Ever wonder exactly how much kinetic energy is released by an arrow, based on its initial speed and distance traveled? Let’s find out.
Other related research:
Crossbow Arrow Kinetic Energy Chart:
We shot 400 grain arrows from 9 different crossbows, achieving nine different initial speeds ranging from 200 to 400 FPS in 25 FPS increments. Using multiple chronographs positioned at varying distances and heights, we calculated the drop in projectile speed at 10, 20, 30, 40 and 50 yards. Finally, using our arrow ballistics calculator (which requires that both arrow speed and mass are known) we determined the values plotted in the chart below.
This is as close as it gets to what your arrows will deliver in the field, though keep in mind that our tests were conducted at an indoor range to avoid wind and weather fluctuations which can impact arrow behavior, so actual kinetic energy values in the field can and probably will vary slightly from what we arrived at in our tests.
The chart below, and the research conducted is copyrighted material and is the property of BestCrossbowSource. You are welcome to share it, as long as you mention www.bestcrossbowsource.com as the original source of the data / research.
Why Did We Use 400 Grain Arrows In Our Tests Above?
Simply put, many of the best crossbows available on the market nowadays (2015) receive their speed ratings based on arrows weighing roughly 400 grain. So when you see a certain crossbow rated at 350 FPS, for example, this was most likely calculated using a 400, maybe 425 grain arrow. As such we decided to use 400 grain arrows, as this was bound to yield results most closely resembling those that a new crossbow buyer will experience when shooting the arrows in their crossbow package.
What If I Use Arrows Heavier Than 400 Grain? How Do I Calculate My Kinetic Energy Then?
Your arrow’s kinetic energy will depend on its speed. The heavier your arrow, the slower its initial speed will be, and the lower its speed will be when it hits the target, resulting in lower kinetic energy. So the important question right now is: how much does initial arrow speed drop as its weight goes up? We have not done very precise tests to figure this out (we will update this post when we do), however a very close estimate would be to assume the following. Please keep in mind that the values below assume the crossbow configuration is not altered, and that the only change is in arrow weight.
*Initial speed = our base speed determined using a 400 grain arrow:
425 grain arrows will fly at ~97.5 of initial speed
450 grain arrows will fly at ~95% of initial speed
475 grain arrows will fly at ~92.3% of initial speed
500 grain arrows will fly at ~90% of initial speed
525 grain arrows will fly at ~87.8% of initial speed
550 grain arrows will fly at ~85.5% of initial speed
575 grain arrows will fly at ~84% of initial speed
600 grain arrows will fly at ~82.5% of initial speed
Here is an example to demonstrate the use of the values above:
Example: let’s say you know your 400 grain arrow will fly at 350 FPS. If you were to shoot a 500 grain arrow from the exact same crossbow, its initial speed would be roughly 90% of 350, or around 315 FPS. If you were to shoot a 600 grain arrow instead, your initial speed would be roughly 82.5% of 350, or around 290 FPS. Knowing how your arrow speed changes with the increase in arrow weight, you can now easily figure out the new kinetic energy:
For instance, if you know your crossbow shoots your current 400 grain arrows with a speed of 400 FPS, our chart above tells you to expect 124.8 ft-lb of kinetic energy at 30 yards. If you’re planning on using 450 grain arrows, the information above suggests that your arrow will leave the crossbow at a speed of around 380 FPS (95% of 400). Knowing the rough speed at which your arrow will fly, you can now use our kinetic energy calculator to quickly figure out your arrow’s initial kinetic energy.
How Much Kinetic Energy Do You Need To Hunt Specific Game?
The chart below demonstrates the minimum, as well as recommended, kinetic energy, for hunting different-sized game. We strongly advise that you stick with the “recommended” values to maintain lots of room for error, particularly if you’re new to hunting.
Observations From The Main Kinetic Energy Chart Above
Here are some interesting things to note, based on the kinetic energy chart at the top of this page:
#1: Kinetic energy drops are fairly predictable. You can assume, roughly, that an arrow’s kinetic energy is reduced by around 3-4% for every 10 yards it travels. So if we have 100 ft-lb initial energy (at 0 yards), at 10 yards you can expect roughly 97-97 ft-lbs. of KE.
This is a good rough rule to remember in case you need to do some quick on-the-spot kinetic energy calculations in the field. Please keep in mind that this approach will only give you close estimates, and that it becomes a less accurate the further out we go (I wouldn’t suggest using it for anything beyond 70 yards, which is far beyond what you’ll be hunting from anyway).
#2: Even a 50 FPS speed difference alters kinetic energy significantly. If you use the chart above to compare kinetic energy between a 350 FPS and a 300 FPS crossbow, you’ll notice that the first one delivers significantly more KE at 50 yards, than the second crossbow does at even 10 yards! That’s a massive difference in a hunting scenario.
#3: Not all crossbows are suitable for hunting. Notice that our recommendation for elk hunting is 65 ft-lbs of kinetic energy (this is a conservative value, you can get away with less). Notice that at 250 FPS this value cannot be achieved. Even at 275 FPS you only get close to it at 10 yards out. It’s only from 300 FPS and up that you get enough kinetic energy at all feasible hunting distances (up to 50 yards) to be able to comfortably hunt elk. As such we recommend that elk hunters stick to 300 FPS or more. For whitetail deer this requirement is lower – any crossbow 250 FPS and up will be a safe pick. Compare expected FPS across over 120 crossbows and decide which one to buy.
Hopefully you find the above charts and information useful in determining what crossbow to buy and how kinetic energy changes based on arrow weight and speed.