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    First build full draw pic and speed test

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    African Archer
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    First build full draw pic and speed test

    Post by African Archer on Sun Dec 09, 2012 1:29 pm

    I managed to spend some time today shooting my crossbow, and decided to see how fast she is. i used my range finder to mark out a 20m to target mark and a half way mark. Recorded the sound and used that to measure speed. Now this is where i get nervous, i did the test quite a few times and even did a 10m to target test and ended up with the same results.
    the crossbow Casts a 500 grain bolt 273 fps ?, Now i was very careful with set up and used 10m as the mike distance ( i also shot over the mike )so to negate sound delay, and worked carefully with a sound program to measure the speed (.24) of a second over 20m. if anyone is skeptical please do the math cause i am, but she dose look dam fast, and penetration is awesome, but i do think i need about a 650 grain arrow just to be safe and drop the noise level.
    Any thoughts guys would be good
    thanks.
    Use the link to see the vid.




    http://s1175.beta.photobucket.com/user/weskuhn/media/IMG_0301.mp4.html?sort=3&o=0
    Photobucket" alt="" />Photobucket" alt="" />
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    Todd the archer
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    Re: First build full draw pic and speed test

    Post by Todd the archer on Sun Dec 09, 2012 2:45 pm

    Looks great. That speed is awesome. Of course the best way to varify speed is with a chronograph. I would have estimated maybe 235 fps. Since you have the horsepower I would definitly go with heavier bolts.

    And again awesome job!



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    African Archer
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    Re: First build full draw pic and speed test

    Post by African Archer on Mon Dec 10, 2012 7:22 am

    Thanks Todd , ya would love to shoot it through a chroni but dont no anyone in my part of the world with one, they dam expensive here to, but would be interesting to see just how accurate the sound method is, iv been working on a 300 grain broad head so will be sticking that on a bolt soon , interested in seeing how a bolt with huge foc flys.
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    Re: First build full draw pic and speed test

    Post by hullutiedemies on Mon Dec 10, 2012 11:14 am

    Speed sounds feasible, just one thing
    is this the 400#/9.2" bow ?
    Because 1800N times 0,23 m stroke means ~210J stored energy.
    30g bolt going 82m/s has only about 100J kinetic energy.
    That is less than 50% efficiency ! You are practically dry firing your bow.
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    African Archer
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    Re: First build full draw pic and speed test

    Post by African Archer on Mon Dec 10, 2012 11:33 am

    Yip thats the one, if you dont mind me asking, how did you come to those figers, im really interesten in prod efficiencies , getting the max out of them.
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    African Archer
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    Re: First build full draw pic and speed test

    Post by African Archer on Mon Dec 10, 2012 11:34 am

    Is anyone else seeing the photos upside down
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    Hotspur
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    Re: First build full draw pic and speed test

    Post by Hotspur on Mon Dec 10, 2012 12:05 pm

    The photobucket album looks great (In the forum I see two photos right side up! Looks like there are some extra characters next to pics).

    Where did you get the prod?
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    African Archer
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    Re: First build full draw pic and speed test

    Post by African Archer on Mon Dec 10, 2012 12:08 pm

    Hotspur thanks, was fighting with the new photobucket, the prod i built from new 8mm spring steel
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    hullutiedemies
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    Re: First build full draw pic and speed test

    Post by hullutiedemies on Mon Dec 10, 2012 12:21 pm

    african archer wrote:Yip thats the one, if you dont mind me asking, how did you come to those figers, im really interesten in prod efficiencies , getting the max out of them.

    Computing projectile KE and stored energy of a harmonic spring is high school physics. At least in this end of the time zone.

    Ek= 1/2m*v^2
    Etot= 1/2 F*s

    m -mass
    v- velocity
    F- force
    s- stroke


    Bow is not allways a perfect harmonic spring. But usually close enough to get a ballpark figure for a feasibility check.
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    African Archer
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    Re: First build full draw pic and speed test

    Post by African Archer on Mon Dec 10, 2012 12:29 pm

    Ya think we must have skipped that one , i was at an agricultural school we learnt the KE of a cow hitting a human at 10m Stretcher Razz
    Thanks for that tho will sit and go through it later
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    olrono
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    Re: First build full draw pic and speed test

    Post by olrono on Tue Dec 11, 2012 7:48 pm

    african archer wrote:Thanks Todd , ya would love to shoot it through a chroni but dont no anyone in my part of the world with one, they dam expensive here to, but would be interesting to see just how accurate the sound method is, iv been working on a 300 grain broad head so will be sticking that on a bolt soon , interested in seeing how a bolt with huge foc flys.


    I don't know if you are from "Africa" or is only your handle? But here in the States we have an outfit called "Cheaper Than Dirt", and they sell a 'crony' for only $79.97 that does 30 to 7000 feet per second. Catalog # GNS-062 Phone # (800) 421-8047 Love that bow!
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    Re: First build full draw pic and speed test

    Post by African Archer on Wed Dec 12, 2012 5:09 am

    Olrono I live on the east coast of SA in the outskirts of a city called durban. Unfortunately we do not have the awesome selection of fishing and hunting gear that is available in the states, but after hunting around I have found some plans for a chrono that is very easy to make and I already have everything to build it , so if it works well and is correct I'll do a build tutorial so everyone can enjoy it , it is available on archery talk but you do need to be a member to access the .pdf and free software. It is called the coil chroni and from tests is very accurate.
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    Re: First build full draw pic and speed test

    Post by kenh on Wed Dec 12, 2012 8:11 am

    Looking forward to that Chrono build. Your bow build certainly is elegant.

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    Re: First build full draw pic and speed test

    Post by Rizzar on Wed Dec 12, 2012 8:32 am

    Hey everyone!

    First again, nice work on that.

    In my opinion you should have a look on the prod mounting it seems a bit out of center.



    I think you should shoot a heavier bolt, too, but I am not agreeing with the formulas used to calculate the energy.
    First of all, you can never apply the same energy stored in the prod into a kinetic mass.
    Especially for crossbow steelprods.

    You must consider the mass of the prod and the string has to be moved, too.
    And you always have a certain amount of friction which decreases energy transfer.
    Even a roller nut from steel takes it´s energy.

    You should try out heavier Bolts in 10g steps (40,50,60 and perhaps 70).
    At least when you built your chrony. (looking forward to it)

    Most of my ressources state to get to 60-65 m/s with a steel prod in safe operating conditions.
    So this seems very fast, lets have a look if the chrony can proof the data.

    How much is the prod weight? 920-960g ???
    Most crossbow prod weights related to the belonging bolts I know from different german sources are in relation of 1/45 or 1/48.
    But the problem ist these were mostly very heavy prods (commonly ~3kg on a "Halbe Rüstung") with higher draw weights than we use.
    It is not that easy to compare with our low weight modern replica bows.
    So with around 3kg prod weight the bolt would have 60-70g.

    Not that easy, you should make tryouts to get into the best speed mass(energy) relation.
    And ft it sounds to loud, it would be a hint of too much dryfire.

    Greetings Rizzar
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    Re: First build full draw pic and speed test

    Post by African Archer on Wed Dec 12, 2012 1:55 pm

    Rizzar the prod is slightly off but not as much as your illustration is showing, the camera is not 100 percent square to the crossbow , so wouldnt give you an actual off set so i will sort that one out, hopefully il knock the chrono together this weekend and give it a go, really keen to see how it operates , as for the bolts, im going to have to find a heavier wood to make the shafts from the ones i have now are beech so maybe ash or oak should do.

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