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Compund bow question (Read 4638 times)
beaverbutt8
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Compund bow question
Oct 8th, 2005 at 6:36pm
 
Hey guys, can someone show me how a compound bow works? I have some scrap metal lying around here, and i thought it might be good for that. What i want to know is how that whole cam system works, and how to make it. This, ofcourse would be just an experiment, to make, say, just a 15 - 20 bow.

I would appreciate your help


Thanks




Mike
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beaverbutt8
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Re: Compund bow question
Reply #1 - Oct 9th, 2005 at 11:07am
 
Uhh, Anyone?
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LKH9
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Re: Compund bow question
Reply #2 - Oct 9th, 2005 at 11:20am
 
I've no idea. Lips Sealed
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Mike_R
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Re: Compund bow question
Reply #3 - Oct 13th, 2005 at 9:08pm
 
It looks simple but it isn't. Modern compounds use cams, they are like an eccentric metal wheel,  mounted at one or both ends of the limbs. The cable goes around like a pulley. That way when you draw back you get mechanical advantage and the force needed to draw is reduced.

You would have to be pretty technical to make one yourself. Try looking online for pics of old compounds. Some one's like old brownings had round wheels and used wooden parts instead of metal.
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ben_banned
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Re: Compund bow question
Reply #4 - Oct 13th, 2005 at 9:54pm
 
this should suit your needs
http://www.huntersfriend.com/bowselection.htm
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beaverbutt8
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Re: Compund bow question
Reply #5 - Oct 13th, 2005 at 10:22pm
 
Complicated  Undecided

BTW, Mike, what do you mean by "eccentric" wheel?
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Curious Aardvark
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Re: Compund bow question
Reply #6 - Oct 14th, 2005 at 8:02am
 
isn't it obvious ? the wheel cackles evilly at the slightest opportunity and wears it's underwear on it's head :-)

Okay an eccentrically mounted wheel is one that is not mounted at it's centre. ie it's mounted off-centre or eccentrically giving a cam like effect. A cam (I understand) is a non circular piece that is usually mounted centrally.

Please correct me if that's wrong :-)
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Re: Compund bow question
Reply #7 - Oct 14th, 2005 at 10:01am
 
LOL!
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Greywolf
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Re: Compund bow question
Reply #8 - Oct 14th, 2005 at 10:34am
 
[quote author=curious_aardvark  link=1128810988/0#6 date=1129291342]isn't it obvious ? the wheel cackles evilly at the slightest opportunity and wears it's underwear on it's head :-)  [/quote]

I just sprayed coffee on my keyboard!  ;D


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beaverbutt8
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Re: Compund bow question
Reply #9 - Oct 14th, 2005 at 1:38pm
 
Well, if theres only a limited amount of string, what enables it to draw out from 0 - 28 inches????
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beaverbutt8
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Re: Compund bow question
Reply #10 - Oct 14th, 2005 at 2:06pm
 
I'm getting suspicious  Tongue Angry

It seems that nothing online will completely explain the system - all the sites say the same, very unclear thing " It works with an exccentric wheel" and will say nothing else of the mechanism. It seems that bowmaking companies are afraid someone might make their own  Angry Roll Eyes Tongue
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Mike_R
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Re: Compund bow question
Reply #11 - Oct 14th, 2005 at 3:03pm
 
The fact that the limbs bend allows it to be drawn. Same as a longbow, the string only has a very small amount of stretch, it is the bending of the limbs that provides power. So picture a compound as a longbow with very short, stiff limbs, with wheels added to allow you to pull it. You could put wheels on a recurve bow to make it easier to pull if you wanted to, they used to sell a kit in the 70's to do just that. Whether it works or not is something I'm not sure of.
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Dale
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Re: Compund bow question
Reply #12 - Oct 14th, 2005 at 3:28pm
 
I, the Archivist and Researcher Extraordinaire, have spent the last <mumble mumble> minutes reading articles ... well... one article ... on compound bows.  But it was a good article.  Very informative.  As far as it went.

So now I'll tell you all I know about compound bows (comprised of what I just read, and my memory of playing with a friend's compound bow some years ago).

The whole idea of the compound bow, is to make it easier to hold the bow at full draw.  That's it.  The string and cam(s) [older bows have two identical cams, newer have one circular pulley and one cam] are arranged so that when you draw the bow, you have to apply more and more force to the string, and then all of a sudden the limbs relax and so can you.  When you loose the string, it "slowly" pulls away from your fingers for a quarter or a half inch, and then suddenly it's being pulled on at full force again, and the arrow takes off.

You're probably saying, Right, I know all that, tell me something NEW!

The secret to all this is the shape of the cam.  The cam will almost always be eccentric (the mounting hole is NOT at the geometric center).  It might be a simple ellipse, but probably has some odd  shape.  The cams I have seen have been odd, mostly.  The cam is shaped, and oriented, so that it is "longest" when you are almost at full draw, then when you pull the string a little farther it rotates so that it is "shorter" and it lets the limbs relax a bit.  Some cams let the limbs relax a lot (like, a 70-pound bow that lets off to about 15 pounds, but that sounds like an extreme case).

Designing the shape of the cam, is not something I am even going to try to explain (because I don't understand it myself).  I do know that the shape of the cam, will be driven by the geometry of the bow it goes with: the length of the limbs, the arrangement of the string, and so on.  It has to be shaped so that the limbs bend smoothly as you draw the string, and then as you finish the draw the limbs relax and pull less strongly on the string.

*sigh* You are probably still saying, OK, tell me something I DON'T know!

The best thing for you to do, besides trying to make sense of my ramblings, is go down to an archery shop, ask to see a compound bow, and play with it.  Do NOT play with a strong bow, play with one that is fairly light.  And DO NOT ever "dry-fire" it!  You'll wind up paying for the bow whether you intended to buy it or not -- "dry-firing" a compound bow typically voids any warranty on it.  I say this because it is very, very easy to get caught by surprise, as you start to let the bow down; to forget that it's about to pull very hard on your fingers.  I speak from near-experience; I was lucky, I didn't quite lose my grip on the string, but it was a near thing.  My friend would not have appreciated me breaking his brand new Christmas present (nor would his wife, whose gift it was).
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« Last Edit: Mar 24th, 2006 at 6:02pm by Dale »  

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Hellfire
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Re: Compund bow question
Reply #13 - Oct 14th, 2005 at 10:34pm
 
I dont think you should make a compound. Hard to make
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beaverbutt8
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Re: Compund bow question
Reply #14 - Oct 14th, 2005 at 10:56pm
 
I agree, but my alternative is using cottonwood to make a bow  Angry Tongue Roll Eyes
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