draw weight vs. speed
11-14-2011, 05:33 PM
#1
Spike
Thread Starter
Join Date: Oct 2009
Location: Illinois
Posts: 25
draw weight vs. speed
How many fps do you guys think I would gain going from 70# to 80# on lets say the Carbon Element. They say 330 fps. So what would my gain be from 70 to 80? Would I see much or is more for kinetic Energy?
11-14-2011, 06:57 PM
#2
Typical Buck
Join Date: Oct 2010
Location: Pennsylvania
Posts: 872
couldn't tell you how much but i could tell you that its probably not needed for any hunting or target shooting situation. you would probably gain both speed and KE. I think it would be tougher on yourself for what its worth. i think extending your draw length gives you more speed than going with heavier poundage but its pointless to extend draw length because its just harder to be consistent. i dont know any ballistic calcs of the top of my head but if you type in arrow speed calculator on google i'm sure you'll find something. Hope you find what your looking for, i'm sure many others on here will be able to help you too.
11-15-2011, 07:39 AM
#3
Spike
Join Date: Nov 2011
Location: Wisconsin
Posts: 11
I shoot the carbon element my self at 67 pounds and 29" and it shoots plenty fast, never had a deer duck or jump my arrow, even out to 45-50 yards, going up to 80# would just end up tearing up your shoulder and regretting the decission
11-15-2011, 12:52 PM
#4
Nontypical Buck
Join Date: Oct 2009
Location: Kansas
Posts: 3,902
Typically, there won't be much change in speed if you use appropriately weighted arrows for a given draw weight (i.e. 5grains of arrow weight per pound of draw weight).
First off, Hoyt bows are tested according to ATA methods, which is exactly 70lb draw weight, exactly 350grn arrow, and exactly 30" draw length. For shorter draw bows, the max draw length is used (i.e. youth or womens bows), and for lighter draw bows, peak weight is used, with 5grain per pound arrows.
Relatively speaking, to calculate the change in KE when changing draw weights, you can get pretty close just by factoring the known KE number. For example, if the Carbon Element ATA/IBO's at 330fps, that means it will throw a 350grn arrow at 330fps when set at 70lbs.
That's a KE of 84.62ft.lbs. If we simply multiply that by 8/7's, we estimate that the NEW KE for 80lbs will be 96.72ft.lbs (84.62 * 80 / 70 = 96.72). However, when set at 80lbs, the bow will now require a 400grn arrow. If I back calculate the arrow speed from the new KE, I get 330fps (sqrt(96.72*450380/400) = 330).
So I wouldn't necessarily expect any increase in speed, but a fairly significant increase in KE.
Now, that said, bows DO get more efficient as you approach peak weight (or LESS efficient as you turn them down from peak weight). So I do expect that the bow will get slightly faster if you have it on 80lbs vs. 70lbs, but not much, maybe 5% more KE, meaning 2.4% more speed.
On the flip side, using the same arrow weight, say 400grn (minimum arrow weight for 80lb draw), you'll see a huge drag in speed at 70lbs. As mentioned, the bow should push 400grns to 330fps at 80lbs, but at 70lbs, it will only move a 400grn arrow to 308fps (based on 84.62ft.lbs. KE).
First off, Hoyt bows are tested according to ATA methods, which is exactly 70lb draw weight, exactly 350grn arrow, and exactly 30" draw length. For shorter draw bows, the max draw length is used (i.e. youth or womens bows), and for lighter draw bows, peak weight is used, with 5grain per pound arrows.
Relatively speaking, to calculate the change in KE when changing draw weights, you can get pretty close just by factoring the known KE number. For example, if the Carbon Element ATA/IBO's at 330fps, that means it will throw a 350grn arrow at 330fps when set at 70lbs.
That's a KE of 84.62ft.lbs. If we simply multiply that by 8/7's, we estimate that the NEW KE for 80lbs will be 96.72ft.lbs (84.62 * 80 / 70 = 96.72). However, when set at 80lbs, the bow will now require a 400grn arrow. If I back calculate the arrow speed from the new KE, I get 330fps (sqrt(96.72*450380/400) = 330).
So I wouldn't necessarily expect any increase in speed, but a fairly significant increase in KE.
Now, that said, bows DO get more efficient as you approach peak weight (or LESS efficient as you turn them down from peak weight). So I do expect that the bow will get slightly faster if you have it on 80lbs vs. 70lbs, but not much, maybe 5% more KE, meaning 2.4% more speed.
On the flip side, using the same arrow weight, say 400grn (minimum arrow weight for 80lb draw), you'll see a huge drag in speed at 70lbs. As mentioned, the bow should push 400grns to 330fps at 80lbs, but at 70lbs, it will only move a 400grn arrow to 308fps (based on 84.62ft.lbs. KE).
