Would You Sacrifice Speed for a Slight Bump in Kinetic Energy?

[jstanton007]

I was looking a different arrow results from a few configurations I fletched up for a buddy's bow as an experiment. I was a little surprised by the results as I would have thought the difference in KE would have been greater given a 54gr weight difference in the two arrow setups and only a 13fps difference in speed.

Bow Setup: Mathews Phase 4, 28.5" draw length, and 65lb draw weight.

Arrow #1: Easton Axis 5mm, 75gr steel half-out, AAE Max 3 Fletch, valley of nock to end of half-out 28.25". Total arrow weight 476gr, including 100gr field point, is the actual weight measured. Clocked the arrow at 258fps and using various KE calculators I get about 70.37lbs of kinetic energy.

Arrow #2: Victory RIP V3 5mm, 60gr steel half-out, AAE Hybrid 3 Fletch, valley of nock to end of half-out 28.25". Total arrow weight 422gr, including 100gr field point, is the actual weight measured. Clocked the arrow at 271fps and using various KE calculators I get about 68.83lbs of kinetic energy.

For me I would go with the Victory RIPs for more speed and only slightly lower KE. I don't really focus on FOC too much and both are about the same FOC sitting around 14.6-14.8%. I actually balanced the arrow and measured it to attain these FOC numbers. I would rather have the flatter shooting faster arrow if I am only giving up 1.54lbs of KE. In hunting situations and judging distances on the fly the faster arrow is more forgiving. Using a drop calculator I found online there is over an 11" difference in drop at 70 yards between these two setups. My buddy is hung up on the most kinetic energy possible, due to shoulder reasons he cannot shoot more than 65lbs, but I think it's not enough difference to really matter on KE. A few more details are this setup is an all around arrow setup for whitetail, mule deer, and elk for which he will be shooting fixed blade Day Six Evo broadheads.

What says the Rokslide braintrust?

 

[Mighty Mouse]

KE shouldn't be a factor whatsoever in choosing an arrow weight. For all practical purposes, KE can be treated as a constant. KE does rise slightly with increasing arrow weight, but the difference is so slight as to be meaningless.

Can't really go wrong with either of those arrows. Practice with both to see what the real world trajectory difference is. Then choose whichever he shoots better and/or has more confidence in.

 

[Billy Goat]

How were you generating the speed numbers?


In my experience with those bows, it should be more like 18 fps different, not 13 tho 5 fps isn't much.



My numbers show the #1 as correct, the #2 arrow would be 276 fps, and closer to my choice.

Personally I haven't liked getting below 280, just because of what I'm use to shooting.


I'd ditch the halfouts, otherwise pick one.

 

[jstanton007]

KE shouldn't be a factor whatsoever in choosing an arrow weight. For all practical purposes, KE can be treated as a constant. KE does rise slightly with increasing arrow weight, but the difference is so slight as to be meaningless.

Can't really go wrong with either of those arrows. Practice with both to see what the real world trajectory difference is. Then choose whichever he shoots better and/or has more confidence in.

Wouldn't KE drop with increased weight since speed would also drop? KE would only increase with heavier arrows if speed stayed the same, right? I tried one of the RIPs with a 125gr field point and got 263fps and 68.67lbs KE. Admittedly with either configuration and different point weights, KE only changed a few pounds either way.

 

[jstanton007]

How were you generating the speed numbers?


In my experience with those bows, it should be more like 18 fps different, not 13 tho 5 fps isn't much.



My numbers show the #1 as correct, the #2 arrow would be 276 fps, and closer to my choice.

Personally I haven't liked getting below 280, just because of what I'm use to shooting.


I'd ditch the halfouts, otherwise pick one.

Initially using a Labradar at my local bow shop and then confirmed them at home with a Garmin Xero C1. Shot each arrow 5 times and averaged the results.

Why ditch the half-outs? I hear some people complain about them but I have had good success personally.

 

[Wilderlife]

For me, a lot of it comes down to pin gaps and a trajectory you can live with.

The common thing you will hear people like Ranch Fairy say is "you need to pick an arrow that's actually going to penetrate when plan B happens, and work back from there."

The other side of the argument will say "if your arrows are going too slow, you'll have way more potential errors when the animal moves and you might miss your spot."

It's also worth keeping in mind that a 50# recurve or longbow with a 500gn arrow going about 170fps (fairly conservative numbers) produces 32.05KE, and anyone who knows anything about trad hunting would say a 50# bow and a 500gn arrow is a recipe for success. So, significantly less energy and trajectory than either of your setups rom your compound.

 

[Billy Goat]

Initially using a Labradar at my local bow shop and then confirmed them at home with a Garmin Xero C1. Shot each arrow 5 times and averaged the results.

Why ditch the half-outs? I hear some people complain about them but I have had good success personally.

They bend, giving you issues with BH flight.


I'm still thinking something off with the speeds, KE wouldn't change that much, and the speed should be pretty linear in that range of 3grs/fps. Or a little off on weights.

 

[jstanton007]

Speeds on the Labradar were within +/- 1fps of my measured vales at home with the Garmin. Weights were measured on my RCBS charge master supreme after the 15 minute warm up and calibration. Are you saying for every 3gr of weight it changes 1fps?

 

[roadrunner]

KE is irrelevant. Don't waste your time by going down those rabbit holes.

 

[Zac]

I couldn’t even finish reading your post. I can’t even believe this much thought goes into an arrow. I just choose a shaft I want to try. Cut it, try to get around 150 in the front and throw 3 Blazers on the back. I don’t ever chrono it, figure out FOC, KE, or any other asinine number. Mental masturbation this is.

 

[Mighty Mouse]

Wouldn't KE drop with increased weight since speed would also drop? KE would only increase with heavier arrows if speed stayed the same, right? I tried one of the RIPs with a 125gr field point and got 263fps and 68.67lbs KE. Admittedly with either configuration and different point weights, KE only changed a few pounds either way.

Speed does drop as arrow weight increases, but KE rises slightly.

A bow is an energy conversion machine; it takes the work performed by the archer while drawing the bow (aka, potential energy) and transfers it to the arrow during the shot in the form of kinetic energy. The amount of potential energy (PE) is a function of draw weight, draw length, brace height, cam profile, and let-off percentage, and PE remains fixed as long as those parameters aren't changed. With modern compound bows, ≈90% of the PE developed during the draw cycle gets transferred to the arrow. The other 10% is "lost" to friction in moving components, acceleration of the string/cables/cams/limbs and continued movement/vibration of those components after the arrow decouples from the string. Though the amount of PE developed in the bow is fixed, energy losses decrease with increasing arrow weight; thus, a heavier arrow receives more energy from the bow than a lighter arrow. The KE increase is relatively small though, and for practical purposes, KE can be assumed to remain constant. Below are some KE vs. arrow weight datasets I've collected over the years to illustrate the typical trend.

 

[SDHNTR]

Accuracy first, always.
Then momentum, to carry the arrow through after impact.
Then speed.
Never KE.

 

[Eastonsgrandpa]

If doing calculations, look at momentum. That’s the energy it has as it impacts and what carries it through what it’s impacting. Speed is good but can be too much when it affects flight. Personally, I’ve settled on a good middle of the road arrow at 435 grains. I get good flight, decent speed, and enough oomph to do the job if I’ve done mine.

 

[Billy Goat]

Are you saying for every 3gr of weight it changes 1fps?

Yes, in that poundage and DW range typically til you get over 600 gr shafts.

 

[jstanton007]

Speed does drop as arrow weight increases, but KE rises slightly.

A bow is an energy conversion machine; it takes the work performed by the archer while drawing the bow (aka, potential energy) and transfers it to the arrow during the shot in the form of kinetic energy. The amount of potential energy (PE) is a function of draw weight, draw length, brace height, cam profile, and let-off percentage, and PE remains fixed as long as those parameters aren't changed. With modern compound bows, ≈90% of the PE developed during the draw cycle gets transferred to the arrow. The other 10% is "lost" to friction in moving components, acceleration of the string/cables/cams/limbs and continued movement/vibration of those components after the arrow decouples from the string. Though the amount of PE developed in the bow is fixed, energy losses decrease with increasing arrow weight; thus, a heavier arrow receives more energy from the bow than a lighter arrow. The KE increase is relatively small though, and for practical purposes, KE can be assumed to remain constant. Below are some KE vs. arrow weight datasets I've collected over the years to illustrate the typical trend.
View attachment 749593
View attachment 749592

Thank you for the detailed explanation, I like diving into the science and math behind things like this. Even if it is an irrelevant factor I still like understanding the dynamic characteristics of arrow and flight!

 

[jstanton007]

Accuracy first, always.
Then momentum, to carry the arrow through after impact.
Then speed.
Never KE.

Like I said originally, I don't really focus on FOC or KE for that matter. I look at properly spined, accuracy, and speed. If I can get a fast moving arrow with 150-175gr up front to hit where I want, I feel good about that setup! My buddy on the other hand has been hung up on KE and maximizing energy. With what I have learned here now, I can better educate him and get him to focus more on accuracy and momentum rather than KE.

 

[jstanton007]

Yes, in that poundage and DW range typically til you get over 600 gr shafts.

I will chrono and weigh them again out of curiosity.

 

[jstanton007]

If doing calculations, look at momentum. That’s the energy it has as it impacts and what carries it through what it’s impacting. Speed is good but can be too much when it affects flight. Personally, I’ve settled on a good middle of the road arrow at 435 grains. I get good flight, decent speed, and enough oomph to do the job if I’ve done mine.

Using the calculator at N1 Outdoors, it looks like the RIP at 271fps and 422gr should give me about 0.507 slugs of momentum while the Axis at 258fps and 476gr would give me 0.545 slugs of momentum. I'm not sure what the scale is on momentum and if this is a significant difference?

 

[jstanton007]

@Mighty Mouse @Billy Goat
I guess the better question, looking at momentum now, is which arrow setup would you lean toward assuming both are equally accurate? Also would the RIP benefit from a 125gr head up from bumping the arrow weight up to 447? I will need to chrono this configuration though to get a speed.

 

[5MilesBack]

Everything is a tradeoff to some degree with archery. Choose your priorities and then build an arrow to accomplish that. I try to keep my arrows between 280-295fps, but I have a long draw length so that's pretty easy even with heavier arrows.