Originally Posted by
Damon
Can you or someone explain how to work the formula? Being a "layman" and not knowing how to get my FPS I guess would pretty much stop the equation right away? I just pulled the "Birth Certificate" of my bow and it says 296 FPS w/ a 350grain @ a 29" draw. I am shooting a GT expidition hunter 5575 with fixed Thunderheads, I BELIEVE 150grain ( cant find the package),28" draw @ 55lbs Bowtech Black Ice. Am I mis-matched? The arrow seems to have good flight and I get good grouping. Thanks Damon
Your bow says 296fps w/ a 350 grain arrow @ a 29" draw. I am not sure what this is "statement" is.
Most companies advertise a bow's IBO speed. IBO speed is the speed your bow shoots an arrow at with the dl set to 30", dw set to 70#'s, and 350 grain arrow (5 grains per # of dw).
The most accurate way to determine how fast your bow is shooting is to shoot it thru a chronograph. But there are some tools on-line that can get you a pretty close estimation by suppling information about your setup.
http://www.backcountrybowhunting.com/articles/tools.php
The above is a quick and easy tool to use. Put your bow's IBO speed in, your dl, your dw, arrow weight, and the weight of anything you have your string (loop, peep). The above tool will give you an estimated speed and ke number. While it is not 100%, it is normally very close. You can double check what the tool is telling you by running the numbers yourself.
Get your bow's speed by shooting thru a chrono and weigh your arrows on a grain scale. Now you have all the information you need to calculate your KE and Momentum. Simply plug those numbers into the below forumulas.
KE - weight(in grains) X velocity(in fps) X Velocity(in fps) / 450,240.
Momentum - weight(in grains) X velocity(in fps) / 225400
Follow the below steps to calculate the your front-of-center (FOC) % for your arrow setup. For fixed blades, I personally like to be around 12-13%
1. Measure the arrow shaft from tip of broadhead to the end of the nock. Divide it by 2 to find the center point (ie. 30" shaft divided by 2 = 15)
2. Mark center point with a felt tip marker. Now balance the shaft.
3. Mark the balance point with a felt tip marker. Divide the distance between the two marks by the overall length of the arrow then multiply by 100. (ie. distance between two marks of 30" shaft is 2.5" then 2.5 divided by 30 times 100= 8.33%)
I do want to add, that while knowing all this information is pretty cool, that any of the newer bows on the market are capable of producing more than enough KE and Momentum to hunt most anything in North America if they are tuned properly.
A properly tuned bow is the single most important factor. This includes ensuring you are shooting an arrow that is spined correctly for your setup and there are several tools out there (Archer's Advantage, TAP, OT2) that are very good with helping with this. While properly tuning a setup can be time consuming, it the only way to ensure you are getting all the performace you can get out your setup. Not to mention, what else are we going to do in the off-season other than playing with our bows?