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    crossbow vibration

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    thorn nork
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    crossbow vibration

    Post by thorn nork on Wed May 01, 2013 8:02 pm

    Greetings, I have made several crossbows, modern and medieval. My problem with my current one is it has horrible vibration/recoil. I made ad aluminum prod for it. All my other xbows I used fiberglass and there was no vibration or recoil. Could this be the problem? It is only 100#
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    Re: crossbow vibration

    Post by B.Cheers on Wed May 01, 2013 9:00 pm

    Well im not a pro on crossbows, but i cant see how you would have much more recoil then my 80 pound crossbow since only time i have noticed it is when firing with out using the stock like a pistol.

    And i have been able to reduce recoil and sound by putting a spring on the bolt that holds the prod in place, so the question is how do you have your prod mounted if its similar to mine, then a damping spring will reduce recoil if not im sure someone esl on here will have other options and answers for you.
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    Re: crossbow vibration

    Post by mac on Thu May 02, 2013 8:29 am

    Thorn,

    We need to distinguish between "recoil" and "vibration". Recoil is a fact of life in any projectile hurling engine, be it a catapult, a crossbow, or a gun. The rearward momentum of the weapon is going to equal the forward momentum of the projectile. http://en.wikipedia.org/wiki/Recoil

    Vibration, on the other hand is a thing we want to minimize. You can think of it as unused energy that is trying to rattle your crossbow apart.

    There are a number of things on a crossbow that might vibrate. The first and perhaps most important place to look is where the prod is attached.

    See if you can post some pics. All diagnoses and prescriptions are more meaningful and accurate if the patient can be examined.

    Mac
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    Re: crossbow vibration

    Post by Todd the archer on Thu May 02, 2013 3:35 pm

    How the prod is tillered can have effect on recoil and vibration. Even a 50 pound hand held bow can have a lot of hand shock if not designed properly. Where along the limb it bends all plays a part. Direction of movement makes a difference as well. Longbows usually have more vibration than recurves.

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    Re: crossbow vibration

    Post by thorn nork on Fri May 03, 2013 11:44 am

    It is tide on to the front of the stock. I'll send pics later. It has so much vibration I cant put my cheek on the stock with out getting a burse.
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    Re: crossbow vibration

    Post by Geezer on Fri May 03, 2013 8:20 pm

    Crossbow vibrating? Something is loose: It could be your prod, or a very heavy bowstring that runs free of the table (top of the stock) The old Jayhawk kits used a heavy steel bowstring that could give you a good buzz. I've also seen at least one medieval bow that had an off-center roller-lock mounted on an axle. That sucker vibrated like anything on release. It was Very Unpleasant to shoot.
    As Mac said: we'd have to know more about your bow to be sure, but a properly tuned crossbow shouldn't VIBRATE on release. It oughta just go 'thunk'. Geezer.
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    Re: crossbow vibration

    Post by Geezer on Fri May 03, 2013 8:23 pm

    A post-script about the vibrating bow. Ralph Payne Gallwey's "The Crossbow" has a long translation of a 17th century Spanish manual on crossbow operation. Somewhere in there, the author notes that the stock should weigh at least as much as the prod. "If not, the stock will recoil against the face, causing wounding and offense." So maybe the relative weights of prod and stock are out of proportion. Geezer.

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    Re: crossbow vibration

    Post by thorn nork on Sat May 04, 2013 11:39 am

    http://s1345.photobucket.com/user/thornnork/slideshow/
    Hope this takes. The prod is 30" knock to knock, brase height is 3.25". The string is heavy duty, I took the string off my combat xbow with a similar prod. This xbow is still a work in progress. It is accurate and dose not misfire.
    Also has anyone made a goats foot for a push pin type of tickler/trigger?
    My combat xbow "SCA" was a belly cocker, 100#, purple heart and hickory. Very deadly!
    Thanks, Thorn
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    Re: crossbow vibration

    Post by mac on Sat May 04, 2013 12:30 pm

    Thorn,

    Here are a couple of things I see....

    ---The brace height is a bit low. This puts the string under greater tension. I know that's counter intuitive, but trust me. A low brace can be useful in storing more energy, but it makes for a greater "shock" to the string at the end of the shot.

    ---The string is twisted too much. This makes the string stretchy. This is certainly robbing you of efficiency. It may or may not be contributing to the vibration issue.

    ---The prod is "limb heavy". This is a "tillering" issue. As it is, is is working mostly in the center, and the limbs are just going along for the ride. Unless you are using a heavy projectile, there will still be a lot of momentum left in the limbs at the end of the shot. This will make the bow unpleasant to shoot. If you do use a heavier projectile, you will get more energy back out of the prod, but you will pay for it in reduced velocity and more recoil.

    If you re-tiller the prod by thinning the limbs you will probably get a sweeter shooting bow and a longer prod life, but you will reduce the draw weight.

    Mac

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    Re: crossbow vibration

    Post by thorn nork on Sat May 04, 2013 5:40 pm

    Thanks! Good observation. Looks like I have some tinkering to do.
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    Re: crossbow vibration

    Post by 8fingers on Sun May 12, 2013 12:14 am

    If the prod isn't loose then my guess would be too much mass in the limb tips. I make a lot of bows and hand shock is often cured by thinning the last quarter of the bow limb.
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    Re: crossbow vibration

    Post by Geezer on Sun May 12, 2013 7:26 am

    First and easiest order of business: Get rid of that 1/4 inch steel cable bowstring and fit something lighter. I recommend B50 Dacron, available from archery suppliers like 3 Rivers Archery, but Fastflite would work as well or better. Stretchy stuff like nylon is too.... stretchy.
    Just changing the string will help a lot, but narrowing or thinning the outer ends of the prod could help as well. The trick is to do one thing at a time and observe the change. Start with the string. Geezer

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