People, people it is not rocket science. A heavier arrow will always shoot slower than a lighter one right? IBO is based om an arrow weight of five grains of weight per pound of limb draw weight. It does not matter if it is a 70 turned down to 60, it is still a 60lb draw bow. If the arrow is heavier that IBO you add 1 fps for every 3 grains over (actually 3.3 grains over).Originally Posted by bl00dtrail
This whole thing about loosing efficiency is BS. I have read a study that shows that reducing a 70 lb bow to 60 lbs results in less than 1% loss of efficiency and considered "inconsequential".
Read the Tune chart, it clearly states that for a 60lb bow deduct 5 fps. If you do not believe me here is a cut and past from the GT500 tune chart (bold is mine) :
GT500 2008
A-A 34 7/8” +/- .1875 Brace 7 1/8” +/- .125
String 58 7/16” Cable 39 ½”
Rev Cam Cam setting @ Brace from Speed Rating
.160 hole to inside of limb #70 350 grain arrow
#1 – 30” .170 gap 336-346 fps
#2 – 29.5” Even with the Edge 329-339 fps
#3 – 29” .010 under limb 324-334 fps
#4 – 28.5” .035 under limb 319-329 fps
#5 – 28” .050 under limb 314-324 fps
#6 – 27.5” .070 under limb 308-318 fps
#7 – 27” .080 under limb 303-313 fps
Deduct 5 fps from rating for #60 peak
Limb Deflection Poundage of Bow Pivots
81 90 Tall
90 80 Tall
105 70 Tall
123 60 Tall
Speed Buttons Qty Location
Black 5 1st one 4” from axle
Spacers .090 string side +/- .005 .035 on cable side +/-.005
Cable Rod 7” Suppressor Rod with bends 5”
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