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    Reverse Draw Crossbow String question

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    SteelTherapy
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    Reverse Draw Crossbow String question

    Post by SteelTherapy on Wed Jun 17, 2015 3:17 pm

    Hi,

    I'm building my Reverse draw crossbow from scratch and I'm at the point of making the strings.  This is my first experience with a crossbow of any kind.  My question is regarding the length of the strings.  I made the limbs out of steel leaf springs.  How much should the limbs be flexed in from unstrung to strung but un-drawn?  I was going to go with 1 inch flex (each limb flexing 1/2 inch) but I thought I should ask from people who know what they are doing.

    Thanks in advance!

    c sitas
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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Wed Jun 17, 2015 5:29 pm

    Hello Steel;  I've never done a steel prod. I would say to make it so the string is good and tight at rest. If it's not tight enough it will follow the bolt way out as in recoil.That is not really desirable. The ones I make out of wood are just good and tight. I don't use any measurement .I'm sure there will be others interested here also.Also you'll have to take your draw length into consideration. Too much preload  will change what your cams or pullies will hold for string,thus affecting the draw.With compounds ,it's a balancing act.Play with the cams, you'll see what I mean.

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    Re: Reverse Draw Crossbow String question

    Post by SteelTherapy on Thu Jun 18, 2015 5:02 am

    Just to add a visual aspect.  Here's a photo from a while back.  It gives an idea.


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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Thu Jun 18, 2015 9:36 am

    Steel; Set your cams so that they would be even and at full rest. Tape them secure to the limbs. Take a string, any long enough string, tie a loop on one end. Do the draw string first. Make the necessary wraps and add the other loop. Now do the same for the take up or power cable. Once you have a slight even tension on both your strings, I would subtract 1" and build them.Looks like you have a beautiful job on the cams and limbs. This , is like most things of this nature, trial and ERROR. You might hit it right on ,maybe not. Also don't forget, you can help your self a lot by twisting the strings . When you make them , I would allow  about 1/2" longer when you set up, then twist the string to get your measurement. On both strings.You can use dacron, you can also use some of the newer super materials.If you've never done a string before , google , make bow string. There are several good vids on this. If I've left you in a dither just come back , I'll try to help you. Good luck, Oh, take your time, don't rush. Rome wasn't build in a day, coarse, I wasn't running that job.I'm sorry , I forgot to tell you how many strands. I would try 24 strands. Now don't forget, that will be 12 complete  loops. That would be using B50 dacron.

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    Re: Reverse Draw Crossbow String question

    Post by SteelTherapy on Thu Jun 18, 2015 9:48 am

    c sitas wrote:Steel; Set your cams so that they would be even and at full rest. Tape them secure to the limbs. Take a string, any long enough string, tie a loop on one end. Do the draw string first. Make the necessary wraps and add the other loop. Now do the same for the take up or power cable. Once you have a slight even tension on both your strings, I would subtract 1" and build them.Looks like you have a beautiful job on the cams and limbs. This , is like most things of this nature, trial and ERROR. You might hit it right on ,maybe not. Also don't forget, you can help your self a lot by twisting the strings . When you make them , I would allow  about 1/2" longer when you set up, then twist the string to get your measurement. On both strings.You can use dacron, you can also use some of the newer super materials.If you've never done a string before , google , make bow string. There are several good vids on this. If I've left you in a dither just come back , I'll try to help you. Good luck, Oh, take your time, don't rush. Rome wasn't build in a day, coarse, I wasn't running that job.I'm sorry , I forgot to tell you how many strands. I would try 24 strands. Now don't forget, that will be 12 complete  loops. That would be using B50 dacron.

    Thanks!  That's how I measured my long string.  I took a single strand of the dacron and wrapped it just how you suggested.  I measured it at 48".  I was going to take an inch off and go with that but I hadn't thought about the twisting the strings.  Thanks for bringing that to my attention.  Now it's  time to build my string jig.

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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Thu Jun 18, 2015 11:25 am

    I'll tell what works good is two pole barn spikes.Drive them in close to what your measurement is and bend them.A lot of people don't serve the loops,myself included. I just make a  serving long enough  to hold the eyes and go around the cam. I quit my strands long enough to bypass the serving  I mentioned. Do you know what burnishing is?

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    Re: Reverse Draw Crossbow String question

    Post by SteelTherapy on Thu Jun 18, 2015 11:29 am

    c sitas wrote:I'll tell what works good is two pole barn spikes.Drive them in close to what your measurement is and bend them.A lot of people don't serve the loops,myself included. I just make a bout 3" long serving to hold the eyes. I quit my strands long enough to bypass the serving  I mentioned. Do you know what burnishing is?

    I don't know what burnishing means in the context of determining string length.  I did plan on serving the entire length that goes around the pulleys in addition to the end loops and center.

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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Thu Jun 18, 2015 1:06 pm

    Steel ; My thing is the lighter you can keep your rotating mass"cams"  the faster the bow will be. That's why I don't serve the loops, it's just not needed really. Also keep your serving that goes around the cam to --just enough.Same with the center serving, don't make more than you Don't make excess length .Put the twists in your string. Now for burnishing. Take a 24" piece of the string your gonna use for your string. Have your string laid out and on the posts, all twisted.. Now take the piece of string and start from your post. Wrap it twice around your string bundle, pull it tight, and slide it down the bundle of string, end to end. .Do this three or four times. This will stripe the excess wax out of the string.It will also give you a nice round bundle to put your serving on.

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    Re: Reverse Draw Crossbow String question

    Post by SteelTherapy on Fri Jun 19, 2015 8:21 am

    c sitas wrote:Steel ; My thing is the lighter you can keep your rotating mass"cams"  the faster the bow will be. That's why I don't serve the loops, it's just not needed really. Also keep your serving that goes around the cam to --just enough.Same with the center serving, don't make more than you Don't make excess length .Put the twists in your string. Now for burnishing. Take a 24" piece of the string your gonna use for your string. Have your string laid out and on the posts, all twisted.. Now take the piece of string and start from your post. Wrap it twice around your string bundle, pull it tight, and slide it down the bundle of string, end to end. .Do this three or four times. This will stripe the excess wax out of the string.It will also give you a nice round bundle to put your serving on.

    I hadn't really thought about the mass of the string.  I do plan on reducing the mass of the pulleys by making it have spokes.

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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Fri Jun 19, 2015 10:01 am

    Steel; That'll be cool .I shouldn't have tell you but, I will any way--Work real careful here and don't go to thin. There is a lot of rebound stress involved. I have seen a few guys dry fire a modern bow and the cams looked like you put them in a vise and squashed them oblong. Totally junk. Just a suggestion, maybe some well thought holes would work good. Just flappin my jaws here , not tryin to tell you what to do. Try to pay attension  to the burnishing part of string making, it'll keep your string from stretching much.You'll be happy with the way it turns into a round cable like shape.

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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Sun Jun 28, 2015 8:49 pm

    Mr Steel; I was looking at and playing with a commercial  reverse draw design. One thing stands out for sure, is the extra mass in the cams. I mean , those things were like twice as massive as a regular draw cam bow. They still bragged the most speed of all the bows in the rack . "  About 10 in all. "

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    Re: Reverse Draw Crossbow String question

    Post by SteelTherapy on Thu Jul 09, 2015 9:12 am

    c sitas wrote:Steel; That'll be cool .I shouldn't have tell you but, I will any way--Work real careful here and don't go to thin. There is a lot of rebound stress involved. I have seen a few guys dry fire a modern bow and the cams looked like you put them in a vise and squashed them oblong. Totally junk. Just a suggestion, maybe some well thought holes would work good. Just flappin my jaws here , not tryin to tell you what to do. Try to pay attension  to the burnishing part of string making, it'll keep your string from stretching much.You'll be happy with the way it turns into a round cable like shape.

    I have to admit.  I didn't think the burnishing would do much but I was wrong.  It turned out really nicely.  Nice and round.  Thanks for the advice!

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    Re: Reverse Draw Crossbow String question

    Post by c sitas on Thu Jul 09, 2015 9:45 am

    Steel ,my friend. I am glad you think that way. It's never toooo  late to learn something. When  I quit learning ,they'll be throwing sand on my face, he he .What you did by burnishing is,--- you took out just about all the stretch out that plaques every body. You see them here, complaining about it.

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