Sharp hooks ( a how to and lots of argument as to their merits )

[DC]

That's why we have mathematic models... But these don't seem to count here 😳

They would count if I had an advanced education. But I don't. For some one like me(and probably others) if I see a page of formulae it might as well be written in Chinese(it does look like it). If I insisted that all my questions were written in Arabic, do you think I would get many answers? I'm 68, it's a little late to change me. Maybe mathematicians and physicists should learn to answer questions in English(or whatever language is being spoken at the time and place)
Sorry, rant mode off

[joachimM]

That's why we have mathematic models... But these don't seem to count here 😳

Maybe mathematicians and physicists should learn to answer questions in English(or whatever language is being spoken at the time and place)

DC, you have an important point here. Maybe it's a good idea to translate some of these papers into laymans' terms, make a kind of simplified resumé highlighting the main aspects. That's what I have tried to do piece by piece in my comments when referring to some papers. But apparently, not well enough.

Frankly, when I read such papers, I rarely go through the entire math section. It would take me days or sometimes even weeks to study these portions to figure out what they exactly or even approximately represent. I largely trust in the scientific peer-reviewing process to control that the math wasn't complete bollocks.
I read what I can understand, and draw my conclusions. Don't let the math scare you 

Joachim

[DC]

I know it's a tough row to hoe trying to put this stuff in laymans terms. Models and pictures work wonders. We need more people like Isaac Asimov (sp?) I do try to wade through the math but when you put in a search like "simple bow mechanics" and the first three hits are full of sqiggly things. Maybe "basic" instead of "simple"? Nope, same stuff. Asimov could explain how the universe worked in simple terms, surely there is someone out there that can do the same for a bent stick.

[willie]

DC-

I was recently researching through some of those kind of papers, and like Joachim, I just try to glean what is pertinent to bow building from the simpler parts of the paper, and rarely look at the math.

In my searching, I often find well written explanations, often from first or second year college texts, but hesitate to post the links,  as there are often comments in the threads, that make you feel as if a scientific or engineering oriented posting is out of place or at least pushing the limits, here at PA. If you have a specific interest. by all means do not hesitate to ask, and as for the Issac Asimov version for wood science, I can recommend  Understanding Wood, by  Bruce Hoadley  , and I may even be able to find a pdf copy if anyone is interested.

[joachimM]

I did some more testing with PVC models, and got some interesting results. Nothing unexpected. Not to me, at least. 
see http://www.primitivearcher.com/smf/index.php/topic,56377.0.html

[PatM]

  Sigh. The deflex  is another variable.  I am purely talking about ONE variable in the SAME bow.

[Danzn Bar]

Sigh. The deflex  is another variable.  I am purely talking about ONE variable in the SAME bow.

It will ware you out, wont it? 

[PatM]

No, I  think I have access to a  a piece of PVC so that I can illustrate what I meant.

[joachimM]

Sigh. The deflex  is another variable.  I am purely talking about ONE variable in the SAME bow.

Com on PatM, you're wrong, IMHO. Not deflexing would have added a variable, namely it would have changed peak draw weight at the same draw length. Of course a bow with higher poundage will shoot faster. That's not the point here. Both bows weigh exactly the same. mass and draw weight at same draw length. Net reflex is identical.
 
You could level the playing field by changing the draw lenght on the less recurved bow, so as to have the same peak draw weight. One at 24", the other at 27" or so. But then the power stroke would be different, and you would object on that.

Look: peak draw weight is identical, but the energy stored in the strongly recurved bow is higher because it is still strained more, in spite of the deflex. What everybody regurgitates is then that this bow should shoot faster, because it stores more energy. Well, theory says it doesn't necessarily. People react by saying: "well that's just theory".
I showed it doesn't in practice. What else do you want??

It's easy to stand by and criticize. You're ignoring hard facts right now. Stored energy (the area beneath the FD-curve) is NOT equivocal to efficiency.
It's your turn now to show, empirically, that I'm wrong and you're right. I have shown hard facts  . You have given your opinion  . Facts versus opinion.
I tend to value your opinion most of the time, but unless you come with equally hard facts, I say you have been trumped 

[scp]

Sigh. The deflex  is another variable.  I am purely talking about ONE variable in the SAME bow.

The variable is the change of angle. We are trying to keep the draw weight and draw length constant. What would be your way of keeping them constant, while just changing the angle of recurve only?

[PatM]

No, you're missing the point. I was strictly talking about people making a bow that is a few degrees sharper than 90 and other people saying that the recurve should be eased off a few degrees in order for the recurve to work at all.

  Would you actually leave the full mass on the recurves in the real world? Reduce both of your bows at the tips as much as you safely can and then test them again.  That's what we actually do when we make bows 
 

[scp]

I was strictly talking about people making a bow that is a few degrees sharper than 90 and other people saying that the recurve should be eased off a few degrees in order for the recurve to work at all.

That's a very good question, but joachimM is not addressing it. Still we can extrapolated from his experiment that it would be better to make the hook less sharp. I hope he is willing to un-bend his sharp hook model slightly, say just 10 degrees, and test it and compared the result with the numbers before un-bending. We can even stipulate that any change in draw weight and length would be within the margin of error.

[joachimM]

No, you're missing the point. I was strictly talking about people making a bow that is a few degrees sharper than 90 and other people saying that the recurve should be eased off a few degrees in order for the recurve to work at all.

  Would you actually leave the full mass on the recurves in the real world? Reduce both of your bows at the tips as much as you safely can and then test them again.  That's what we actually do when we make bows 

PatM: and what will you compare then? Arrow speed of bows of different poundage?

Next the full mass: well I explained you cannot tiller PVC that way. Despite this, the highly recurved bow performed much worse.

The point I was trying to make is that a force-draw curve is not a good proxy of performance. Bows with fat hooks have fat FD-curves. But they are deceiving. They store more energy but do not necessarily impart more energy to the arrow. I have explained the mechanics of the reason for this.
Two bows with the same peak draw weight, one sharply recurved the other weakly: which one will shoot a 10 gpp arrow faster (or transfer its stored energy most efficiently)? Probably the bow whose FD-curve resembles a straight line the most.

I wasn't even talking about a few degrees above 90. The point is that whenever you have a recurve that's not lifting off during some part of the draw, that reduces efficiency of the bow. So how would you test this then?

Recurves, especially in highly strained designs like the various asiatic composites, make the bow unstable (flip-flop tendency) unless the entire lower limb is made very wide and the ears are made very stiff. One way to reduce this lateral unstability a bit is by allowing the ears to be set back a bit more than a contact ear at brace.
The hungarian composites lift off immediately (contact recurve with long ears), but the lower limbs are very wide, and the ears are stabilized laterally with bone, for its very high stiffness. The ears are rather heavy, but the lift-off is immediate.
Turkish composites have rather short and light ears, and are set back a tiny bit. This setback adds stability, and lift off is very early in the draw. So it concedes a tiny bit of efficiency for stability. Idem for the korean bow.
The Chinese Manchu composite has ears set back a high degree. They lift off only at about 22". But when drawn to 36 or even 40", this is even relatively speaking a fast lift-off. These bows weren't made for high efficiency, they were made very heavy to shoot war arrows the size of broomsticks. They didn't care about 10 gpp or high dry fire speed.

So how do you define "in order for the recurve to work at all", and how would you test this without comparing apples to pears? And mind you, it will show exactly the same as what I've shown if you can make a level playing field.

[Pappy]

Performance aside, which is smoother to draw[less stack] and shoot [stable]? That is what I mainly look for. another can of worms.
 Pappy

[joachimM]

Pappy, very good question, but to a large degree a matter of personal preference. My shooting was more consistent with the longer bow. In general, long bows shoot more stably and have less stack. I admit that the lift-off of a good recurve at the end of the draw eases the archer (more leverage), but that's like saying "it feels good when the headache stops". The leverage was there all the time for the longer bow, not only at the end.