I agree that KE increases with arrow weight. I consciously ignored that fact for simplicity, but including it would only widen the gap in calculated KE between comparable compound and single string bows. I re-did the calculations assuming a KE increase of 1.5% per 100 gr of additional arrow weight, and the point still stands: the compound generates much more KE. In my testing of various compound bows, 2-3% KE gain per 100 gr is typical at lower arrow weights then declines to ≈1% beyond ≈500 gr. My speed estimation method is fairly simple:
- Adjust IBO speed (measured at 70#/30"/350 gr) to actual DW/DL using 2.5 fps/lb and 10 fps/in rules of thumb.
- Calculate KE using adjusted speed from step 1: KE (in ft-lbs) = Arrow Weight (in gr) × Arrow Speed² (in fps) ÷ 450,240
- Use KE from step 2 to calculate speed at actual arrow weight. Apply KE adjustment factor if so desired.
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Yes, that calculator is very inaccurate. As shown below, it shows KE decreasing with increasing arrow weight, which we have both acknowledged is not accurate. This calculator is better (or the method I described above):
https://www.ashbybowhunting.org/arrow-speed-calculator
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I agree that the sample single string draw curve I posted is oversimplified and would not be perfectly linear in reality. However, a single string bow still isn't going to store as much energy as a compound at the same draw weight/length.
Just to clarify, these are the specific claims I'm disputing:
Compound
do outperform that by a significat margin...by 40-50 fps in the examples above.
Either the calculator is flawed (the one you linked to above definitely is) or you're not comparing apples-to-apples on draw length/weight between the compound and single string .
False. A lightweight arrow at 300 fps out of a compound will have far more KE than a 10 gpp arrow out of a single string (at the same draw weight/length). Exactly how much more depends on several factors, but I would guess it's on the order of 40-50%.