Re: why you need high poundage on your bow
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: flounderv2</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: dogmessiah</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: flounderv2</div><div class="ubbcode-body">Actually with a bow, youre wrong. Velocity with a rifle yes, due to the wound channel created by the shockwave. With a bow its all about KE (Kinetic Energy) i.e. Energy delivered on target is directly proportional to the arrows ability to pass through hide/meat/bone. There is no wound channel created by arrows other than the cutting surface of the broadhead.
There are a lot of archery KE calculators out there, check em out. </div></div>
You are wrong too. KE goes up as the square of velocity,
KE=mv^2, with a bow what you really need is penetration, which you get from momentum, p=mv, so holding either mass or velocity constant and increasing the other will increase momentum. The problem with shooting real fast bows is that you normally drop arrow weight to get the higher speed. I shoot heavy (slow) arrows with a heavy pull weight and have blown through both shoulder blades of deer. My arrow would of penetrated on that shot.
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I'm a little confused what both of you are arguing towards here but based on a some calculations that I did, which were then backed up with some arrow calculators the KE changes very little for a given bow. With my bow it was as only about 5-8% change from 300gr to 540gr arrow weights.
The amount of energy a bow imparts is dictated by the limbs and draw length. If you change nothing but the arrow weight and leave the limbs/draw weight set, there's very little change in KE when you change arrow weights.
The limb flexture gives you the force, the draw length gives you the distance.
Work = F*D, Change neither, you can't get more work from the system.
Sounds a little wrong, but when you actually test a couple different arrows through a chrono, no other changes to the bow, they come out with small % differences in KE. Here's the data on my 2008 Commander at 29" with 60# limbs. I'm currently shooting 70# limbs backed to about 64-65#.
300gr - 61.2FPE
360gr - 63.6FPE
420gr - 64.6FPE
480gr - 63.2FPE
540gr - 60.6FPE
There's a thread on here from October that I started when I shot my first deer with a bow. Neck shot with an OLD Mathews QD-32, pulls 54# @ 27" and she dropped like a rock when it hit the spine.
The next one I shot was my own bow, a 64/65# @ 29" draw commander.
Pass through with lung and aorta, 2 broken ribs from 42yd away.
I'm more of a "heavy @ medium speed" kind of shooter because I know it gives me good solid punch but there are LOTS of deer killed with light, medium velocity arrows from young archers shooting 40# bows.
I don't need 90# of draw weight because even at 40yd I get pass-throughs. Sitting in a stand gets cold and stiff like Aaron said, but the animals in PA are a lot smaller than in Skinney's neck of the woods. </div></div>
Josh, You may want to revisit your calcs.. they are off significantly! Guessing you fat fingered something in Excel. But ill use this against you forever going forward.
Using KE=mv^2 with a static 275FPS arrow speed we get:
300gr - 50.3 KE .36p (p=Momentum)
360gr - 60.3 KE .44p
420gr - 70.4 KE .51p
480gr - 80.5 KE .58p
540gr - 92.1 KE .67p </div></div>
You're right that the KE goes up if you fix the speed and raise the mass but that's <span style="font-weight: bold">fundamentally wrong</span> in how the system works. Heavier arrow, lower launch velocity.
The velocity doesn't stay constant when you change arrow weight.
For my bow the weight to velocity relationship is ~3.3gr more arrow weight is 1fps loss.
You leave the bow poundage set.
You leave the draw weight set.
You shoot the 300gr arrow through a chrono, record the velocity.
You shoot a 360gr arrow through a chrono, record the velocity.
Etc.
If you could just jack the arrow weight and get a constant arrow speed then we'd all be shooting 1000gr arrows from 25# bows and killing elephants, but that doesn't work.