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    First submission

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    War Song
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    First submission

    Post by War Song on Sun Mar 12, 2017 11:13 pm

    My third crossbow, but the first two had been junks I am too embarrassed to post here XD. Still not as pretty as the other submissions I seen here I'm afraid, but I hope it's passable.

    Delrin nut, stock made from oak and maple mill cuts from Home Depot, a polypropylene rope for a bow string, and a bunch of scrap parts and hardware I had laying around cobbled together into surprisingly comfortable shooter. Loose laminate prod from fence tension bars (thanks @kenh).

    The new prod (old one broke) is working like a charm. was able to check the draw today with a bathroom scale and it weighs in at 65 to 70 pounds at 13" powerstroke.

    Performance is currently a disappointment, clocked a 500 grain bolt at 102 fps, or 12 ft.lbs, on the chrony Neutral. I suspect my 1 inch brace was just a tad too low to be practical, so I shortened the string for a 2 inch brace. Sun went down before I can clock it again though. 



    Excuse the rubber band, it's acting as a temporary return spring for the trigger mech. Would replace later with a proper coil spring.




    Nut and side plate removed here. Just a simple sliding pin to lock and release the nut, actuated by a lever (which I bent into a sort of grease gun trigger to accommodate a pistol grip). 



    If you're wondering, the clip spring was bent from an aluminum gutter hanger. Have a lot of those laying around from my old job as a roofer.
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    kenh
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    Re: First submission

    Post by kenh on Mon Mar 13, 2017 5:42 am

    We all have to start somewhere!  That's an interesting build; at least this one shoots!

    A couple of comments not intended to be critical, but rather constructive:

    Polprop for a string is 'way, 'way too stretchy and probably a major contributor to the lack of performance.  You'd be better off with a string made from a strand or even two  #36 Mason's Line (has a breaking strength in excess of 700 lbs)  Mason's line stretches initially but then stops; polyprop just keeps on stretching.  
    My feeling is that you've got the loose laminates too tightly bound together -- they're supposed to slide on each other.  Original Chinese crossbows of this design used 4-12 bamboo slats, with draw weights in excess of 400#s, and held the slats together with a couple of wraps of crushed bamboo or rattan twine on each side.  All that mass of excess wrapping is stealing efficiency.

    I don't actually know the scientific effects of brace height on efficiency; but it can't be good.  Ten of thousands of years of practical archery experience by millions of people, does not have modern champion archers or crossbowmen using such a low brace height, probably for very good reasons.   "If it ain't broke, don't fix it."
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    War Song
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    Re: First submission

    Post by War Song on Mon Mar 13, 2017 12:23 pm

    kenh wrote:We all have to start somewhere!  That's an interesting build; at least this one shoots!

    A couple of comments not intended to be critical, but rather constructive:

    Polprop for a string is 'way, 'way too stretchy and probably a major contributor to the lack of performance.  You'd be better off with a string made from a strand or even two  #36 Mason's Line (has a breaking strength in excess of 700 lbs)  Mason's line stretches initially but then stops; polyprop just keeps on stretching.  
    My feeling is that you've got the loose laminates too tightly bound together -- they're supposed to slide on each other.  Original Chinese crossbows of this design used 4-12 bamboo slats, with draw weights in excess of 400#s, and held the slats together with a couple of wraps of crushed bamboo or rattan twine on each side.  All that mass of excess wrapping is stealing efficiency.

    I don't actually know the scientific effects of brace height on efficiency; but it can't be good.  Ten of thousands of years of practical archery experience by millions of people, does not have modern champion archers or crossbowmen using such a low brace height, probably for very good reasons.   "If it ain't broke, don't fix it."

    I wasn't aware of that fact with poly :O. I just knew it had been strong enough for my needs, and the mason lines at the store aren't marked with their breaking strength so I had been reluctant to try it. Though I'm not sure about that 700 lb number, since I had these mason lines snapped while my father in law and I were using them to line up our new fencing. I do have a spool of #49 Dacron laying somewhere...

    As with the binding, I'll see if I'm getting any improvements with the higher brace height first. I understand you had a 3.5 inch brace for the 28 inch prod with your pinlock build? Mine is at 2 inch for 30", so I think I'm still on the low side.  

    Thanks for the pointers Ken Smile.
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    Anatine Duo
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    Re: First submission

    Post by Anatine Duo on Tue Mar 14, 2017 5:51 am

    Plus one on the string and the wrap.  

    Turns out to be very easy to make a string... I started making them on break at work.

    I didn't find much change going from 2" to 3" brace on the Chrony, but it is generally accepted that for a given lath, higher brace = lower energy 

    Certainly there is a lot of evidence of high brace crossbows but I don't think it will help in your case.
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    OrienM
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    Re: First submission

    Post by OrienM on Tue Mar 14, 2017 10:04 am

    Cool! I like your bow, it shows a lot of ingenuity. The trigger mech is particularly interesting. Love the rubber band "return spring", I've done that before myself... Wink

    Brace height...you often see a much higher brace on crossbows than you would in archery bows; my take is that it helps with geometry, putting the string in a better position over the deck. Low-braced prods tend create a lot of downwards pressure and friction on the string. Theoretically speaking, a higher-braced bow is less efficient (it naturally shortens the powerstroke), but in crossbows the reduced friction seems to offset the loss of efficiency.

    I suspect any weak cast is from low overall prod weight...70# just isn't that much power in the world of crossbow prods. Is 13" the total draw length (prod to nut), or is it the powerstroke (brace height to nut)?
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    War Song
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    Re: First submission

    Post by War Song on Thu Mar 16, 2017 1:25 pm

    @OrienM

    13" is the powerstroke, measured from the back of the string to the back teeth of nut.

    From a thread on the forum, I read that a higher brace would result in a higher bolt speed and the ability to use a lighter bolt. Hence, why steel prods are strung with such high brace heights, to offset the weight of the prod tips.

    The friction on the bolt tracks have lessened significantly after I restrung the prod for a higher brace. Poundage is also 5 pounds heavier now. Perhaps the prod wasn't under enough tension for a efficient cast? It's been snowing and overcast these couple of days so haven't been to clock the new prod.

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