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    Need some help with a Reverse Draw Crossbow

    JacobL
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    Post by JacobL on Wed Jan 02, 2019 12:41 am

    So I’m up super late right now and I’m really quite frustrated. I’ve been designing a crossbow/launcher hybrid for launching random objects and so on and I figured that a reverse draw bow would best fit the bill.

    Anyhow, it’s having the same problems I’ve always had with this design. Using normal pulleys and cocking I have reached up to 130 pounds in draw weight which is what I’m looking for but once I moved to actual cam testing I was getting figures lower than 30 pounds. No matter what I did to change the eccentrics of the cam I would not achieve anything higher than 30 pounds.

    Need some help with a Reverse Draw Crossbow  0dfc7a10[url=https://servimg.com/view

    As you can see the limbs are definitely not weak. They’re made from a 5/16” thick leaf spring which is really a poor choice but meh it’s all I have that would fit the bill.

    Is this something that is common when using cams? I don’t have a picture of my cams but they are the same as those used in any other reverse draw build.
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    Post by c sitas on Sat Jan 05, 2019 9:04 am

    Your preload is maybe not high enough.Looks like you have a lot of limb length. Reverse draw stuff is mostly pretty compact and super strong.
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    Post by JacobL on Sat Jan 05, 2019 1:16 pm

    Ahh well that’s the thing. This needs to be long; it’s part of the design. The crossbow has a 3” wide rail for launching big projectiles. It’s a pretty crazy machine LOL. I couldn’t have the bow draw past the riser like most Scorpyds so I needed long limbs like a Horton Vision/Armcross Leopro.

    I’ve switched to normal pulleys for now, but I’m still wondering if cams give and intense drop in draw weight or something.

    Thank you for the reply C sitas
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    Post by FrenckBrambo on Fri Jan 11, 2019 7:02 am

    Preload is one thing. You haven't showed your cams (curious), but with relatively long limbs you need bigger cablecams. If you increase the preload and you still do not have enough draw-weight, you might need bigger cablecams. I assume your string-cams have the right string-cams for the draw-lengt you need.
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    Post by JacobL on Fri Jan 11, 2019 11:08 am

    I have tried cams from 2”-4” in diameter and haven’t had success above 40 pounds so I went with simple pulleys. About 1.5” in diameter and they have a bearing through the center. It gives me a good enough velocity testing with a lightweight arrow and the poundage can reach up to 168 pound.
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    Post by c sitas on Sat Jan 12, 2019 7:44 pm

    Jacobl'  what your doing will work fine. Thing is , your pulling on the limbs the same as you would a long bow. Your not magnifing  your pulling advantage.Your using the pulleys for draw length.  There is no let off on the power. Myself, I prefer this 
    way but you have to use more spine for the arrow.Don't try to shoot real light arrows, guaranteed to break the bow somewhere.
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    Post by JacobL on Sun Jan 13, 2019 10:37 am

    Yeah I prefer it as well, I just don’t understand cams. This works fine. However I’ve actually had to leave this design alone. The crossbow I am making is a general purpose launcher of sorts, so it has to handle lighter projectiles as well as heavy. The weight of this bow, coupled with the poor velocities I tested, and the fact that I can’t cock it by hand (extreme string pinch), caused me to leave it alone. I’m working on a recurve style bow from a Skidoo suspension; it will work much better for the launcher I am building.

    Thanks for the help though, if I ever use limbs I will use those rollers but honestly I think my next designs will be spring powered haha.
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    Post by c sitas on Sun Jan 13, 2019 7:55 pm

    Jacobl, winch shouldn't be out of the question. I make mine to mount and leave right on the stock.I make them direct drive ,no gearing.They'll pull three hundred pounds with just a little extra arm work.What you have there would'nt be much of a tussel. Handy for decocking when you don't want to shoot also.
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    Post by JacobL on Sun Jan 13, 2019 9:21 pm

    That’s actually what I was doing. The crossbow was to have a winch built into the stock. A lever extended out the bottom sort of like a trigger and there was a handle below the stock for support.

    It had the ratcheting wheel and used a piece of steel pipe as the main shaft. To get it to work properly though it had to be really wide, and I ended up with 3” of pipe on each end hitting right into my shoulder.

    I know of a lot of different cocking devices to make but none really work well with a 3” wide barrel. I’d much prefer to just go the simple route because I think it’s the only option that keeps things light and easy.

    I’m rambling though haha. Do you have any photos of this type of winch? If it’s really easy to produce I may consider it on a future bow.
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    Post by JacobL on Sun Jan 13, 2019 9:35 pm

    Also I’m curious as to your thoughts on a rope cocker? To me it seems far easier to produce and I’m sure it’s not all that slower to use. I believe it could suit this project much better.
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    Post by c sitas on Mon Jan 14, 2019 10:56 am

    I'm kinda at a picture lose here. Do you know how to string the rope? It might work if your stroke isn't toooo long. You can only pull so much rope you know.Also ,on my winches , nothing hardly sticks out the side. You can take the handle off if wanted.Not enough to bother any thing. My winch pulls on both sides of the stock. I hook on to the string with two hooks ,one on each side of the edge of the stock. I made a sled that rides the stock and grips the string. That way it's always the same pull and catch at the lock.Do you have email at home?
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    Post by JacobL on Mon Jan 14, 2019 4:02 pm

    What I mean by rope cocker is the pulley-equipped cocking hooks sold for most high draw weight crossbows. It uses two hooks which are equipped with pulleys to halve the draw weight of the bow. Im considering that over a crank.

    Im interested as to how you made it so slim yet able to coil up well. How wide of a shaft do you use for the rope to wrap around?

    My email is jacoblesann@gmail.com


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