Kinetic energy

[Ace12]

Got my bow checked over today, was having time timing issues. Was curious to see how fast it was shooting I’ve never actually tested my setup. And was shocked to see 303fps. For reference I’m shooting a Triax @70lbs and maxima hunters weighing 385 grains. Just curious what some of you guys who have ever measured it were shooting

 

[Mighty Mouse]

I assume you mean that you were shocked by the "low" measured speed compared to the Triax's IBO rating of 343 fps. Possible explanations include:
• Arrow weight: IBO rating is based on a 350 gn arrow. Most bows lose 1 fps per 4-5 gn of arrow weight, so 7-9 fps can be attributed to your arrow weight.
• Draw length: IBO rating is based on a 30" DL. Rule of thumb is 10 fps loss per inch shorter DL.
• Draw weight: IBO rating is at 70 lbs. Rule of thumb is 2 fps loss per lb lower DW. Even if your limb bolts are bottomed out, actual peak DW can vary a few lbs if string/cable lengths are a little out of spec. Have your shop measure your DW with a scale if you're curious.
• Extra string weight: IBO rating is based on a bare string. Peep sight, D-loop, silencers, etc. slow your string down a bit. I don't know a rule of thumb for fps loss per gn of extra string weight, but I think it would be a minor effect compared to those previously mentioned.

Regardless of what the chrono says, your bow delivers plenty of energy to kill whatever you want. I'm personally in the heavy arrow/speed-be-damned camp, so I'd recommend adding 100-200 gn to your arrow depending on what you're hunting, but there's more than one school of thought out there.

 

[Shawn_Guinn]

Momentum matters not kinetic energy. I want a heavy arrow carrying energy through the animal.

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[Btaylor]

The thread about real world hunting speeds is full of the info you are asking about.

 

[5MilesBack]

I've never shot the Triax before, but at 303fps you've got speed to give and could easily add some weight to your arrows.

 

[SummitsCalling]

@Mighty Mouse and @Shawn_Guinn hit the nail on the head imo. I am not expert, however I believe your falling into the trap of speed numbers. A slower fps will also make your bow quieter. No animal is gonna notice the difference between 303fps and 280. Don't fall into the fps trap, go heavy arrow like previously mentioned and add a great broadhead, you'll punch through.

 

[UglyJow]

I've written several posts on this in the "other" archery forum. Your bow at your draw length and draw weight is producing roughly 78 ft·lbf of kinetic energy. That number is a characteristic of your bow, meaning that it won't change very much when you change arrows, regardless of what the archery calculators tell you. Now that you know you have more than enough KE, you need to build an arrow that uses that KE to maximum lethal effect. The more massive the arrow, the more difficult it is to slow down; its best to build as heavy an arrow as you are comfortable with as this will increase your likelihood of getting passthroughs even on poorly placed shots.

The only reason that velocity should concern you is if you are afraid of making mistakes in range estimation and if you are unhappy with the arc of your arrow flight. It is not a concern when it come to penetration. Lots of energy and a massive arrow are the two prime factors that will increase arrow lethality.

If you are willing to shoot 270 fps, which I think most on rokslide would agree is plenty of arrow speed, you can up your arrow to 480 grains. That's a 25% increase in arrow mass for only an 11% decrease in velocity. Again, your energy will be at a minimum the same (it may very slightly go up, in fact). This will be a great arrow for whitetail and a good arrow for elk, though I would personally add another 70–100 grain on top of that for an elk arrow (I'd recommend 550–580 grains for an elk arrow with your setup; I'd personally probably run about 680–700 grains for elk).

 

[roadrunner]

KE is irrelevant in archery for killing requirements and p is a feel good number anyway...

 

[UglyJow]

KE is irrelevant in archery for killing requirements and p is a feel good number anyway...

I wish the myth of KE being irrelevant for archery would go away... but p is a feel-good number, for sure.

 

[roadrunner]

I wish the myth of KE being irrelevant for archery would go away... but p is a feel-good number, for sure.

KE in archery is irrelevant. It describes the work the bow does to get an arrow of a certain weight downrange. The by product is the speed the arrow flies. Has nothing to do with the "killability" of the setup...

 

[jmez]

Read some of jow's post on AT on the subject.

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[Blockcaver]

Sure a lot of guys that think you need very heavy arrows to kill North American big game with the bow. Have at ‘em. I am totally happy going about killing everything I hunt with a 65# 29” draw bow with 475 gr arrows. No penetration issues on our largest game with this combo (or ones quite similar)....fixed or mechanical that are razor sharp and flying perfectly.

 

[UglyJow]

KE in archery is irrelevant. It describes the work the bow does to get an arrow of a certain weight downrange. The by product is the speed the arrow flies. Has nothing to do with the "killability" of the setup...

KE quantifies all the work that the arrow can do. That includes penetration. And also any "killability" the arrow has.

KE tells the arrow everything that it could do.

Out of everything KE tells the arrow it could do, mass tells the arrow what it will do.

 

[RosinBag]

@Adam12, your set up is fine for anything you want to kill. It’s all personal preference. Heavy and slow or light and fast, there isn’t any deer or elk that won’t die with a shot in the vitals.

For dozens of years KE was the go to stat for hunters and now it’s all about momentum. Reality is if you can accurately shoot your set up, that is the most important factor in all the equations. Speed has its pros and cons and heavy arrows also have their pros and cons.

 

[5MilesBack]

Sure a lot of guys that think you need very heavy arrows to kill North American big game with the bow. Have at ‘em. I am totally happy going about killing everything I hunt with a 65# 29” draw bow with 475 gr arrows.

At 7.3gpp your arrows are heavier than mine at 6.9gpp for draw weight.

 

[Brandon_SPC]

Sure a lot of guys that think you need very heavy arrows to kill North American big game with the bow. Have at ‘em. I am totally happy going about killing everything I hunt with a 65# 29” draw bow with 475 gr arrows. No penetration issues on our largest game with this combo (or ones quite similar)....fixed or mechanical that are razor sharp and flying perfectly.

I'm a huge heavy arrow advocate but in all honesty I would rather see someone shoot a well put together 475 grain arrow that has amazing arrow flight vs seeing someone shoot a 600+ with okay arrow flight especially if you have confidence in it. Personally me I will go as heavy as I can as long as I am around 270 fps from my (give or take 5 fps).

 

[Keepkickin_19]

I agree with a heavier arrow but also I believe you need to have more FOC(front of center) if you have more weight to the rear of your arrow if you nick a branch, your arrow will deflect with major consequences if you have more weight say in your broadheads the extra weight will pull the arrow through on the same course

 

[roadrunner]

KE quantifies all the work that the arrow can do. That includes penetration. And also any "killability" the arrow has.

KE tells the arrow everything that it could do.

Out of everything KE tells the arrow it could do, mass tells the arrow what it will do.

Absolutely not. A .22 rimfire rifle has more KE but hardly has enough of anything to kill. KE matters for hemorrhagic shockwave ability in ballistics to kill. KE has nothing to do with the slicing and clean cut severing of tissue and arteries. That is all in broadhead design and functionality.

If KE truly mattered, then no youth or small frame person setup would be adequate to bowhunt elk with...

 

[UglyJow]

Absolutely not. A .22 rimfire rifle has more KE but hardly has enough of anything to kill. KE matters for hemorrhagic shockwave ability in ballistics to kill. KE has nothing to do with the slicing and clean cut severing of tissue and arteries. That is all in broadhead design and functionality.

If KE truly mattered, then no youth or small frame person setup would be adequate to bowhunt elk with...

And what is the mass of that .22LR?

Take some time to get educated on the subject. And not from "archery" enthusiasts. Pick up a college text on physics. Be willing to learn. It takes very little energy to produce significant penetration—if the arrow is right.

Yes, I am educated in the subject. Yes, I know what I am talking about.