Increase in Arrow weight, drop in velocity - Do you think the KE/Momentum/Penetration power equal out in the end?

[ndbuck09]

I'm in the process of testing some new arrow weights. I have shot a black eagle x impact at 425 grains for a few years, which according to software for archers based on my sight gap, was at 290 fps. I have now bumped up my arrow weight to 485 (arrow is shorter now to make up for the increased insert/outsert/point weight and it shoots a 1.5 fixed head at 60 with field points). I shot in for 30 and 60 and my new fps calculation is 267. Now the KE for the 425 arrow is 79.42 and for the 485 arrow its 75.7. Momentum for 425 arrow is .547 and for the 485 arrow its .57.

It's really got me pondering....I just can't see how guys running these realllly heavy arrows at 500 plus grains are really getting better killing arrows....???

What are your perspectives? Am I too much into the data and not the real world on this?

 

[Billy Goat]

I'm not into the heavy arrows. I'm playing right now with a heavy arrow setup. It's a 460 grain arrow from a 63# bow. I feel like it's similar to 525 out of my 72# bow but in one that I can shoot more without fatigue. Issue I'm dealing with is the trajectory. At 65 yards I was hitting branches on my course that I had never paid attention to. Several elk I wouldn't have gotten if I didn't have a flat trajectory setup. Makes shooting in deadfalls a lot easier I feel like. I don't need a tall window. I'm planning on using this heavy setup on whitetail around home this year. Going to stick to my 415 grain arrow at around 300 fps for elk at this point. So far I haven't had a penetration issue. I have seen lots of sub 400 grain arrows zip through elk.
Everyone has different experiences tho.

Shoot what you have confidence in, and confidence comes from shooting, a lot.

 

[Trial153]

Who ever entered KE into the picture for killing animals with a bow didnt do the sport any favors. It's a pretty worthless number for practical applications.
Momentum isnt much better truthfully. As an indicator for performance, because honesty the number doenst translate well to what you will see in perfomace..


Personally i feel you need a pretty big gap in weight over a good size sample of game killed to start appreciating how well a heavier arrow performs on the killing end of things.
If you think your going go from a 400 grain arrow to a 450 grain arrow and think you will witness magic your probably mistaking.
However spread that gap out to say a 400 grain arrow to a 550 grain arrow and apply to say a dozen kills of various game with each arrow and in think you will start to draw a different conclusion to the benefits and limitations of the heavier or lighter set up.

 

[*zap*]

Flatter trajectory is also a big consideration.

 

[elkstabber]

Do KE/Momenum/Penetration equal out in the end?

Here is an analogy. You have a nail that you want to drive into wood. This is a penetration test. You have two tools, a 1/4 lb kids hammer and a 2 lb hammer. The kids hammer doesn't weigh much so you can swing it much faster. The 2 lb hammer is much heavier and you can't swing it nearly as fast. If you've ever tried to use a kids hammer I'm sure you will agree that the heavier hammer drives the nail easier.

Either hammer could be used. The kids hammer swings faster and you could probably hit the nail 2-3 times for every time that the 2 lb hammer hit the nail. But, the 2 lb hammer will get the job done faster.

Its interesting to note that the kids hammer, which is much faster, probably has more KE and the 2 lb hammer has more momentum.

In the end momentum is more important than KE for penetration (within normal bowhunting equipment ranges).

You may disagree with my analogy and you can bring up physics formulas if you want. I've taken lots of physics classes too. But the real work experience of many bowhunters over a long period of time have shown that momentum is more important than KE for penetration.

But its important to be reasonable. You want your arrow to penetrate through the animal, but if it still penetrates 24" into the ground then you wasted a lot. You don't need a 900 grain arrow to shoot through a whitetail (but you might for a Cape Buffalo). You want your arrow to penetrate through the animal, but not go much further. Choosing a broadhead with more/wider blades will reduce penetration. This is why I use 3 and 4 blade designs on deer and have used 2 blade designs on bigger game.

 

[5MilesBack]

What are your perspectives? Am I too much into the data and not the real world on this?

"Real world" will show that going up in arrow weight and tuning to that new arrow will always give you more KE and MO than a lighter arrow. That's been proven over and over with actual testing. The problem is......people use archery calculators that don't take bow efficiencies into account as arrow weight increases, and they put out bad numbers. But there's a reason why very large game hunters use really heavy arrows........they will absolutely out penetrate lighter arrows out of the same bow.

The reality for North American game is that keeping it within reason will suffice on normal non-heavy bone hits.

 

[Woodrow F Call]

The energy equation puts a premium on the velocity as if you do the calculation, velocity is squared while mass is not. It's a good tool to compare cartridge performance, but it's not perfect. I think John Linebaugh said something along the lines of "Velocity is a constantly diminishing variable, mass is not."

Both have their pros and cons, both are important, in the end you want enough penetration to wound the vitals..... I'd think that if you were going after tougher game, you'd want a heavier arrow.... if you needed to shoot longer ranges, you'd probably want the velocity.

In the end it's finding the right tool for you specific job. Both arrows would have their place, you are going to have to determine which one is right for your hunting.


Edit to add:

K.E. = 1/2 mv^2

Velocity is squared.

 

[2Stamp]

I have a short draw length so I've never shot "fast". I'm very comfortable with the arrow trajectory at 250-265ish. I went heavier because I didn't like the results I was getting on Muleys with a lighter set up. Do I believe you need a 650gr arrow for deer? No. The proponents of that are planning for a worst case scenario (hitting a shoulder for example). Shot placement is still key, but a heavier arrow may give you an extra penetration advantage on a marginal shot. There is a reason trad shooters go heavy. With lighter poundage, they need more momentum. And they usually blow through animals. What I think is important is a sharp cut on contact broadhead, a well tuned bow and arrow, and really good arrow flight. You can have a sharp as hell broadhead, but it the arrow isn't hitting the animal straight, it's not going to penetrate as well.

There is another advantage to heavier arrows. It can make your bow quieter. I've noticed it, my friends have noticed it, on my bows and theirs The heaviest I've shot is a 540gr arrow, currently my arrows are at 502. I noticed that when my arrows hit 500gr and over, my bow was quieter. Way it was explained to me is a heavier arrow helps the bow work more efficiently. Whether you call BS or not, I noticed a difference.

 

[justin davis]

I like heavier arrows for a few reasons. They are quieter and they penetrate and hit harder.

 

[Topo_trekker]

On broadside shots, absolutely not.

Guys really up arrow weight so they can take quartering shots on elk more confidently. You’re almost doubling the distance arrow has to penetrate. You want to be able to take advantage of opportunities ethically. I don’t think I would take a quartering away shot with a 425 arrow on elk. If your just a couple inches off at 30yrds and hit the hay bail that is their gut bag that arrow will not reach the vitals and little to no blood. I’ve seen it happen even with fixed blade. Now if youre Gillingham and can hit the spot every time, go for it shooter. Up it to 550, elk will be down in 50yrds.

The heavier arrow will hold its speed and momentum for longer upon penetration.



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[Wrench]

In the broad scheme of things 30 grains isn't anything.

If you really want a deadly arrow that passes through, go with as heavy of arrow as you can make work for your shot distances that you can tune as well as possible and top it with a cut on contact head. Push a sharp COC head through a critter and a sharp non COC head....you'll wonder why they even make non COC heads.

 

[willfrye027]

Physics equations are almost always oversimplified so that they can make intuitive sense. They take into effect all kinds of “assumptions” to make it clean like that. As someone mentioned above bow efficiency is obviously not factored in anywhere, although if you took actual chrono velocities that would control for it.

In the end the principles are still valid but “killing” is way way more complicated than your arrow weight and broadhead. I would take accurate over anything else...maybe quiet as a close second

 

[Beendare]

Your KE numbers are wrong. Simple physics....somethings not right.

KE and MO continues to increase when you up your arrow weight. Its simple, the heavier arrow absorbs more of your bows energy...and as Justin mentioned- its turning the excess energy exhibiting itself as noise and putting that energy into your arrow.

FYI, your trajectory isn't going to change much. Set your sight to the heavier arrow...then shoot one of your old arrows at 40 yds. It will be maybe 1" higher is all...

...

 

[dkime]

I'll add to the pile here, but start with the short answer for those who want to duck out early. A slower heavier arrow (from my testing) aids in accuracy at known distances. The largest benefit you will see by going to your heavier setup is accuracy with fixed heads and forgiveness. Also, ignore KE because it's not a vector.

Deeper dive, I don't think you're getting caught up in the data, I just don't think you have enough of it. Things that most people don't like to discuss are drag and impulse. Because of our super small niche of a hobby, there's not going to be much data available for the builds we like to speculate over. So when we start with your "faster" arrow we're talking about an arrow that is faster by 8%. What does that gain you? Well according to your numbers "gains" you a negative 4% on your momentum calculation. Real world trajectory, it may gain you a few inches per yard after 50yds. This is something that I think we as archers and bowhunters need to do a better job of discussing when it comes to arrow selection. However, I think this will eventually prove to be a wash anyways because with an increased initial velocity also comes an increase in drag. (Fd=1/2pu^2CDA) Drag increases as velocity increases. What's interesting about this formula is it takes into consideration the surface area of the projectile with a fixed Drag coefficient, I say interesting because our arrows are far from having a fixed coefficient. We've all seen the paradox that arrow shafts experience, creating a dynamic coefficient at random distances until the shaft is stabilized. While surface area remains constant, the shaft is wiggling and writhing it's way through the air and bleeding energy due to it's shitty flight. So we rely on vanes to correct this, an increased velocity can create a larger distortion in these vanes (Check out the firenock vanes, Dorge covers this pretty well) As the vanes distort and compress the moment (torque) between centerline of the shaft and tip of the vane decreases, thus lowering angular velocity, thus lowering spin rate, thus lowering stability. Arrow stability (in my opinion because no one gives a shit enough in the scientific community to actually study this lol) has more to do with penetration than either of the two formula's you we're considering when we are talking real world applicability. A stable arrow is delivering all of it's energy through the centerline of the shaft, which is why MOST people shoot 3 fletch because they unknowingly do not have competing forces on opposite sides of the shaft. Putting all the speculation aside, everyone (whether it makes you feel good or not) should recognize that a heavier arrow (with all variables being equal) will always out penetrate lighter arrows.

Skipping over impulse, but a heavier arrow will allow for more energy to be conserved. Longer lock time through the bow, and more resistant to exterior forces.

So let's come full circle on this, what is the real world POI shift you are seeing between the two arrows? A 460gr arrow with a velocity of 290 will impact 1" higher at 20 and 2" higher at 40 when compared to a 512gr arrow at 272 for my setup. When I take into consideration my shooting ability at 40, the two arrow groups will have a 1" overlap most days which makes this comparison a wash in my mind.

Is the 4% increase in momentum going to save your ass on an animal? Doubt it

Is the extra 8% in initial velocity going to limit your yardage errors? Doubt it

Heavier arrows at slower velocities are more resistant to forces and seem to stabilize faster. (There's a reason why most people claim fixed blade heads tune "better" at lower velocities)

Thanks for letting me rant

 

[Mighty Mouse]

"Real world" will show that going up in arrow weight and tuning to that new arrow will always give you more KE and MO than a lighter arrow. That's been proven over and over with actual testing.

My testing agrees with what 5MilesBack and others have stated. With every setup I've chrono'd, KE and momentum have increased with increasing arrow weight. Attached is KE data calculated from chrono'd speeds of arrows weighing 375 to 500 gn. The data shows a consistent gain of 0.02 to 0.03 ft-lbs of KE per grain of arrow weight for that particular bow (Mathews Heli-m at 70 lb draw weight with draw length at 26.5" and 28").

 

[ozyclint]

dkime brings up very valid points on the drag involved and it's effect.

if you plotted momentum values of a bow arrow setup on a bell curve where you had a given bow and kept increasing arrow weight the momentum value would increase with arrow mass until the peak of the curve was reached at which point momentum values would begin to fall again. however, consider two arrows with the SAME momentum (the same point either side of the peak of the curve) one must be lighter and have more velocity the other more mass and less velocity. the slower arrow will penetrate more because it encounters less drag as a result of it's lower velocity. resistance increases as a square to the velocity. double the speed= quadruple the resistance.

a plane like the SR-71 blackbird could fly at speeds of 2000mph. but it had to do it at 80000ft where the atmosphere is so thin that the drag was low enough to permit such speed

 

[MattB]

I've tested a bunch if bows and KE has always gone up with arrow weight as a heavier arrow absorbs marginally.more of the bow's energy. I too question your results.

 

[Piscatory_4]

I'll add to the pile here, but start with the short answer for those who want to duck out early. A slower heavier arrow (from my testing) aids in accuracy at known distances. The largest benefit you will see by going to your heavier setup is accuracy with fixed heads and forgiveness. Also, ignore KE because it's not a vector.

Deeper dive, I don't think you're getting caught up in the data, I just don't think you have enough of it. Things that most people don't like to discuss are drag and impulse. Because of our super small niche of a hobby, there's not going to be much data available for the builds we like to speculate over. So when we start with your "faster" arrow we're talking about an arrow that is faster by 8%. What does that gain you? Well according to your numbers "gains" you a negative 4% on your momentum calculation. Real world trajectory, it may gain you a few inches per yard after 50yds. This is something that I think we as archers and bowhunters need to do a better job of discussing when it comes to arrow selection. However, I think this will eventually prove to be a wash anyways because with an increased initial velocity also comes an increase in drag. (Fd=1/2pu^2CDA) Drag increases as velocity increases. What's interesting about this formula is it takes into consideration the surface area of the projectile with a fixed Drag coefficient, I say interesting because our arrows are far from having a fixed coefficient. We've all seen the paradox that arrow shafts experience, creating a dynamic coefficient at random distances until the shaft is stabilized. While surface area remains constant, the shaft is wiggling and writhing it's way through the air and bleeding energy due to it's shitty flight. So we rely on vanes to correct this, an increased velocity can create a larger distortion in these vanes (Check out the firenock vanes, Dorge covers this pretty well) As the vanes distort and compress the moment (torque) between centerline of the shaft and tip of the vane decreases, thus lowering angular velocity, thus lowering spin rate, thus lowering stability. Arrow stability (in my opinion because no one gives a shit enough in the scientific community to actually study this lol) has more to do with penetration than either of the two formula's you we're considering when we are talking real world applicability. A stable arrow is delivering all of it's energy through the centerline of the shaft, which is why MOST people shoot 3 fletch because they unknowingly do not have competing forces on opposite sides of the shaft. Putting all the speculation aside, everyone (whether it makes you feel good or not) should recognize that a heavier arrow (with all variables being equal) will always out penetrate lighter arrows.

Skipping over impulse, but a heavier arrow will allow for more energy to be conserved. Longer lock time through the bow, and more resistant to exterior forces.

So let's come full circle on this, what is the real world POI shift you are seeing between the two arrows? A 460gr arrow with a velocity of 290 will impact 1" higher at 20 and 2" higher at 40 when compared to a 512gr arrow at 272 for my setup. When I take into consideration my shooting ability at 40, the two arrow groups will have a 1" overlap most days which makes this comparison a wash in my mind.

Is the 4% increase in momentum going to save your ass on an animal? Doubt it

Is the extra 8% in initial velocity going to limit your yardage errors? Doubt it

Heavier arrows at slower velocities are more resistant to forces and seem to stabilize faster. (There's a reason why most people claim fixed blade heads tune "better" at lower velocities)

Thanks for letting me rant

I think Alaska bow Hunter guy did a test on this with a hooter shooter and various arrow weights with tips from 85 to 200 or something like that. At 30 yds 125 and 150 shot same, 175 dropped a little from there, etc. The arrows seemed to group by 2 weights then make a step change so to speak. It's on YouTube, a few years old, maybe 2011 or so.

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[Brandon_SPC]

Have you actually chrono your bow? Generally the heavier and arrow get the father off these online calculators are. For example my Elite energy 35 at 30" draw and 70lbs 330 ibo.

-Calculator predicts a 525 grain arrow at 265 fps when over the chrono it was 268 fps.
-It also says my 600 grain arrow should do about 240 when shot through a chrono it was 251 fps.

If we look at momentum and KE values:
- 350 grain arrow at 330 fps Momentum is .512 slugs and KE is 84 foot lbs
-525 grain arrow at 268 fps momentum is .610 slugs and KE is 79 foot pounds
- 600 grain arrow at 251 fps momentum is .668 slugs and KE is 83 foot pounds.

As I get heavier my momentum value increases. The only time heavier arrows really matter is if they come into contact with bone even Ashby himself states this. If you look at the 12 penetrating factors this is on the last part of the 12 points until heavy bone is encountered. If arrow flight is great, structural integrity of your arrow set up isn't great it doesn't matter how heavy you go.

I would rather a hunter shoot a 450 grain arrow with decent FOC (15-18%) that has perfect arrow flight and is built well vs a 550 grain arrow with crappy arrow flight that is built terribly. What I generally will preach is go as heavy as you can with the trajectory that you like. It doesn't hurt to have extra weight but with that develop a good arrow set up and tune it well.

https://www.grizzlystik.com/Top-12-Arrow-Penetration-Enhancing-Factors.aspx

 

[5MilesBack]

The only time heavier arrows really matter is if they come into contact with bone even Ashby himself states this.

And yet we see TV hunters all the time shooting an elk broadside with nothing but ribs in the way, and there's 15" of the fletching end of the arrow still hanging out of the bull. I think they could have benefitted from a heavier arrow........or a more efficient BH.........or more draw weight......or a combo of any or all three.