AfterShock
Reality Strikes 3 - AfterShock Broadheads
By
Nov 7, 2005, 10:03
 

Reality Strikes Series: Volume 3 – Speed and Energy at target.
by Aftershock Archery


This series of articles has been written by AfterShock Archery’s R&D Engineering quality team to reduce the BS factor when it comes to Bowhunting and broadheads in particular. We’ll separate the marketing buzzwords and deceptive advertising from reality and proven physics you can test yourself.

Speed, speed, speed! It seems everyone is selling speed. I can’t believe how many bowhunters think it’s crucial to get higher speed from their arrow/broadhead combination by reducing the weight of the arrow and broadhead. I was talking to a hunter that looked like he could pull back a hundred pound bow without issue. That hunter wanted to know if we made a 75 or 85-grain HyperShock. I said it’s in the works but who is it for? He answered that it was for him and he pulls a 90-pound bow with a 75% let-off. Jokingly I asked if the reason was that he wanted reduced blade area to give him pass-through capability on elephants. Nope! He wanted more speed!

I had to ask him the next question, “what are you trying to accomplish by doing that?”

He looked at me like I couldn’t understand the obvious and said, “I want to increase my energy so when it hits the game I have more speed and power to drive the broadhead through”.

Like many others who fall into the marketing hype, this man does not understand what’s really happening and what “Energy at Target” really means for him. He only seems concerned with speed out of the bow.

Kinetic energy is really quite simple; it is the energy of motion. Any object, which has motion, has kinetic energy. There are many forms of kinetic energy - vibrational (the energy due to vibrational motion like your bow at release), rotational (the energy due to rotational motion like the cams and pulleys on your bow), and translational (the energy due to motion from point A to point B). To keep matters simple, we will focus upon
translational kinetic energy for the arrow/broadhead combination and just call it kinetic energy for this discussion.

The amount of kinetic energy, which an arrow has, depends upon two things: the mass (m) of the arrow and the speed (v) of the arrow. For people who like to see formulas, the following equation is used to represent the kinetic energy (KE) of an object.

Where m = mass of object and v = speed of object. Or more simply, ½ the weight of your arrow times the speed squared.

For the sake of our discussion, some simple facts come about. First off, if you reduce the mass of your arrow assembly, your bow is going to have an easier time accelerating it to faster speeds. So far so good eh?

Did the kinetic energy also go up? Maybe, maybe not.

Most would think that it might stay the same since the reduction in arrow mass is made up by the increase in speed. Too bad the bow’s limbs, string and pulleys have to deal with the starting and stopping of motion themselves and have limits.

The problem is simple; Diminishing returns. The bow can’t keep upping the speed of lighter and lighter arrows forever. The bow has a max string speed even if you dry-fire it!

This tells us that every bow setup must have its own perfect arrow assembly weight that delivers the most kinetic energy to the arrow at release. Many would say, “Hey stupid, just keep testing different weight arrows, log the speeds and use the formula”. As you’ll see in a minute, (unless you’re using nothing but field points) this means very little when you really care about “Energy at Target” and not just at the bow.

So now you say the faster arrow has a flatter flight and is more accurate. It may have a flatter flight and be more accurate if no outside issues (like wind, broadhead planning, fletching drag, etc.) come into play. On the other hand, the heavier arrow will be less prone to having its course changed than the lighter one and may in many cases be more accurate in the real world. Now you’re saying the lighter arrow is going faster and could match the kinetic energy of the slower heavy arrow. So what’s the problem? The problem is the aerodynamic drag on the broadhead, arrow shaft, fletching and nock is going to chew up more kinetic energy from the light arrow than the heavier one.

Said another way, if you had two identical arrows, but one was 50 grains heavier than other, that heavier arrow (even though it left the bow slower) will lose less of its total kinetic energy on its way to the target than the lighter one. Why? They both are dealing with the same amount of drag from the air, but the heavier arrow has a better mass to surface area ratio than the lighter one and will keep more of its initial kinetic energy.

In other words, even though it was slower than the light arrow, it won’t slow down as quickly as the lighter one.

Now we get to the broadhead issue and energy at target. All that matters is how much speed and kinetic energy is left over when that broadhead contacts the game. Plain and simple, if you have a vertical drop when you take off the field points and screw on your same grain broadhead, your energy at target just dropped too.

We have done tests between field points, fixed blade broadheads, mechanicals and the HyperShock. We took one arrow and fitted 125-grain versions of each and saw the speed out of the bow was the same.

No surprise there.

At the target 30 yards away was a big difference. The HyperShock and the field point were so close in speed (energy at target) that the resolution of the measuring device couldn’t separate them (not to mention they were flying into the same hole). The other popular mechanical was slower and did drop at target relative to the HyperShock and field point, but nothing like the vertical drop and loss in speed (kinetic energy) the fixed blade had.

Want an even worse scenario we found? How about fixed blade broadheads attached to arrows with fletchings that spin the arrow. The exposed broadheads blades try to fan the air instead of cutting through it and lose even more kinetic energy!

On top of that, there are fixed blade broadheads that have part of their blades kicked, lifted or bent to impart spin. Since most of the blade area is straight, one part of the blade is trying to spin while the straight parts drag themselves through the air and reduce energy at target worse than a standard fixed blade with equal exposed blade area! Not only is it fighting itself to get spin, it is also fighting the fletchings trying to make them into worthless fans also.

I personally would take a straight arrow and a well-tuned bow, but if you must have fletchings that spin the arrow, the HyperShock will work well since it does not have much exposed blade area to act as a fan and chew up precious speed and energy.

In the future, when our lawyers allow us to name the other broadheads and testing equipment, we’ll reveal it all. But for now, don’t take our word for it, you can test and judge for yourselves. If your broadhead drops below your field point at target, you lost speed and energy at target.

So when your spending big bucks to get another few feet per second out of the bow, start thinking about the bigger picture; speed (or energy) at target.

We hope this article gives you some things to think about and please don’t forget to join us next month for another very penetrating article.
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