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

[sleek]

Ok. I will chime in again. If at full draw there is a portion of the receive the string is still laying on, what is that portion doing? My impression, nothing. But I don't know if it affects string tension.  Now that leads me to wonder what string angle at full draw is desired?  If 90 degree is best should a recurve be cut down until the angle desired is reached and the rest is therefore unneeded weight? Of course you don't need to cut the recurve down, you could open it up some or make it work open at full draw. What are the performance differences?

All speculation but I would love to know for certain. 

[joachimM]

So that two inches of lift off at the end of the draw is enough to make the bow pay for all that added mass?
   Many others would say it's the increased string tension early on that is the beneficial part.

I'm sorry, I think you got me wrong or I expressed myself wrongly. Lift-off for the last two inches will not outweigh the added mass IMO. Lift-off should happen as soon as possible during the draw.
The increased string tension certainly is beneficial. But if you have a 60" stave with 5" 90° hooks at each side that don't lift off during any part of the draw, you might as well just make a straigth stave 50" bow (or cut off the hooks). it will have the same string tension at brace and have the same force-draw curve. But it will have lighter tips. Now, as Badger stated: any recurve will show at least partial lift-off, however; it's only the dead-mass part that doesn't lift off (or too late) that should be cut off.

The cost of the higher string tension at brace is the higher tip mass. the benefit of a recurve is first the higher string tension, because it's functionally a shorter bow at brace (the functional length defined by the length of the string between the contact points of the recurves). A short bow will stack earlier and heavier. When recurves lift off, they functionally lengthen the bow, compensating for the higher stack.  Basically, recurves with complete lift-off let you start with a 50" bow, and let you end with a 60" bow. Stack is essentially a reduced leverage because the limbs' functional length shortens during the draw (the below fig should clarify this).
The later the lift-off, the less distance the limbs travel back during wich the recurves function as levers. In some sharp recurves (with late or even without complete liftoff), the advantage of higher string tension at brace doesn't outweigh the disadvantage of the added mass. This is the trade-off to watch out for., this is what makes the extreme Hickman recurve less efficient that it's fantastic FD-curve suggests.
 
The longer the limbs, the later the lift-off of sharp recurves, and the less net gain there is. Also, the longer the limbs, the less stack (percentage reduction in effective leverage length) there is, and the less need for compensation of stack.

There are more subtle ways to get higher string tension at brace, with less cost of added mass of sharp recurves. Deflex-reflex bows like Marc St-Louis' (http://www.primitivearcher.com/smf/index.php/topic,52801.0.html) or other low-stack designs
Anybody has a good picture of Karpowicz's low stack design? Basically a straight setback eiffeltower bow with an elevated handle. At brace the string lies nearly flat on the entire limb, with immediate lift-off during the draw. Drawn, the bow resembles angular bows.

And please, those who haven't done so, do an attempt to read (parts of) Kooi & Bergman's paper. http://www.bio.vu.nl/thb/users/kooi/kobe97.pdf
It will clarify a lot of things that are questioned over and over again here.

Joachim
 

[PatM]

So that two inches of lift off at the end of the draw is enough to make the bow pay for all that added mass?
   Many others would say it's the increased string tension early on that is the beneficial part.

I'm sorry, I think you got me wrong or I expressed myself wrongly. Lift-off for the last two inches will not outweigh the added mass IMO. Lift-off should happen as soon as possible during the draw.
The increased string tension certainly is beneficial. But if you have a 60" stave with 5" 90° hooks at each side that don't lift off during any part of the draw, you might as well just make a straigth stave 50" bow (or cut off the hooks). it will have the same string tension at brace and have the same force-draw curve. But it will have lighter tips. Now, as Badger stated: any recurve will show at least partial lift-off, however; it's only the dead-mass part that doesn't lift off (or too late) that should be cut off.

The cost of the higher string tension at brace is the higher tip mass. the benefit of a recurve is first the higher string tension, because it's functionally a shorter bow at brace (the functional length defined by the length of the string between the contact points of the recurves). A short bow will stack earlier and heavier. When recurves lift off, they functionally lengthen the bow, compensating for the higher stack.  Basically, recurves with complete lift-off let you start with a 50" bow, and let you end with a 60" bow. Stack is essentially a reduced leverage because the limbs' functional length shortens during the draw (the below fig should clarify this).
The later the lift-off, the less distance the limbs travel back during wich the recurves function as levers. In some sharp recurves (with late or even without complete liftoff), the advantage of higher string tension at brace doesn't outweigh the disadvantage of the added mass. This is the trade-off to watch out for., this is what makes the extreme Hickman recurve less efficient that it's fantastic FD-curve suggests.
 
The longer the limbs, the later the lift-off of sharp recurves, and the less net gain there is. Also, the longer the limbs, the less stack (percentage reduction in effective leverage length) there is, and the less need for compensation of stack.

There are more subtle ways to get higher string tension at brace, with less cost of added mass of sharp recurves. Deflex-reflex bows like Marc St-Louis' (http://www.primitivearcher.com/smf/index.php/topic,52801.0.html) or other low-stack designs
Anybody has a good picture of Karpowicz's low stack design? Basically a straight setback eiffeltower bow with an elevated handle. At brace the string lies nearly flat on the entire limb, with immediate lift-off during the draw. Drawn, the bow resembles angular bows.

And please, those who haven't done so, do an attempt to read (parts of) Kooi & Bergman's paper. http://www.bio.vu.nl/thb/users/kooi/kobe97.pdf
It will clarify a lot of things that are questioned over and over again here.

Joachim

  Will you grab a piece of PVC and confirm all of that?
  It really does seem like something is being overlooked here if you examine those old records.
 And without going all Pearlie on you, how many statics have you experimented with?

[dragonman]

Levers that dont lift off are simply not just dead weight, its silly to say so.

I appreciate that Pat, but I dont  think the leverage principle can be seperated from what is being discussed,its part and parcel of how all types of recurves function...

[joachimM]

PatM, I appreciate we are staying civil here 

I might try to show it with PVC, if you promise to read Kooi & Bergman's paper 
But I have the feeling I could come up with tons of data, scientific evidence supporting the point I've been making, and still some people won't believe it, because their gut feeling says otherwise.
To each his or her own.
I could as well return the argument and say "prove me wrong", and let you do the hard work. 

Sure I havent made a ton of bows, certainly not of static recurves. I bank on other people's experience (among which some peer-reviewed scientific papers), common sense and logic.
I don't see anyone taking the logical arguments to show that I made a critical mistake in some reasoning. All I read is "I don't think so" or some variants thereof.
I don't know what hard facts to present to you other than what I have shown already, about the Hickman recurve, about the efficiency of different kinds of bows (straight, recurves, asiatic composites, extreme recurve, ...), both theoretic and in practice,  etcetera. There's been a ton of threads on this and other archery forums on the trade-offs of recurves. What I wrote about is not my work, my experience... I'm just tying different ends together.

My goal in life is not to convince you. I might build the PVC examples. You could still say "well, that only counts for PVC" or some other arguments. 

If we agree to disagree that's fine with me.
Sorry Sleek for abusing your thread here. Hope it's been at least to some use.

Joachim

[PatM]

You know I'm pretty sure it was me that originally steered you towards the papers of Kooi  over on Paleoplanet
 I have read them.
 Sometimes it's hard to tell if those guys actually built real bows though or just drew diagrams and did the math.
 I do know that their names are absent on those old records.
 I keep hearing about better ways to achieve performance and yet all of those better examples don't deliver at the salt flats.

[sleek]

Some use..... Im out of popcorn.

[Badger]

    The only benefit I coud see to a recurve that didn't open up would be the amount of reflex it introduced. I would have to agree that if it didn't open up it wouldn't add any benefits of a recurve. Joquin, did you see my post above on the Tim Baker bow you mentioned? I have done a lot of testing on bows but I have never tested one that did not open up. Aside from that I have never seen a straight limbed bow anywhere near as fast as a good well made recurve. The Baker example given above was only par for a well made straight limbed bow.

[Bryce]

Oh jeez not this tired debate again.......
You can find this same subject, with at least 100 results in the search bar with the same arguments.


I say...MORE HOOKS!!!! Let's see em!

[RyanY]

Sounds like we need to see some force draw curves that correspond with where the string is on the recurve. I'd like to see if there's a moment of change when there's significant lift off of a recurve.

[wizardgoat]

I'm no mathematician, but I have built a fair amount of statics.
I went through a phase where I was FD charting every bow, about 9 or 10 bows, and all but one were recurves. Depending on length and degree of curves, I could always see on the graph where the string lifted off, but I will admit it was never as drastic or "wheelie bow-ish" as I would of hoped.
All my top performing bows have been shorter statics
I've never built a bow where the string did not lift off, and I don't think I've ever seen one, modern or artifact, but I would be curious to see those numbers as well

[joachimM]

Well folks,
since I came up with the freaking PVC, it's time I delivered...
So here 's a mini test bow out of electric PVC pipe. it's 1/2", thin walled and will probably pull 10-15 pounds at 24" or so. it's 45" with 6" hooks (a bit more than the intended bow this thread originally started with). It took me some time to make the recurves non-working...
I made it with big and sharp hooks so they won't open up completely at full draw. its 23:50h right here, so I wont be testing it anymore tonight.

And yes PatM, that's you who sent me the link to the papers! I nearly forgot about that. Thanks again, you see at least I'm trying to use them in a sensible way. This means you already did your part of the bargain I proposed, so I really need to do my part too 

So just to make sure we agree on the setup:

This particular test bow, I'll put on the tree and get a FD-curve. Next, I'll shoot it with arrows that fit 10 gpp as close as possible and chrono it. But the lightest carbon arrows I have lying around are 26" 210 grain so let's hope I get a bit higher draw weight. I'll try to take 20 measurements, in order to average out variation caused by differences in release (I'm not exactly a drawing machine). I will see to where the recurves open up and to what extent they seem useful to me, and I'll shorten them next. I'll shorten the string for the same brace height, and recalculate the FD-curve. If I'm correct, the FD-curve should be identical.
Next, I'll chrono it again with the same arrows 20 times. I do a standard t-test (a statistical test to find out if there is significant difference between two groups of observations) to test if the bow with shorter hooks shoots any faster, on average. If it does, it supports the argument that the rest of the hooks are dead mass not contributing to string tension and other performance.

Badger: yes I read about it. Thanks for correcting me there. It's not just a matter of opening up (partly or completely), its also to what extent they open up. When projected on the X-axis, how much net limb length does it give? If the angle is still sharp, the horizontal projection of the recurve will be less than its actual length (but with the same mass, of course).
But do you agree that the fastest 10gpp wood-only bows so far don't have extreme (90°) hooks and shoot about 190 fps? 

cheers
Joachim

[joachimM]

And here it is strung. it's bending too much in the handle section and draws 10# at 22" but what do you expect from electric pipe...
So I'll really have to find even lighter arrows. Maybe I can check for my longest chop sticks and fletch them 

Joachim

[paulsemp]

This officially belongs in around the campfire. We have PVC posted in the bow section, blasphemy. Sleek I do not have much experience in Statics but the few I've done boil or heavy steam and have your jig ready and bend them in one shot.

[PatM]

It would be neat if you could get a variety of degrees  of recurve from the same piece and also chop them off entirely and test it as a short bow strung across the points of string contact.
 That might introduce too many variables with heating and getting the material tired though.

  The hollow outer portions should be enough to compensate for the fact that we do still reduce tips as much as we can.