid Hill style and allow the bow to move a bit in my hand, I shoot arrows that are dynamically spined at least 10# over my bow weight and can shoot up to 15# - 20# over my bow weight depending on how I build my arrows.  For these discussions I'll keep it at my usual arrow spine ranges.

    An arrow that is purchased commercially is spined and sorted and sold in a 6# spine group. If I buy for my bows of 57# @ 27", this would make my initial spine range in the 55-60# group.  However, because I know I shoot a fluid dynamic style I start out with 65-70# shafts.  These are spined for 28", which is the normal setup for wood arrows and to which most, if not all, wood arrow suppliers adhere.  This means my arrows are spined 65-70# at 28".  Here's how to figure dynamic spines:

     For every inch of arrow that is longer than 28" (including the amount of arrow that is inside the head via the taper) you will weaken the shaft by 5# of spine range (dynamic). For every inch of arrow that is shorter than 28" (including the shaft in the head) you will stiffen the shaft by 5# of spine range (dynamic).  For every 25 grains of point weight you add in excess of 125 grains, you will weaken the shaft by 5# (dynamic) spine and for every 25 grains of point weight under 125 grains you will stiffen the shaft by 5# (dynamic) spine.  For typical Hill style longbows, using a low-stretch string material will usually cause the need for 5# stiffer spine arrows than when using B-50 or B-55 string material.  (However, I use thick 16-18 strand B-50 and with those strings I don't need to vary my arrow spines compared to using low-stretch strings).  I also don't use string silencers and for those who do, this can alter the arrow spine range needed. This will have to be tweaked based on personal preference, but my recommendation is to dispense with the string silencers on a Hill style longbow.  They are quiet shooters and don't need them.  A dynamic pull through release will increase the need for stiffer arrow spine and a static dead hand release will need softer spined arrows.

     That's it.  Here's how it works.

      I shoot 57# at 27".  I cut my broadhead arrows at 28" total arrow shaft length.  They are 68#-70# in my grouping.  With the broadhead taper, my back of point is 27 1/4".  I use 125-135 grain broadheads. This is my basic setup and will shoot the same arrows from 3 different bows.    If I was to use a 150 grain head, that would reduce my dynamic spine by 5# and I'd have to increase my initial arrow spine range by 5# to 73-75# to reach the same dynamic spine range as my basic setup.  If I was to lengthen my arrows to 29" total arrow (28 1/4" bop) that would weaken my arrow by 5#, the same as if I use a heavier head.  So varying from my basic setup, there is two ways I'd need to use 5# stiffer arrows....one: using a heavier head by 25 grains.  Two: lengthening the arrow by 1".  If I add both of those factors together...the 25 grain heavier head and the 1" longer shaft, I've increased the need for 10# stiffer arrows to shoot well, dynamically.

      This is why it gets so confusing in making wood arrows for new arrow makers when there is so much internet and Youtube talk about using heavy foc arrow broadheads or other points.  Adding a 190 (or 200 grain) head to an arrow will weaken the dynamic spine 15#.  Add the fact that they are using an arrow that is 3" longer than their draw bop plus the 1" of arrow in the head which drops dynamic spine 20#, you can see that they are using arrows that are grossly underspined if the arrow is in the spine range matching their bow weight. To be able to shoot an arrow this underspined, they must...MUST...shoot with a stiff bow arm, tight grip and low stretch string to try to stiffen up some of the dynamic spine they have lost in the arrow.  And...this is a big And...they have to have a supremely good release.  A weak arrow or one that has a lot of FOC is very picky of a good release. To make this work decently, they must shoot arrows spined very much over the weight of their bow, i.e. a 75-80# shaft from a 40-45# bow. This becomes more critical of perfect shooting form as the heavier weight of the shaft is trying to push the bow aside, yet the extra heavy point weight is making the shaft bend more than necessary. There is no reason to go down that path...

     This scenario doesn't sound very forgiving of an arrow, does it?  The Hill style of longbow, of shooting style and arrow choice is all about forgiveness when shooting under the varied circumstances found in the field while shooting at game.  

     This is why I've harped on getting good shooting form ingrained and understanding good arrow balance.  It makes good arrows easier to build and shoot.   Keep it simple. Know your exact draw length while shooting. Cut those arrows as close to the back of the bow as possible, try for a balanced arrow.

     These are the basic principles of dynamic arrow spine and we'll go into more detail as we start building arrows and with them, learning to...

     Shoot Straight.