Mechanical Broadheads and Kinetic Energy

[TheCougar]

I was listening to a podcast (either GB or Kifarucast) and Snyder mentioned NAP Killzones as good mechanicals, except that they took a huge amount of KE to open and they weren't a good choice unless you were shooting a bow/arrow setup that could stand to lose that KE. I don't shoot a setup that should have problems with KE, but I do want to maximize penetration and minimize KE loss. Are there particular broadheads/mechanical designs that minimize energy loss when they open? Over the top vs rear opening, etc? I guess it is something I never thought of, and it got me wondering. Is there any empirical or anecdotal evidence that a certain design or company is better for maintaining KE? Thanks.

This is not intended to be a forum to tell me that mechanicals suck and fixed blade broadheads take 0 KE to open.

 

[Sharp Things]

I wonder if KE is the proper yardstick? Momentum is what drives the projectile through the other side. Adding momentum really improves the odds of the projectile will continue to pass through despite the losses to blade deployment.

 

[TheCougar]

I’m totally going to nerd out on this. KE= 1/2m*v^2 and Momentum (p) is p=mv (mass x velocity). Both are valid measurements for my purposes, but KE is more sensitive to changes in velocity (by a factor of 2), in this case as a result of an arrow slowing due to the force required to deploy blades. For instance, a 450gr arrow at 300fps or a 300gr arrow at 450fps, momentum is the same for both, but the KE of the 300gr arrow is significantly higher than the heavier arrow that is traveling slower. Because I am treating mass (and pretty much everything but velocity) as a constant, and I am considering the deceleration of the arrow to be an instantaneous event in time, velocity is the variable in question. If you really want to nerd out, momentum is an angular value, so when guys talk about tuning their bow for best penetration, they are really talking about the preservation of momentum -100% of the force behind the arrow is going in the direction of shaft travel- and not KE. Momentum is the force that carries an arrow through a target and increasing mass (and arrow tuning) is a very effective way to increase momentum. KE is the energy created by the shot and is a "snapshot" in time, in this case the instant it hits the animal.

 

[Brendan]

I wonder if KE is the proper yardstick? Momentum is what drives the projectile through the other side. Adding momentum really improves the odds of the projectile will continue to pass through despite the losses to blade deployment.

True - but KE and momentum are correlated. Also - it's not really relevant to his question. He's asking what mechanical design is the most efficient in retaining KE (or momentum, if it makes you happy) when it opens. For example - a 2" front opening, vs a 2" rear opening...

OP - In regards to your initial question, I've heard anecdotally that a rear deploy (e.g. Rage) uses less energy than a front opening. Haven't seen any definitive testing though so I'm not sure if that's just a manufacturer claim. Would be hard to test as you'd need comparable front and rear opening heads with same number of blades, same blade type and length, same tip design...

 

[TheCougar]

True - but KE and momentum are correlated. Also - it's not really relevant to his question. He's asking what mechanical design is the most efficient in retaining KE (or momentum, if it makes you happy) when it opens. For example - a 2" front opening, vs a 2" rear opening...

OP - In regards to your initial question, I've heard anecdotally that a rear deploy (e.g. Rage) uses less energy than a front opening. Haven't seen any definitive testing though so I'm not sure if that's just a manufacturer claim. Would be hard to test as you'd need comparable front and rear opening heads with same number of blades, same blade type and length, same tip design...

Yeah, I figure it is hard to quantify this, because there are so many other variables beyond just the action of opening blades. Primarily forces produced by friction (blade angle, sharpness, tip design) that also slow an arrow on impact, whether fixed or mechanical. I have also read the propaganda on the broadhead packaging for rear blade designs, but I don't know if it is true. The NAP Killzones are rear deploying BTW, which is what surprised me the most.

 

[Nick Muche]

I don't get into the crazy math side of things, but here's a musk ox I killed with a Rage 2 Blade and a 680 grain arrow out of a 60lb bow. Plenty of KE or Momentum to get the job done.

IMG 2269 1 - YouTube

PS.. How in the hell do you embed a youtube video so it shows up instead of a link?

 

[Sharp Things]

Was that a heart shot? It got the job done but it looks like you got about 15 inches of penetration?

 

[Nick Muche]

Yeah, it was a heart shot. Based on my experience 15" of penetration is more than adequate for any animal in North America.

 

[roadrunner]

To start, KE means nothing as applied to the penetration debate, and is a snapshot of the result of momentum (well said Cougar).

So, to not get too nerdy, let's use a little common sense. What holds the blades in place on a mechanical? How easy is it to break the retaining band? How easy do the blades fall open with no retaining band? Mechanicals use little to no (comparatively) energy to open.

Choose one and go hunt.

 

[Trial153]

I don’t see energy as a factor in opening a mechanical Broadhead. Most open very easily. But to expand I don’t feel that the opening is a yard stick that you can measure performance either.
So take a look at the design of the open head it’s size, blade angle and materials will be a better indicator for performance..ie penetration.

 

[Beendare]

There are many tests on energy required with the various heads.
[video=youtube;tpHpVR5SUX8]https://www.youtube.com/watch?v=tpHpVR5SUX8[/video]

Plug 'broadhead test' into youtube to see hundreds of BH's shot into everything imaginable.

I base my opinion from my experiences in the field.......there is less penetration with the over the top mech heads....and the short wide chisel heads. It doesn't much matter much if all you hunt is thin skinned whitetails........but it can be a factor on heavier animals. Then add the other disadvantages with those designs...they aren't for me.

 

[wapitibob]

I shoot Spitfire 125's where legal. 60# and 425 gr arrow and all but one Bull were pass thru's.

 

[Nick Muche]

Spitfires are one of my favorites, haven't shot them in years but they were great on whitetails. I can't imagine them not working very well on other critters. I did however have one crazy experience with them but it did result in a dead deer. He was quartered away with his head down sniffing where a doe I just shot had pissed, as I shot, the blade must have caught a rib and only one opened. The arrow glanced off the side of his body and cut him open like a scissors from mid rib to nearly his ear. He ran 15 yards and I had to shoot him again.

I am excited to shoot the new crazy looking rage at some bears this spring. That thing is going to be hell on them.

 

[Brendan]

I don’t see energy as a factor in opening a mechanical Broadhead. Most open very easily. But to expand I don’t feel that the opening is a yard stick that you can measure performance either.
So take a look at the design of the open head it’s size, blade angle and materials will be a better indicator for performance..ie penetration.

Not sure I agree with this. Take a spitfire. When they hit the animal and start to rotate backwards you get a situation where the leading edge of the blade isn't sharpened and is pushing backwards to open the blades up and over before they start cutting.

Spitfire and killzone for comparison.

 

[Trial153]

Not sure I agree with this. Take a spitfire. When they hit the animal and start to rotate backwards you get a situation where the leading edge of the blade isn't sharpened and is pushing backwards to open the blades up and over before they start cutting.

Spitfire and killzone for comparison.

You don’t agree with it then a simple test will prove you correct. Shoot a undeployed Spitfire into medium and measure. Then shoot the same Spitfire with the blades fully deployed into the same medium and measure the difference. If it takes significant energy to open the head then you should see a significant difference in penetration. However From what I’ve seen it’s a barely measurable difference.

 

[Brendan]

Have you actually done that test, or are you just supposing too? What is "barely a measurable difference" when shooting two identical broadheads like that (one deployed and one not) into the same medium? I'll admit I haven't shot a deployed mechanical of any sort mainly because the blades wouldn't clear my bow / rest without building longer arrows. Only tests I've done are non-deployed including scapulas hung on a foam block target...

Physics though would indicate that you get a force pushing from that unsharpened edge down the blade and backwards through the arrow, but you do get the same with the Killzone... How much difference it make, who knows...

 

[Sharp Things]

I have tested MANY MANY broadheads over the years for puncture force to breach a consistent media. The slip cam (rear, sliding to deploy) take far less force to get the blades into the final position than do the swing out styles. All my evaluations have been done at slow speed with a meter to capture puncture force. Also I have killed game with mech heads. My testing and first hand kill experience made me move away from mech heads. Certainly they will kill and have killed many animals of all species but we all make personal choices and there is a long list of reasons I am a fixed blade fan.

As to comments about just how easy it is to break the band or dislodge seated blades to get them to open, that suggests simply tripping them by hand to get them to open and discounts the effort required to make them fully deploy while passing through hair, hide, meat and bone.

The 3Rivers video posted here in an earlier post is pretty telling. I did one that is similar but included a popular rear deploying broadhead and filmed it. (yes I know it appears unsafe but the heavy leather welding gloves protected me and no harm came to me)

[video=youtube;c2WRsNXgzR0]https://www.youtube.com/watch?v=c2WRsNXgzR0&list=UUrDKSMxGGaa59V0uZPABJ6Q&index=17[/video]

 

[Sharp Things]

I just looked back at my broadhead puncture evaluation data from 2009. The Spitfire took nearly 3 times the force to breach the media than did the Rage. Both with a magnitude of force over that of simple fixed blade heads. All the pictures and video of each head evaluated would be a great undertaking to try and post here and I think its against the rule to link away to personal web sites.

 

[roadrunner]

Don't confuse the actual work required with initiating the opening of the blades and fully deploying them with resistance perpendicular to the line of action. If a mechanical doesn't penetrate deep, there are other factors involved. Pushing a mechanical into anything slowly and by hand to watch a scale or gauge for "opening force" is not the same thing as it traveling at 265 fps. Back in the day, the famous test was pushing a piece of stretched elk hide over a stationary broadhead fixed in a vertical position to test (or to refute the validity of mechanical's, and at the time it was the Punchcutter) which broadhead took the less force to penetrate. It was one of the biggest follies of tests ever done and didn't prove a whole lot.

 

[Trial153]

I just looked back at my broadhead puncture evaluation data from 2009. The Spitfire took nearly 3 times the force to breach the media than did the Rage. Both with a magnitude of force over that of simple fixed blade heads. All the pictures and video of each head evaluated would be a great undertaking to try and post here and I think its against the rule to link away to personal web sites.

So you confirming that lack of mechanical advantage in the head has as the determining factor? For example would a open rage head performed just as poorly because of its poor design(bad blade position and angle and lack of leading cuting edge)?
The “opening” isn’t the culprit so much as what it opens too. Witness the slick Trick.