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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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5 posters

    Measuring projectile speed

    Ivo
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    Measuring projectile speed Empty Measuring projectile speed

    Post by Ivo Wed Dec 15, 2010 8:45 pm

    Hello Everyone,

    I've had this question on my mind since reading a few topics where people didn't have chronographs to test their equipment performance. I'm aware that chronographs such as "Chrony" are not all that expensive these days, being in the range of $70-100...still it is of great interest to me and I'm sure a few of you might also be interested.

    http://en.wikipedia.org/wiki/Ballistic_pendulum

    The item in question is a Ballistic Pendulum, it's principal of operation and perhaps, if possible, it's construction...and most importantly - can it at all be useful in measuring the speed of a long projectile like an arrow?

    If not...perhaps there are other alternatives to electronic chronographs.

    Thanks,

    Ivo
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    Post by Basilisk120 Thu Dec 16, 2010 12:41 pm

    Ballistic pendulums would work well for approximating the speed of an arrow. Make it from a decently heavy piece of wood and suspend it from a point with fishing line. Arrows would work well because they would stick in to the wood and stay.
    4 issues to consider when building one.
    1. Appropriatly heavy mass target. The mass needs to be light enought to move a messurable amount but not so light that it moves in an unstable erratic pattern. I would aim to have the mass rotate about 30° or so. Just taking a guess at a good number nothing sacred about it.
    2. Making the pendulms arms (or string) as light as possible so that the mass can be ignored. Fishing line would work well. The math gets considerably more difficult if the arms aren't of negligable mass.
    3. Having a way to measure the distance the combined mass (target mass + projetile) traveled. That adds negligable friction. The ribbon methon mentioned in the Wiki was interesting. Others might include using magnits or a pen.
    4. Having accurate masses of all the componants.
    I thought I had a forth issue but drawing a blank. those should be enought for now
    Yeah the above issues have all been solved at different times in different and creative ways. Just throwing them out there to think about.

    Other ways to measure arrow speed? well there is the Mythbusters standard of the high speed camera and a calibrated background. But thats not really cheaper or easier than buying a chronograph.
    The Wiki article mentioned a couple other methods that were used but I don't think the rotating paper tool would work (arrows and bolts are too long)
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    Post by jake-owa Sat Dec 18, 2010 5:02 pm

    I did something different to measure arrow speed. I measured out 100' from my target and recorded my shots. Then I took the audio into a digital editor and found the bow release sound and the sound of the target being hit and measured the time between the two. Then I subtracted 11% for returning sound at 1100 fps and I got the time from transient to transient, how long it took the arrow to actually fly 100'. for example 0.5 seconds over 100 feet=200 fps.

    Now this won't tell you the "muzzle" velocity like shooting point blank through a chronograph will but it will give you a good idea of your average speed over a distance. If I did a 50' test my average speed would be better but it's a compromise.
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    Post by Ivo Tue Dec 21, 2010 1:02 am

    Thanks guys, all points taken. Smile

    Magnet retained arm(finally some use out of those headphone magnets) or simply the use of marker for recording the motion of the pendulum is great.

    And suspension by means of fishing line, I was thinking something with less stretch, perhaps some dacron or a few strips of really thin sheet metal to keep pendulum from rotating.

    Two questions come up...

    So far I've seen a few different versions of pendulum suspension - 1. by a single rod...usually in a cylindrical pendulum; 2. by four strings connected at four corners of the box shaped pendulum.



    The first one is pretty much straight forward as to the point of the pendulum to take the reading from(assuming it's along the axis of the suspension cable/string/rod), but with a pendulum attached in four points the motion is somewhat linear? or am I mistaken?

    Any way the above mentioned can be figured out during assembly...what bothers me the most is this...
    Material of the pendulum, will a foam insert be a good idea? or will it dampen the impact throwing off the transfer of kinetic energy and as the result give an incorrect reading? Reason I'm asking...well shooting a block of wood might break some arrows...especially long ones....any ideas?

    PS: Jake-owa...I've heard of a slightly different method, but still very similar to yours...two frame stretched papers with microphones connected to them were used to record and plot the time of impact on each. Smile
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    Post by Basilisk120 Tue Dec 21, 2010 2:11 am

    The material the block is made out of shouldn't matter much. The main concern is that it doesn't break apart and the projectile stops in the block.
    The goal is to have all of Kinetic Energy of the projectile to be imparted in to the pendulum. The pendulum itself can be considered a black box. What happens inside doesn't matter as much as what happens with the pendulum.
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    Post by Ivo Wed Dec 22, 2010 6:32 pm

    Well that settles it then. Very Happy

    I'll still try both the block of wood and foam insert with some cheaper arrows...not that I don't trust an engineer, just there's that little superstitious man inside me that needs something to chew on...plus it'll add a bit of fun to my midnight experiments.

    Thanks for the help guys, I'll post the results along with my update on my brother's crossbow build.

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    Post by robert.collard.5 Sun Mar 17, 2013 4:16 pm

    I found some software called arrowmatcher which is available in nearly all formats. You can figure out ballistics for individual arrows as well as make up tables for your crossbow. Depending upon your computer type, aka: Windows, Mac, or, Linux, look it up on Google.
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    Post by rolynd Mon Mar 18, 2013 11:09 am

    If you already have a smartphone (android based) ther is a free app called chrono connect lite which was developed for airgun speed measurement. Bill Hayes tested it against a calibrated chrony F-1 with his slingshot and found it to be reasonably accurate to within 5% of the claibrated chrony. It should work for crossbows too if your shot isn`t totally silent on the launch.
    Maybe an option for the smartphone users out there, Should give you at least a fairly good idea of the ballpark figure you are dealing with.
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