Primitive Archer
Main Discussion Area => Bows => Topic started by: AndyTurner on September 08, 2020, 05:24:10 pm
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Hi. It would be nice to compare bow performance using arrow speed for a common arrow mass/ poundage
For example, this bow shoots 100fps at 10grains/lb. That said I don’t want to make special test arrows.
What I would do is measure arrow speed with lighter or heavier arrows and then crunch some numbers to make an estimate of arrow speed at 10grains/lb.
Is thee a universally accepted equation already that everyone else uses?
Thanks,
Andy
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I could be wrong ( I often am ) but I think speed of cast has more to do with arrow speed than poundage.
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I know there are formulae out there but the problem with what you are trying to do is that arrow speed is a result of what happens when you release the string. Draw weight happens as you are drawing it. If you have a poorly made bow it will not release the stored energy properly. If you have two 40# bows, one with nice skinny tips and one with great clunky tips the skinny tip one will out shoot the clunky one every time.
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What is the reason you would use this data? I will assume its to make decision in some type of specific shooting application.
Those types of data answers are prolly out here currently.
Shawn~
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Is thee a universally accepted equation already that everyone else uses?
If the two arrow weights you want to compare aren't too far apart then you can assume the kinetic energy for both will be the same right after release and use the known arrow weight and velocity to calculate the velocity for the 10gpp arrow. This assumes the bow efficiency doesn't change much, which often only holds for similar arrow weights and spines. If there is a large difference between the two arrow weights then the efficiency will not stay constant and the calculation becomes inaccurate.
KE = 1/2 (mass) (velocity2)
KE1 = KE2
1/2 (mass1) (V12) = 1/2 (mass2) (V22)
V2 = (m1/m2)0.5 (V1)
Mark
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starting from 10 grains per pound arrow speed will go down as you increase arrow weight and the kinetic energy will go up. Conversely arrow speed will go up and kinetic energy will go down for lighter arrows. A rule of thumb I often use is that for a 50# bow it usually shoots about 15 fps faster if I reduce arrow weight by 100 grains. If I increase arrow weight by 100 grains it will only shoot 10 fps slower because heavy arrows are more efficient. A good estimate might be just using this number proportionately.
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It is an awful lot easier to make a test arrow for each bow. 1 arrow might take 20 mins to make. You are physically only going to be able to shoot a certain range of bows so just make a few different arrows.
The beauty of this is it is very simple and totally accurate :)
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Reason for this thread? I want to compare what I make with others to see how efficient my bows actually are. I want to make good bows in terms of both efficiency and reliability. This helps me measure the efficiency aspect.
It’s really really intreating what you said about heavy arrows (proportionately) are more efficient. I’m guessing it’s because they stay in contact with the string longer and soak up more energy. They also bounce off trees better. Lighter arrows of the same material expload more! 😂😂😂
Cheers guys! 😎
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Have a look at this:-
https://bowyersdiary.blogspot.com/2011/11/mass-velocity-and-energy.html (https://bowyersdiary.blogspot.com/2011/11/mass-velocity-and-energy.html)
Del
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I like the way you did that Del, I used to use extremely heavy shafts to calculatate stored energy instead of building force draw curves.
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I usually see which kill and penetrate better. Light ones have left me hungry a few times. Heavy ones make me tired, tired of dragging critters up and over the ridge.
Like i was sayin. Depends what yer doing. A slug selfbow shooting 575grns 100fps will keep an mans family fed in the right hands. Then again game don't hardly know they died of an arrow doing 100fps or 165fps.
But....
If your flight shooting selfbows it may matter a great deal.
Shawn~
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What do you think would be the minimum speed for a 600 grain arrow for Whitetail.
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DC,, I dont know the minimum, but I shot some deer and hogs with bows that would shoot a 500 grain arrow around 150 fps,,
I was using about 600 grain arrows out of those bows and they,, did well,, that formula Badger said would put that about 140 fps,, right,,??? for the 600 grain arrow,, so I think you could go slower,, but the only problem,, is even with good aim,, at slower speeds sometime the deer not there when the arrow gets there,, I never killed a deer with anything less than that,, so would just be guessing,, but if the deer would stand there,, im sure a much slower arrow would work,,
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DC, your bow would likely be in the 170's or at least 160's. with a 600 grain arrow and a fast flite string at 40#. More than enough power. At just over 40# you shot a 450 grain broadhead 222 yrds at the flight shoots and that was using a very fat 10 strand linen string.
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It was a hypothetical question. I was just curious about how crummy a bow could be used for hunting stupid(for Brad's post) deer ;D ;D
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Hi Del. I’ve just found time to read the post with graph in.
“Good Job Sir!” - that was a really interesting and informative experiment. Well done you! 😎
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nice shooting on your flight shoot,, congrats
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They had a deer killed hear with a 12lb bow at 10 feet.
110 be enough at close range with a heavy arrow. The only if is: whitetails down south are not whitetails of Maine or Kansas in size.
Shawn~
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Thanks HH
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DC - remember mass belongs to the arrow. It never decreases. So a 600 grain arrow is always 600 grains. A 1000 grain arrow is always 1000 grains. Velocity decreases as soon as it leaves the bow - and it decreases quite rapidly when it hits somethings. Remember momentum is Mass x velocity. If you have the same momentum - a heavier arrow going slower and a lighter arrow going faster - the heavier arrow will always penetrate better because it does not shed mass. It does shed velocity. So if the mass of the arrow makes up the bigger portion of your momentum equation it will retain more momentum when it hits something... .. I hope that makes sense. Basically, so long as a deer doesn’t jump the string a really heavy slow arrow will do as much if not more penetrating as a faster lighter one - from a given “power” bow...
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DC, if the crummy bow is like my 43 # bow I built 8 yrs ago getting into this, I agree w Brad, in that on unsuspecting deer, it'll work.
Shot a small coues deer after waiting the two deer out for 30, 45 minutes. At 12 yards, that 550 grain arrow took enough time to allow the deer to go from broadside to taking the arrow in guts, through liver, a lung and stopping above heart. Great penetration, slow arrow.
This year, shot. Mule deer. Thought I missed. But, sitting in blind days later, and doing some later CSI, surmised that buck dodged. 20 yards. I should have waited, dumb Stickmark :( . (That was a great flat Sawn red oak bow but not trapped, so it failed cause of knot I sinewed caused chrysal down lower limb. Another "dead bow", and another "I am an idiot" t- shirt).
I admire your work. You actually build 40#ers I would hunt with.
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Thank you. :D