Today I decided to do some arrow comparisons. I tested two different arrow setups out of my bow. Single cam, set at 60# draw weight and 29" draw length. Arrow 1: My current setup. Beman ICS 400 spine, 29", 100 grain tip. Total weight 387 grains. Calculated speed 248 fps. Arrow 2: Beman ICS 340 spine, 29.5", 100 grain tip. Total weight 422 grains. Calculated speed 237 fps. I shot groups of four arrows into a Rinehart cube target that contained two arrows from from each size at 20, 30, and 40 yards. Results: 20 yards: All arrows grouped together, the lighter weight, lighter spined arrows penetrated the target 2-3 inched deeper than the heavy, stiffer arrows. 30 yards: All arrows grouped together but the group was not quite as tight as the 20 yard group. Again, the light arrows penetrated 2-3 inches deeper than the heavy arrows. 40 yards: The heavy arrows start to show significant drop compared to the light arrows but the respective arrows grouped together. Light arrow out penetrated the heavy arrows again by about 2 inches. The difference in POI was about what I expected to see considering the difference in weight and speed. The main result that is confusing me is the fact that the light arrows are out penetrating the heavier and stiffer arrows. It's exactly opposite of what I think I should be seeing and I don't understand why. I know that it is what it is and I suppose it's possible that my bow just has a sweet spot of 6.5 grains of arrow per pound of draw weight of something like that but that still doesn't answer my question of why. Anybody else have any thoughts or ideas to offer?
How? Perhaps confirming with a chrono may be insightful. A few other thoughts: Fieldpoints/broadheads? Sound differences? May be interesting to setup a camera/phone with slow motion to see if there are other insights (arrow flex, non obvious release differences, rest performance, etc) if broadheads fixed or mechanical? Mechanicals, opening early?
Same spot for the groups but I rotated the target between groups. After talking to a couple of friends, here's the best theory I've heard so far: At the close ranges, extra speed trumps the extra weight and allows for the lighter arrows to out penetrate the heavier arrows. Moving past 40 yards there will be a point where they will penetrate the same and then the results will be reversed and the extra weight and momentum of the heavy arrow will trump the speed of the lighter arrows. Keep in mind that this theory applies to field points and a foam target and may or may not transfer to a broadhead cutting through hide, flesh and bone. Another factor to consider is that my bow is tuned to shoot the 400 spine arrows which could have an undesirable effect on how it shot the heavier arrows so the test results may not be completely fair. Anyway, His theory sounded good to me. What do you guys think?
Would be interesting to see actual clocked speeds are. Not sure how much difference the change in spine is going to cause. Better comparison would have been of different types of arrows both in the same spine and differing gpi. Like a ICS hunter 400, with an Easton bloodline 400.
These assumptions would be correct. There comes a certain point when KE and momentum from a heavier arrow will outperform the speed of a lighter, faster arrow. I have found this mark to be anything further than 37 yards for me. This is when tighter groupings can be observed with my heavier arrow and my lighter arrows start to drift apart. Like you mentioned, your bow is set up for the 400 spine arrow, so the drop can't really be factored in here, that will be a given. Especially when your bow is producing less than 275FPS. The speed your bow is producing is better suited to the lighter, faster arrow. FOC wasn't mentioned on these two arrow configurations, that might be something worth looking into. Personally, I would say the 387grain arrow is better suited to your particular setup. I would steer clear of that 422 grain arrow for hunting purposes if you can. This opinion has many debatable factors too, but in my experience, you're going to see more animals jumping your string with that 422 grain arrow. Whitetails have a very keen sense of hearing and can detect sound faster than most people expect. I'm not a speed freak by any means, but I try to keep my bow producing 280FPS whenever possible. If my setup changes and I'm getting less than 275FPS, I go with a lighter arrow and add FOC. This is what works for me. Not saying your should do the same, but use what works for you. In this case, the 422 grain arrow did not work, steer clear. I'm happy to see however that you are doing multiple arrow tests and questioning results. Keep up the good work and good luck on finding that sweet spot.
I would like to have a real speed from a chronograph as well instead of the mathematical estimate. I wonder how close the two figures would be. I know my tests were far from scientific and there could have been a lot of things done differently if my goal was to pick a new arrow. Truth is that I was looking more towards a confirmation that I already had the best arrow when compared to my previous choice. I recieved the confirmation I was looking for. This is my first foam target so this was the first chance I've ever had to do any kind of reliable penetration comparison. On a side note, the target results matched the hunting results I had this past season with the two arrows in question. The heavy arrow resulted in poor penetration and a lost deer where the faster arrow resulted in in quick kill and easy track. Thanks everyone for the advice and ideas.
I did a similar test a few months ago, but through my chronograph. Arrow weights were very similar to yours, 400 and 430 respectively. The chrono, of course, showed faster speeds with the lighter arrow. The chrono was set up about 2 yards in front of the target and I shot at 10 yds out to 40 yds. I was wanting to see which arrow decelerated at a faster rate, and out to 40, neither did over the other to a real noticeable amount. Physics will show in equation the faster something goes, the faster it will slow down because of drag, or the opposing forces to the forward motion. In pretty much all hunting scenarios with arrow weights of comparable likeness in weight (400 grs vs 430 grs, NOT 400 grs vs 750 grs) you're not going to see enough of a difference to matter at most bowhunting shot distances. The lighter arrow likely penetrated better because the shafts overall weight is less and any weight up front will increase FOC. This changes the effective or dynamic spine and gives the appearance of it being "stiffer" as you are changing the center of gravity from the center to a different location on the arrow towards the front. This changes the way the arrow flexes as it is not necessarily flexing at it's natural axis (dead center) anymore, making the point not oscillate as much as it enters the target. The tip of the arrow is basically entering at a straighter "line of sight" than if the flex or axis point were farther back on the arrow. Bows and arrows are mechanical devices and as mentioned, there is an optimum to each setup based on several factors such as draw wt., draw length, arrow weight, FOC, release technique, etc. You'll find there is no simple rule of thumb for everything, and if you really fine tune your setup for what you have at that optimum, you may just find you'll out penetrate a heavier arrow from the same bow with a lighter arrow every time. My setup is a bow set at 72#, shooting a 400 gr arrow at 309 fps at the bow. This same arrow at 40 yds is traveling at 287 fps, KE is 73.1 (if it means anything to anyone) at 40 yds - which is way more than plenty to punch through any elk on the mountain. I like a flatter trajectory, that's all...
