deflex and reflex theory

[Badger]

Halfbow, I think we are talking about string tension in the context of a bows stability at brace, not string tension as it affects the draw. Any bow resembling anything like a "C" when braced will likely be stable. If it resembles a triangle it will be much less stable because of the forces pulling the limbs together have no where to go without distorting the limbs, there is no clear path the limbs have to follow as in a bow with a "C" shape. Kind of like pushing straight down on a poll, you have no idea which way it is going to bend. I tried building some deflexed inverted limb bows just to see how they would do. The string tension was still too high at brace for the bows to be stable and the bows had no reflex at all.

[Selfbowman]

Steve the pole thing is what I was thinking also. Did not know how to say it though. Well done. Arvin

[Halfbow]

Badger, I'm with you there. Pulling down on a pole is what I was trying to describe when I asked if it was because the limb was running quite parallel with and close to the string for so long at brace.

I'm unsure about how much that kind of braced profile increases initial string tension, as would be measured by a scale, but I can see how the profile could even make less string tension too much. Revised design that I think will be more stable, coming soon.

[willie]

...... But I'm on conceptual mission for the fps here. 

Half,
 I am kinda curious for what purpose you you seek max fps with primitive mat'ls?

hunting?
flight?

[Badger]

Badger, I'm with you there. Pulling down on a pole is what I was trying to describe when I asked if it was because the limb was running quite parallel with and close to the string for so long at brace.

I'm unsure about how much that kind of braced profile increases initial string tension, as would be measured by a scale, but I can see how the profile could even make less string tension too much. Revised design that I think will be more stable, coming soon.

It is a huge amount of string tension, I have had it break strings as soon as I braced them that were fine for normal 50# bows.

[Halfbow]

Half,
 I am kinda curious for what purpose you you seek max fps with primitive mat'ls?

hunting?
flight?

Time travel of course. If you ever find yourself stuck in the stone age, you ain't going to be building computers, planes, or lightbulbs. But you might be able to build the most efficient bow the world has ever seen.

Really though, primitive materials because I like knowing I can make things from my environment. I'd love to get in to flight archery, but really the main reason is I think it's an unusually fascinating puzzle, and I like puzzles. A bent stick with a string seems so simple... until you start really thinking about it.

Also, I think that understanding concepts is inherently valuable. Even if the most efficient possible design is torture to shoot, understanding the concepts behind it will be helpful for making knowledgeable decisions about what trade-offs to make, and for making better usable bows.

It is a huge amount of string tension, I have had it break strings as soon as I braced them that were fine for normal 50# bows.

That's very interesting, and it does expand my understanding of string angle at the tips. (Though I stand by my point about reflex mattering a lot for string tension too)

[Halfbow]

Alright here's mkII. It may not look like much, but this was quite difficult to arrive at. Playing around with design becomes a real pain when you have an end goal as precise as straight limbs at full draw.



Fair warning, skip the next 3 paragraphs if you don't care about fiddly details. Here's my thought process:

My first thought for how to increase stability was to get the string contacting the outer limbs at brace. Where the string contacts the limb is mostly taken out of the stability equation, and the bow behaves like a shorter bow until the string gets lifted off the limb during draw.

But I also knew I wanted to the limbs to be straight at full draw. It's not especially easy to achieve both these things. If I had just made the limbs more dramatically curved at brace, I could've gotten good string contact, but then the brace height would get too low. This meant I had to increase the deflex at the handle, as this way the limbs are angled more directly toward the string and can contact it sooner. But that meant I had to make the bow shorter. If I didn't, the limbs wouldn't bend far enough to be straight at full draw. So the challenge was to deflex the handle enough to get good string contact while maintaining brace height while avoiding making the bow absurdly short.

After I got the string to contact the outer limbs, I started to worry about efficiency losses there. The string doesn't have much leverage on the limb when it's right up against it. With it contacting so much of the limb, that could turn in to a real problem. When shooting, after releasing, when the bow hit brace, the string wouldn't have enough leverage to stop those outer limbs. They'd just continue moving forward past brace position, violently stretching the string. A lot of energy would go in to the bow instead of the arrow. To deal with this, I added some string bridges along the outer limbs. This is kind of like thickening the outer limbs. They'll be much far stiffer for the price of the minimal mass of the bridges. They give the string much better leverage over the limb. They also offered the benefit of lengthening the string and raising the brace height. This longer string meant I had to lengthen the limbs, or the draw length would be too long, which meant I had to decrease the deflex in the handle, which was convenient because I now had some extra room to do so thanks to the extra brace height. So adding string bridges caused a slightly longer and less deflexed bow, which is what I was wanting. Thanks string bridges.



With it on top of my old design, you can see how much shorter it is. You can also see it's suffered a significant downgrade to the string angle at the fingers. But on the other hand, we've added the benefits of less mass from shorter limbs, and the string let off. The string coming off the bridges during the draw should fatten the f/d curve.

I'm honestly not sure which of those 2 bows to expect to be better performance-wise. I think that string angle is really important, but the little bow has strong advantages on its side too. I want to experiment. But, I am at least pretty confident this new bow is more stable.

I drew in quite a lot of unbraced reflex again. But note that that is the easiest part of the design to change. It's basically the only thing you can change without throwing everything else in to disarray. So if it's too much reflex for you or your materials, you can just lessen it. Unbend those working areas a bit. If you went far enough unbending them, you could even make this bow have the same braced and unbraced profile. Get that slack string, if that's what you're in to.

Note that with a bow this short, I think wood would have a hard time even without the extreme unbraced reflex. I think we are in horn/sinew or modern material land here. ..But possibly a modified version of this design could be a wooden paddle bow?

But that said, I think this odd short little guy would be good. Excellent string angles for its length, light limbs (especially if you keep a lot of unbraced reflex), fat f/d curve, and more stable than my previous design. It would need a hefty string though.

Tell me what I'm missing guys.

P.S. Yes, I know the handle is a pinnacle of ergonomics. 

[Badger]

  There is another issue I am going to throw at you. When you go to tiller the bow at brace you will have high string angles coming out of the handle so that will want to bend first, you will have a tendency to weaken the stiff area with low string angles to get them bending and then suddenly when they start to bend their string angle will increase and they will bend too much. It is a real balancing act trying to get it right

[DC]

Good post. I like it because it parallels kinda where my thoughts were going. I was thinking about  a lot of string contact as a good thing but I hadn't thought about how the unsupported outer limb would behave when the bow was released. It would basically be a dry fire. As much as I don't like bridges(aesthetics mostly) they do seem like an answer.

[DC]

  There is another issue I am going to throw at you. When you go to tiller the bow at brace you will have high string angles coming out of the handle so that will want to bend first, you will have a tendency to weaken the stiff area with low string angles to get them bending and then suddenly when they start to bend their string angle will increase and they will bend too much. It is a real balancing act trying to get it right

I was/am having a lot of trouble with this. What I find helped was using a long long string. It puts some strain on the outers while tillering and kind of keeps them in line. You have to be careful and conscious of what a long string can do but it helped me with the "all of a sudden" issue I was having.

[Halfbow]

There is another issue I am going to throw at you. When you go to tiller the bow at brace you will have high string angles coming out of the handle so that will want to bend first, you will have a tendency to weaken the stiff area with low string angles to get them bending and then suddenly when they start to bend their string angle will increase and they will bend too much. It is a real balancing act trying to get it right

Thanks, definitely something to watch out for. I'm sure such a thing takes practice, but you may have saved me some frustrations.

Good post. I like it because it parallels kinda where my thoughts were going. I was thinking about  a lot of string contact as a good thing but I hadn't thought about how the unsupported outer limb would behave when the bow was released. It would basically be a dry fire. As much as I don't like bridges(aesthetics mostly) they do seem like an answer.

DC, a test you did awhile ago helped me to understand the geometry of bending limbs. You may remember when you were tracing the arc various limbs made at the tip as they bent. That was really interesting, and is what eventually lead me to realize the benefits of straight limbs at full draw.

[willie]

Really though, primitive materials because I like knowing I can make things from my environment. I'd love to get in to flight archery, but really the main reason is I think it's an unusually fascinating puzzle, and I like puzzles. A bent stick with a string seems so simple... until you start really thinking about it.

Also, I think that understanding concepts is inherently valuable. Even if the most efficient possible design is torture to shoot, understanding the concepts behind it will be helpful for making knowledgeable decisions about what trade-offs to make, and for making better usable

Half
Lots of good ideas being explored here, and the concepts are challenging. Since we lost Stuckinthemud after he third post, I don't suppose it would be inappropriate expand the discussion to point out that raw speed does not always equate to long cast or accurate arrow placement.

BTW, are these conceptual bows "geared" towards lighter arrows? Or arrows of any particular type?

[sleek]

Alright here's mkII. It may not look like much, but this was quite difficult to arrive at. Playing around with design becomes a real pain when you have an end goal as precise as straight limbs at full draw.



Fair warning, skip the next 3 paragraphs if you don't care about fiddly details. Here's my thought process:

My first thought for how to increase stability was to get the string contacting the outer limbs at brace. Where the string contacts the limb is mostly taken out of the stability equation, and the bow behaves like a shorter bow until the string gets lifted off the limb during draw.

But I also knew I wanted to the limbs to be straight at full draw. It's not especially easy to achieve both these things. If I had just made the limbs more dramatically curved at brace, I could've gotten good string contact, but then the brace height would get too low. This meant I had to increase the deflex at the handle, as this way the limbs are angled more directly toward the string and can contact it sooner. But that meant I had to make the bow shorter. If I didn't, the limbs wouldn't bend far enough to be straight at full draw. So the challenge was to deflex the handle enough to get good string contact while maintaining brace height while avoiding making the bow absurdly short.

After I got the string to contact the outer limbs, I started to worry about efficiency losses there. The string doesn't have much leverage on the limb when it's right up against it. With it contacting so much of the limb, that could turn in to a real problem. When shooting, after releasing, when the bow hit brace, the string wouldn't have enough leverage to stop those outer limbs. They'd just continue moving forward past brace position, violently stretching the string. A lot of energy would go in to the bow instead of the arrow. To deal with this, I added some string bridges along the outer limbs. This is kind of like thickening the outer limbs. They'll be much far stiffer for the price of the minimal mass of the bridges. They give the string much better leverage over the limb. They also offered the benefit of lengthening the string and raising the brace height. This longer string meant I had to lengthen the limbs, or the draw length would be too long, which meant I had to decrease the deflex in the handle, which was convenient because I now had some extra room to do so thanks to the extra brace height. So adding string bridges caused a slightly longer and less deflexed bow, which is what I was wanting. Thanks string bridges.



With it on top of my old design, you can see how much shorter it is. You can also see it's suffered a significant downgrade to the string angle at the fingers. But on the other hand, we've added the benefits of less mass from shorter limbs, and the string let off. The string coming off the bridges during the draw should fatten the f/d curve.

I'm honestly not sure which of those 2 bows to expect to be better performance-wise. I think that string angle is really important, but the little bow has strong advantages on its side too. I want to experiment. But, I am at least pretty confident this new bow is more stable.

I drew in quite a lot of unbraced reflex again. But note that that is the easiest part of the design to change. It's basically the only thing you can change without throwing everything else in to disarray. So if it's too much reflex for you or your materials, you can just lessen it. Unbend those working areas a bit. If you went far enough unbending them, you could even make this bow have the same braced and unbraced profile. Get that slack string, if that's what you're in to.

Note that with a bow this short, I think wood would have a hard time even without the extreme unbraced reflex. I think we are in horn/sinew or modern material land here. ..But possibly a modified version of this design could be a wooden paddle bow?

But that said, I think this odd short little guy would be good. Excellent string angles for its length, light limbs (especially if you keep a lot of unbraced reflex), fat f/d curve, and more stable than my previous design. It would need a hefty string though.

Tell me what I'm missing guys.

P.S. Yes, I know the handle is a pinnacle of ergonomics. 

I'm very certain that your string bridges will do the exact same thing as deflex.

[PatM]

String bridges won't stiffen the tips.  They may stiffen  tiny parts of it but not the entire area.


 Are you aware that Karpowicz did a fair bit of experimenting with this design and Hickman also in the past?

 Someone on ATARN made this design  also.

[Halfbow]

Half
Lots of good ideas being explored here, and the concepts are challenging. Since we lost Stuckinthemud after he third post, I don't suppose it would be inappropriate expand the discussion to point out that raw speed does not always equate to long cast or accurate arrow placement.

BTW, are these conceptual bows "geared" towards lighter arrows? Or arrows of any particular type?

Definitely. As with most things, optimizing for one thing means trade offs in other areas (though not always). But again, I think these underlying concepts are useful to understand.

As to your question about arrows, I too am curious about people's thoughts on that. My thoughts are to take the dry fire speed of the bow (something you'd never want to test, but nonetheless a real and interesting data point) then figure out how easily it gets slowed down by the weight of an arrow. I suspect this bow will have a very fast dry fire speed, but due to the light limbs it may be more easily slowed down by heavier arrows. However, I'm not sure how much the extreme reflex affects this, but I bet it does.

I'm very certain that your string bridges will do the exact same thing as deflex.

The same thing as deflex? I don't follow. Can you explain more?

String bridges won't stiffen the tips.  They may stiffen tiny parts of it but not the entire area.


Are you aware that Karpowicz did a fair bit of experimenting with this design and Hickman also in the past?

Someone on ATARN made this design  also.

Stiffen tiny parts of it? Hahaha that's an amusing thought. Yes, I agree, the bridges themselves don't do much to stiffen the limb. It's the elevated string under tension.

Think of a cable backed bow. If you put bridges along the back to elevate the cable, the limbs get much stiffer, and not just tiny parts of them. I know this not just because it makes sense (which it does, you're essentially thickening the limb by moving the tension side farther out), I also know it first hand because I've done it. My bow here uses the same concept flipped around.

On a braced bow, the only thing stopping it from returning to its unbraced profile is the string and its tension. Elevating the string off the limb gives it better leverage over the limb, and makes it harder for the limb to bend away from it. This is useful at the end of the powerstroke when the outer limbs have a lot of momentum. Without string bridges the string would be right up on the outer limbs, and thus wouldn't have much leverage to stop them. At best the string would get stretched, reducing the efficiency of the bow.

Stiffness may be a poor choice of words on my part here. Obviously this "stiffness" only applies in one direction, and only while the string is actually on the bridges. The bridges do nothing for the stiffness during most of the draw, which is fine.

I was unaware of other people trying things like this. That's kind of awesome. I did some searching, but was unable to find Karpowicz's version, or the one on ATARN. I did find a drawing of the Hickman design. I can see why you'd make a comparison, mine and his both have severe deflex in the handle and straight limbs at full draw. But I wouldn't call it the same design. That's as bad as saying all straight bows have the same design.

Just so everyone knows what we're talking about:



A big difference is that his design has very similar braced and unbraced profiles. Little extra unbraced reflex. I'd expect that design to have much lower string tension at brace. Also my design has a smaller working area and stiff outer limbs (even without the string bridges). I think small working area and stiff outer limbs are significant for all the reasons Badger says they are.

Also, to me that design is just begging to have string bridges. I'd expect that bow to have a fantastic f/d curve, but to be inefficient at putting that energy in to the arrow because the string can't effectively stop the limbs' momentum.

I do, however, think he's on to something with the long unbending handle. This gets you the string angles of a longer bow but with the short light limbs of a shorter bow. I was thinking of doing something like that on my design, but decided to leave it for another time.