KE vs. Momentum

evan williams

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I have always been more of a Kinetic Energy guy so lets hear some arguments for momentum...

KE = [M(V)^2]/450,240 these numbers are used for English measurements of speed in feet per second and mass measured in grains

where....

Momentum = MV

What does everyone use to build their set-ups?
 

RosinBag

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KE....more universal measurement of set ups. Plus I don't know enough about the momentum side. Seems heavy and slow would out do light and fast though. But either way if you put it in the right spot, dead is dead.
 

Nick Muche

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I use a heavy arrow and a 125 grain broadhead. This coupled with a fast shooting bow gets the job done. I personally think there are more important factors to consider when building a good setup than crunching numbers to figure out KE and Momentum. If you have a 400 grain arrow or heavier, you will have more than enough of both to take down any animal in NA. With that said, I like to worry about the correct spine, total arrow weight and accuracy.

Many people get all wrapped up in KE, Momentum, FPS, FOC, etc. etc.... The list goes on and on... Archery and Bowhunting are very simple concepts, so I like to keep it that way througouht the entire process.
 

larryschwartz

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Speed has always been one of the ways bowyers/bow mfrs have used to describe and differentiate their bows from other peoples bows. It is also the key variable in KE. As a result, KE has become the key way that bow mfrs use to say my sh't is better than your sh't. Speed also helps when it comes to range estimation as a faster bow will reduce the amount of elevation error if you misjudge the distance to the target/animal, so it became a popular factor for 3D shooters and folks who like to take shots out past 40 yards. But if you use a rangefinder that should not be as much of an issue.

Momentum has more to do with how much of the energy the arrow stores from the bow when it is shot and how quickly it looses that energy after the shot. A heavier arrow, one with more mass, will store more of the energy from the bow when it is shot and will also loose energy/bleed energy at a slower rate than a light arrow. As a result you will have more energy when the arrow gets to the target/animal with a heavier arrow and will have more energy left to get through bone or other "obstacles" once it gets downrange.

So, the optimum arrow configuration is probably one in the middle and not one on the extremes. You want an arrow that has enough mass (probably around 8-10 grains per pound of draw weight) to store as close to all of the energy from the bow while still flying well while still flying fast enough to help minimize range errors. As Nick mentioned above, the thing to pay the most attention to is getting an arrow that has the right spine/stiffness to fly well from your bow at whatever weigth you shoot. If it is flying sideways when it gets downrange all the KE or Mo in the world won't help you get a passthrough.
 

bz_711

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if talking purely penetration I prefer Momentum...but when talking hunting situation, as long as you use common sense I don't think either matter too much. Most any hunting arrow, out of a decent bow will kill an animal...I would recommend spending the time on tuning that arrow to fly correctly in order to retain all KE and Momentum...and then practice like crazy to become best shot you can be. A 35-40lb bow with sharp broadhead will get the job done...anything more than that just comes down to how good a shot can you place.

Good Luck!
 

Titaniumman

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I think that heavier arrows help in quieting down your bow. I am shooting a 430 gr. arrow @ 278 fps which gives 73.81 pounds of KE. Plenty of smack for anything I will ever hunt in North America.
 

mattstanton

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I shoot the heaviest arrow I can while staying above 280fps. This year it is an 80lb athens afflixtion flinging a 517gr arrow at 290fps. Gets me right around 96lbs KE.
 

Backpack Hunter

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I really don't pay much attention to it. I make sure the arrow is correctly spined and go from there. Current arrow is 400gr but I have been tinkering with a couple in the 425-450gr range just for the fun of it. To be fair the 400gr arrow has taken out everything it has been shot at.
 

OR Archer

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I don't worry about the numbers. I want my arrows to be accurate above all else. I also try and make sure they weigh in to at least 400 gr. Gives me a good balance between speed and arrow weight. Put em where they need to go and youre going to have a dead animal on the ground.
 

Steve O

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I am a momentum guy. Throw a ping pong ball and a golf ball at the same speed at a snow bank...what goes in farther?

Give me a heavy arrow with some decent speed over a screamin ultralight arrow any day. But I shoot thru stuff with 250g broadheads with stickbows!
 

Yukondog

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Two years ago I was writing down numbers, doing equations and tweaking my set up. Then I realized that the only shot that was not a pass thru was a quartering to me buck at 40yds. Arrow ended up going thru his liver after passing thru his front shoulder. I figure my set up is good. 412gr arrow at 295fps. Being able to cut vanes at 60yds with BH's is more important than extra speed or mass. The arrow I shot seems to fly well out of my bow, which I think should be the goal more so than achieving a certain number. I need to be shooting more to get back to that accuracy and confidence. Life has been extra busy and practice has be sacrificed.
 

weaver

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I used to really get into the technical side of archery. Now i just shoot my heavy arrows at slooow speeds and focus on the accuracy part.
 
R

rebecca francis

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I agree that the very most important thing is accuracy, because you can have the greatest set up there is and if you make a bad shot, you lose the animal, or chase him for a long time. However, due to the weight I pull, I have to pay very close attention to the KE and momentum when I am shooting dangerous game that requires a lot of penetration. I did a lot of testing on my own set up so that I could be adequately prepared for my hunt. I read everything I could get my hands on about KE vs. momentum. There are a lot of very good theories out there regarding both of these issues. I agree that I would rather get hit by a fast ping pong ball over a slow golf ball….BUT in my testing all my lighter faster arrows penetrated further than my slower heavier arrows. I was told I needed at LEAST 80 ft. lbs KE for this hunt, which is hard for a woman to achieve. So I added weight, changed broadheads, changed lengths, increased poundage and did everything I could to reach that requirement. The bottom line….I needed penetration and my lighter arrows unequivocally penetrated better than my heavier arrows in every type of target I shot into. I needed my broadhead to reach the vitals and do the damage. If my heavier arrows wouldn't penetrate to the depth of the vitals than I was going to have a very angry animal on my hands. Interestingly enough, it follows the bell curve of the mathematical equation of KE and at the high point of the bell curve just before it drops off, you have the highest amount of velocity. That is also the velocity to weight ratio at which you achieve the greatest penetration.
I shot everything from 300 grain arrows up to 1100 grain arrows. But I found that when I got above 600 grains it started to reduce penetration due to the reduction in velocity.
Everyone has their own idea behind what works….that is just what I found in my personal research.
 

Darin Cooper

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I agree that the very most important thing is accuracy, because you can have the greatest set up there is and if you make a bad shot, you lose the animal, or chase him for a long time. However, due to the weight I pull, I have to pay very close attention to the KE and momentum when I am shooting dangerous game that requires a lot of penetration. I did a lot of testing on my own set up so that I could be adequately prepared for my hunt. I read everything I could get my hands on about KE vs. momentum. There are a lot of very good theories out there regarding both of these issues. I agree that I would rather get hit by a fast ping pong ball over a slow golf ball….BUT in my testing all my lighter faster arrows penetrated further than my slower heavier arrows. I was told I needed at LEAST 80 ft. lbs KE for this hunt, which is hard for a woman to achieve. So I added weight, changed broadheads, changed lengths, increased poundage and did everything I could to reach that requirement. The bottom line….I needed penetration and my lighter arrows unequivocally penetrated better than my heavier arrows in every type of target I shot into. I needed my broadhead to reach the vitals and do the damage. If my heavier arrows wouldn't penetrate to the depth of the vitals than I was going to have a very angry animal on my hands. Interestingly enough, it follows the bell curve of the mathematical equation of KE and at the high point of the bell curve just before it drops off, you have the highest amount of velocity. That is also the velocity to weight ratio at which you achieve the greatest penetration.
I shot everything from 300 grain arrows up to 1100 grain arrows. But I found that when I got above 600 grains it started to reduce penetration due to the reduction in velocity.
Everyone has their own idea behind what works….that is just what I found in my personal research.
That's very interesting Rebecca... I've never experimented with lower draw weight/shorter draw setups and very heavy shafts, but I've never seen KE or Momentum numbers decrease as a result of adding weight to the arrow. In my experience - both increase with mass - always... and frictional forces that stop the arrow in a target decrease with reduced arrow velocity. Your setup seems to defy the laws of physics regardless of which theory you subscribe to.

Were you forced to use different arrow diameters in your testing to achieve different arrow weights? Arrow diameter is a HUGE factor in penetration so could that have skewed your results?

Interested...

Coop
 
R

rebecca francis

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That's very interesting Rebecca... I've never experimented with lower draw weight/shorter draw setups and very heavy shafts, but I've never seen KE or Momentum numbers decrease as a result of adding weight to the arrow. In my experience - both increase with mass - always... and frictional forces that stop the arrow in a target decrease with reduced arrow velocity. Your setup seems to defy the laws of physics regardless of which theory you subscribe to.

Were you forced to use different arrow diameters in your testing to achieve different arrow weights? Arrow diameter is a HUGE factor in penetration so could that have skewed your results?

Interested...

Coop
Yep Coop it makes sense that heavier arrows would have better penetration, but in preparation for my dangerous game hunt, we bought the Alaskan Grizzly sticks along with 315 grain tips and shot them out of my bow and my husbands bow that has more draw weight. We shot for months recording all our results by video and on paper. The bottom line is….when you increase the weight of an arrow... there is going to be a point at which the velocity begins to decrease, which in turn decreases KE. You can see this when you run the KE values.

With a 60lb bow shooting a 300 grain arrow, IBO is 330, draw length 28 in. on the KE calculator the resulting KE is 60ft. lbs and speed is 299 ft/sec.

In turn, same bow, same set up but 400 grain arrow the resulting KE is 64 ft. lbs KE and 269 ft/sec

425 grains is 65 ft. lbs. KE and 262 ft/sec

430 grains is 65 ft. lbs. and 260 ft/sec

435 grains is 65 ft. lbs. and 259 ft/sec

440 grains is 65 ft. lbs. and 257 ft/sec

445 grains is 65 ft. lbs. and 256 ft/sec

500 = 63 ft. lbs. KE and 239 ft/sec

600 grains = 58 ft. lbs. KE and 209 ft/sec

700 grains = 50 ft lbs. KE and 179 ft/sec

So referring to the bell curve….there is a sweet spot where it plateaus and reaches the optimum KE and speed before it starts to drop off. In this set up it would be between 414 grains and 448 grains. The KE is at the plateau of 65 ft. lbs. with 414 grain arrow shooting 265 ft/sec and 448 grain arrow shooting 255 ft/sec.

We found with our penetration tests that penetration mirrored this mathematical bell curve.

You can use these same calculations to set up a bell curve with your bow.

I would be happy to send you the Alaskan grizzly sticks with the 315 grain broadhead for you to test and see what you think (send me a PM with your address). We were told that this was the way to go for the best penetration. But that is not what we found. The results were not what we expected either, but we watched it happen shooting in to various targets. I am still testing MANY different arrows, broadheads, weights, etc..because I am determined to find the set up that will provide me with the most KE and velocity that will give me the most penetration. This is crucial for me because I cannot pull 80 or 90 lbs. I have to test all of this. I do not want to go hunt dangerous game and not be completely prepared.

So it may seem that my set up defies the law of physics….but try it out yourself.

As far as the arrow diameter. I am using VAP arrows because of the small diameter. The small diameter decreases drag upon penetration, and they ALWAYS penetrated better for me. The grizzly sticks have the same idea because it is tapered front to back, but it is a much larger diameter which will increase the drag.

I am currently working on a very in depth article to show what I found. Everyone is still going to have their own theory and that is how it should be. But this is what I found not only by physically testing it, but backing it up with the KE calculations.

This subject causes a lot of debate among archers, and the way I feel about it is…..you need to find the set up that performs the best for you based on what you are looking for. My normal bow set up is very different than my dangerous game set up. It may be speed, it may be KE, it may be penetration.

Its a fun subject! I would love to hear what other people have found in their testing. But as Nick said, archery is a very simple concept and most people really don't need to worry about these details. ;)
 

BuckSnort

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Rebecca, I also found that more times than not the lighter faster arrows penetrate more than slower heavier ones...Back in the mid 90's when bows were just starting to get fast I noticed this... The newer bows shooting the newer lighter carbon arrows were out penetrating the heavier older slower aluminum arrows..

Kinda like how a 22-250 will penetrate more mild steel than a 30-06
 
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