KE question and penetration

[DIY]

When I run some KE calculations I get the following scenario: 540 grain arrow @ 267 fps gives me 85.5 KE. If I were to reduce the 40 grains and shoot a 500 grain arrow @ 280 fps it gives me 87.06 KE. My assumption is that a heavier arrow will give me KE and penetrate better, sort of confused on this. According to this the 500 grain is a better choice? Looking for some direction on this if possible. Thanks in advance

 

[jmez]

Are your numbers coming from a calculator or actual chrono numbers from your bow?

If it is a calculator they likely aren't accurate. Heavier arrows are more efficient and absorb more energy from the bow, they also retain speed better downrange than lighter arrows. The calculators don't factor this in.

You should also look at the momentum, it is higher with your heavier arrow. IMO that is a better number to maximize vs KE.

Heavier arrow should penetrate better but when you hit a certain point, robbing speed changes the numbers and they begin to go the other way. Velocity is squared in the KE equation so small changes in speed, either way, have a large effect on the final number.

 

[DIY]

this are speeds from a chrono, then I typed in the specs into a KE calc.

 

[Rizzy]

Makes sense, you get more acceleration with a lighter projectile.

 

[larryschwartz]

You get more acceleration with a lighter projectile is true, but....once you hit your target the penetration is based on the momentum of the arrow. It is a tradeoff and we each need to decide which is more important to us. Do you want or need a faster arrow to minimize the affect of errors in range estimation, OR, do can you live with being an inch or so off but getting better penetration.

 

[DIY]

You get more acceleration with a lighter projectile is true, but....once you hit your target the penetration is based on the momentum of the arrow. It is a tradeoff and we each need to decide which is more important to us. Do you want or need a faster arrow to minimize the affect of errors in range estimation, OR, do can you live with being an inch or so off but getting better penetration.

That's where im at now.. wonder which is better. Im elk hunting and my range is 70 yard max

 

[MattB]

Typically, the heavier the projectile, the higher the KE (within the "reasonable" range), as the arrow captures more of the energy imparted by the bow. It sounds like your bow is of a design that is more efficient with lighter arrows. KE is also a poor predictor of penetration, so don't get too wrapped up in it.

Stepping back, both arrows will be fine for anything in North America, so spend the time and mental energy elsewhere because there will be very little practical difference (trajectory, penetration) between the two in the field.

 

[jmez]

Since your numbers are accurate you operating right around peak efficiency for the bow at 500grns. Beyond that the arrow just has more weight and doesn't take up any more energy. I would opt for the heavier arrow. I tend to be a momentum is more important type rather than KE.

Out of curiosity what kind of bow is it? I think some guys on AT did some testing a few years back with the Bowtech CPXL and that particular bow actually lost efficiency as arrow weights went up for some reason.

 

[larryschwartz]

If you are taking shots out to 70 yards then I hope that you use a rangefinder for every shot, which will make your range estimation a non-issue. Also, at that distance your arrow has a lot of time to bleed energy out so I would suggest you go with more mass. With more mass you get a number of benefits. First, the arrow will absorb more of the energy that they bow transfers to it. Second, it will loose less energy on its downrange trip. Third, the extra mass will help to "hold" the energy and give you better penetration.

 

[DIY]

PSE Freak 31.5 inch draw

 

[Screamrider]

Momentum is what gets you penetration, based on the following equation: Momentum = Mass * Velocity.

KE heavily favors velocity by squaring it in the equation: KE= 1/2 * mass * velocity squared

Your heavier arrow at 267 fps will give you more momentum, and more penetration, even though it has less KE than the lighter faster arrow.

 

[Jeff Martin]

When I run some KE calculations I get the following scenario: 540 grain arrow @ 267 fps gives me 85.5 KE. If I were to reduce the 40 grains and shoot a 500 grain arrow @ 280 fps it gives me 87.06 KE. My assumption is that a heavier arrow will give me KE and penetrate better, sort of confused on this. According to this the 500 grain is a better choice? Looking for some direction on this if possible. Thanks in advance

This make sense as KE is based on Mass and Velocity. People always assume if one keeps adding weight you achieve more KE. The reality is that it is a somewhat inverse relationship (their is a sweet spot) but based on your example, you are talking less than a 2% difference. I would not get caught up in <2%.

I applaud you going with the heavy arrow but you have MORE than enough for any NA big game animal. I would say from here, go with what is performing the best out of your bow. I would focus my efforts elsewhere. I purposely did not go into the momentum stuff because it was covered well above.

 

[5MilesBack]

I shoot a 500gr arrow at 285fps and don't have any issues with penetration.......unless of course I hit the wrong part of the shoulder.

I'd go the 500gr route. I too have 540gr+ arrows but I rarely ever shoot them. I'm glad to see someone post some real world numbers for the Freak. I have one sitting here that I haven't set up yet, but figured I'd get around 290fps with my 500gr arrows at 32 1/2" draw. With your 280fps at 31 1/2" draw and the same weight arrow......the 290 checks out.

I really like shooting 280-290 with BH's.

 

[tipsntails7]

When I run some KE calculations I get the following scenario: 540 grain arrow @ 267 fps gives me 85.5 KE. If I were to reduce the 40 grains and shoot a 500 grain arrow @ 280 fps it gives me 87.06 KE. My assumption is that a heavier arrow will give me KE and penetrate better, sort of confused on this. According to this the 500 grain is a better choice? Looking for some direction on this if possible. Thanks in advance

KE is not a terribly good metric to use for penetration. technically KE is a non directional force. Momentum is really what you want to use as a penetration metric.

In an easier terminology, rifles kill with KE, they apply shock (energy) to the vitals they do not rely solely on hemorrhaging like arrows do which momentum provides. Neither are the be all end all but KE is used WAY to much in my opinion an has become a marketing tool more then anything.

Honesty 500,540, either one will work fine I doubt you could tell the difference. So whichever shoots better or you have more confidence in.

Your broadhead and the angle of the blades also plays a role in your penetration so keep that in mind as well

 

[charvey9]

Keep in mind that speed and mass are both variables in the KE equation. Generally accepted is that increased weight adds KE, which is true to a point. However, decreasing speed also decreases KE. So, when applied to the shooting of a bow, the two varibles act against eachother. You can only increase weight to the point, before the loss in speed starts negatively effecting the KE.

I'm not a physicst, but in playing with the KE numbers, speed has a greater impact on KE than weight of the arrow in common setups and ranges of weight and speed we are dealing with in the modern compound.

You could do a lot of math and figure out what the optimal KE for your setup is, but it is just one number, and as others have said does not neccessarily equal better penetration.

 

[MattB]

This make sense as KE is based on Mass and Velocity. People always assume if one keeps adding weight you achieve more KE. The reality is that it is a somewhat inverse relationship (their is a sweet spot) but based on your example, you are talking less than a 2% difference. I would not get caught up in <2%.

In almost every instance that I have tested (multiple bows/manufacturers in arrows up to ~600 grs.) the more arrow weight, the more KE. There is certainly a point beyond which adding more weight decreases KE, but I personally have not achieved that in the weight range I have tested. I don't make this comment to be argumentative, but more to clarify around the inverse relationship comment. There is such a relationship beyond a certain arrow weight, but I haven't found that with the set-ups I have tested with arrows up to ~600 grains. The OP did with his, but I haven't with a number of Darton and Hoyt models over the years.

It is the case that some cam systems are more efficient with arrows in different weight ranges (heavy or light in more generalized terms).

Probably most salient was your point about the difference in KE between the two arrows being ~2% - it isn't material or worth worrying about.

 

[Screamrider]

Keep in mind that speed and mass are both variables in the KE equation. Generally accepted is that increased weight adds KE, which is true to a point. However, decreasing speed also decreases KE. So, when applied to the shooting of a bow, the two varibles act against eachother. You can only increase weight to the point, before the loss in speed starts negatively effecting the KE.

I'm not a physicst, but in playing with the KE numbers, speed has a greater impact on KE than weight of the arrow in common setups and ranges of weight and speed we are dealing with in the modern compound.

You could do a lot of math and figure out what the optimal KE for your setup is, but it is just one number, and as others have said does not neccessarily equal better penetration.

I wouldn't say "generally accepted" that increased weight increases KE, it's in the formula below and is actual fact. And you're 100% correct that velocity has more impact on KE, as velocity is squared in the KE equation. This is why KE isn't the best metric for hunting, as it favors velocity heavily over arrow mass. Momentum is far more appropriate, especially when penetration is a concern (which it always should be for hunting).

Momentum = Mass * Velocity.

KE= 1/2 * mass * velocity squared

 

[charvey9]

I wouldn't say "generally accepted" that increased weight increases KE, it's in the formula below and is actual fact. And you're 100% correct that velocity has more impact on KE, as velocity is squared in the KE equation. This is why KE isn't the best metric for hunting, as it favors velocity heavily over arrow mass. Momentum is far more appropriate, especially when penetration is a concern (which it always should be for hunting).

Momentum = Mass * Velocity.

KE= 1/2 * mass * velocity squared

I used that term because it is a fact, but only to a point. Taking it to extremes, increasing arrow weight to the point your velocity drops to zero will not increase your KE. Only point I was trying to make was that there is a limit to how much weight you can add before seeing a decrease in KE. Yet, we "generalize" that more weight will yield higher KE.

I think they are both good indicators, but wouldn't build a bow/arrow setup based on results of either equation by themselves....and still believe that overall speed is an important critera as well for hunting.

 

[Jeff Martin]

Help us, we have fallen to the KE discussion, and we cant get up !

 

[Rolo]

this are speeds from a chrono, then I typed in the specs into a KE calc.

What were the differences between the 2 different shafts other than weight? I'm guessing there more differences than weight, and the bow was not tuned to the different arrows. The heavier arrow should have more KE and MO assuming the bow was tuned to it, and all else was equal.