Trad Gang
Main Boards => PowWow => Topic started by: ChuckC on January 19, 2016, 10:17:00 AM
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It is winter, I am bored.
Can someone who KNOWS please explain to me this whole aspect of powerstroke ( no, not the engine).
In a lay persons mind (mine) there May, or may not be something to this in the pie in the sky working of things, however,
the reality is ( again, in my mind), the limbs will recover in the speed that they recover.
This will send the projectile out / away at a particular speed. No matter how long the powerstroke is, a certain speed is developed, and that is the end result that matters. A power stroke of one mile that develops a particular speed will not send an arrow any faster than a power stroke of 20 " if the same speed is developed.
Once the arrow is off the string there is no more added energy. It is all it can be.
The way it appears to me, the length of time the arrow is on the string is not near as important as the recovery rate of the limbs, and that would be influenced by the geometry of the limbs and the physical make up of the limb materials.
Some limbs simply recover faster than others.
Additionally, some limb designs might have changing limb recovery rates, depending upon their geometry, with working / nonworking recurves, single and double reflexes / deflexes, short limbs, long limbs etc etc. The degree of angle of the limb compared to the riser or center / launch point might even impact the rate of recovery.
An analogy could be in rifles. Different powders yield different recovery rates (speed).
So, how does this all work... I mean.. really
ChuckC
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Chuck rifles with longer barrels shoot faster than rifles with short barrels when they shoot the same ammunition, bows are similar. A longer power stroke allows the arrow to receive more of the speed the limbs produce. If you take two bows of the same draw weight and one is has belly mount limbs and one has back mount limbs the back mount limbed bow will be faster due to the longer power stroke. I make both and use the same limb with both. There is a significant difference in speed and energy between the two.
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Chuck,
You are asking a VERY complicated question. One that can really only be answered with advanced physics and mathematical descriptions. The study of ballistics.
Simply, speed is a RESULT of a force applied to a mass for a given time period (the power stroke). With trad bows, the force applied to the mass (arrow) decreases from the moment of release to arrow separation from the string. How much it decreases is quite complicated and is based on many factors from bow design to draw length (power stroke again). The force application of a compound bow is different, there, the force actually increases over the power stroke and is the main reason C bows shoot faster than trad bows.
A mass attains a given speed based on how long a force is applied to the mass. Let's use your gun example but we have to compare apples to apples. It is the length of the barrel for a GIVEN load that determines the speed of the bullet. Take a cartridge and shoot it out of a 10 inch barrel and then shoot the same cartridge out of a 20" barrel. Which one will give the highest speed(velocity)? In this example, barrel length is the power stroke and the longer barrel keeps the force applied to the bullet for a longer period of time -- hence a higher velocity.
Changing powders can change the velocity of the bullet but that is because the force applied to the bullet is different (changes differently over time). AND!! Presumes the same barrel length (again, power stroke).
So it is not really the recovery rate of the limbs themselves, it is the force those limbs apply to the string and then the arrow.
Anyway, a great question but one that has no easy answer. Physics classes in "mechanics" address this but can take several semesters of study to get to the level of understanding. We are addressing mass, inertia, energy, force, acceleration, velocity, etc., and the mathematical relations of all of them. They all are connected.
This is also why there is so much misunderstanding out there about common concepts of kinetic energy, potential energy, momentum and all that "stuff."
So, dig out the physics and calculus books and you won't be "bored" anymore. :>)
Arne
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The way I understand it, no matter what your limb geometry, when you release your arrow/string it gains speed until it reaches it's maximum, then loses speed after that point. Ideally, you want the arrow to leave the string at the maximum point of this speed/energy bell curve. If the arrow leaves too soon, the energy is wasted. If it leaves too late, the arrow loses speed/energy because it is still carrying the string after it reached peak speed.
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Ooh, Arne, my brain is hurting now!!!
Chuck, you are bored!!!
Bisch
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LOL Bisch!!
NothingHappenedToday,
You KIND of have it BUT... As long as there is a positive FORCE (pushing the arrow) there will be a velocity increase. Has nothing to do with string speed, just force on the arrow. IF, the nock is too tight on the string, then there will be a decrease in velocity when the string stops and impedes the separation of the arrow from the string. But that is a negative force that we try to avoid.
What I'm trying to say is that "speed" is a result of force whether on the string or arrow and as long as that force (of ANY magnitude) is positive (pushing the arrow) then we have acceleration. To be as correct as possible, force on a mass produces acceleration. Then at any given point (this is where calculus comes in) a velocity (speed) can be determined.
Arne
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Moebow -- thanks for the clarification. makes perfect sense
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Originally posted by ChuckC:
No matter how long the powerstroke is, a certain speed is developed, and that is the end result that matters. A power stroke of one mile that develops a particular speed will not send an arrow any faster than a power stroke of 20 " if the same speed is developed.
This is correct. If you could figure out a way to achieve a certain initial velocity with only a 10" draw, the downrange trajectory would be the same as a bow with a 30" draw that achieved the same initial velocity, given the same arrow.
The way it appears to me, the length of time the arrow is on the string is not near as important as the recovery rate of the limbs, and that would be influenced by the geometry of the limbs and the physical make up of the limb materials.
Some limbs simply recover faster than others.
This is also mostly correct, but there is no way, given current limb materials, to achieve a fast enough recovery rate to where a bow drawn to 10" would have the same initial velocity as a bow drawn to 30", assuming the bow weight is the same for both bows at their respective draw lengths. In order to achieve the same initial velocity, the bow with a 10" draw length would have to have a substantially higher draw weight at 10" than the 30" bow did at 30". Unless, I suppose, the 30" bow's limbs were made from some really doggy material with a recovery rate much less than any bow currently being made
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I see what is being said, but always being the devils advocate, a rifle barrel 30 feet long will not increase the speed over one, say 24" long ... will it ?
(Unless the amount of powder used in creating the force is not fully burned and the gas is still expanding after all that volume has opened up, which is why the short barrel loses, because that all expanding gas is dumped out the barrel end before it can make the projectile reach maximum speed).
My point is, is there a never ending, all encompassing length matters ( OMG) thing going on here ?
Is that force ALWAYS pushing with the same... force, from release to detachment of arrow (or exit of bullet from barrel) ?
Doesn't design.. which in a bow creates that force, throw a big kicker into the play ? As you pointed out the positive force generated by the bow pushes the arrow, but is that force ALWAYS the same from release to finish ? Why are some bows faster than others, no matter what the power stroke is ?
ChuckC
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Yes.... bored ! :bigsmyl:
ChuckC
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Chuck C, I would suggest a good game of Pick up Sticks, or maybe Euchre.
As you pointed out, maximum velocity is achieved when barrel length is matched to optimum powder charge for that cartridge. Don't hold my feet to the fire over the exact number, but I believe a .22LR reaches maximum velocity in around 16" of bbl length. Beyond that, the force of friction actually reduces velocity. Change the cartridge to .22Mag and that length changes quite a bit.
The force is not constant with either gun powder or bow limb. There will always be some form of curve if you graph the pressure applied to the rear of the bullet or arrow. The bow's forward acceleration ends when it reaches the static braced height. The only way to increase the total force applied to the arrow for a given draw weight and limb design is to increase the rearward travel before release.
Me, I kind of like the idea of a good Euchre game when I don't want to get out in nasty weather.
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Chuck, if you could somehow eliminate the friction, then yes, a 30 foot barrel would yield a faster bullet than a 24-inchbarrel, all other things equal.
Arne's explanation is right on. Look at it this way, if you plot a force/draw curve, the area encompassed under the curve (the energy or force to be imparted to the arrow) will be larger with a reduced brace height because the draw becomes longer.
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Chuck, You're pretty much spot on with the limb recovery speed. The faster the limbs straighten out after release the better performance. And concerning power stroke. I have found that a bow that can be shot with a lower brace ht does have a bit of an edge on performance.
Another other thing, is that the longer the limbs the less distance the tips have to travel on release, so (to a point) longer limbs and lower brace ht. makes it even better. Lower brace ht, giving the arrow a bit more distance on the string before it's launched.
But...
If the limbs are too long for the size of the riser, it makes the working limb more massive and can slow things down. (when it comes to riser length:total bow length:working limbs length, there is a "sweet spot" I've found)
Ideally the limbs need to be as light as possible to achieve a certain draw weight to perform the best, especially at the tips. That's why the Adcock ACS bow performed so well, the way he did things enable him to make limbs about half as thick as other bows.
The mass of the limbs as I stated, is evident when comparing self bows to laminated bows, take one of each, 50# each 62" long and the laminated bow having less mass to the limbs will have the edge, if you get into things like carbon and foam limbs and it gets even more evident.
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"The force application of a compound bow is different, there, the force actually increases over the power stroke and is the main reason C bows shoot faster than trad bows."
Hey Arnie, That was a pretty good explanation of the power strokes effect on arrow speed in layman's terms... :thumbsup:
But regarding your example of the compound bow increasing in exceleration as the string moves forward, vs Traditional bows that don't.... i think you'll find that with some traditional limb designs, that the exceleration does increase in the last few inches that arrow stays on the string..... If you look closely at the pounds per inch energy storage on a DFC chart, you can see that some designs with higher preload store a lot of the limbs total energy storage in the first few inches....... These type bows have much more tension on the string at brace height which stops the forward limb motion cleaner, and transfers more of the stored energy into the shaft......
For example we could take two 55# bows. one of them is a long bow only storing 12 #'s in the first 4" of the draw cycle with a nice even 3 pounds per inch draw right from the start..... This bow is very easy to string and has very little preload.... Unfortunately you get a lot more hand shock from this design because the string isn't tight enough at brace to stop the limb travel.TA lot of stored energy is wasted and stays in the bows limbs.
The second bow is a RC design with high preload. It gains 16# in the first 4 inches of the draw cycle. These bows need a stringer to get strung up and a lot more pressure is on those limbs at brace. (Pre-load)..... Now take these 2 bows and shoot the same weight arrow from both bows at different draw lengths..... You are going to see an amazing difference on how the power stroke effects the velocity....btw.... the high preload bow transfers MUCH more energy to the arrow shaft
here's a chart you guys can look at to help understand what i'm talking about.
(http://i171.photobucket.com/albums/u301/kirkll/Bow%20Testing/Jeff%20Grant%2010%20gpp.jpg) (http://s171.photobucket.com/user/kirkll/media/Bow%20Testing/Jeff%20Grant%2010%20gpp.jpg.html)
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If you understand the statement I am going to make the answer is so simple that I can actually answer it in one sentence.
At an equal power stroke distance and an equal poundage the quicker the limbs return to brace the faster the bow.
God bless, Steve
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It is too easy to mix things up. Yes, a gun will have a maximum "efficiency" (highest possible velocity for that particular load) IFFF the powder charge is completely burned AND the pressure of the gasses produced allow the projectile to leave the barrel while there is still a small amount of pressure behind it. That point would be where the volume of gas produced by the burning powder is just slightly greater than the volume of the barrel.
But that is NOT the point.
The velocity of a projectile is produced simply by the amount of force applied over time. The longer that force PUSHES, the faster the item will go. To be sure, that force must be greater than any resistance force (friction, hysteresis,etc.) but as long as the "push force" is greater than the "resist force" the item will accelerate.
A short barrel will not allow the force to act for as long a time as a longer barrel will. Not saying a 30' barrel, let's be realistic. Force for a time is what produces velocity. So for a given load, you will get less velocity from a 10" barrel than you will from a 20" barrel. That is NOT necessarily the max velocity available for the load, just relative velocities in comparison to each other and using longer or shorter sections of the force curve.
So for the bow, we've all seen force draw curves for bows, the farther you pull the string back, the LONGER it has to push the arrow. Yes, brace height will help here too but there we are usually talking about an inch where for full draw it could be as much as 5 or 6 inches depending on your comparison points. Longer distance to travel allows the force to act for a longer period of time hence higher velocity.
Can't explain it any better. As long a FORCE is pushing an item and as long as that force is greater than ANY resistance force, the item will accelerate. So, if it PUSHES for a longer time you will get a higher speed at the point it quits pushing.
Arne
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Kirkll,
I didn't say acceleration, I said force. In general, the FORCE produced by a trad bow will decrease as the string returns to brace height. In a C bow the force increases to a max after release to it's max. Obviously the force curves are very different between bows. But it's still force applied over time/distance that produces speed.
Arne
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AHH winter time discussions, a long way to spring.
As has been stated the way I understand it the longer the power stroke on the same bow ie. shorter brace height, gives more time to accelerate the arrow. According to my chrony, not much velocity gained. At my hunting distances not enough to matter.
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I took up trad because of the simplicity and you guys are ruining it!!! :smileystooges:
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The shooting/tuning part is simple. But when it comes to design there's actually a lot to it, so much that could make two bows that look similar, shoot as different as night and day.
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ummm, go shoot your bows, fellas
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Originally posted by Mike Mecredy:
The shooting/tuning part is simple. But when it comes to design there's actually a lot to it, so much that could make two bows that look similar, shoot as different as night and day.
Amen brutha.... Amen....
As has been stated the way I understand it the longer the power stroke on the same bow ie. shorter brace height, gives more time to accelerate the arrow. According to my chrony, not much velocity gained. At my hunting distances not enough to matter.
The reason for that is typically the optimum brace height on any bow is when the tension on the string is greatest.....
by running your brace down to 6" you may get a longer power stroke, but the tension on the string is not as tight as it is at 7" and it doesn't transfer the the same amount of energy into the shaft......
Are we splitting hairs? You bet we are. :biglaugh:
but that higher preload on the string that stops the forward motion of the limbs mitigates that extra inch of power stroke. Same thing goes for lengthening your brace beyond the sweet spot, only you are loosing both preload & power stroke....
Clear as mud eh? :p
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Certainly agree with you Kirk as we have had this discussion before. Set bow at best brace height ie. maximum tension which yields quietest and least vibration, and leave it alone. Use other variables to tune. :thumbsup:
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Also, there are some of us that actually like to discuss such things, we get a lot from each other, especially those of us that sell bows. Reading things might make a little light bulb come on, we try it, it improves what we already build, and it transfers on to those that buy from us.
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OK, Look at the force draw curve Kirkll posted earlier. There if you look at the brace height entry, it starts at 9". Now look at the LITTLE triangle formed between 9 inches and 10 inches. That triangle is the energy difference between a brace height of 9 to 10 inches. See how small it is??
Now look at the 28" draw length and the 29" draw length. See how much larger the area below the curve is?
My point is that with trad equipment, draw length has a MUCH greater effect on the energy available to produce a force than a similar change in brace height. One inch vs. one inch.
Now we get into individual shooter characteristics. It is self defeating to TRY to artificially increase your draw length. Your draw length is your draw length! That leads to all kinds of form problems and loss of accuracy. But with trad equipment, a brace height change is nearly negligible for velocity change. Since the energy change is so small at brace height compared to the full draw end.
Arne
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Arne, if I'm reading Kirk's chart correctly, the brace height is 7 1/2". This is measured from the string to the inside of the grip. Draw starts at 9", with zero force, because draw is measured from the string to (approx) the back of the bow, a difference in this case of 1 1/2". The energy stored in the little triangle between 9 and 10" is the energy stored in the first inch of the draw, which is different from the effect on total stored energy of increasing or decreasing brace height by 1". To bend the limbs by the same amount they would be bent by increasing the brace height by 1" would require much more than 1" of draw. So we're really talking apples and oranges here. Increasing the brace height by 1" would require a whole new force draw curve, starting with zero force at 10", and the effects would be measured over the whole length of the curve.
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That's not how I'm interpreting it but you may be right. Still, the area under the curve at OR NEAR brace height is very small compared to the area under the curve at 28/29". My point still is that the brace height adjustment has a very small effect on the force applied to the arrow compared to a full draw length change.
Arne
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McDave,.... You are correct. If we changed the brace height on the same bow i used as an example on the DFC chart from 7.25" to 6.25" The weight measurement in the first inch of draw would be lower, and would change the whole chart a bit. The amazing part, or the part that makes you scratch your head is that you would see a lower velocity coming out of the bow..... Granted... its a rather small amount. maybe 2-3 FPS. but you can feel the buzz in the riser caused by the vibration....That vibration is lost energy that stayed in the limbs.
(Btw... that chart should have read 7.25" on top for actual brace height instead of 7.5")
This one is a real hard one to wrap your head around, but sometimes just watching the difference in slow motion what that string & limb is doing after the arrow leaves the bow can be an eye opener.
check out some of these video clips. i've got a bunch of different bows on photo bucket that i filmed..... i'll have to post that link later ... photo bucket is down right now.
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Some of you over think a fun sport' This is suppose to be simple and fun.
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Well Grey Buffalo.... if somebody isn't interested in the physics of how these bows work, they would all be the same and no improvements would ever be made... To a lot of us... this IS fun....
Here are some interesting slow motion videos you might enjoy.
https://youtu.be/rqO6WkXJ87A
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ChuckC,
Let's go beyond the bow and look at the arrow.
You thought about the gun barrel length and the unburned powder and gas temp and such. Let's think about the arrow.
The arrow bends at the release of the string. To some extent, it will continue to bend to a point (in time) at which it starts to "un-bend". It can "un-bend" three different ways (if we only consider one dimension - lets say "out" from the sight window).
Scenario 1: the arrow never completely "un-bends" at the time it leaves the string. This will probably be an "inefficient" scenario.
Scenario 2: the arrow completely "un-bends" and then starts to bend "the other way" before leaving the string. Again, an "inefficient" scenario.
Scenario 3: The arrow completely "un-bends" or unfurls to be perfectly straight at the exact moment that the arrow leaves the string. This is, in my own speculation, going to be the MOST efficient scenario.
This is just one small bit of arrow tuning that could effect the overall "efficiency" of your system.
Now, all above being said by everyone, you could create the most efficient bow / arrow combination but find that it is so sensitive to changes in release that it's impossible to hit the "sweet spot" without a machine. You then have to cut some corners on efficiency to improve overall shootability.
Interesting question(s). Would be fun to sit around a table and discuss over a beverage or two.
homebru
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Homebru,
Scenario 2 & 3 are not going to happen because the arrow doesn't quit flexing until it comes off the string.... as long as the force is applied to the end of the shaft its going to stay in paradox....
watch this video... its got some real slow motion showing two different arrow spines coming out of the same bow.
Another thing to take a close look at is how the most force put on the arrow is in the last 2" of the power stroke.
https://youtu.be/rqO6WkXJ87A
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Originally posted by Kirkll:
Homebru,
Scenario 2 & 3 are not going to happen because the arrow doesn't quit flexing until it comes off the string.... as long as the force is applied to the end of the shaft its going to stay in paradox....
Not saying your incorrect or that you've done anything wrong......always room for lots of armchair speculation on all of this.
The point I was trying to make is that there will be a "most efficient point" in the paradox of the arrow. The Goldilocks principle. Not too stiff, not too soft. Just right
homebru
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In the video, at the very end, is the slow motion of the recovery of the bow limbs, Does that string look as if it is going in circles instead of just back and forth ?
Watching each of the arrows, during that last two inches prior to release from the string... it makes the arrow jump sidewise, most so with the heavier spined arrows. At first I thought it (arrow nock end) was bouncing off the riser.
What is up with that ? Why ? Maybe nocks too tight ? Maybe that string "spin" I mentioned ?
Interesting video there, thanks
ChuckC
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I don't think you would see that jump sideways as much with a fingers release because the string is rolling off your finger tips instead of coming straight off the caliper release jaws at a slightly different angle.
I saw a hooter shooter release one time that pushed the string off the end of a steel rod that matched a fingers release a little closer than the caliper release does.... Maybe i should look into that closer and see what the difference shows.
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I saw a slow motion video done with a shooting machine years ago showing the paradox with a compound and a longbow. With the compound there was a minimal, if any inward bend at the initial release, even less than you get with yours. However, with the longbow there was considerable inward bend at release. I assume that is because the bow is not center shot and the arrow resisting acceleration, it has no choice but to bend inward. A simple test, take a fishing rod set the tip off to the left and then push it straight forward, it will bend to the right every time. I was told that initial inward bend is because of the fingers. The Hooter Shooter proved it was because of the outside of center arrow angle. More center shot adds to arrow spine forgiveness. I wonder if having an arrow outside of center adds something like a stable launch point where the arrow positively gets its flex going. Recurves for many years were all outside of center. Dick Robertson told me that it helps with longbows to have something for the arrow to fly around. Could that be because of the dynamic differences from wood arrows to carbon arrow?
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This is way more interesting than the average bare shaft tuning posts so keep it coming.
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Hey Pavan.... You brought up some good points on the cut to center issue. What i need to do now is set my ILF rig up in the hooter shooter and shoot some film at true center shot with different spine arrows.....
You mentioned the "Spine" being more forgiving as you get closer to center shot. This i agree with. you can use your strike plate thickness to fine tune the arrow flight... But lets throw something else out there just for speculation fun....
Lets take a bow that is cut an 1/8" past center and tune it with the stiffest shafts we can....
Now take the same bow, and tune some weaker spine arrows by shimming the strike plate out to accommodate more paradox flex....
Would a weaker spine arrow that goes into more paradox be more forgiving to a less than perfect release using fingers than the stiffer shaft closer to center?
This question could stimulate some interesting theories..... What do you think guys?
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Would a weaker spine arrow that goes into more paradox be more forgiving to a less than perfect release using fingers than the stiffer shaft closer to center?
This question could stimulate some interesting theories..... What do you think guys?
If you agree that a bow cut closer to center will be less sensitive to spine variations than a bow not cut as close to center, then I would have to think that it would also be less sensitive to release errors. Just my guess, of course.
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Of course a lot of variable come into play when any bow is hand held. Also, shooting a bow that is far out from center, even, though I have had very good successes with such a bow, does rely on something different from the arrow that a center shot bow can avoid. In theory the center shot bow should win on all counts. Providing I am taking care to control my release, I can stack arrows tighter with more deliberate procedures with a center shot bow. When things start going off, a whole series of variables comes into play, so I would guess that different aspects would show either a better or worse reaction through the bow limbs and/or the arrow, again depending what went off. It would be very interesting to see in slow motion what happens when I have my more typical screw up with either type of bow. My own conclusion from the difference from a cedar arrow to a carbon arrow is that the carbon arrow, in my hands, seems to have a narrower happy window than a cedar arrows. What I do not know is, if it is me, the arrows or the bows.
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Anyone know of any high speed videos of arrows shot thru a very center shot bow ( not a compound) using fingers ?
That would be an interesting treat, to be able to see both a center shot and a non center shot and see if we can SEE any difference in action. Granted, both take different arrows and are different tools.
ChuckC