String angle

[DC]

I started a new post because this is going away from the reflex/recurve thing I think.

Stacking starts when the the distance from the handle perpendicular to the string starts to go down after reaching whatever its maximum is based on the bow geometry.

I've never heard it put this way. In my head I can see this is close to the 90° string angle so often mentioned but maybe a little more precise. Is this tied in with the mechanical advantage the string has over the limbs? If so is it the distance you mentioned compared to???

Edit-I didn't change anything. I was just trying to get the edit box in the right place. I failed Got it

[mmattockx]

I've never heard it put this way. In my head I can see this is close to the 90° string angle so often mentioned but maybe a little more precise. Is this tied in with the mechanical advantage the string has over the limbs? If so is it the distance you mentioned compared to???

Since that was my comment, I might as well carry on with it.

I don't know if it always happens at 90degrees or not, but I expect the point it happens is different on different bows because they don't all bend the same when drawn. You could likely do a decent comparison by taking good pictures of bows on a tillering tree, taking a picture for every inch of craw and then comparing to an FDC to see what the string angle is when stack begins. I don't have a large enough selection of bows to do much with this, though.

It is definitely the leverage the string has over the limbs, as measured from the grip location. This is why string tension goes down as you draw, at brace the string only has a moment arm length equal to brace height. As you draw the moment arm (ie - leverage) increases quite rapidly, giving lower string tension even though the force required to bend the limbs is increasing. At some point in the draw the string to handle distance will hit a maximum. Drawing further will cause the distance to go down and string tension to begin rising again.

EDIT - Further thought has me wondering how this interacts with the string angle at the fingers and the draw force we feel because the angle is combining with the string tension to produce the actual draw force. Let me look at some numbers and maybe do up a spreadsheet to compare tension to draw force and string angle. I think I can get most of the info off Super Tiller output, but I haven't explicitly looked at this before to know for sure.


Mark

[willie]

At some point in the draw the string to handle distance will hit a maximum. Drawing further will cause the distance to go down and string tension to begin rising again.

Mark,
where on the string does this maximum distance occur?

[DC]

Draw a line at 90° to the string going to the handle. I think that's it

[DC]

I tried to duplicate this and the 90° line never started to get closer. I think I need pictures with circles and arrows telling what each one is about.

[mmattockx]

Mark,
where on the string does this maximum distance occur?

Draw a line at 90° to the string going to the handle. I think that's it

DC has it, it is drawn from the center of the handle perpendicular to the string. Wherever that hits is the point.

I tried to duplicate this and the 90° line never started to get closer. I think I need pictures with circles and arrows telling what each one is about.

It's been a rainy day so I have had time to look at this more. At the string angles we see on typical bows you are correct, the moment arm does not start to reduce. After some more tinkering, it looks to me like it is a combination of the string angle, string tension and geometry of how much the nock point moves per inch of draw (the 'gear ratio').


Mark

[DC]

How do you figure the MA of the string? The best I could come up with was the draw length compared to one limb length. ie at 6" brace on a 66" bow  the MA would be 5.5 to 1. At FD 28" it would be 1.17 to one. Is that even close to right? They kicked me out of school in grade 11 so I don't have much to fall back on

[willie]

I tried to duplicate this and the 90° line never started to get closer. I think I need pictures with circles and arrows telling what each one is about.

If there are geometrical solutions, I also think a sketch would help.


PS. I have a different perspective in mind, but hesitate to mention it until Marks idea is visualized better.

[mmattockx]

How do you figure the MA of the string? The best I could come up with was the draw length compared to one limb length. ie at 6" brace on a 66" bow  the MA would be 5.5 to 1. At FD 28" it would be 1.17 to one. Is that even close to right?

MA = maximum angle?

They kicked me out of school in grade 11 so I don't have much to fall back on

You're hiding it very well. 


Mark

[Badger]

  I don't accept that the string angle should correlate with the handle. I have alwasy heard and it makes sense that the string angle correlates to the outer limb. 90 degrees is usually the point where stacking becomes pronounced.

[DC]

How do you figure the MA of the string? The best I could come up with was the draw length compared to one limb length. ie at 6" brace on a 66" bow  the MA would be 5.5 to 1. At FD 28" it would be 1.17 to one. Is that even close to right?

MA = maximum angle?

They kicked me out of school in grade 11 so I don't have much to fall back on

You're hiding it very well. 


Mark

Mechanical advantage

[DC]

90 degrees is usually the point where stacking becomes pronounced.

It's pretty hard to argue that. I thought that mmattockx maybe had a geometric explanation. I could imagine it working but when I tried a mockup I couldn't get it to work.

[mmattockx]

I have alwasy heard and it makes sense that the string angle correlates to the outer limb. 90 degrees is usually the point where stacking becomes pronounced.

I believe Tim Baker relates this to the the point where the gear ratio becomes 1 or less (1" of draw=1" or more of tip movement) and the limb gains the upper hand in leverage (TBBV1 in the design section?). Perhaps what I saw geometrically is all rolled into the gear ratio concept, meshing together the string angles and leverage of the string over the entire limb. The gear ratio concept would also explain the string tension rising when stacking starts.


Mark

[willie]

The gear ratio concept would also explain the string tension rising when stacking starts.

Does anyone feel up to the challenge of writing a simple explanation of the concept?

Or perhaps continue last years thread...

http://www.primitivearcher.com/smf/index.php/topic,65348.0.html

[DC]

Opinion here. I don't think changing ratio quite covers it all. The changing ratio of the string is a constant thing. It changes from being to your advantage to being against your advantage. As far as I know it's a constant linear change. People always talk about stack as coming up against a wall. I don't have a lot of experience with stack as I don't make short bows(yet) so I'm going by what I've read. It seems like there may be two or more things that come together at one time to form the wall. I do know that the lever of the limb gets shorter as the limb bend gets tighter. Maybe the loss of mechanical advantage from this combined with the loss of mechanical advantage from the gear change combine.  I'm pretty sure that mechanical advantages are multiplied rather than just added so if they happen at the same times it might make the wall.  I'm quite willing to be proven wrong.