Bow design and mechanics testing with PVC models

[joachimM]

Plan B: point taken. I will do the tests with heavier arrows.

[scp]

joachimM, most people are not disputing your result. They are just curious about other aspects of similar experiments.

As with PlanB, I also would like see the same experiment using a heavier arrow, say 15 gpp. We do appreciate your effort a lot. Thanks in advance.

[PlanB]

Thanks Joachim.

Also just as a point of interest in conversation, I believe that hand shock is to some degree a result of the arrow absorbing little energy after the initial acceleration. That energy stays in the limbs, and has to go somewhere when the bow reaches brace height, so it overshoots -- a little like dry firing.

People say that it s caused by heavy limbs, and in a way that is true. But not directly The heavy limbs are slower and so an arrow can absorb less energy. A light arrow will only absorb mostly initial energy, leaving a lot in the limbs.

I think that it would be easy to prove it is an energy transfer problem, not just heavy limbs, by taking a bow with bad hand shock and trying a heavier arrow and seeing if the hand shock goes away or reduces. The heavier arrow would be able to absorb more of the bow's energy and there would be less left over at the end of the shooting cycle to transfer down the limb.

That's a guess...

[willie]

So I say: prove me wrong! I feel like I did my fair share of empirically testing the mathematic models. Come with your own hard data instead of opinions. . Shoot two bows which have the same peak draw weight at the same power stroke, and which are strained the same amount (the mass principle is a good starting point to know this). And prove me that a bow with sharp hooks/ bow with a fat FD-curve shoots faster at 10 gpp than one with "flipped tips" / bow with nearly straight FD-curve.

Seems like your work has stood the test, at least until a doubter presents something different, however I am not a doubter

 I recently got a chrony and even found some pvc, and am following this thread to see if anyone thinks that there may be a potential use of pvc pipe models, scaled up in size, to improve designs for selfbows.

  Of course we might have to incorporate some way to monitor the acceleration of the arrow, if efficiency is the goal.

In short, my question about pvc pipe is not to discount it's use for simple design evaluations as you have done here, but to see if its up to adding to our already nuanced understanding of bow/arrow dynamics.

[bubby]

You know the original question was not non contact vs string that doesn't leave but when the string loses contac during the draw and what point of the draw lift off would be more efficient you seem to have morphed the test completely away from that. Why?

[willie]

Bubby-

Don't know about the original question part (have not gone back to reread everything), but that might be a worthwhile question to test with pcv, especially in light of Plans B;s inquiry of "at what relative arrow weight". Do you have an arrow weight in mind? or even better, bend up a bow with 1/2 pvc and post the specs, I can try to duplicate and verify with the chrony if you do not have one available.

willie

[joachimM]

mmh, I didn't think I moved away from that model, I just didn't explicitly link it up with when the lift-off of the recurves. Basically, theory indicates that lift-off should be as fast as possible so as to have as little as possible dead mass during any part of the draw. All the tip mass beyond the contact point with the string will be dead mass from that point onwards during the return of the limbs.
The real advantage, IMO, of recurves is that they increase string tension at brace, not the change in leverage (causing the fat FD-curve). There are other ways to increase tension at brace (see the angular low-stack designs with slightly reflexed/recurved limbs) without the cost of dead tip mass. Basically these are exaggerated variants of working contact recurves.

So if you think I didn't answer the right question, I'm up for any sensible challenge one might propose. Basically, make me a good proposition, set out the rules you think are good to test the difference in performance (plus a how you define performance) and I can try to make model bows to test hypotheses and shoot them according to the rules you set up. If we agree these are good rules to compare bows.

So shoot

[bubby]

No chrony here, I've made a few plastic bows but use 3/4" or 1" but that's besides the point

[joachimM]

Plan B and PatM got me thinking about a few things that might explain why or when sharp recurves could actually beat other bow designs, namely for flight shooting.
A sharp recurve can be viewed as a shorter bow with higher tip mass, but with extra leverage at the end of the draw (and the start of the return).
Maximum strain on a sharp recurve is attained at a shorter draw length than with less recurved (or straight) bows (all else being equal). Say the recurve will have its max strain at 24" and the straight bow at 28". Total energy stored is smaller in the sharp recurve, but the slope of the change in energy transfer is steeper. 

The straight bow will store more energy and be more efficient overall, but the short bow with extra levers during the first few inches of release could have a higher dry fire speed. Only the potential energy of the last few inches of the draw (when the recurves lift off) may matter for flight shooting if the arrow is so light that it doesn't decelerate the tips. A heavier arrow would decelerate the tips, and the speed of the arrow at exit would be influenced by the acceleration during the entire return of the limb, not just the start.
PatM wrote "there's no priZe for efficiency" (got the orthograph right). For flight shooting there isn't. For hunting with heavy arrows I guess there is.
But at least I can now get my mind around why some very fast flight bows are short static recurves with sharp hoooks.

[PatM]

I said no prize, not no price.
 But yes there still has to be a reason for the 500 plus yard Osage statics having  tips close to 90 degrees. They did make the tips as close to non-existent as they could.
  No question that mass at the tips is the biggest enemy of the static but fwiw I have shot statics with still chunky tips way farther than the most minimal tipped straight bow.

[PlanB]

Joachim, you elucidated what I was thinking. The shape of a tip speed curve is important to what weight arrow it is attuned to.

It's unfortunate that we have a word for a rate decrease for speed (deceleration) but not one for a rate decrease for force (depression?!). A plotted out dynamic "depression" curve, along with the mass of an arrow would give you it's terminal velocity, and also the location in the release cycle where terminal velocity occurs.  After that point the arrow is decelerating from friction. On a fast tipped bow with a reasonably massive arrow, that might be at or near brace height. On a very light arrow with a highly stacked bow with low brace tension, it might be well before that.

Since short limbs at high draw weight can have fast tip speed, I'm not surprised that fight shooting with light arrows works well with them. If a fast tipped bow also has low stack at the same draw weight, that would be ideal. A bow shape with short limbs might stack more than a similar bow with longer limbs, but the shorter limbs might have less inertia. Since two variables are present stack, and inertia, it's likely that two bow types will produce good results under different conditions (ie arrow mass). A longer version optimized for lower inertia where possible and extremely low stack, and a shorter version optimized for lower stack where possible and extremely low inertia. Then one or the other will be appropriate for a particular arrow type.

A very short bow with extremely low limb inertia, and some stack, shooting a very light arrow will depend on acceleration only in the early part of the release cycle. Because of this the results will be relatively inconsistent, and it will be highly dependent on the archer's release, bow condition, wind direction and resistance and many other factors.

A longer bow with extremely low stack, and some additional inertia, shooting a more moderate mass arrow will depend less on the early part of the release cycle. Because of this the results will be more consistent, the heavier arrow will gain more energy and moving inertia, be less affected by the above factors.

It may not shoot as far however as the best shot from the less stable alternative bow system. I don't know about actuals. In one case occasional records may be favored. In other cases consistent wins may be favored. It depends on whether you favor extremes or averages. Overall points, or one time maximums.

It seems to me that gentler recurves would likely reduce stack, and static and lever systems would increase initial force  (accpression???!) early in the cycle.

It would be really useful to have a curve of limb tip speed. If that could be summed with the traditional F/D curve, a new curve could be generated which is much more characteristic of a bow. And if the arrow mass was known, the point at which terminal velocity could probably be determined, as well as total energy absorbed, etc.

These are just thoughts -- nothing proven. But just what seems reasonable to me.

[joachimM]

It would be really useful to have a curve of limb tip speed. If that could be summed with the traditional F/D curve, a new curve could be generated which is much more characteristic of a bow. And if the arrow mass was known, the point at which terminal velocity could probably be determined, as well as total energy absorbed, etc.

I think there is, it's just damn difficult to get it. it's the dynamic force-draw curve, as opposed to the static force-draw curve. The DFD shows which energy is effectively transferred to the arrow, but I've only seen it in mathematical models 

Good thinking, but we need some of these other flight shooters to chip in, next to Pat.

[Urufu_Shinjiro]

Just to throw my two cents worth in, I concur with the mathematical models, but I suspect they are missing a variable. The proof as they say is in the pudding and I too can't get past the fact that these sharp 90deg hooked bows were throwing these arrows farther than anything else out there, there has to be a reason for that, it can't be despite the hooks.

[willie]

I do not claim to be a flight shooter, but could some of the "excess" mass in the tips, be beneficial in the last few inches of the powerstroke?

Presuming an early liftoff design,  the contact point is rapidly changing at the end of the powerstroke.
Could extra tip mass and their corresponding momentum, be delivering the extra oomph needed as they come around and shorten the string?  Boosting  the arrow acceleration, right at the last?
 
The excess mass  delivering at a more effective moment, making up for the penalty they impose on  the limb earlier in the powerstroke?

[PlanB]

Joachim, let's call it the Force/Release curve. I'm pretty sure a set of them if they could exist, would explain all.

Basically you need a high speed camera, and measuring nock positions at set intervals on a set weigh arrow. Plot that in a curve, and you've got it.

The nice thing is you could do it for the limb tips at the same time.

Casio makes some cheap high speed cameras, available used on ebay for the $100-$150 range that do a 1000 fps video -- but it's small format and the quality might not be good enough, dunno. Might have to wait a few years til cameras catch up.

To be honest, this stuff is interesting to discuss, but I get tired of it eventually and want to grab a piece of wood and just whittle while thinking of almost nothing at all.