Stupid question

[Jim Davis]

Del, you are certainly right about the angles affecting the forces. And, like you, when it gets real complicated, I won't bother to try to work the math. I just go  try something to see if it works.

[upstatenybowyer]

You've never asked a stupid question DC. You ask stuff that I wish I could ask, but can't find the right words to put it. 

[DC]

It's tough to find the right words. My Mom was and old school English teacher. Maybe she did drill something into me. She would be pleased to hear that

[Mad Max]

I have clamped 1 limb (with siyahs) on the tiller tree and use 2 ropes and pulleys to keep the center of the string going down the center of the tiller tree.
For checking siyah angles, and glue joints.

That would be 20#

[Marc St Louis]

The one limb may be exerting 20# of force on the string but so is the other limb, I say go with 40

[Selfbowman]

Stupid question two. What is siyahs? I would think that if you hang a bow string on a scale and pulled the bow to 40# . The scale would be holding forty pound and you would be pulling the same forty pounds. Is that correct? Arvin

[Selfbowman]

Oh ! Siyah is one of those jacked up Japanese bows. Arvin

[Jim Davis]

I would think that if you hang a bow string on a scale and pulled the bow to 40# . The scale would be holding forty pound and you would be pulling the same forty pounds. Is that correct? Arvin

Yes

[DC]

A siyah is kind of a glued on recurve. It's mostly used in composite(horn) bows I believe. I just stole the idea to make a bow longer and recurved. It's been done before.

[simson]

I have no clue how you will do the testing. Clamp the bow in a vice and attach weight vertically on limb or tip?
This is not how the real forces are working on a bow. In fact you get a changing momentum in the handle when a bow is drawn.

When you attach a scale between the two halves of a string, you will notice that string tension is highest at brace height (or near brace height). It is about 2,5 as high as draw weight and will decrease when the bow is drawn!

I would suggest: make all your parts as exact as possible and use good well mixed epoxi and your fine!

[DC]

I was thinking I would make a mock-up of a siyah and it's "V" joint and do just as you suggested. Clamp it in a vice and hang weight on it. OS is incredibly strong and I thought that by doing this I could get a very rough idea how small I could go with these parts. At the moment I have no idea how small they could be. They may be 3 or 4 times too big and they may be entirely the wrong shape. I would rather break a couple of mock-ups than a bow

[Ballasted_Bowyer]

To do your math modeling, sketch a bow of the type you want to build at full draw. Fill in the major forces. The sum of all forces should equal zero unless the bow is being thrown. The force always acts on the bow in a direction. There are basically two forces on the bow, the bow hand force pushing the bow away from the archer at 40lbs and the string hand force pushing the string towards the archer. Now draw a free body diagram of the point where the string is being drawn. You have a 40 lb force pulling at zero degrees. You know that strings can only pull in the direction of the string so you have two other forces at other angles. For the sake of illustration let's say the angle is 60 degrees from horizontal top and bottom. So, the horizontal component of these forces has to be equal to the 40 lbs going the opposite way. The string leaves the draw point in two directions symmetrically which makes it mathematically easy. 2*20-40=0. However that is not the total force on the string. That is the force on the string in the horizontal direction. To find the force on the string draw a triangle with the horizontal leg being 20, the vertical leg being unknown. The hypotenuse is unknown in length as well however, the angles are 30,60,90 with the 20 lb leg going from the 60 to 90. From this we can use trig functions on the calculator to find out that the magnitude of the tension in the string is 40 lbs. now draw a new free body diagram at the tip of the bow to calculate the forces on the tip. Once you have these you can draw a free body diagram of the segment and work your way around the bow. The technique for doing this is a full quarter in engineering school. If you want to do this kind of research pick up an old book on statics at a university library and dig in. You'll want to have at least trig under your belt to get started. Also, the numbers I used came out even for illustration. A real bow would be far more difficult. I have the math to do this. I but never do. Archery is fun not school.

[bradsmith2010]

wow

[Jim Davis]

wow +1

I understand about that kind of math but not how to do it. Would love to see you write an article for Primitive Archer working over the forces at work in the launching of an arrow from a primitive bow (no shelf please and not cut near center.) Article should be as plain spoken as the math allows and replete with illustrations. Or maybe write a follow up to Klopsteg's "Archery, the Technical Side."

[willie]

I have the math to do this. I but never do. Archery is fun not school.

the math is interesting, insomuch as bows are a fascinating study in statics and strength of materiels. but as said above, not too much fun to do

now if Jim or anyone else, were to come up with an really interesting challenge for an article........any other suggestions out there?

Article should be as plain spoken as the math allows and replete with illustrations.

that's actually the hardest part.