Possible Benefits Of Zero Brace Heights

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Possible Benefits Of Zero Brace Heights

Tim Baker
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June 2nd, 2018, 12:33 am #1

 
Please check this thinking for errors.

Two identical bows, one braced at 6", the other at zero brace height, have essentially the same draw weight when drawn 10" or 28"
 
but the zero-brace limbs bend about 6" less to reach full draw length and weight, so are far less strained--the energy stored in limbs at normal brace height stays in the bow, straining the limbs but not available to the arrow.
 
Stored energy is essentially the same in both cases, but the zero-braced, less-strained limbs will have lower hysteresis,
 
Draw weight at all points along the strings' return are essentially the same for both, until the zero limbs approach 6". At that point the braced-bow's limbs are stopped suddenly, causing typical limb vibration; but the zero limbs once inside 6" are stopped slowly, likely reducing limb vibration.
 
If this thinking is valid then a zero brace height bow should have lower hysteresis and lower limb vibration, the two largest energy stealers.  Possible a 10% or so increase in efficiency could come from a zero-braced bow bow.
 
Pure center-shot designs would prevent departing-arrow trauma despite zero bracing. A recessed or front grip would prevent hand injury by the returning string.
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Jim Hamm
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June 2nd, 2018, 12:47 am #2

Sound logic. If the bow is under less strain, it could be made shorter, or with pronounced whip-tiller, either of which would lower moving limb mass and increase speed.
Are you making one for testing as we speak?
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Tim Baker
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June 2nd, 2018, 6:20 am #3

People have been making bows for over 10k years so it seem that if zero brace heights increased efficiency it would be broad common knowledge now, so all bets say the thinking here is wrong. Still, it's interesting and likely educational to go through the steps of figuring out the why of it.

A knowledgeable bowmaking friend holds that zero bracing yields inefficient bows. Part of his thinking is:
 
"...  the string can not bring the limb tips to zero speed if the string is equal  length to the bow."
 
Here's my response, which all bets say must be in error, so help find any if your can:
 
This seems to be his bottom-line case against the zero-brace case. And it's true in an absolute sense. But I think it's not fully relevant here for two reasons:
 
Imagine an absolutely stiff string applied to the bow such that if one jot shorter the bow would have a brace height of one millimeter. Such tips would be brought to zero speed. (bowing forward limbs would shorten the bow a touch, allowing the tips to go past the zero-brace point, but that's a separate issue)
 
There are no absolutely stiff strings, so use the best real-life string and apply it short enough to almost create brace height.The tips won't be brought to a dead stop upon returning to zero brace position, but they almost will, meanwhile having more inches of travel during which energy is passed to the arrow. In this way, part of the energy normally retained in the bow as brace energy can go to the arrow.
 
It doesn't matter if the limb tips fully stop when they otherwise should.. As long as the arrow continues to accelerate for any distance beyond normal brace-height distance.
 
Is there any reason such continued acceleration won't occur?
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French Crow
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June 2nd, 2018, 8:51 am #4

Tim Baker wrote:Two identical bows, one braced at 6", the other at zero brace height, have essentially the same draw weight when drawn 10" or 28"
...
Draw weight at all points along the strings' return are essentially the same for both, until the zero limbs approach 6".
If the bows are identical, the zero brace height bow (0BHB) will have a slightly lower draw weight at full draw than the 6BHB. Despite this, the 0BHB should have higher draw weights during most of the string return:

courbe poids-allonge-bands.gif
Tim Baker wrote:Stored energy is essentially the same in both cases, but the zero-braced, less-strained limbs will have lower hysteresis,

It seems that the 0BHB stores much more energy than the 6BHB, which is kind of counter-intuitive even if we know that bracing a bow lower provides a better cast.
I agree 100% with the expected benefits.It might be even more beneficial to make the 0BHB slightly stiffer than the 6BHB, so that they have the exact same draw weight at full draw.
Bruno
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Tim Baker
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June 2nd, 2018, 10:40 pm #5

Bruno:

Don't know why the numbers here differ so much from what your computer supposed would be the case, but forgive me for feeling secret evil glee--the subtle secrets that bows harbor can only be teased out with equally subtle exploration... I hope.
 
Likely off a half-pound or so on the below numbers, but it's clear that both bows store essentially the same amount of energy.
 
If the measurements had been made with a new, never drawn, no set, no string follow bow, the early numbers would be a shade higher. Still, not enough energy would be stored in those early numbers to significantly continue accelerating the arrow after passing normal brace distance. So it seems that advantages of zero bracing, such limbs being less strained, would be low hysteresis and string follow..
 
It seems worthwhile efficiency increase would come from zero bracing, still, if so it seems this effect would be common practice. The chronograph will tell all. To come.
 
Another set of measurements  will be done with a no-string follow bow, first at zero brace, then at 7". I'm guessing that energy storage will again be about the same, but won't bet on it.
 
 
72" bow
1.5 string follow
 
Drawn "-- Pounds
 
'Zero'/ 1.5" braced bow:
6 -- 1
8 -- 4
10 -- 7.5
15 -- 16
20 -- 22
25 -- 30 
28 -- 35.25
 
 
5" brace from belly::
8 -- 4.5
10 -- 7
15 -- 16
20 -- 22
25 -- 31
28 -- 35.5
 
Comments, suggestions and criticism welcomed.
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Tim Baker
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June 3rd, 2018, 12:09 am #6

Bruno:

New numbers from a 48" bamboo bow with 3/4" general reflex, brought straight with a quite taut string, so a bit more severe than pure zero bracing. Pure zero bracing should give F/D curves something like half way between these and those earlier posted for the 72" bow with 1.5 string follow.

Very low weight bow, so more chancy when weighing, but the F/D lines are surprisingly smooth so I trust the numbers to a few %..

This time the zero braced bow shows a bit more energy storage, but essentially no more if both bows' draw weigh was the same. There's essentially no energy storage difference per full draw weight pound.

Note these number and F/D lines were drawn by carbon-based computer, the way the bow gods intend. The bow gods apparently dislike straight lines.

Zero brace:
5 --  1.3
7 -- 2.2
9 -- 3.5
11 -- 5
14 -- 7
20 - 11.3

4" brace
7 - 1.9
9 -- 3
11-- 5
14 -- 6.5
2- -- 11
DSC01672 20 .jpg
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French Crow
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June 3rd, 2018, 6:51 am #7

I should have guessed that curved part of F/D for the 0BHB in the 1st inches...
It doesn't make sense the 4BHB has lower maximum draw weight though, even if it's only by 2.7%.
Bruno
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Tim Baker
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June 3rd, 2018, 8:33 am #8

Bruno:

It doesn't seem to make sense because the computer drew its lines based on false assumptions:
 
Likely it didn't know that zero-brace limbs are less strained at target draw weight than braced limbs; and even if it did know this, and knew how to make accommodations, there are key other things it didn't know:
 
It was a new bow and had never been pulled to full draw length and weight while braced at 4", only while zero-braced, so the computer couldn't anticipate that the bow wold take more set when 4" braced than when zero-braced. The zero-braced measurement were taken first, the 4" numbers last, the small additional set likely accounting for that small difference in draw weight. 
 
If you hadn't expressed puzzlement over the sense of it, likely the 2 and 2 of it wouldn't have been put together, not until or unless several future bouts of such tests made it obvious.
  
This is a good reminder to chrono tests at zero brace first, and with a fresh no-set bow.
 
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Patrick St M
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June 3rd, 2018, 11:10 am #9

The essentially zero brace bow was a thing in a very localized area.   There seems to be just one advantage to the technique.   Lower strain and the ability to keep the bow "strung" at all times.
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Tim Baker
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June 3rd, 2018, 6:51 pm #10

Patrick St M :
 
" The essentially zero brace bow was a thing in a very localized area."
 
Do you mean this literally, that a tradition of zero-bracing existed somewhere at some time?
 
An always-strung version might not be desirable: for best performance the string needs to be taut at zero brace, this putting perpetual low-level strain on the limbs. Then again, such low strain might not be much of a problem. Two benefits would remain in either case, reduced set and reduced hysteresis. 
 
Another thought worth pondering: the reduced set resulting from zero bracing might allow slight general reflexing, in turn allowing extremely taut strings at zero brace, yielding elevated early draw weight for a fatter F/D curve. 
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Tim Baker
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June 3rd, 2018, 10:31 pm #11

Remeasured the 48" bow, now that it has the same set during  both zero-brace and 4"-brace measurements. Small and diminishing weight advantage for the zero bow out to 8", but past that measurements are identical, to my capacity to measure them accurately, a challenge for such a low weight bow. This will have to be repeated this with a normal weight bow.  Now a true center-shot, no-set bow has to be chronographed, the zero-brace shot first, before high-brace induced set is taken. 
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Tim Baker
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June 5th, 2018, 12:56 am #12

More ponderings:

As a bow string approaches its brace position after release, the string now pulled almost straight, is trying to accelerate the arrow to thousands of feet per second, in the process using up every last jot of stored energy put into the bow as the string was drawn to full draw length. However, the energy stored when bracing the bow remains in the bow, unavailable to the arrow;
 
A 1"braced bow stores a bit more energy than say a 6"-braced bow, especially in its early inches of draw.
 
A one-inch-braced bow, as its string approaches its 1" braced position, now also an almost straight string, also trying to accelerated its arrow thousands of fps, also draining every last jot of the bow's energy, which unlike the 6" braced bow, also includes the additional energy stored during its early zero-to-just-past-normal brace height inches.
 
If none of the givens here are substantially wrong, how could the 1"-braced bow's arrow leave at a slower speed than from the 6"-braced bow?
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Little Bob
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June 5th, 2018, 8:40 am #13

If the limbs have zero mass and the string has absolutely no stretch, then the machine is 100% efficient and what you say should happen.

The big killer will be limb mass.

In a braced bow, as the arrow nock approaches its brace point the forward speed of the nock is greatly increased by the speed at which the limb tips are moving apart vertically (this is geometry in the triangle formed by limb tips, arrow nock and brace point). A little bit of vertical tip movement causes lots of forward speed at the nock.

In the zero-brace bow the limb tips are hardly moving apart at all towards the end of their travel, so the string can't be pushed any faster than the limbs themselves are moving forward. If limb mass prevents them from going ever-faster then a point is reached where they can't keep up with the arrow. The string nock slows down and the arrow parts company with it before the nock reaches zero. Some of the energy remains in the moving bow limbs and the limbs flex forward beyond vertical. Loads of handshock I expect.

Bracing the bow to a normal height allows the string nock to go faster than the forward speed of the limbs approaching brace. The arrow can be pushed harder and get closer to the brace point before it leaves the string and efficiency is correspondingly better.

A stretchy string I think gives an advantage to the zero-brace bow if the string is pre-stretched at brace (instead of being slack). Extra string tension will lift the early part of the F/D curve, so this ought to make a zero-bow measure better than it really is.

Any good ?  I don't think the dynamic behaviour challenges your measurements, it's just what happens next. Recurve archers spend lots of time optimising their brace height and there is definitely a height at which arrow speed peaks.
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Fundin
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June 5th, 2018, 10:03 am #14

The african deflex tip bows are approaching this concept to some extent, with very low strain in the bow at brace height, albeit with other disadvantages. Those extreme deflex handle reflex limb bows you show in TBB as well.  The tricky part I think is what happens when the arrow is about to leave the string, the last 6" of beeing pushed.....


Also there is a question of comfort to shoot an accuracy that are usually attempted to be optimized besides efficiency in traditional archery.
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Tim Baker
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June 6th, 2018, 2:09 am #15

Little Bob:

"If the limbs have zero mass and the string has absolutely no stretch, then the machine is 100% efficient and what you say should happen." That's similar thinking to the below.

All:

Here's, for most, surely a boringly long series of emails on the subject, but it's an important issue so worth the reading for others:
 
My question to Woodbear and other engineers.
:
As a bow string approaches its brace position after release, the [ string] now pulled almost straight, the string is trying to accelerate the arrow to thousands of feet per second; in the process draining every last jot of stored energy put into the bow as the string was drawn to full draw length. However, the energy stored when bracing the bow remains in the bow, unavailable to the arrow;
 
A 1"braced bow stores a bit more energy than say a 6"-braced bow, especially in its early inches of draw.
 
A one-inch-braced bow, as its string approaches its 1" braced position, now also an almost straight string, also trying to accelerated its arrow thousands of fps, also draining every last jot of the bow's energy, which unlike the braced bow, also includes the additional energy stored during its early zero-to-just-past-normal- brace height inches.
 
If none of the givens here are substantially wrong, how could the 1"-braced bow's arrow leave at a slower speed than from the 6"-braced bow?
 
Am I not getting something?
 
His reply:
Tim,
 
What you are missing is the massive string tension required to accelerate the arrow at the end of the stroke.
 
Model results one inch brace: A bow with a 3/4" thick handle has a 1.75" brace height from the back and 1 inch from the belly.
Computations of draw/acceleration stroke are referenced to the bow back.
 
IF all the bow arm  kinetic energy is to be extracted from the limbs:
At 3.05in "draw" the string tension is 470#  if no string stretch. At this point a stretch of only 0.030"  will make the string the same length as at brace.
At 2.31in the tension is 2,060#. Now the string only needs to stretch by 0.007" to match the braced length.
At 2.01in the tension is 4,864#. Now the string only needs to stretch by 0.0017" to match the braced length.
 
To extract the energy from the bow limbs at this late part of the stroke with the string almost straight it will take one or two tons of string tension. With this much force the string will stretch, and/or the bow limb will buckle just enough to prevent a lot of the lomb kinetic energy from being transferred to the arrow.
 
 
My response:
Dave:
 
Thanks for that. It's really very clear and sensible. Neither people or computers can know what they don't know, therefor haven't accounted for, so there's always the possibility of error, so the computer's thinking has to be submitted to real-life testing. One benefit of such will be to nail down just how low bracing can be, and how high string strength/mass can be before diminishing fps begins. As far as I know this has nerve been done. It should be fun, and not too difficult. Mainly an appropriate center-shot bow will have to be made.One difficulty is that zero-braced bows are less strained so take less set and have lower hysteresis than normally braced bows, so the bow can't be first tested at normal brace height then at progressively lower heights. And it can't be first tested at zero brace then upward. Something of a dilemma. Suggestions welcomed.
 
PS: Is it ok to post your below on PP where I've been talking on this subject?
 
His response:
Tim,
 
Feel free to put the response up on PP.
 
Comparing a 1" brace to 6" brace bow (from the belly not the back as is my norm), in addition the showing the intense string tension required IF the bow transfers 100% of limb kinetic energy, I also get predicted arrow speeds, and strain levels. (The programs main cause for existence is to predict strain so I do not break bows.)
 
So..........the 1" brace bow has a strain of 0.6605% at 28" full draw, while the 6" brace has .06968% strain. This is only a 5.5% increase in strain. I suspect that this will not make a significant alteration in the internal hysterisis...moves it in the right direction, but not a lot of difference.
 
The 100% efficient arrow goes 192.8fps from the 1" brace, and 184.1fps from the 6" brace. (Actually true 90%efficient because I assume all bows loose 10% to hysterisis in the program. But 100% of what is available to the arrow, and 0% kinetic energy left in the arms.) That is ~5% faster (10% more energy) for 5% less strain.     BUT/HOWEVER, that speed assumes that 24% of the arrow energy comes from kinetic energy reclaimed from the moving bow arms in the last 3.5 inches of the stroke. More than 1/2 of that reclaimed energy would have to have been transferred via string tensions in the range of 500# to 5000#. It is beyond the explicit ability of the program to get the exact dynamics, but I expect that there is no way that even 14% of that 24% will actually make it to the arrow, to be able to match the speed of the 6" brace bow.
 
But as you say the proof is in the test. Please  do make the test, and get real numbers.
I think you can do the test with the 1" brace bow, and then up the brace to 6" and test at that level, so long as the bow starts life at the low brace. (Note that my program re-tillers the width to maintain the weight. This can be done in practice as well, as width changes should not effect the strain levels already experienced in the wood.)
 
Dave
 
 
So now a center-shot test bow is being made. 

Here's a don't-do-it-this-way test of a too simple way to do so:
 
Just applying a taut string at zero height caused the limbs to slide toward each other, shortening the bow. When dry fired at half draw they slid more.
 
So a one-piece no-slip version is under construction.
 
DSC01710 15 .jpg
DSC01718 15 .jpg
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Tim Baker
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June 9th, 2018, 7:06 am #16

Attention high I.Q. types:

Without two absolutely identical and unstrained bows to work with it's likely impossible to get uncontaminated draw length, weight, and tip-advancement numbers from both 1" and 6" braceings. Higher bracing strains the limbs such the the benefits of zero bracing are compromised, even if testing at 1" brace before each 6" test, and advancing to longer draws inch by inch. From tentative testing with the below center-shot bow it's clear that zero-brace bows store the same energy with less tip advancement--a good measure of limb strain--through early and mid draw, but likely loose this advantage late in the draw, so assumed advantages of reduced hysteresis and set might be substantially lower than hoped, and possibly impossible to ever determine with precision. So suggestions for decontaminating testing procedures welcomed. 
 
The numbers gotten so far are too tentative to report--this first test bow's weight is too low for best weight-measuring accuracy. A higher-weight version is in the works. Nope, no zero or 1" braced shots were made. 
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Patrick St M
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June 10th, 2018, 2:39 pm #17

Tim Baker wrote: Patrick St M :
 
" The essentially zero brace bow was a thing in a very localized area."
 
Do you mean this literally, that a tradition of zero-bracing existed somewhere at some time?
 
An always-strung version might not be desirable: for best performance the string needs to be taut at zero brace, this putting perpetual low-level strain on the limbs. Then again, such low strain might not be much of a problem. Two benefits would remain in either case, reduced set and reduced hysteresis. 
 
Another thought worth pondering: the reduced set resulting from zero bracing might allow slight general reflexing, in turn allowing extremely taut strings at zero brace, yielding elevated early draw weight for a fatter F/D curve. 

  The Yukon bow used a lashed on piece of wood to achieve brace height without really stringing the bow.   That clears the hurdle of having the bow at zero brace height.  You need somewhere to get your hand on the bow and string clearance on the loose.   Plus the string would crack like a whip if it was slapping on the whole length of the bow on the loose.
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Tuomo.e
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June 11th, 2018, 6:54 pm #18

Tim Baker wrote:  So suggestions for decontaminating testing procedures welcomed. 
Easy solution - use ****** bow. Very low hysteresis and accurate measurements are possible.

I made measurements and got following result:

Brace height 190 mm, stored energy 48,13 J (40,50#@28")
Brace height 58 mm, stored energy 56,30 J (38,50#@28")

Brace height measured from belly. I used accurate and good scale and checked all the measurement two times.

So, 17 % difference in energy storage between the two. Obviously low brace height have some advantages - more stored energy, less strained and possibly more arrow speed.
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Tim Baker
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June 11th, 2018, 11:45 pm #19

Tuomo e:

Excellent! Thanks for that.
 
Agreed, a glass bow is likely the best approach. I'd like to replicated straight wood bow geometry with a taut string at 1" brace but so far I've not located a quality straight glass bow with zero string follow or slight reflex and no outer-limb or tip recurve..
 
Would you show or describe the unbraced side profile of the bow you used? Recurves would give the low-braced version an advantage over the high-braced, but can still be useful.
 
And give a sense of how taut the string was, say by the twanginess of it? Best way would be to mount a thins scale in the string, but likely that's not available.
 
It's a bit of work, but if you have the time could you also say how far the tips advance when drawn 28", for both high and low-braced versions? This will give a good sense of limb strain.
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Tuomo.e
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June 12th, 2018, 6:12 am #20

It was a mild deflex-reflex bow. String was almost as taut as high braced bow. See attached draw force curves.

A second easy option is to use a lot used wooden bow, which has at least 1000 shot behind. Its hysteresis is high but it is always the same. First, 30–50 full draw as a warm up before measurements and then it is ready for accurate measurements. Then you can test low and high brace heights easily so no need for ***** bow.

Draw force curves.png
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