The way in which I determine the line between light and heavy is purely mathematical. The element I use to determine the line is Momentum. I am not interested in Kinetic Energy or speed when I draw the line to separate light and heavy arrows. To determine which arrow will work best out of a particular setup one needs to:
1. Shoot some arrows of different weights through a chronograph and keep track of the speed and arrow weights.
2. Calculate the momentum for each of the arrows used and plot it on an “Arrow weight vs (Momentum x 100)” polynomial graph.
3.Calculate the Kinetic Energy for each of the arrows used and plot it on an “Arrow weight vs Kinetic Energy” polynomial graph.
4. Subtract 10% of the maximum momentum to determine what the separation between light and heavy arrows will be. separation between light and heavy arrows will be.
As an example, I have done some tests with 2 bows: A Bear Assault set at 70# with a 29” Draw Length and an IBO of 328ft/s and a Bowtech Insanity CPX set at 80# and 28.5” Draw Length with an IBO of 355ft/s. The table below indicates the data recorded for the test arrows from the Bear Assault:
By using the data in the table above one can plot the graph below. Subtracting 10% off the momentum on the graph leaves us at 60 on the vertical axis and the red line and an arrow weight of 475gr. For this particular bow setup a heavy arrow will be any arrow 475 grain and heavier.
The second bow I have tested is a Bowtech Insanity CPX with the below test data collected:
Using the same approach as with the Bear Assault the below graph can be plotted. I have not plotted any heavier arrows since this bow interestingly seams to get better in both KE and Momentum as the arrows get heavier. Taking the available data collected 10% on the graph will be at 75 on the red line and vertical axis. The corresponding value on the arrow weight axis through interpolation comes to 647 grain.
It is important to note:
• Momentum and arrow stability goes hand in hand and can be mathematically proven. The higher the momentum the more stable the arrow will be.
• Kinetic energy in an arrow is in my personal opinion not worth much, which is the reason why I use momentum as a guideline to determine optimum arrow weight.
• No arrow is fast enough to prevent a stringjump on a wary animal, but one needs to consider arrow speed when hunting to some extent. My personal lower limit for arrow speed is 240ft/s out of my own compound bow setup.
• One can shoot an arrow that is too heavy for a particular bow where the momentum will start to decrease due to the lack of speed on the arrow. It is therefore important to select an arrow on the incline of the curve.
• This method can be used for any type of bow (traditional or compound) at any draw weight and draw length.