I found this a couple of years back and have used it for some of my classes. I wanted an opinion from the forum if you believethe formulasare accurate.
I know that chronographs have anerrormargin of + or - 2%.
I just don't want to distribute bad information........
***It seems that the table that I copied from wont paste correctly....please excuse the confusion.
Estimating Arrow Speed
Shoot a 5-arrow group at a spot on a LARGE target at 20 yards (use your 20 yard pin). Mark the center of the group with a pen/marker.
From 40 yards, shoot a 5-arrow group at the same spot USING YOUR 20 YARD PIN. Mark the center of the group with a pen/marker.
Measure the distance between both center marks and refer to the following table for an approximate arrow speed;
Approximate Arrow Speed (fps)
Distance between Groups (inches)
220
28.7
240
24.2
260
20.6
280
17.7
300
15.5
320
13.6
330
12.8
To calculate kinetic energy, weigh arrow in grains. Use approximate arrow speed and follow the following formula;
KE = mass of arrow X velocity (fps) X velocity (fps)/ 450240.
Example; a 400g arrow traveling at 270fps
400 x 270 x 270 = 29160000
2916000 divided by 450240 = 64.76 ft/lbs of KE.
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Mighty Mite VFT, Muzzy ZE Rest, 260g Gold Tip XT' s/Feathers, Magnus Stinger 100g, No Peep with a bored peep, King Cobra sight, limbsavers, Cat Whisker string silencers, Vibracheck stabilizer, String loop, Scott release, 29 inch DL, 62 lb, 283 fps.
It depends a bit on how high your peep (or general line of sight) is above your knocking point. The chart will be independent of weight however...beyond that I cannot comment because I have not tested it and am not feeling motivated enough to run the numbers right now.
I checked this out a couple years ago to see how it compared to actual chrono tests. Sighted in for 20 yards and using that pin to shoot 40 yards my groups averaged about 16" low. This should indicate a speed of just about 294-295 fps. The bow actually chrono'd at 264. Therefore I deduce that the hart is malarky.
As for Kinetic Energy, the only way to calculate this is to know the mass of the arrow and the speed of that arrow at a given distance. Just from practical experience I've learned that for deer sized game any properly spined arrow for any bow of about 50# is enough to do the job---period. I don't need bone splitting energy because that's not how broadheads do their job.
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Martin Silver Star Shooter
UBP Life Member
PSAA Life Member
Worldwide Scuba Diver
Opinions on estimating arrow speed.
