Will a stiffer arrow penetrate better?

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You have two arrows, same weight, same O.D. One has a static and dynamic spine of 250, the other static and dynamic at 300. Both tuned perfect to the same bow.

Which one will penetrate farther?

Will the 300 shaft flex more upon impact causing less penetration or will it be the same?


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Stiffer should penetrate better. Probably a lot of factors, but the lighter spine flexing on impact is going to absorb impact force. For every action there is equal reaction. That's why when you hit something hard it flings the nock off. That's energy leaving the arrow in the other direction.
 
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I second Billy Goat’s response in theory. Never tested anything though, but I thought the exact same as him.
 

fatlander

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Billy goat is probably right. However, I think that you’d find the small difference from a 250 to 300 to not be statistically significant.

Maybe a 250 to 500 or 600.

But then one arrow is going to be tuned and one isn’t so you’ve got different issues.

I think a well tuned system is more important than the spine.


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Beendare

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The BH would be the determining factor....and whether you hit bone.

Maybe if you are shooting an over the top mech head...or a short wide chisel point head that compresses the hide on impact arrow flex might be a tiny factor.[ Dale from 3 Rivers has a video illustration this]

I know its not a factor with a leading edge cut on contact head like any of the 2 and 3 blade leading edge heads....those penetrate effortlessly with minimal pressure so its essentially nothing transmitted to the rest of the arrow.

I had a case many years ago shooting a big Black bear while on a stalk that turned back right as I released the arrow. The arrow caught him right on the heavy part of the front shoulder [Beman Hunter 300 about 440gr with a Slick Trick BH] and you could literally see the arrow flex and it bounced back out. [maybe 3 " of penetration]

I've shot elk through both shoulder blades with COC heads but that bone was only about 5/8" thick.

Hit something really hard and all bets are off!

_____
 

Wapiti1

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Given the parameters you lay out, they will penetrate the same as long as the weight added to the 300 spine arrow is at the point end and all other factors are held constant (broadhead, medium being shot into, etc).

Impact stability would be resistance to buckling of the shaft. The point end has no influence on buckling of the shaft upon impact. Only the weight of the shaft, fletching and nock are part of that. i.e. FOC is not relevant either.

So, a 250 at say 11 gpi verse a 300 at 9 gpi due to thinner wall at same OD. Since energy is highly dictated by mass, you can see that the energy generated per inch is lower on the 300 than the 250. The 300 shaft is thinner/weaker, but it doesn't have to withstand the same force as the 250 either. In other words, it has adequate stiffness for the energy it has to handle. If that isn't the case, the 250 is hosed as well since it should a linear mass/stiffness change for each spine.

Remember arrow spine deflection at the shot is because of pushing on the nock. Impact is the opposite reaction to the arrow.

This also assumes the arrows maintain geometric constants as well. A fatter shaft will take more force to buckle even at a reduced wall thickness.

Now, if you add a bunch of weight to the nock end, the shaft has to deal with that, and you could reach a buckling limit. Just like you get weaker spine when you add point weight and put more force on the shaft to get it moving at the shot.

Jeremy
 
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Given the parameters you lay out, they will penetrate the same as long as the weight added to the 300 spine arrow is at the point end and all other factors are held constant (broadhead, medium being shot into, etc).

Impact stability would be resistance to buckling of the shaft. The point end has no influence on buckling of the shaft upon impact. Only the weight of the shaft, fletching and nock are part of that. i.e. FOC is not relevant either.

So, a 250 at say 11 gpi verse a 300 at 9 gpi due to thinner wall at same OD. Since energy is highly dictated by mass, you can see that the energy generated per inch is lower on the 300 than the 250. The 300 shaft is thinner/weaker, but it doesn't have to withstand the same force as the 250 either. In other words, it has adequate stiffness for the energy it has to handle. If that isn't the case, the 250 is hosed as well since it should a linear mass/stiffness change for each spine.

Remember arrow spine deflection at the shot is because of pushing on the nock. Impact is the opposite reaction to the arrow.

This also assumes the arrows maintain geometric constants as well. A fatter shaft will take more force to buckle even at a reduced wall thickness.

Now, if you add a bunch of weight to the nock end, the shaft has to deal with that, and you could reach a buckling limit. Just like you get weaker spine when you add point weight and put more force on the shaft to get it moving at the shot.

Jeremy

Wait why would the weight be added to the front of the 300? It wouldn’t be tuned. The 300 is higher gpi.

The 250 to be tuned to the same setup has the weight added to the front and is lower gpi.

Both total weight the same.

Let me rephrase so I understand.

When both are tuned to the same setup with the varying point weights and gpi but both static spine and dynamic matches(250-250 300-300) they would flex the exact same amount on impact? Correct?

The only case where a stiffer spine would out penetrate a weaker spine is if they both were 250 static but one was dynamic 260(stiff) and the other 240(weak)?


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Wapiti1

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Wait why would the weight be added to the front of the 300? It wouldn’t be tuned. The 300 is higher gpi.
One of the parameters you stated in the initial post was that both arrows are the same weight. A 300 spine shaft has a lower gpi than a 250, so you have to add weight somewhere.
The 250 to be tuned to the same setup has the weight added to the front and is lower gpi.
300 is lower gpi than 250. You add wall thickness to get a stiffer spine arrow. I'm pretty sure you know this and are just mixing it up. Spine only matters in this excercise at the launch, not at impact. I'll explain below.
Both total weight the same.
Right. I'm also assuming they are otherwise identical for length, fletching, etc.
Let me rephrase so I understand.

When both are tuned to the same setup with the varying point weights and gpi but both static spine and dynamic matches(250-250 300-300) they would flex the exact same amount on impact? Correct?

The only case where a stiffer spine would out penetrate a weaker spine is if they both were 250 static but one was dynamic 260(stiff) and the other 240(weak)?

Outside of extreme examples like a 32" 500 spine shaft, spine ONLY matters at the launch of the arrow from the bow. It dictates flex and recovery characteristics. The string imparts the bow's force to the nock, the shaft transfers it to the point, and overcomes the inertia the point has to stay stationary. This flexes the shaft and creates the need to pick a stiff enough shaft for flex (buckling) control.

Upon impact, the force is reversed. The force of whatever you just shot the arrow into acts on the tip, then transfers down the shaft to the nock. The nock end has very little inertia to keep moving because it is light and the shaft itself doesn't weight much either, so it only has to support it's own weight so to speak. As long as the point penetrates, there is very little flex of the shaft to create drag. If the point stops suddenly, though, all that energy gets expelled somewhere and it flexes a ton.

Where you would get penetration issues from spine is if you shoot an out of tune setup or the arrow impacts before it recovers. In these two cases, the arrow point will not impact with the shaft in line with the force vector. The shaft is flexing, or fishtailing and energy is expended straightening that out as it penetrates.

Jeremy


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I know in general if you use same brand and/or model of arrow that is this case. Higher spine equals less gpi. I’m saying specifically to use a 2 different makes and models where this isn’t the case and the sizes are the same.

Example
Black Eagle X-impact 250 - 9.5gpi with more point weight

and compare it to

Easton 4mm FMJ 300 - 12gpi with less point weight
-OR any other high gpi arrows types of different brands, don’t care.
-AND/OR use weed wacker cord inside 300 shaft to raise total gpi. It doesn’t change spine or foc/balance of said shaft. It’s an old traditional archery hunting mod.

However you want to do it but specifically using lower gpi 250 and higher gpi 300 for this comparison. Modified or not, don’t care.

...and that’s where the hypothetical elk bone comes in.(Shoulder blade, center of farthest forward rib). The hard impact where the shaft is 100% definitely going to flex and continues to vibrate back and forth as it enters the elk completely.

On this hard bone impact and sub-sequential full penetration into the elk will the 250 spine flex/vibrate/buckle any less than a 300? Even if it’s very small, hypothetically 1% less, will it flex less?

Again the low gpi 250 and high gpi 300 are perfectly tuned to same bow with different point weights, nock weight, vanes, wraps, etc so flight is perfection.

Or will they flex the same amount?

Thanks


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mag1

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Check out the Ranch Fairy on Youtube. he has a good video explaining the effects of spine weight on impact. Better yet, here is a link to the one to start with.

 

Beendare

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Those Ranch Fairy vids are hard for me to watch...but he does have some good footage and makes a good case for an arrow around 20% FOC. It seems to me that building an arrow in that range is pretty stiff spine.

The one thing he keeps saying which will drive the physics guys nuts is that the heavy point is "pulling the arrow through the animal". Arrow force is a push....no pulling involved.



_____
 

Wapiti1

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I know in general if you use same brand and/or model of arrow that is this case. Higher spine equals less gpi. I’m saying specifically to use a 2 different makes and models where this isn’t the case and the sizes are the same.

Example
Black Eagle X-impact 250 - 9.5gpi with more point weight

and compare it to

Easton 4mm FMJ 300 - 12gpi with less point weight
-OR any other high gpi arrows types of different brands, don’t care.
-AND/OR use weed wacker cord inside 300 shaft to raise total gpi. It doesn’t change spine or foc/balance of said shaft. It’s an old traditional archery hunting mod.

However you want to do it but specifically using lower gpi 250 and higher gpi 300 for this comparison. Modified or not, don’t care.

...and that’s where the hypothetical elk bone comes in.(Shoulder blade, center of farthest forward rib). The hard impact where the shaft is 100% definitely going to flex and continues to vibrate back and forth as it enters the elk completely.

On this hard bone impact and sub-sequential full penetration into the elk will the 250 spine flex/vibrate/buckle any less than a 300? Even if it’s very small, hypothetically 1% less, will it flex less?

Again the low gpi 250 and high gpi 300 are perfectly tuned to same bow with different point weights, nock weight, vanes, wraps, etc so flight is perfection.

Or will they flex the same amount?

Thanks


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OK, I see where you are going. If they are both moving at the same speed, the 300 FMJ will flex more. There is more kinetic energy and inertia to dissipate from the increased shaft weight on the 300 with less stiffness to counteract it. The 250 will recover faster as well.

This is, of course, in a perfect world so the forces are all neatly lined up.

Jeremy

That Ranch Fairy video might be the most annoying thing I've ever watched. His reasoning is also somewhat flawed. He doesn't address the velocity difference between the arrows in the "test".
 

KyleR1985

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All variables being fixed, with the exception of the spine, with a perfectly straight arrow flight/entry into the target medium, and a monolithic target medium, there will be no measurable difference in penetration.

If there is a difference in penetration, all of the above is not true.
 

dkime

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All variables being fixed, with the exception of the spine, with a perfectly straight arrow flight/entry into the target medium, and a monolithic target medium, there will be no measurable difference in penetration.

If there is a difference in penetration, all of the above is not true.

Disagree; What you are saying is that the shaft does not flex upon impact which we know to be incorrect.
 
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In an animal, of course yes, stiffer will penetrate more. There can be exceptions, like when you hit bone with the side of a vane and the flex of the weeker arrow allows it to change direction and move around to the side of the bone. But on average, definitely stiffer will penetrate more.

Having said that, 250 vs 300 will likely have no appreciable difference in kill success.

Shoot a wet noodle into piece of paper, then shoot a pencil of the same weight. 😁
 

KyleR1985

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Disagree; What you are saying is that the shaft does not flex upon impact which we know to be incorrect.

I didn't say, nor did I imply an arrow shaft doesn't flex upon impact. A more careful reading of what I said would reveal an important point. There will be no measurable difference, assuming all variables, with exception of 250 verus 300 spine, are fixed. Key word being measurable.

What I said was rather succinct, but I'll put it even more bluntly: It would be nearly impossible for the average person to develop the test I described. But if you did, the difference in penetration between the two spines, all other variables controlled for, would likely not be measurable.
 
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Like said before, the stiffer spine should be better. Less flex on impact that said, optimum spine is more important to me


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