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 Home made compound bow to crossbow project

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JoergS
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PostSubject: Home made compound bow to crossbow project   August 16th 2011, 3:36 am

I still have my Razor Edge compound bow. Not a bad weapon, but ever since my Liberty 1 arrived, it really only caught dust.

So I had this idea to use it as a part for a home made slingshot crossbow, much like the medieval style rubber powered model I am so happy with.

Took a whole afternoon, but it is now very functional. Medieval style trigger and rolling nut meets modern compound technology! The low letoff (15%) allow very easy trigger pull weights, making the weapon very accurate.

Finished the weapon, now I just need to attach a sighting system.





Here is a video that shows it in action:

http://www.youtube.com/watch?v=3_8AI3KeLE0

It takes NO practice to hit with this thing, whereas a bow requires much training.

I also made another "Six Circles" shooter from Maka Mong burlwood, came out nice. See the pics in my blog!

http://www.slingshotchannel.blogspot.com

(If you leave a comment in the blog, you can win a slingshot)
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PostSubject: Re: Home made compound bow to crossbow project   August 16th 2011, 2:21 pm

Can't believe I get to comment on this before Ivo

Anyway,

That's pretty slick dude!

Since the compound bow is very modern you should put a modern scope on it. I love the medeival scope you have on your slingshot crossbow, but I think this one could look like an assassin's crossbow.

What is the weight on that bow, it looked pretty low when you loaded it, then again, you are a beast with arms the size of christmas ham.
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JoergS
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PostSubject: Re: Home made compound bow to crossbow project   August 16th 2011, 4:05 pm

The bow is 60 lbs. Remember the power stroke on this crossbow is 29", most commercial crossbows have about 15"... the bow shoots at 308 fps (IBO). The speed comes from the draw length. Imagine what a 180 lbs crossbow could do at 29"!

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PostSubject: Re: Home made compound bow to crossbow project   August 17th 2011, 4:50 am

|

I only ask because I have a 67# compound bow that I'm very proficient with.

Now I know what kindof destructive power this crossbow of yours has.

Also remember that you are shooting an arrow instead of a bolt. Things that weigh more can always hit harder.

A perfect example is gonna be your slingshot cannon. I wait see it man.

What are you gonna shoot with it?



P.S.

I humbly ask if you've seen this before and if it all makes sense and is true...

I found this website when I was looking up slighshot ammo weight info

http://www.melchiormenzel.de/info_ammo.html

I dunno if all of it is true cause I'm not the sligshot god, as you are. it all sounded pretty legit though.
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PostSubject: Re: Home made compound bow to crossbow project   August 17th 2011, 10:12 pm

Quote :
Can't believe I get to comment on this before Ivo

Welcome back man. pirat

Quote :
The bow is 60 lbs. Remember the power stroke on this crossbow is 29",
most commercial crossbows have about 15"... the bow shoots at 308 fps
(IBO). The speed comes from the draw length. Imagine what a 180 lbs
crossbow could do at 29"!

Looks like Joerg has entered the craze of the archery crowd - beat the 400fps barrier.

But...Let me clear something up a bit first.

power stroke vs draw length

Powerstroke is the distance from lock(or fully drawn position) to the strings resting point. While draw length is the distance from full draw to the riser.

Meaning...

if the draw length is 29" then you need to subtract the brace height(varies with different bows) and you are left with the distance that is called powerstroke.

The term has become popular since it describes the distance that the string travels accelerating the arrow. Past that point the string comes to a halt and simply vibrates creating the "thump" noise - no longer contributing to the projectiles acceleration.

The average powerstroke of the modern crossbows is 12"-13" with brace height of around 3", total draw length being in the average 15". While vertical bows usually have adjustable draw length (repositioning a few bolts in the cam) ranging from 28" to 35"(depending on the shooter and technique), subtract the average brace height 5'"-8" and it's possible to get the 29" you are talking about (again...depending on the setup).

The powerstroke factor is also present in slingshots. Past a certain point of the rubber contracting the rubber is no longer contributing to projectiles acceleration. Assuming the draw length is 30" and the length of the rubber used is 8"-10"... 30" minus 8" (or 10") and the 22" (or 20") is your powerstroke...again this is rubber so I'm just pulling an educated guess here. Razz

From there you can draw FDC(Force draw curve) recording the poundage per every additional inch of draw and make your estimations of how "efficient" the design is. tongue

And as opposed to archery where the string stops, in the basic slingshot design the rubber keeps traveling past the fork alongside the projectile.

That last one may be a small factor, but it's something I don't like. A small personal *ekh* pirat However some of Joerg's designs I really like, can't post a link (computer broke and I'm on my phone), but it's the one where rubber is hidden inside the hollow fork stems - exiting the fork through the top and riding on the rollers as the rubber is drawn. That may be a design that wears our rubber and robs energy due to friction in the rollers, but I like...it's just more of my style of thinking.

As a last note...The above build has several names : Draw-lock Bow or Inline Crossbow. It's great in many ways, where crossbows are not allowed or you are die hard vertical archer(and have a disability, but want nothing to do with crossbows) the draw-lock offers the ability to use the compound like a crossbow. And the Inline Crossbow is a relatively new product...It's about the size of a Liberty Compound that you have, but is also equipped with a rail to which a pistol grip based trigger is attached. This hybrid that can be used as a bow and a crossbow depending on how you hold it, has been serving as a loophole for countries where crossbows are illegal, but bowhunting is welcome....so there you have it...a "Draw lock" or "Inline" Bow and you can avoid disputes with authorities without having to call your compound a slingshot for cover.

I wish I could post some pictures, since this project is good, but all I can offer to help is this...

Go the Archery Talk forum, and enter Inline Crossbow in the search. Topic is written by a friend of mine (Moon) who owns one and has been fiddling with the thing like crazy (scopes, sights, different grips/shooting styles). you do need to register to see the pictures, but it's well worth it - a really cool little shooter. And yeah...some serious draw with little weight...result: 300+ speeds. tongue

God luck on your speed craze Joerg...the arrow rests will be getting hotter as you dig deeper.

Ivo
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