Testing arrow penetration with 2 different setups

[monterey]

OK, put #8 on the scale again and drew it four times.  It averaged 36.5#.  Next, I checked the scale with a 22# weight (a sender international metric 10 KG 22# weight plate). It scaled at 21#.  That puts the scale at 1/2% error which would correct the draw weight to 38.3#.

So, using that draw weight and shooting over the Chrony again will produce a different profile.  I will do that today, but first I have to paint the trim around the front door.

[Nativestranger]

My ballistic gel is ready. Hope to do some testing with broadhead soon. This is costing me money and time but I am willing to do it for the sake of Scientific research.

[monterey]

Does not look good for getting any arrows across the Chrony today. Some bad weather slowed the paint work.

You should send those two arrows to me to try.

[Nativestranger]

Those are just regular GT carbon shafts. Velocity 600 28.8" 100g point and traditional hunter 1535 30.5" 125g point

[tomsm44]

For the life of me, I can't figure out why anybody would test penetration on anything other than a dead cow.     :D

[Nativestranger]

Because dead cows are harder to kill? Wait you can't kill a cow that's already dead.  

[Nativestranger]

Ok first round of test done. I used 20% density ballistic gel. Feels firmer than human flesh. Friction on the shaft is high compare to real living tissue with blood. Since I only have 1 type of broadhead in weight of 100g, I only tested with one bow shooting both arrows. The the lighter arrow 300g. Heavier arrow 375g. Result: The lighter arrow penetrated about 1/2" more than the heavier one. Not a whole lot there. It could be the margin of error in draw and release.  
       
       
     
 
Broadhead used: Cabelas copperhead with 3 replaceable lutz blades
     
 
Bow used: Grozer biocomposite Scythian bow 37# @28"    

[Nativestranger]

Repeated the tests 2 times. Moving the back stop further away. The heavier arrow out penetrate the lighter one by 1/3" on the last attempt. Given over 17" penetration, there's overall almost no difference in the ability of both arrows to penetrate the test medium.
       

     

To continue with the testing I will thicken the ballistic gel by another 10% to make real tough and shoot a mix of broadhead and field point.

[Zradix]

I'd suggest stacking 2-3 gel blocks in a row and shooting them.
..so the arrow never penetrates out the other side.

...or make the gel in something longer..like a cooler or something.

[JimB]

I personally think the density you have is good.It allows the arrow to penetrate deeply without the fletching contacting the medium.

If I were going to go through that much trouble though,as I said before,I would want the shaft finish and diameters to be exactly the same.Just the shaft finish alone can show that much difference in penetration.You would also be surprised how much effect a few thousandths of an inch diameter will make in penetration in mediums where the arrow is stopped by friction on the shaft.Again,because a maker calls a shaft 5/16" doesn't mean it is exactly that.They vary a lot and either calipers or more specific specs should be used to make sure they are exactly the same if you want a true comparison.

Years ago,Fred Bear did some ingenious testing to determine what effect arrow weight had on impact.A weighted pendulum was used to test impact.He tested 6 different arrow weights out of eight different bows,shot from a machine.The beauty of this type of test is that it eliminates any question about test medium and penetration and also eliminates issues like shaft diameters,finish etc which plague penetration tests.His testing isolated impact and the effect that arrow weight had on it.

In all cases,the heavier arrow impacted more-with every bow.He even goes so far as to note that in one instance,a 45# bow with a 17.7 GPP arrow came within 1% of the measured impact of a 60# bow and 10 GPP arrow.

He settled on a preference for 9-10 GPP as a balance of trajectory and impact force.

[Nativestranger]

Good suggestions. Will try make anotger block of gel. As you can see from photo.  Both shaft are flush with the standard gt insert. Small difference in shaft diameter should not matter for soft elastic medium like ballistic gel. It still grips the shaft. Ok shaft finishing do matter. But I don't think there's much difference in friction here. Both finish are smooth but not shiny type.

[JimB]

The diameter does matter.It's about friction.The way to rule it out,is to use two shafts identical in diameter.As have others,I've tested both aspects.it makes a difference.

[Nativestranger]

Tested with 30% ballistic gel. I was able to move the back stop way back. So both arrows penetrate deeper and not stopped by the back stop. This time the lighter arrow penetrated about 4.5" deeper and almost to the feathers. Though I am not fully convinced. I have to re melted and test again with 2 blocks.

 

[Nativestranger]

Latest test results in. Used a 30% density gel block and a 20% block lined up together. The 30% block really grips the shaft hard so I am not able to pass through both gel blocks. The bow used is 40# @28 drawn to 26.5". This time the 375 grain shaft penetrated about 1/4" more than the 300 grain. Given about 11" total penetration that's about 2.5% difference. Easily explained by the drop in KE on the lighter arrow.
     
     

[Nativestranger]

Final test results in. Used the lighter 37# bow with single block of 30% ballistic gel this time. Both arrows penetrated about 10.5". The lighter arrow about 1/8" more.

   

     
     

Conclusion: There seems little to no correlation between arrow weight/ momentum and the ability to penetrate ballistic gel with a broadhead. Kinetic energy seemed to determine penetration.

This is my interpretation and opinion of the results:
Penetration as tested depends on the total energy in the moving arrow. Which makes sense since to cut a wound channel, work has to be done and the amount of work done depends on the total energy available. The amount of arrow energy is determined by the stored energy of the bow multiplied by the efficiency factor. And that's why in my case the heavier bow always penetrated better than the lighter bow regardless arrow weight. Now that's not to say for your setup you won't get better results with a heavier arrow. But if you did, that is probably a function of the bow being more efficient and therefore having slightly higher kinetic energy with the heavier arrow. Thank you for looking and have a nice day.

[Scott Barr]

Jimmy,  I would not take the comments you are getting as negative criticism. Just good points to add to the thinking on this issue.  I very much appreciate all your effort.  Experiments of all kinds and all topics tend to draw healthy scrutiny.  Well done.

[Nativestranger]

Thank you Scott. I never took the comments negatively. Not at all. I welcome construction criticism and healthy scrutiny. Without the same mindset I would have just accepted commonly held beliefs and not be bothered to do any actual testing of my own.

[monterey]

Ok, finally got an opportunity to reshoot the bow #8.

I used three arrows this time.  One 450 grain, one 525 grain and one 560 grain.

Remember that I retested the actual bow weight and came to 38.3# at 28".  I also checked my actual draw length with that bow and determined it to be an actual 28" draw.

I replaced the battery in the chronograph just for good measure.  I also used a better quality scale to weigh the arrows.  

Each arrow was chronoed six times for the averages.

Here are the results;

68" slightly reflexed long bow 38.3# @28"

450 grain arrow

161.9
162.5
159.4
162.7
164.0
163.5
----------
162.33


525 grain arrow

155.9
155.2
155.9
155.4
154.1
156.3
---------
155.47


560 grain arrow
15.08
15.06
150.0
148.5
150.3
150.6
--------
150.13

I'm still pretty well convinced that you are arriving at erroneous conclusions regarding differences between KE and momentum as applied to penetration.

You large difference in velocity between the two arrows is problematic.  Notice that with the lightest and heaviest arrow above the difference of 110 grains results in a difference of only 12.2 fps.

Granted, the percentile difference in weights is not exactly the same, but the difference betwen percentage difference between arrow weights and differences in velocities shows a very large differential.  There, I believe lay the source of your conclusion that KE outweighs (pun intended) momentum in penetration.

If you are shooting the asiatic style bow in the previous picture, I think it may explain the large velocity differential.  That type of bow is known for preferring a lighter arrow and was in fact designed by our rather brilliant (if not engineering educated) predecessors to do just that.

OTOH, the bow I am shooting is known for it's ability and effectiveness in hurling heavier arrows.

So, IMO, one problem in your test results is that your arrows are flying at a way disproportionate velocity.  So disproportionate that the penetration results are nearly an even draw.

I'm pretty sure that the effects of variables in the surface of your two arrows along with other differences cited above are not enough to produce the near even up penetration results you are getting.

If you have another bow available that would produce velocity differences of 5 to 10 fps with those same two arrows I think you will find the 400 grain arrow out penetrating the 300.

[Nativestranger]

Thank you for taking the time to test your bow. I think your results are typical for longbows. And my results are typical for shorter recurve bows. The big difference in speed in first test is due to the heavier bow shooting the lighter arrow vs the lighter bow shooting the heavier arrow. When the same bow shot the 2 different arrows speeds are closer. The 40# bow (drawn 36# @ 26") shooting the light arrow (300 gr 8.3 gpp) was averaging 185 fps 22.8 ft.lbs. Shooting the heavier arrow (400g 11.1 gpp) was 164 fps avg 23.9 ft.lbs. So that's 19 fps difference between 11.1 and 8.3 gpp. But that's 33% difference in arrow weight. Please note that as gpp goes down the speed goes up more rapidly, at higher gpp speed difference is less. Your arrows are ranging from 11.7 gpp to 14.6 gpp. 12 fps difference. 7% difference in KE with 24% difference in arrow weight. Longbows with long working limbs tend to be more efficient with heavier arrows. While recurve bows with short working limbs tend to shoot both more or less as efficiently. Horsebow or not the difference just in style of grip. I have a Pinnacle recurve with black max medium limbs and the performance is similar. But I don't shoot that anymore since I switched to right hand. Please do take a look at the test data I posted of this 60" one piece recurve tested by Arctradonly. This particular bow gained 25 fps going from 11 gpp to 8 gpp. And there's nothing special about that bow. Just a regular 1 piece hunting bow with typical recurve profile limbs. Also noticed the energy and efficiency curve is quite flat from heavy arrows to light arrows indicating the bow is as efficient shooting a wide range of arrow weights.  
                     

For my case, if i have the 300gr arrow and 400 gr arrow being only 10 fps apart like you said. Let's say 164 and 174 fps. The resulting difference in energy is nearly 20% less for the light arrow. That's unlikely for well designed modern bows. Maybe we can see that behavior with very slow and over built D shaped longbows. But these are my results with my bows. May or may not be applicable to you. The point of this test was originally to test if Momentum or KE is the determining factor in penetration NOT whether a light or heavy arrow is better. For me both arrows penetrate about the same for each bow. The arrow with higher KE always out penetrate the one with higher Momentum. In your case you have better penetration with heavy arrows. Your results doesn't contradict my results at all because your heavy arrows also have 7% more KE than your light arrows. It just means your particular bow is better at imparting it's energy into the heavier arrows. As you can see this test data for Great Plains 68" longbow is quite similar to the results you are getting. About 13 fps difference between 11 gpp to 14 gpp. Efficiency difference between light and heavy also about 8%. Compare this to the data for the recurve above. Much less fps difference between arrow weights when compared to the recurve. Efficiency dropping much faster with lighter arrows. All these are just the difference in arrow weight vs efficiency between the styles of bows. Doesn't mean one must be right while the other is wrong.  

   

My conclusion is Kinetic energy is important in penetration while momentum is irrelevant. Its easy for you or anyone with a chronometer to prove me wrong. All you need is shoot a lighter arrow with high KE, low momentum from one bow and a much heavier arrow with higher momentum lower KE from another bow 5# weaker draw weight. Both arrows same foc same diameter. See which one is better in ballistic gelatin or plywood or whatever medium that's uniform. If you can prove that a higher momentum arrow can consistently penetrate better than a higher KE arrow then I may have to revise my testing procedures see what went wrong.