Calling all Shade Tree Physicists I was lying in bed one morning not too long ago and thinking about arrows. OK, all you folks about to make snide remarks about poking things and other suggestive comments, here is the place to get it out of your systems. At any rate, I was thinking about arrows, the physics involved in impact drivers and energy transfer pulse/time intervals…OK, you can make a couple of more jokes here too. You won’t be the first (or last) to make fun of me…and I began to wonder about an experiment with the following conditions: 1. With a “standard” arrow weight of 500 grains striking a target at a “standard” velocity of 250 fps obtain an energy transfer pulse length as a “base line”. In other words, obtain a measurement of the length of time the arrow actually delivers its energy to the target. 2. Then, as a starting point, construct an arrow with a free floating, internal weight, occupying the rear one third of the inside of the arrow weighing 50 grains as a starting point. 3. The Forward of Center (FOC) of the rear-weighted shaft would be 15% FOC. 4. Then, the test arrow would be shot into the target at the same initial velocity of 250 fps and the energy transfer impulse time would be measured again. My belief is that the weighted arrow’s transfer time would occur over a more prolonged period than that of the un-weighted shaft. If so, the effect would be similar to an impact driver, assisting in penetration due to the act of the internal weight transferring its energy to the tip of the arrow. In effect hitting a “double tap” upon impact. My question is: Should I spend valuable sack time in more productive pursuits or might I be onto something? Thanks, K
In reality I think the tuning issues would far out weigh any benefits of the secondary impact of the additional 10% of total weight. Its not as if the arrow stops instantly, it slows on impact which would lessen the effectiveness of the additional weight.
The problem would be keeping the weight back when shot. If the weight is truly free floating, it may or may not be at the rear of the shaft when shot. This would mean it would slam backwards upon the shot and in turn , negate and may actually reverse any potential advantage. Now, if it were magnetic and was stuck to the rear of the shaft until impact, then the arrow would act as a dead blow hammer and have increased penetration potential. That is until it hit all the way forward and would rebound into the backend of the arrow. Maybe having a magnet in the front would cure this as well.:what: I have actually given this some thought before. Sent from my SAMSUNG-SGH-I317 using Tapatalk 2
Add to this that 500 gr arrow would be a very hard thing to achieve out of "normal" setups due to the weight needed to have the foc at 15% while having extra weight added to the rear and feon of the arrow. This would require a very high energy bow to achieve 250 fps with as well. Sent from my SAMSUNG-SGH-I317 using Tapatalk 2
This was my thought as well. I've given is some thought also. I think the 500 gr, 15% and 50 gr are just theoretical values, they would have to be refined into something more feasible as well. I have been toying with the idea of a good penetration test. I haven't had much search time but finding a cost effective and consistent material is the key. Something more precise than the arrow went 6 inches deeper in my shot up foam target... I think this would be something better to tinker with, it would help give a more valuable and realistic measure If you have a high speed camera to calculate velocity on impact that would be awesome as well. I'll talk to my dad about your question though, he actually is a physicist.
The internal weight I am going to use in testing is shot shell buffering media. It is non-hygroscopic, light spherical plastic, free-flowing and does not stick to anything. I currently have some on order and as yet do not have a good idea of its density in grains per cc. As it happens, I use alloy shafts in a modern bow and the internal finish is mirror bright and smooth. While it (the buffer media) will easily be moved to the rear of the shaft by tilting the arrow up, as soon as deceleration due to target contact is encountered, it will begin moving toward the front in one extended (hopefully) “pulse”. My primary issue (as I have already decided to proceed with experimentation (assuming I can get appropriate weights before running out of internal space) has been in the manner in which quantitative data is to be collected. I have imagined a crude method would be the use of ballistic gelatin or perhaps clay. First shooting a “solid” arrow into the medium and then shooting into a fresh block with the movable weight arrow, both tipped with the Saunders Combo-point. One sees the use of ballistic gelatin in firearms testing as a standard media for projectile/load experimentation. It would seem to me that the same media should give excellent results at least giving an “apples to apples” comparison.
I've already played around with water in a shaft. To see any positives, I had to ensure the shaft was filled completely and void of air. However,this was not a real scientific test and to be honest, a bit dangerous. I used tapered shafts that had a high spine range and thought it could handle it. I did have a shaft explode in mid flight after several penetration test in hard medium. My point is just to be careful. Sent from my SAMSUNG-SGH-I317 using Tapatalk 2
I think you have something, A mercury recoil reducer in a rifle is the same principle, only you would be trying to increase recoil instead of dampening it. It makes sense. Keep thinking and try something.
The genesis of the idea came from considering rotational imbalances in an arrow and the means to dynamically balance it in flight. I was just playing “video tapes” in my mind, trying to visualize what would take place during launch, flight and impact. The most accurate long distance shafts are tapered at both ends (Easton refers to them as “barrel tapered”) to reduce oscillations during flight. Of course, thicker walls would have the same effect but the additional weight affects velocity even though the thickness would produce the dynamic spine characteristics desired. That’s when I too considered a fluid. I figured there would be no way for me to adequately seal the shaft and that’s when I thought about the Slime product used in off road tires. Besides being messy and the shaft would have to be water tight the fact that I would have to be able to change weights easily and accurately lead me to a fine powder. That’s when I remembered the shot shell buffer used in buck shot loads. We shall see what we shall see…
I see one other potential issue with having something moving inside the shaft. When an arrow impacts something, it typically flexes noticeably. This would entail that whatever was coming forward from the back end of the shaft would have its forward motion slowed or even stopped before it could make it to the front of the arrow.
250 feet per second and 500 grains is going to pass through pretty much any animal it hits. So the weight sliding forward when the arrow stops its initial forward movement may succeed in driving your arrow deeper into the ground on the other side. I can't think of any real benefit that will provide.
