Arrow FOC

[ImBillT]

To clarify my comment of it must be a soft spot was purely sarcasm and meant no low blows to @FoodIsMemories. And as far as the rest of the Ashby stuff wasn’t most of or all his testing done with traditional tackle in mind? Not compounds.

It only matters if it was done with a traditional bow or a compound if he was making the claim that high FOC was necessary to get the job done. All of the advantages of high FOC occur with compound bows. Compound bows can get the job done with much worse arrows due to the increased energy storage that compound bows are capable of. I’m impressed however that hunters have found ways to have an arrow stick halfway out the impact side of a deer even with a compound. All of Ashby’s penetration factors should behave exactly the same when using compounds and improving on those factors should increase penetration even when using compounds. The difference between compounds an trad bows is that a crappy arrow that only penetrates 6” from a trade bow might blow clean through a south TX whitetail from a compound.

 

[ImBillT]

@ImBillT you’ve got a wildly wrinkled brain and I’m intensely intrigued. It’s hard to decipher the bread from the bologna; the shit from the shinola here… I hope I didn’t offend you personally. I look forward to learning from all of ya!!

No offense taken. I was just surprised that you thought I was knocking you. I was doing the opposite.

 

[Ought Six]

From what I can tell, one of the more difficult things about high FOC arrows is good flight in the wind. I’m not sure whether or not it’s possible to get an arrow with really high FOC to fly straight in high winds, especially at low arrow velocity. Im sure a missile guy could answer that question with very little thought required. Even if it’s possible to get a really high FOC arrow to fly properly in high wind, I’m almost 100% certain that it’s easier to get a more balanced arrow to fly properly in the wind. Remember, there is a difference between wind drift and an arrow shaft that is aligned with the direction of flight. I feel like higher FOC should result in reduced wind drift, but worse shaft alignment to flight path. I may be totally wrong. I don’t remember anything from fluids. That said, I’ve heard a podcast or two involving complaints about high FOC and high wind. An arrow shaft that is misaligned with the arrow’s flight path is going to rob a lot of penetration.

Very detailed and in depth explanations. I thoroughly enjoyed reading through it, took me back to my college days. You’ve obviously went through the theory to prove to yourself that spending that extra money to increase FoC in your setup was worth it, I like it.

As far as wind flight, I shoot in the wind all the time, and at 15-20mph, even at 30 yards, my 8% FoC 490 grainers going 295-297 have a noticeable tail. Wind catches the fletchings.

After reading through all that theory, etc., would you say that increasing FoC on a setup that is already heavier and faster than a majority of all hunters is worth the money and headache?

 

[Ought Six]

What’s that mean? Lol it was commercial grade core bond I had the insulators at work fill for me in a box. Stops a broadhead almost instantly, field points make it a little further. These are with broadheads and you can see it was easily 6-8” deeper when I added an additional 50 gn to the front, W/O adjusting anything else.

But animals aren’t made out of foam, and a heavier, high FOC setup seems to handle the variables of an animal better than medium to light and fast

 

[ImBillT]

All of this assumes a perfectly spined arrow. In an arrow that is spined correctly there would be no discernable difference in penetration of a high FOC setup and an average one

Correct arrow spine does nothing at all to prevent the shaft from flexing on impact.

 

[JDH]

Correct arrow spine does nothing at all to prevent the shaft from flexing on impact.

Isn't that one of Ashby's main things? Getting your arrow under control so it enters a deer straighter so it penetrates better and they don't really know what hits them? I haven't really followed his stuff.

 

[ImBillT]

But animals aren’t made out of foam, and a heavier, high FOC setup seems to handle the variables of an animal better than medium to light and fast

I plan to watch the whole thing, but don’t currently have time. I watched up to 3:36.

First, I’ve heard of Joel’s test before, but could not find anything about it on the internet. Maybe I was spelling his name wrong. Seeing his graph, hearing a bread overview of what he tested and what his results were, and seeing one chart for the 35lbs stick bow, his results A) don’t violate my understanding of physics, and B) aren’t quite what you said, or what I’ve heard other claim.

You basically said(if I’m not mistaken) that going for speed instead of a heavier arrow resulted in better penetration. If that’s not what you meant I apologize. In my replies, I basically said that sacrificing momentum to gain velocity does not increase penetration, or if it does, it probably only does so in commercial foam targets for some strange reason. I have a hard time believe that it does, but I’ve heard the claim before, and I haven’t tested it myself.

Now for some physics, and why Joel’s data(from what I’ve seen of it) doesn’t equal speed being more important than momentum. In my physics classes, and in many others, it is taught that momentum better describes a collision than kinetic energy. When it comes to rifles, kinetic energy got popularized by non-physicists as a means of compared things by people who wanted to be scientific, but they should have been looking at momentum. To this day everyone cares about a cartridge’s energy. Somehow in the archery world, people actually talk about momentum, which it seems like they should. With rifles, lighter faster bullets yield a higher kinetic energy, but a lower momentum. KE=MV^2 Momentum=MV. With a bow, that doesn’t happen. With a bow, decreasing arrow mass does not increase velocity enough to increase KE. Decreasing arrow mass increases velocity a little, but KE and momentum both decrease. Joel’s chart for the 35lb bow shows exactly what physics would expect from a bow. When he increased arrow mass, velocity decreased, but penetration increased. That’s what everyone has always said about bows. If you decrease arrow mass, you’ll decrease penetration. What is interesting about Joel’s findings is that kinetic energy seems to be a better indicator of penetration than momentum, BUT every time he got more penetration he had more momentum. If you decrease arrow mass on the same bow, you’ll loose momentum. Any time you increase arrow mass, using the same bow, you always get a decrease in velocity and an increase in momentum and kinetic energy. Joel’s data doesn’t contradict that. It is interesting that it shows energy as a better predictor of penetration than momentum, because they teach the opposite about collisions in physics classes. They’re not teaching a scientific law when they teach that though. To discover that energy better described a situation in which certain things collide would not violate physics in a any way. What I will be interested to see when I watch the whole video is whether Joel’s data suggests that when changing bows, he can get increased penetration, with increased kinetic energy but DECREASED momentum. It’s not possible to create a situation in which you increase kenetic energy but decrease movement in the same bow, but perhaps by changing bows you could, and if so, I’d like to see what he gets for penetration there.

FOC isn’t mass. FOC is a measure where the mass is. You can shoot a 450gr arrow with 8% FOC and a 450gr arrow with 30% FOC. They both have the same mass. The 30% FOC has been shown to penetrate animals better than the 8% FOC even at the same mass. So there’s no such thing as “speed is better than heavy/high FOC arrows” because you have high FOC without reducing speed at all. That said, increasing arrow mass to gain FOC gives you two penetration improvements. 1) Increased FOC increases penetration, and 2) increases arrow mass(even if you left FOC the same) gives you increased penetration.

 

[ImBillT]

Isn't that one of Ashby's main things? Getting your arrow under control so it enters a deer straighter so it penetrates better and they don't really know what hits them? I haven't really followed his stuff.

Yes it is, but the shaft will still vibrate on impact even with perfect arrow flight. That’s why high FOC improves penetration. High FOC reduces the amount of energy in the rear of the arrow where most of the vibration occurs. That’s an oversimplification, but FOC increases penetration by reducing the percent of the arrows energy than can go someone other than forward.

The guy I quoted was arguing that as long as the arrow flight was proper, then FOC didn’t matter because a properly spined arrow would not flex on impact. That’s completely wrong. A perfectly spined arrow will still flex on impact. It will do so primarily due to the fact that we cannot have perfectly orthogonal impacts with a perfectly planar target very often, BUT, if you had such an impact, the shaft would still vibrate in a compression wave from front to rear, and higher FOC arrows would still do so to a lesser degree.

 

[TheTone]

Isn't that one of Ashby's main things? Getting your arrow under control so it enters a deer straighter so it penetrates better and they don't really know what hits them? I haven't really followed his stuff.

His stuff is based on shooting dead african critters with longbows and recurves.

There are plenty of other people out there that kind of contradict what he holds near and dear. Some of the engineers at companies making bows seem to not really completely buy into it with tests they have done with efficiency of their product. I do think his principle of having durable arrows is great

 

[Ought Six]

You basically said(if I’m not mistaken) that going for speed instead of a heavier arrow resulted in better penetration. If that’s not what you meant I apologize. In my replies, I basically said that sacrificing momentum to gain velocity does not increase penetration, or if it does, it probably only does so in commercial foam targets for some strange reason. I have a hard time believe that it does, but I’ve heard the claim before, and I haven’t tested it myself.

Now for some physics, and why Joel’s data(from what I’ve seen of it) doesn’t equal speed being more important than momentum. In my physics classes, and in many others, it is taught that momentum better describes a collision than kinetic energy. When it comes to rifles, kinetic energy got popularized by non-physicists as a means of compared things by people who wanted to be scientific, but they should have been looking at momentum. To this day everyone cares about a cartridge’s energy. Somehow in the archery world, people actually talk about momentum, which it seems like they should. With rifles, lighter faster bullets yield a higher kinetic energy, but a lower momentum. KE=MV^2 Momentum=MV. With a bow, that doesn’t happen. With a bow, decreasing arrow mass does not increase velocity enough to increase KE. Decreasing arrow mass increases velocity a little, but KE and momentum both decrease.

I agree that momentum is probably more important than KE as the mass of the arrow stays constant while the velocity decreases, so a higher mass setup will maintain higher momentum while slowing upon impact.

Velocity is squared in the KE formula while mass is constant, however, so increasing velocity increases your KE at an exponential rate. Velocity is very important to KE.

My original comment to @FoodIsMemories was from anecdotal evidence on various YouTube videos where guys were shooting different setups at foam and ballistics gel. There are a lot of issues with those tests, but I was more than anything playing Devil’s advocate on the speed versus weight stuff.

I have a background in physics, engineering, and calculus as well, but those classes were a long time ago and I haven’t put as much thought into it as you. I like your points a lot, as I too like to put math and meaning to things.

In the end, as Aron Snyder says, pick an arrow, buy 3 dozen, get off social media, and go shoot your bow… a well placed shot will mean more than anything. This has been a good discussion though I like hearing everyone’s point of view and I like tinkering

 

[Ought Six]

But, let’s get back to the purpose of the post. I’m not worried about penetration one bit with my setup. Will FOC lead to a more forgiving setup when shooting fixed blade broadheads? I’m not sure that can be proven with anything other than anecdotal evidence and me experimenting with my setup.

Will I see an increase in broadhead forgiviness adding 50-75 grains upfront, and decreasing my speed from 297 to 280’s? The guy at my archery shop says he likes to be in the 290’s for speed, and I won’t see a noticeable difference increasing weight to decrease speed to try to create a more forgiving setup

 

[ImBillT]

Increasing FOC will give your fletching a longer steering arm. So maybe.

 

[tjones]

His stuff is based on shooting dead african critters with longbows and recurves.

There are plenty of other people out there that kind of contradict what he holds near and dear. Some of the engineers at companies making bows seem to not really completely buy into it with tests they have done with efficiency of their product. I do think his principle of having durable arrows is great

This.

I have been bowhunting since 1982 and one thing I learned is fretting over equipment is a waste of time if you aren't a good enough hunter to get close enough to get a shot. Lots of guys agonize over gear and then rarely get a shot at a critter.

 

[Howie]

The fellow who did extensive research on this subject, I believe he is a Dr. Ashby, has a ton of deep dive info. Google it. It can get dry but it is informative. He did most research in Africa on Big big game but it all is relative.
I've gotten good results. I shoot a 45#@29" recurve. 31" 2215 easton XX75 shaft with single bevel 2 edge broadhead. 550gr@ 19% FOC

 

[Mighty Mouse]

'Tis rare to find such learned interlocuters willing to investigate the minutest of archery minutiae, so I shall take advantage of the opportunity and offer the following theoretical analysis of an arrow's behavior in crosswind—and the effect of FOC thereupon—and solicit the critique of my fellow nerds. @ImBillT I'm particularly interested in your thoughts on my line of reasoning.

1. In reference to the sketches below, the arrow is being viewed from above. Original arrow trajectory is in the positive X direction (left-to-right). Crosswind is blowing in the positive Y direction (bottom-to-top).
2. Crosswind creates drag forces across the broadhead (FY1), shaft (FY2), and fletching (FY3) acting in the positive Y direction
3. Motion of the arrow through the air creates drag forces across the broadhead (FX1), shaft (FX2), and fletching (FX3) acting in the negative X direction
4. The forces described in 3 and 4 can be simplified into resultant forces FX and FY acting at the arrow's center of pressure (CP)
5. FX applies torque acting counterclockwise around the arrow's center of gravity (CG)
6. FY applies torque acting clockwise around the arrow's CG
7. The torques described in 5 and 6 cause the arrow to rotate clockwise and reach equilibrium at angle A in the horizontal (XY) plane
8. The magnitude of the torque described in 5 is calculated by multplying FX by the distance between CP and CG perpendicular to the direction of FX (aka, the moment arm). The length of this moment arm is calculated by multiplying the distance (D) along the arrow shaft between CP and CG by the sine of angle A. The magnitude of the torque in the counterclockwise direction is therefore equal to FX•D•sin(A)
9. The magnitude of the torque described in 6 is calculated by multplying FY by the distance between CP and CG perpendicular to the direction of FY (aka, the moment arm). The length of this moment arm is calculated by multiplying the distance (D) along the arrow shaft between CP and CG by the cosine of angle A. The magnitude of the torque in the counterclockwise direction is therefore equal to FY•D•cos(A)
10. At equilibrium, net torque acting around the arrow's CG is zero (aka, clockwise and counterclockwise torques are equal); therefore FX•D•sin(A) equals FY•D•cos(A). The distance (D) along the arrow shaft between CP and CG appears on both sides of the equation and cancels out, meaning that the location of CG (and therefore the amount of FOC) does not affect the angle (A) the arrow adopts in the horizontal plane. The calculation of A resolves to the inverse tangent of the ratio of drag forces, atan(FY/FX), meaning drag forces alone determine angle A regardless of CG or CP location (or by extension, amount of FOC).

 

[brocksw]

 

[ImBillT]

'Tis rare to find such learned interlocuters willing to investigate the minutest of archery minutiae, so I shall take advantage of the opportunity and offer the following theoretical analysis of an arrow's behavior in crosswind—and the effect of FOC thereupon—and solicit the critique of my fellow nerds. @ImBillT I'm particularly interested in your thoughts on my line of reasoning.

1. In reference to the sketches below, the arrow is being viewed from above. Original arrow trajectory is in the positive X direction (left-to-right). Crosswind is blowing in the positive Y direction (bottom-to-top).
2. Crosswind creates drag forces across the broadhead (FY1), shaft (FY2), and fletching (FY3) acting in the positive Y direction
3. Motion of the arrow through the air creates drag forces across the broadhead (FX1), shaft (FX2), and fletching (FX3) acting in the negative X direction
4. The forces described in 3 and 4 can be simplified into resultant forces FX and FY acting at the arrow's center of pressure (CP)
5. FX applies torque acting counterclockwise around the arrow's center of gravity (CG)
6. FY applies torque acting clockwise around the arrow's CG
7. The torques described in 5 and 6 cause the arrow to rotate clockwise and reach equilibrium at angle A in the horizontal (XY) plane
8. The magnitude of the torque described in 5 is calculated by multplying FX by the distance between CP and CG perpendicular to the direction of FX (aka, the moment arm). The length of this moment arm is calculated by multiplying the distance (D) along the arrow shaft between CP and CG by the sine of angle A. The magnitude of the torque in the counterclockwise direction is therefore equal to FX•D•sin(A)
9. The magnitude of the torque described in 6 is calculated by multplying FY by the distance between CP and CG perpendicular to the direction of FY (aka, the moment arm). The length of this moment arm is calculated by multiplying the distance (D) along the arrow shaft between CP and CG by the cosine of angle A. The magnitude of the torque in the counterclockwise direction is therefore equal to FY•D•cos(A)
10. At equilibrium, net torque acting around the arrow's CG is zero (aka, clockwise and counterclockwise torques are equal); therefore FX•D•sin(A) equals FY•D•cos(A). The distance (D) along the arrow shaft between CP and CG appears on both sides of the equation and cancels out, meaning that the location of CG (and therefore the amount of FOC) does not affect the angle (A) the arrow adopts in the horizontal plane. The calculation of A resolves to the inverse tangent of the ratio of drag forces, atan(FY/FX), meaning drag forces alone determine angle A regardless of CG or CP location (or by extension, amount of FOC).

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Don’t have as much time today. I’ll try to read it sometime and I’ll tell you what I think, BUT I took fluid from a professor that handed out passing grades. I slept through class when I went. I don’t remember any of it at all. I actually had to learn statics and dynamics to get through those classes, so they’ve stuck with me much better, even though I’ve still forgotten more than I remember.

My thoughts on wind drift are going to be less reliable than on other thibgs.

 

[BackofBeyond]

View attachment 262323

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Still enjoying the conversation, good reminder on why I'm not using my degree for it's intended purpose.

 

[ImBillT]

@Mighty Mouse is correct.

I think. Lol. Convinced at a glance. Probably will remain so.

 

[Wind Gypsy]

Goodness gracious, ranch fairy strikes again.