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Compound bow or crossbow theory


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Old 07-15-2017, 01:38 PM   #1
BoyntonStu
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Lightbulb Compound bow or crossbow theory

I would like to share what I just discovered about how a compound system operates and why a 50# compound bow shoots much faster than a 50# recurve bow, etc.

Consider a 2x4 with a pivot between the ends, like a see-saw.

If the longer portion is 2 times longer than the shorter portion you have a choice of a 2:1 mechanical advantage or a 1:2 mechanical disadvantage depending on which end you apply the force.

With 2:1 if you apply 10# you can lift 20#.

However you must push 2 units of distance to lift the other end 1 unit.

Speed: If you push the smaller end at 10 fps the longer end will go 20 fps.

Almost there:

2 cams on a compound bow. A large cam and a small cam. They are bound together.

That is where the mechanical advantage and mechanical disadvantage come into play.

The draw string is on the larger cam.

There are 2 cables, one on each smaller cam that is attached to a fixed position on the other limb side.

As you draw the string, the mechanical advantage allows you to pull a 150# limb bow with only 75# of force (assuming 2:1). As you draw, the attached smaller cam pulls the limbs together,

When you release the string, all the bow force is only on the smaller cams, The small cams unwind in a 1:2 mechanical disadvantage and the arrow will travel at twice the speed of a no cam bow.

When you see a compound bow advertised as a 70# bow, you must understand that the bow limbs are at least 140#.

I hope that I made myself clear.


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Old 07-16-2017, 08:47 AM   #2
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Yea its well known Compund bows are made that you can hold 80 lbs at full draw, and only hold a small a small part of that force. Makes it easyer to release.
But i like recurve better you can do this with it:


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Old 07-19-2017, 02:51 PM   #3
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Yea its well known Compund bows are made that you can hold 80 lbs at full draw, and only hold a small a small part of that force. Makes it easyer to release.
But i like recurve better you can do this with it:
https://www.youtube.com/watch?v=GtpH_4ExmG8

Nice find!

How would it work on a tree?

I prefer this for a tree:

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Old 07-19-2017, 05:10 PM   #4
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When you have tree its no problem to build that!
Or do you mean that crossbow-bow hybrid?

As much i know both recurve and compund bow can use that platform.
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Old 07-19-2017, 08:59 PM   #5
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Both of these bows just seems so blasphemous to archery.

The first one seems plain unless to me. You don't need a fancy device to shoot multiple arrows. It's plenty possible to fling 5 regular arrows from a recurve. Try it for yourself. Find a field or otherwise open and safe place to shoot (but not a real archery range. They probably won't like this) and nock a couple arrows to a weak recurve bow. You might have to change your grip on the string but the rest should be the same. The arrows will obviously go slower and you will lose some accuracy but it works. Best try with a big target too.

So is that second one fired with a release or a trigger? I would worry about accuracy at longer distances primary with the compromised draw length. I get it, a reverse bow is supposed to have a very short draw for maneuverable but I doubt it achieves the accuracy of a full size compound. It seems like the bullpup of bows.
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Old 07-19-2017, 10:26 PM   #6
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Originally Posted by Daedric-Panther View Post
Both of these bows just seems so blasphemous to archery.

The first one seems plain unless to me. You don't need a fancy device to shoot multiple arrows. It's plenty possible to fling 5 regular arrows from a recurve. Try it for yourself. Find a field or otherwise open and safe place to shoot (but not a real archery range. They probably won't like this) and nock a couple arrows to a weak recurve bow. You might have to change your grip on the string but the rest should be the same. The arrows will obviously go slower and you will lose some accuracy but it works. Best try with a big target too.

So is that second one fired with a release or a trigger? I would worry about accuracy at longer distances primary with the compromised draw length. I get it, a reverse bow is supposed to have a very short draw for maneuverable but I doubt it achieves the accuracy of a full size compound. It seems like the bullpup of bows.
The brace height of a reverse bow is zero inches. If you subtract the 6-7 inches of a normal bow, my guess is that the actual power strokes are similar.
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Old 07-19-2017, 10:28 PM   #7
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When you have tree its no problem to build that!
Or do you mean that crossbow-bow hybrid?

As much i know both recurve and compund bow can use that platform.
Imagine climbing into the dense branches, short bow or long bow?
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Old 07-20-2017, 05:17 AM   #8
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Quote:
Both of these bows just seems so blasphemous to archery.

The first one seems plain unless to me. You don't need a fancy device to shoot multiple arrows. It's plenty possible to fling 5 regular arrows from a recurve. Try it for yourself. Find a field or otherwise open and safe place to shoot (but not a real archery range. They probably won't like this) and nock a couple arrows to a weak recurve bow. You might have to change your grip on the string but the rest should be the same. The arrows will obviously go slower and you will lose some accuracy but it works. Best try with a big target too.
You sound like those guys at the my archer club What you see at the first video is a over draw! Normaly you use it to shoot small arrows with full power. Also its alows you to shoot the Arrows of your enamy back. With a over draw you can put weaker shooter at the frontline.
Shoot more arrows from the bow without Persian Panjegan is mostly usless i heard. Woudnt even be possibel on my horseman bow.
You need to flip the bow when you shoot so you can compensate the arrow paradox.
Also most peopel in my club say Compound bows are blasphemous You make evrything mechanical. No skill needet to shoot bullseye at 100 meter. Its possibel the traditional archer are jealous

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Imagine climbing into the dense branches, short bow or long bow?
Not a problem when you learn thumb release! You can turn the bow at direction. Works best with a horsemen bow they are very handy.


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