Because most bow manufacturers base IBO ratings at 70# I wondered if a 60# bow have a lower IBO even though your still shooting an arrow at 5 grains per pound?
I though I read somewhere here on AT that a 60# bow would IBO a little less than a 70# rating because of the energy difference?
IBO is 5 grains per pound. if you maintain the 5gpp ratio between the limbs, you theoretically shouldnt lose anything. reality doesnt always play by the same rules.
manufacturers use the 30"/70lb parameters because it looks better in numbers......and numbers sell bows.
My XLR8 lost 1 -2 fps going from 70# to 60# as the poundage went down, and I maintained 5gpp.
I have a thread here somewhere where I show the draw weight, arrow weight, and chrono reading.
Over on the Mathews forum it was explained to me that as the poundage is lowered, the dl gets longer, and the brace lowers, accounting for the low loss in speed.
I didn't do that part of the experiment, but I didn't notice the measurement changes by feel at the time.
After I found my comfortable DW, I tweaked it a little, and found another 4 fps anyway...
When you decrease poundage by backing the limb bolts out, you will increase draw length and also INCREASE brace height. They will both increase because the limb tips move back as you back the bolts out which also moves the string back. If your brace height decreased your draw length would shorten. I would think the increase in brace and draw would cancel out any speed you'd gain but alot of it has to do with cam timing and rotation. You can sometimes over or under rotate cams for increased speed.
To the OP, if you shoot the same arrow at 70 and 60 lbs, the 60 lb draw will be about 20 fps slower. I got the following from Backcountry Bowhunting:
For every inch of draw length you go down you will lose 10 FPS
For every 5# draw weight you go down you will lose 10 FPS
For every 3 grains of weight your arrow is over IBO ( 5 grains per pound) you will lose 1 FPS
For every 3 grains of weight you add to your string you will lose 1 FPS.
Hmm interesting. It does make since though that it takes less force to move a lighter object the same speed. I need to find me some 275gr TW arrows for 3D
FSUBIGMAC said:
Well i know when I changed out the limbs on my old Marquis, at 72# 360gr it was shooting 328, at 61# 305gr it shot 331.
there is going to be some drop in efficiency,your moving the same parts on the bow with less draw weight so something gets lost there,however due to the increase in dl and brace you loose less of it.if dl and brace stayed constant like say going from 70lb max limbs to 60lb max limbs you should loose more fps due to the brace and dl staying the same.i think kevin strother told me 6-10 fps if i remember correctly.
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