Crossbow Hunting: Arrow & Broadhead Tuning
Written by: Kim Lockhart, Copyright.
1. Three Blade Broadhead Alignment: Aligning broadhead blades with the arrow fletching alows air to flow over the entire arrow evenly. There are several companies that make aluminum, steel, nylon and rubber washers or spacers that will allow you to align the fletching with a good three blade broadhead. These will also aid in setting your (F.O.C.) correctly by fine tuning your weight.
When looking down the arrow shaft from the nock end your three blade broadhead and fletching should look like the below diagram.
2. Concentricity: The wobble test as it is known through out the world of archery is spinning the arrow in v-blocks to see if the broadhead is in true alignment with the arrow shaft and then visually make a judgement by eye only, whether it is concentric or not, to the best of my machining knowledge the human eye can not discern the difference of a few thousandths of an inch in measuring what is known as runout.
It is my belief that this is where most accuracy issues arise with bowhunters in tuning their broadheads.
The tool to the left is the RCBS Case Master gauging tool used in ammunition reloading, it is a very useful tool for tuning arrows as well.
By placing the arrow in the v-blocks you simply lower the run-out gauge down to make contact with broadhead ferrule where it is attached to the arrow, spin the arrow and observe the run out gauge for the run-out and make adjustments accordingly by loosening and retightening the broadhead until it is in perfect alignment with the arrow shaft.
3. Front Of Center: Adusting (F.O.C.) or "front of center" weight distribution of the assembled arrow with broadhead installed. There is no perfect F.O.C. for each individual arrow and broadhead setup. Common F.O.C. for hunting and field archery is 10% - 15%.
The balance of an arrow can be modified by adding weight or reducing weight to the front of the arrow as needed.
In order for an arrow to fly correctly, the center of mass or (balance point) must be located somewhere between the tip and the middle of the arrow of its entire assembled length in other words its entire length from the tip of the broadhead to the end of the nock or center of its total (Over All Length).
If the center of mass or (balance point) is located close to the tip, the arrow will have good stability but will drop quicker and have less penetration because of the heavy nose.
If the center of mass or (balance point) is located close to the center of the shaft, the arrow will have good range and good penetraton, but arrow flight may be unstable.
How To Calculate The F.O.C. Of An Arrow:
1st: Measure the full length of your arrow from tip to nock, write this number down. Divide this measurement by 2, this is the exact center of your arrow. Make a mark on your arrow.
(Example) 21 3/16" convert fraction to decimal divide the 16 into the 3 = .187 so the overall length is 21.187" now divide this by 2 = 10.593" place a small mark on your arrow at this measurement with a marker.
2nd: Now balance your arrow on a pencil or whatever you have and measure from your balance point back to your mark that is center of your arrow. Now take this number and multiply by 100. Write this number down.
(Example) 2" x 100 = 200
3rd: Now take your second number and divide by your arrows total over all length number = % F.O.C.
(Example) 200 divided by 21.187 = 9.439% F.O.C.