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    Split limb and Draw length

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    80Eight
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    Split limb and Draw length

    Post by 80Eight on Fri Nov 20, 2015 3:16 pm

    Question for the Veterans and Experienced: Is there a "formula" to determine the Draw Length for a Split Limbed Compound Crossbow? I'm in the brainstorming stages and before I get carried away, I'd appreciate any info.

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    Re: Split limb and Draw length

    Post by c sitas on Fri Nov 20, 2015 3:55 pm

    80; your draw length would be determined by the string wrapping on the cam or pullies or what ever your using. I make a lot  of crap to shoot. What I mean is ,I'll take almost any compound bow and use the limbs and cams. make my riser and go from that. I never worry too much about draw length when working like this . I would prefer to short draw the cam though,cause I don't need a let off. Doing so this way gives a little more of a power stroke, plus a lot of movement in the draw length for gaining position needed. I'm no scientist here ,all I want to do is hunt and kill humanely.They do that and then some. I work on the cheap, build my strings , triggers stocks and what ever it takes. I have more time than money,he-he.
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    Re: Split limb and Draw length

    Post by Onager Lovac on Fri Nov 20, 2015 4:07 pm

    Hello and welcome, generally the formula i fallow for making wooden longbows is: 1/10th the size of the bow is the brace height, and the bow itself can be drawn half it size minus 1/10th, so a 66 inch bow braced at 6.6 " can be drawn 26.4 safely, this is the formula for self longbows in the book "The bowyers bible vol. 1", mind you this is the "general safe" formula if you are inexperienced and trying to make board bows and in reality you could make a bow much smaller and with a longer draw length if you make a bow wide, in a proper piramyd shape and with proper materials. For a crossbow the formula is diferent since a bow is drawn and shot generally in less than 5 seconds while a crossbow is drawn for minutes or hours and sometimes you cannot reduce the brace height to much or the bow might reverse itself or the string might get stuck in the front of the stock, but for wooden prods i generally fallow the safe formula but try to reduce the stress on the bow a bit example: 40" prod, 4" braced, 16" draw length(includes brace height 12" power stroke). What i do: 40" prod, 3" braced, 11" draw length, i also add a fiberglass backing and a nilon cable to increase resistance and power. If your compound is going to have a 2 to1 mechanical advantage you could reduce the prod size to 20" or make it proporcionally bigger to give it a longer draw length. ps: a "safe formula" bow could still break if improperly made though.
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    Re: Split limb and Draw length

    Post by 80Eight on Fri Nov 20, 2015 4:23 pm

    A much clearer understanding and a base to start from. Thanks for your replies.

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    Re: Split limb and Draw length

    Post by FrenckBrambo on Mon Jan 02, 2017 8:09 am

    Hallo,
    I am new here, and just started building a reverse compound CB. Many questions come up, and a want to put 2 of them here:
    1: If you want to connect the cable that comes from the small cam (on a reverse compound bow) near to the axle of the cam on the other side, than you must connect it UNDER the cam, because a connection on the both side of the axle will interfere with the power-cable. On my test-bow, that caused the limbs to bend upwards. How come that that doesn't happen with bows that have cables fixed like that?
    2. How is the draw-weight of a compound bow measured, on the main-string, or on the limbs. And if it is the latter, is the draw-weight the sum of the force on eacht limb?

    I am very curious to the answers. I will post pictures later.

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    Re: Split limb and Draw length

    Post by c sitas on Mon Jan 02, 2017 3:47 pm

    Hello Frenck; total pounds pull is measured at full draw just slightly before full cam roll over.This of coarse on main string.If you were to pull test your bare limbs,with out cams, you'd see a much higher pull.That is yet another way to make a bow with out any let off.I have several made this way.
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    Re: Split limb and Draw length

    Post by FrenckBrambo on Mon Jan 02, 2017 6:50 pm

    Hallo Sitas, thanks for the answer (on my 2nd question). I never used a compound crossbow, but it is hard te believe anyone can span a crossbow of 150 pound or more, because it is usually done with a spanning cord (I don't) know if that is the right term) and that has a leverage of 1:1..

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    Re: Split limb and Draw length

    Post by c sitas on Tue Jan 03, 2017 7:47 am

    Hello Frenck;   150 is kids stuff.I was my buddies cock 225 all the time with a rope. I've never tried it but,they do it easily.
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    Re: Split limb and Draw length

    Post by OrienM on Tue Jan 03, 2017 8:08 am

    A cocking/spanning rope gives a 2:1 advantage, due to the pulleys. I can't cock my 160# by hand, but I can easily using the rope.
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    Re: Split limb and Draw length

    Post by FrenckBrambo on Tue Jan 03, 2017 10:16 am

    OrienM, rethinking it; you'r right, it's 2:1.  Smile

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    Re: Split limb and Draw length

    Post by alan hernandez on Wed Mar 15, 2017 10:50 am

    hello im new here and im actually making a reverse draw compound but im using pvc limbs now but im trying to get information about spring steel limbs because pvc is weak and i need to change the limbs often   Sad

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    Re: Split limb and Draw length

    Post by c sitas on Wed Mar 15, 2017 6:52 pm

    Hello Frenck, I will answer your number one question in your first post. I am sure if you just thought about it for one second, you would answer it your self. When you would hold a regular compound bow  by the handle , there is nothing in the way to obstruct the string. Take the same bow ,mount it on a "stock " . Now you have something in the way of the string.Not all, but most normal crossbows have this condition. The string rides the stock to help it launch the bolt correctly. To say that it doesn't bother isn't true, but it doesn't bother that much.Also if you put your bow under one foot in a stirrup,and bend your knees slightly , and pull the string with both hands and your knees . It don't take much to cock it. Especially  if you were to wear a light pair of leather gloves.
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    interference of cables strings on RDT

    Post by FrenckBrambo on Sat Mar 18, 2017 4:20 am

    Hello Sitas, I didn't mean the interference of the string with the stock, but of the string with the cables, on a RDT crossbow.
    But I am quite sure to have figured it out now. There are 3 ways to solve this problem:
    1. connect the cables behind the cams (like Scorpyd does), the advantage of this is that the connection can be in the middle of the split limbs, so the limbs do not bend upwards.
    2. connect the cables to the cam-axle, under the cams (like Horton), this causes the limbs to bend upwards, and a solution for this has to be made (e.g. place the cambs in an opposite angle and/or make the lower limb thicker then the upper;
    3. make the cables split connected to the top and lower end of the axle of the cams (like on Bear Fisix). With cams that rotate inwards when cocking the string this causes the problem my question was about. And the solution for this is to have the string on the outside of the cams, that makes the cams turn the other way round (outwards when cocking and inwards when shooting) like on the new Bear Fisix.

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    Re: Split limb and Draw length

    Post by c sitas on Sun Mar 19, 2017 7:55 am

    Frenck,   All the compound type bows, be they crossbows ,or regular hand bows,  all use some type of cable guard . All it does is pry the" cables "away from the string.The string cannot be obstructed,it has to have an all clear path. This is a product that some even modify . Seems like no one ever has the perfect mouse trap.In the case of the crossbow ,I consider the stock as part of this system ,as the string will ride tight to the stock in most cases.Still, maybe not always.

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    Re: Split limb and Draw length

    Post by Andy. on Sun Mar 19, 2017 2:58 pm

    The new Ravin crossbow has alleviated cable interference. Very interesting design....

    http://ravincrossbows.com/

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    Re: Split limb and Draw length

    Post by c sitas on Sun Mar 19, 2017 5:18 pm

    Andy, At the price of that thing, you might as well buy a Weatherby  .I can't justify that kind of money, and have it last for maybe 200 shots. That's the problem with high performance though. If you shoot real light bolts , it probably won't last that long. A young friend just got a new crossbow. With the noise it makes at the shot , I don't give it much chance. Bolt is tooooo light , not pulling the energy  out of the bow . It's like your dry firing it.I love studying designs though, some are realy well thought out

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    Re: Split limb and Draw length

    Post by Andy. on Sun Mar 19, 2017 11:19 pm

    Agree, no intention myself on purchasing or suggesting anyone buy. FrenckBrambo may be interested in the design.

    It is a fairly unique crossbow, and does not have cables crossing beneath the stock, and therefore the problems with drag etc associated with this.
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    Re: Split limb and Draw length

    Post by Anatine Duo on Mon Mar 20, 2017 2:30 am

    That is interesting technology... I had independently scrawled on note paper the Helicoil system years ago.  I'd be surprised if I was the first... I often wonder if those patents really hold water.
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    Re: Split limb and Draw length

    Post by FrenckBrambo on Mon Mar 20, 2017 7:42 am

    That's indeed interesting, but did anyone notice this helicoil is missing something? It is round, so it has a linear draw-strength. The string-cam is a bit excentric, but I think it wil miss a lot of the advantage of an excentric cablecam, that inverses the draw force in some extent. Am I right?

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    Re: Split limb and Draw length

    Post by c sitas on Mon Mar 20, 2017 8:25 pm

    Anatine, maybe you could be the first to try out the patents mith here? I think I have  more use  for my money . I detest lawyers, having several in my family, all grand kids . Still detest them, " and they know it".That's the trouble with this whole world.I, have a good acquaintance on here from the UK that can tell you about patients, and so on.He has one hell of a machine , that's the trouble . All seems like fun and games, "until"  you try to sell the idea. I do agree that with all the "high tech crap out there", walking about without banging into someone is,  at best, a crap shoot. Have fun for your self, just don't try and sell it.

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