RE: KE vs Momentum
 

I've intentionally been avoiding this one, Frank. ;)

Ashby says an arrow with a momentum figure of .57 is adequate for all game up to the size of zebra. Of course, most KE buffs pooh pooh Ashby's arrow lethality report because they're hunting puny, wimpy little whitetail deer, not that tough African game, and say momentum is meaningless for deer.

That's just as wrong as saying KE is meaninghless. You don't have momentum without also having KE. You don't have KE without also having momentum. KE simply tells how much impact energy the arrow has. MOMENTUM is what determines how that energy will be used and how deep that energy will drive the arrow.

To get a 400 gn arrow up to .57, you need to move that sucker along at 321 fps. To get a 700 gn arrow to .57, it only has to travel at 183 fps. The 700 gn arrow would carry 52 ft lbs of energy. The 400 gn arrow would be stroking along with 91.5 ft lbs of energy!

You wanna get a dinky little 350 gn arrow up to .57?? Easy! All you gotta do is fire it at 367 fps and 104 ft lbs.

So, we've got a 350 grain difference in arrow weight, a 52 ft lb difference in energy and a 184 fps difference in arrow speed, all to get exactly the same momentum. Or, said differently, the same penetration potential.

Certainly, you don't need anywhere near the levels of energy and momentum these figures represent to blow a broadhead arrow through a deer's ribcage. On the other hand, hopefully you can see why looking only at speed and KE when choosing an arrow weight for hunting is a big mistake. And why those "minimum KE" charts, for hunting this or that animal, are not to be relied on as the absolute last word.

Momentum formula: Arrow weight in grains X speed/225213.8

225213.8 = 7000 grains (per pound) X 32.1734 (gravitational constant)

That formula should be just as well known as the KE formula.

Now, I have never seen a minimum momentum figure for hunting whitetail deer. I'd hazard a guess and say that .33 would be a fair bottom end number. I know there are guys out there hunting with selfbows and 500 gn arrows at 150 fps that are doing quite well on deer.

Oh, by the way, don't forget that arrow speed, energy and momentum are constantly decreasing as the arrow goes downrange. How much you have at the bow is secondary. It's how much oomph your arrow has when it hits the animal that's important.

Here's a link to Ashby's arrow lethality study. It's a very interesting read, whether you agree with him or not. He covers momentum, KE and mechanical advantage very thoroughly in Part IV.

www.tradgang.com/ashby/

RE: KE vs Momentum
 

By the way... .33 pound seconds... That's assuming a cut on contact type head. Use a punch type tip or a mechanical and I'm sure you'll need more.

RE: KE vs Momentum
 

I would think shaft diameter or friction would play an important role also !

RE: KE vs Momentum
 

I think any friction advantage smaller diameter arrows have is a minor advantage at best, and is way overtalked and overblown. IMO, the lubricity of body fats and fluids would minimize any real advantage smaller arrow diameters would have.

Also, when you start talking smaller diameters, that generally means carbon vs aluminum. That also generally means the smaller diameter arrows are significantly lighter arrows. Significantly lighter arrows carry significantly less momentum. So, the friction advantage of smaller diameters is even more offset by their lower momentum values.

Comes pretty close to being a wash, as far as I'm concerned.

Now, if you weight those skinny arrows up to weighing in about the same as a 2216 to get their energy and momentum values up... Then we can start looking at arrow diameter as more of a major player when it comes to penetration.

RE: KE vs Momentum
 

If you want a lot of momentum without a complete pass thru use a 2514 arrow with a 125 grain head on it
if you want less momentum with a complete pass thru use a speed pro max with a 55 grain head on it.

the point I am trying to make is that you Can have an arrow that carries a lot of momentum but if it has a weak spine the arrow will stop dead because of all the flexing it it does and the friction it produces on impact of its target.

while a arrow with less momentum and a stiffer spine and smallet diamiter will penetrate deeper because momentum is lot lost due to flexing of the shaft .
other thing to consider that the cut diamiter the head produces will allow a smaller diamiter arrow to pass thru much more easily opposed to a larger diamiter shaft.

all this momentum and KE equations work good on paper. but the proof is in testing the arrows aginst each other.
doing a side by side compairison of different brands of arrows you will be shocked to find what actually out penetrates another brand.

just because you shoot a heavy arrow does not always mean you are shooting the most energy efficient arrow.

RE: KE vs Momentum
 

Mike from texas

what I consider an acceptable level is enough to kill the animal ASAP!

RE: KE vs Momentum
 

I have yet to see any slo-mo video of an arrow penetrating an animal's ribcage. Bigpapascout, if you have the visual proof that arrows flex when passing thru an animal's ribcage, I'd really like to see it. Otherwise, I consider that nothing but a myth.

I have seen so called tests that are set up specifically to 'prove' small diameter, light carbon arrows outpenetrate heavy, large diameter arrows. They've all been conducted on artificial mediums that are designed to stop an arrow by friction. I do not find it shocking in the least to learn that a large diameter arrow with lots of surface area is going to penetrate less in that kind of medium than a skinny arrow with little surface area. In fact, anybody with any sense would EXPECT those results. (That's why they 'tested' in that kinda stuff anyway - to produce 'shocking' results to baffle the unwitting and sell the advertisers' products.)

I'd like to see Ashby's study repeated using one specific brand of broadhead and bow as constants, with the variables being point of impact, arrow diameters, materials and arrow weights. With those results, we would be a long way toward blowing all the theory, mythology and outright BS totally out of arrow selection.

RE: KE vs Momentum
 

Art, I assume you know he's in the process of doing a new study, right? I have no idea what the parameters are this time, but I'm pretty sure he's using a wide variety of modern day heads.

I think it's still a few years from completion, so, keep the faith :)

RE: KE vs Momentum
 

Arthur p

I dont have any slow mo videos
but I have shot animals with large diamiter heavy grain flimsy aluminum arrows with a 70 pound bow and the arrow not completley pass thru the animal
I have NEVER experienced this using Stiff carbon arrows, I have always got a complete pass thru with carbons.

just because you dispute the facts changes nothing.

RE: KE vs Momentum
 

Conversely, sir, just because you said it doesn't make it fact.

Did you use different broadheads? Different bow? Did you simply make better shot selections with better placement at more reasonable distances? You leave out way too many details. Maybe you didn't even consider the details, yourself.

Range... I heard he was preparing to do another study in Australia. Didn't know he was underway with it. The man does his lethality testing the right way though. Several years of in depth work on actual game animals vs an hour in the backyard with a foam target. ;)