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

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    Post by Frode Wed Feb 15, 2012 7:45 pm

    I was testing the new Alchem prod bow this evening, and this happened. I pulled the string and it popped back, but the brace seemed a bit lower. On the next shot the string broke at one of the loops. The marshal, a crossbow man himself, hadn't seen one do this before. Any thoughts?
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    Post by Geezer Wed Feb 15, 2012 8:11 pm

    No, that's not supposed to happen, but strings do fail occasionally, even new strings. Sometimes bows built with too much down-pressure on the string (frinstance, those with prod mounted too low in the stock) will roll the string off the top of the stock. Once you get it into the position illustrated in your photo, there's a good chance the eye-loop is already compromised. Once you break one strand in your string it's toast. Geezer
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    Post by Ivo Wed Feb 15, 2012 8:16 pm

    Wow...that's wicked! Shocked

    You sure that's a 90# prod there?

    I remember there was a topic where someone had an issue following a similar pattern...only not as extreme. If I remember correctly the culprit was low brace height and prod socket angle was too steep...but I could be wrong (my memory is crap when I'm busy in real life). Hope someone can remember a little better.

    [Was writing up a post when Geezer beat me to it, still need to go look for the topic though Smile ]

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    Post by Todd the archer Wed Feb 15, 2012 8:20 pm

    What did you have it braced at? I believe alchem recomends 3 1/2".



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    Post by stoneagebowyer Thu Feb 16, 2012 3:10 am

    That happend to me with my first project, when it was at too low a brace height. It took out the bolt rest in a spectacular fashion, as well. I popped it back, and it was no worse for wear.

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    Post by mac Thu Feb 16, 2012 7:30 am

    Frode,

    One way or another, the loop of the string got over the upper shoulder of the nock. As Geezer says, the geometry of the prod-stock-string system may be at fault. This can also happen if the upper shoulders of the nocks are too small, or not sufficiently square. It can be exacerbated by having too big of a loop on the string.

    The time to check these thing out is when the bow is strung. Look for any tendency for the loop to want to get over the upper shoulder of the nock.

    Looking at your pics from the "stringing" thread, I am inclined to agree with Dane about the brace height. Low brace height can make dodgy geometry turn into an "incident". Raising the brace height would probably have kept this from happening. On the other hand, you don't want to take that too far. That prod has quite a bit of recurve, and no deflex. If you brace it too high, you will be bringing the limbs closer to their elastic limits when fully drawn. At some point you will start significantly reducing the number of loading cycles the prod will endure.

    That the string did not survive the ordeal is not that surprising. A string is under its greatest tension when at brace. (this is counter intuitive, but apparently true) During the reversal, one of the loops will have been pulled forcible over the unyielding corner of the nock shoulder. Some damage to the strands is likely under these conditions, and as Geezer suggested: that's the "beginning of the end" of the string.

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    Post by stoneagebowyer Thu Feb 16, 2012 9:31 am

    Mac always gives great advice, Frode.

    Looking at the pics from your stringing thread, the loops seem really large. Did you make that string? They are not terribly hard to make, so you should take the time to learn. I'm transitioning to B50 for my strings, something you will be used to working with as an archer. And making extra strings at will is a nice thing to be able to do. Strings and bolts are consumables in my opinion. I just had muy frst William Tell this morning at my club's indoor range shooting my little Horton Scout. A couple of guys were there and witnessed it, and although it is cool, there goes two more bolts. Smile

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    Post by Frode Thu Feb 16, 2012 11:03 am

    Thanks for the response, all!
    Let me see if I can answer everything (and I have some postmortem photos of the stringbelow)...
    Ivo, based on his experience the group crossbow shooter says it's probably more like 110#, in which case the string was probably a bit light.
    Todd, Dane, and Mac, I believe brace was closer to 3". I'm guessing now that a longbow is a little more forgiving of low brace height than a steel prod? I didn't make either of the strings here, but I'm going to start! What do you recommend for serving crossbow strings?
    Dane and Mac, looking at all the loops below, I see that the loops do look large (the all black string came with the prod, and was too long. The yellow and blue string was made on site at an event by a merchant per my request for shorter, and for a 100# prod (I thought I was giving myself a little extra margin, oops!)
    My only other observation, and that of my fellow shooter, is that it kicks like a mule!
    Thanks,
    Frode


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    Post by stoneagebowyer Thu Feb 16, 2012 12:05 pm

    You know as well as I do what can happen to a wooden bow if the string breaks or it slips off at full draw...not good, generally!

    For serving, serving thread you use for archery bow strings I think would work just fine. I have used thick linen thread, heavily waxed, and that works well. Others may have other choices and suggestions.

    I dont recall the breaking stength of artificial sinew, but when I was making strings from that material, I was using 100 windings. B50 has a 50# breaking strength, so you are looking at 5000# at 100 windings. I dont think the rule of four (4x draw strength) applies in crossbow archery. As well, the loops I put into my crossbow strings are small on inside diameter, and fit nice and snug up to the little prod shoulders.


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    Post by pacer Thu Feb 16, 2012 2:54 pm

    I'd make sure the metal of the prod wasn't cutting into my string or the problem will just repeat.
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    Post by Frode Thu Feb 16, 2012 3:53 pm

    Thanks Dane and Pacer,
    The nocks seem OK, pretty smooth in fact, with no sharp edges, but I'll be augmenting them with leather pads.
    Here's a picture of the servings, center and loop. The centers are served with something much heavier, that seems to be coated (waxed?). Hopefully my little camera shows the details enough.
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    Post by Ivo Thu Feb 16, 2012 7:21 pm

    As with vertical archery - you can *feel* the right brace height...or in other words it is not too much nor too little kick.

    In crossbows there is another thing that kinda bothers me, but I never get enough patience to sit down and find a way to calculate it properly. Limbs with different mass have different driving force - light limbs of wood or wood/glass laminate accelerate faster, but put a heavy load behind them and they don't perform what so ever. Then take a steel prod - it accelerates slower (thus not so good for lighter projectiles), but at the end of the power stroke *it's got the force of a locomotive*.

    So I also believe what Dane believes...crossbow archery is a bit different since we deal with smaller bows, more aggressive forces behind materials with heavier mass moving at high speeds. If I'm not mistaken, Robin Allen once mentioned that with Crossbow prods you need a string that is approximately 5 times the draw weight, but I have also heard arguments of strings having to be as much as 7 times the prod #.

    Spoiler:

    If I'm not mistaken, the 5x ~ 7x the draw weight is just a quick guide
    line that we have been using blindly when the real deal is behind the
    power stroke,prod mass, and velocity of the limbs achieved at the end of
    stroke. Which would give us a different strand count for the same prod depending on the brace height chosen...since there is more energy in a longer power stroke for the string to deal with and it must be built accordingly.

    Any volunteers from the crazy scientist department? Laughing



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