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    String slips off nock center (Prod angle issue)

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    barsod123
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    String slips off nock center (Prod angle issue)

    Post by barsod123 on Mon Jul 04, 2011 10:50 am

    Hello, I thought I saw this mentioned somewhere but I'm having a hard time finding it. I tested the crossbow and the string pops off the top nocks.Can iI fix this by wrapping the ends of the prod with string to keep it on?( kind of like the plastic end pieces the fiberglass prods have) or is this a matter of something else that needs to be fixed?
    Help in figuring this out, or directing me to somewhere else is appreciated
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    jake-owa
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    Re: String slips off nock center (Prod angle issue)

    Post by jake-owa on Mon Jul 04, 2011 3:17 pm

    Well, a pic of the crossbow prod and how the string ends up would help. It sounds like there may be too much angle on the bow. I don't know what you mean by "top nocks". I am assuming it's the top string notch on both sides of the bow.
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    Re: String slips off nock center (Prod angle issue)

    Post by mac on Mon Jul 04, 2011 4:48 pm

    Barsod,

    The short answer is that something else indeed does need to be fixed. However; without a picture, we can't even begin to guess what that might be.

    Mac
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    string pops off top nock

    Post by barsod123 on Mon Jul 04, 2011 7:59 pm

    I sat and looked at it, and i think the string has to be more in the center of the prod limb. Its more towards the top. I'll try that and maybe get a pic up. Yes jake you said it better,thats what happens. I used the cord bound plans from alchem, but has a prod for darkwood.


    Last edited by barsod123 on Mon Jul 04, 2011 8:37 pm; edited 1 time in total
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    Re: String slips off nock center (Prod angle issue)

    Post by Todd the archer on Mon Jul 04, 2011 8:16 pm

    Sounds like your prod needs to be tipped up more. The string should just touch the top of the deck without undo pressure.
    Of course pics will help us better.

    Todd
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    Re: String slips off nock center (Prod angle issue)

    Post by jake-owa on Mon Jul 04, 2011 8:20 pm

    Todd the archer wrote:Sounds like your prod needs to be tipped up more. The string should just touch the top of the deck without undo pressure.
    Of course pics will help us better.

    Todd
    Right of course, I mistakenly said too much angle but its much more likely not enough angle.

    If the string loop ends are too large the string can move also. For all these reasons, a picture is worth a thousand words.
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    Re: String slips off nock center (Prod angle issue)

    Post by barsod123 on Mon Jul 04, 2011 8:43 pm

    built it at 6* maybe cut it at 12* will be better its hard to know. Thanks for helping everyone.
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    Re: String slips off nock center (Prod angle issue)

    Post by Geezer on Tue Jul 05, 2011 9:51 am

    Geezer here on the popping-string. From the photo, it looks like you've got an Alchem or a Darkwood 28 inch prod. The size of your string-loops is about right and the string is approximately the right diameter. You may indeed have too much down-pressure of string on the table, which tends to peel the string off at the end of the throw, particularly if you're using rather light bolts. I can't see how much brace/fistmele you have. I would recommend 3-4 inches of brace for this prod... I usually aim for the middle... 3.5 inches, and will twist/shorten a string if the brace stretches lower than 3.
    If you have excessive string pressure on the top of the stock, you'll need to pitch the top of the prod forward to reduce the drag. If you over-pitch the prod, you'll get string-hop, which will result in misfires instead. The golden-mean is hidden somewhere in the middle. When you' have built a few bows, you'll get things worked out so this seldom happens.
    Keep making... Geezer..
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    string pops off

    Post by barsod123 on Tue Jul 05, 2011 6:16 pm

    I did have too much pressure on the deck, I removed the prod today and tilted it foward more resulting in the string just resting on the deck, so now the prod is at 12 degrees . The prod is from darkwood and the string aswell. When i first strung the prod it was a heck of a time. From the prod to the string when strung (brace) was like a 1 inch i think. I know thats not right.
    Geezer stated to twist the string to reach the 3" or so mark, does twisting the string 1 inch shorter, result in the brace becomming 1 inch larger?
    I'm using three fletched, 16.75" tip-nock 125gr. bolts. suggested by a gander mnt. archer employee. Do I need heavier?
    Thanks again for everyones input and suggestions.
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    Re: String slips off nock center (Prod angle issue)

    Post by Todd the archer on Tue Jul 05, 2011 7:44 pm

    Twisting the string 1" shorter will not necessarily result in raising the brace height 1". However excessive twisting should be avoided. More importantly the prod angle can only be set when the prod is set to the correct brace height.

    Also I could not tell from the picture if the groove on the deck is deep enough for a third fletching, as most medieval crossbows use only two fletchings.

    Todd
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    Re: String slips off nock center (Prod angle issue)

    Post by Basilisk120 on Tue Jul 05, 2011 7:50 pm

    barsod123 wrote:,
    Geezer stated to twist the string to reach the 3" or so mark, does twisting the string 1 inch shorter, result in the brace becomming 1 inch larger?
    I'm using three fletched, 16.75" tip-nock 125gr. bolts. suggested by a gander mnt. archer employee. Do I need heavier?
    Thanks again for everyones input and suggestions.

    Its not a 1 to 1 ratio for reducing brace height to shorting strings. It depends on the shape of the prod and other things.

    As for the bolts. They seem to be a fine weight but Todd mentioned I'm not sure a three fletched bolt would work if the vanes are 120° apart. Most fletches on the bolts I make are 180° apart.



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    string pops off top nock

    Post by barsod123 on Tue Jul 05, 2011 7:59 pm

    The groove is deep enough, I wasn't going for a replica off of history just something to have fun at the range with some store bought bolts slightly modifitied. I have shot the crossbow once with the bolt and it worked and hit the target, thats how I came upon this string problem.
    So if the string and prod are not a 1:1 ratio how does one happen upon getting the 3 inch brace? Does it come down to string lenght to get that brace? I have not altered anything, just plugged and play once.
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    Re: String slips off nock center (Prod angle issue)

    Post by Todd the archer on Wed Jul 06, 2011 2:28 am

    There is no formula for how much to shorten your string. It is trial and error. Which means twist it, restring it, and check. Repeat until you get the correct brace height. This can be a lot of work considering how hard it is to bend the prod. Also best to brace slightly on the high side as a new string tends to stretch abit until it settles in.

    You can see why people are discussing ideas for making a "bow press" to make this job easier.

    Todd
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    string pops off top nock

    Post by barsod123 on Wed Jul 06, 2011 4:55 am

    Well, I got a little work to do then, thanks for the help and information everyone.
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    string pops off top nock

    Post by barsod123 on Sat Jul 09, 2011 10:38 pm

    Hi, did some changes and did a go at it. Here is a pic of what i mean.

    I had the string just touching the deck, and the brace was at 3.25" but shrank to 2.5 after a hour and a half. Prod is angled at 6 degrees. Is this still a brace issue causing this? Or a prod issue-like the nock not ground down enough?
    I'd appreciate more help.
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    Re: String slips off nock center (Prod angle issue)

    Post by mac on Sat Jul 09, 2011 11:25 pm

    Barsod,

    A picture from the side would be very informative.

    Mac
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    string pops off top nock

    Post by barsod123 on Sun Jul 10, 2011 4:54 am

    Mac:
    From the side down the prod?The string? what exactly should I be focusing the camera on?
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    Re: String slips off nock center (Prod angle issue)

    Post by Todd the archer on Sun Jul 10, 2011 5:03 am

    I think Mac means a side view.

    Did the problem stop when braced at 3 1/2" and then return when the brace dropped down? Looking at that picture, it may be a prod nock issue. Does it always happen on the same side?

    If you are experienced with making string nocks, you might be able to correct that using a chain saw sharpening file.

    I have tweaked my prod from Alchem with a file to round the edges better so that string life is increased. Will try to post a pic later to show what I have done.

    Todd
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    string pops off top nock

    Post by barsod123 on Sun Jul 10, 2011 5:36 am

    I never had time to try it at 3.5". I work on it at work. I was thinking the nocks too were not deep enough, or even have a angle greater of 90 degree> see photo


    I've had it on both sides and just one side in the 3 times shooting it. Im about to cut the binding and re-brace it to 4.25" on monday so if it settles its at 3.5" or so,
    Todd, a pic would help and maybe how much to file off.
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    string pops off top nock

    Post by barsod123 on Sun Jul 10, 2011 6:22 am

    Here is another thing I noticed. I dont know how much of a difference it makes. Besides the dimensions, on the left side there is a space from the loop to where the string intersects planes with the prod( the top of the prod showing just right to the loop. (With the string re-strung)On the right side the string is parallel to the top of the prod and is no space. I hope i explained that right.
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    Re: String slips off nock center (Prod angle issue)

    Post by mac on Sun Jul 10, 2011 7:26 am

    barsod123 wrote:Mac:
    From the side down the prod?The string? what exactly should I be focusing the camera on?

    I think that we need to see a side view that shows how the prod is situated with respect to the tiller. The pic should include averylthing from the prod to the lock, and be focused on the tiller. One pic with the bow braced, but not spanned, and another with the bow drawn to the lock would be good.

    It looks to me like part of the problem is that the string loops are too big, and the "shoulders" of the knocks (at least the right one) are not square enough. There may also be a bigger problem in the geometry of the weapon its self. A couple of side view pics should show us what we need to see.

    Mac
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    Re: String slips off nock center (Prod angle issue)

    Post by Todd the archer on Sun Jul 10, 2011 8:00 am

    Here's pics of my "tweaked nocks:





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    string pops off top nock

    Post by barsod123 on Sun Jul 10, 2011 8:35 am

    Mac I can take a picture of the tiller, nut and prod. But if the brace is not correct would the spanned and unspanned picture help at all?
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    string pops off top nock

    Post by barsod123 on Sun Jul 10, 2011 9:05 am

    Thanks for the pics todd, I think I have to shape them a little.
    Mac heres some more pics, the brace has shrank to 2" now. So I need to redo that again.


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    Re: String slips off nock center (Prod angle issue)

    Post by mac on Sun Jul 10, 2011 3:12 pm

    Barsod,

    I keep hoping for something that shows more of how the prod is situated with respect to the tiller; but I think I can see what's wrong from what you have shown us.

    I think it is a combination of three things....Low brace height. Over sized loops. And sloping shoulders on the nocks.

    I think that the string is making contact with the bellies of the recurves, and its momentum is carying it "up the ramp", so to speak. At that point, the loop can get over the shoulder of the right nock, and thus come undone by sliding up onto the limb. The momentum of the string then carries the left loop off its nock pin, and the string swings around to the right and is carried off the right limb as well. The string ends up on the ground several feet in front of the bow and slightly to the right.

    Here is my prescription....
    --File the shoulders of the nocks like Todd showed in his pic (above).
    --Make a new string with slightly smaller loops.
    --Have the new string be enough shorter that it does not make contact with the bellies of the recurves.

    I say a new string, rather than just twisting the old one to make it shorter, because I can see from the pics that the string is already too tightly twisted. Too much twist makes the string stretchy, and robs energy. The string should have, at most, two or three full twists.

    Mac

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