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Old 03-23-2004 | 05:08 AM
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Mike from Texas's Avatar
Mike from Texas
Nontypical Buck
 
Joined: Feb 2003
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From: Arlington TX USA
Default RE: ? about calculating KE and TRUE FPS

ORIGINAL: JoshKeller

I'll also take a stab at it..

IBO speed is 308.

308 - 10 fps for the ultra nok (aluminum is heavier than a string loop, and makes more noise)

298 fps. - 8 - 10 fps for the kisser and peep.

288 fps. - 5 fps for the whiskers.

283 fps

I, too, will assume xx78's and that you use the uni bushing.

I came up with a 534 grain arrow with 7.4% FOC.

I've generally found that 5 grains = 1 fps.

534 - 350 (ibo) = 184/5 = 37 fps.

283 - 37 = 246 - 250 fps, for a total of approx 70 - 73 ft lbs of KE.

Id estimate the AMO speed of this bow at 230 - 235 fps. Added your 10 lbs of draw weight, I think this is fairly close. I think you'd gain approx 10 fps by loosing the ultranok.
I'm curious as to how you and jmac_or are coming up with such heavy arrow weights. I'm basing mine off of the following:

29" arrow length x 11.67 gpi = 338 grains for arrow shaft only
I used 16 grains for the insert
I used 13 gains for the nock
I used (3) 4" vanes for 27 grains
I used 10 grains for the uni bushing
I used 100 grains for the field tip/broadhead

I come up with a total arrow weight of 504 grains.

For speed deductions:

-4 fps for peep sight.
-6 fps for Ultra Nock (figured about the same deduction as 2 brass nocks)
-6 fps for the whiskers.
-3 fps for the kisser.

I come up with 238 fps based on an IBO rating of 308fps.
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