Is there anyone on here smarter than I am (not hard to do) that can explain to me how adding weight to the string in various places can make a bow faster?
How do speed nocks work?
sagecreek
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First off, great post to illustrate how speed nocks work.nuts&bolts;1098933561 said:
I'm curious to the above portion though. Adjusting the speed nocks for the "fixed speed", does that mean you need to adjust the speed nocks if arrow weight changes? In other words, different arrows weights produce different speeds so does that mean with the change in the "fixed speed" do you also need to adjust the speed nocks when arrow changes are made?
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Good question !!!
Anyone wanna take a "STAB" at the answer ?? I need to know this for argumentative purposes.....:wink:
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I would say yes. . I could be completely wrong but I see the nocks as an extension of the cams. Picture casting a fishing pole with a 1/2 oz weight tied on the end. Start your cast with the weight reeled all the way to the tip of the pole. . Then make the same cast [same degree start and stop poaitions] with 3ft of line out. You are going to have different trajectories and distance casts even though the same force and movements were made. These weights are finding the sweet spot to get the "longest cast" or optimum speed. . With that said there is also the weight of your arrow and dloop etc. Acting as mass across the string. Change the arrow weight and the Nock weight or position needs to compensate.
I could be way off, but this is how I picture it in my head.
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makes three of us..
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Or.....
What is the reasoning of the staggered groups of speed nocks ??
PSE and Mathews (maybe more) did this and still do. The X-Force, when it came out, had like 16 or 18 staggered speed nocks on the string. Is this better than having three in a bundle on each end ?
The sling analogy ( a sling shot is something altogether different) doesn't really work for me. Probably me just not getting it again. A sling relies on accelerating a fixed end point at the end of a lever which in turn is rotating about its center (the other end). Back to swinging buckets around. A speed bead (what is the proper term anyhow) is not that. It is a point in space between the multiple forces at work. You have the cam (ignoring the limb), you have the arrow nock which is dividing the work of both cams, the string, and the other cam. If you ignore half the string/bow you are calculating a lever whose length is the distance between the nock and one of the cams (to take one instant in time), and at some point along that lever is a speed bead or three (or 4 or whatever). Lets say each speed bead weighs 5 grains, three make it 15 grains which conveniently is approximately 1 gram of weight for all three speed beads. For the sake of simplicity lets attach the speed beads 1/2 the way along the lever (1/2 of the distance between nock and cam along the string). Our lever is defined as the distance between cam and nock and the beads are half way along the lever. When we release an arrow we know that these points are accelerating along an arc which means that the end of the lever with the arrow nock is traveling the furthest, and the end with the cam traveling the least distance. Yes this is a simplification but a speed bead should work on a recurve or longbow as well as compound so its easier to think of the cam as being the other end of the lever. All good, easy to calculate. To put it all out there, lets say this is a 40" ATA bow and the draw length is 28", and the BH is 8". (see what I did there). Subtract BH from draw length and when you draw the bow you have a right sided triangle whose 'a' and 'b' sides are an equal 20" in length. This makes the length of our lever ~28". With the beads half way along that lever they are 14" from cam bead or 14" from bead to nock.
Back to our arc which ultimately defines the amount of travel for the different points on our lever. If the nock travels 20" (DL - BH) then the speed bead will travel roughly 1/2 that distance. I think that all make sense. Yes, I believe it does because of my convenient 45' angles. Anyhow, lets say that our mythical bow shoots at 300fps, that means that in the same amount of time, the speed bead is traveling at 1/2 the velocity or 150fps. If you care to, you can plug those numbers into your favorite online calculator to get the momentum and kinetic energy of both (350 grain arrow at 300 fps vs 30 grains of speed bead at 150fps). You have to remember that the arrow calculators consider the entire force (both cams) so its only fair to double up the speed bead weight to account for them on both halves of the string, but I digress.
The point is that what matters here is the force that 'might' be added by the speed bead. To get that we have to convert the additive force generated by the speed beads. Since we know the velocity (150fps) and the weight (30 Grains) and we get a ke result ((v2*m)/450435)) or 1.5 ft/lb. Ok, that's more than I would have thought but lets add that in. The bow is generating 69.93 ft/lb of kinetic energy so the speed beads have a theoretical improvement of 2.14%. Since we can use that information in reverse, we should be able to apply that efficiency into the arrow speed, as it results in an improvement in energy transfer efficiency (KE/SE), at least I think it works that way and that translates back into arrow speed so our arrow gets a whopping efficiency gain of 2.14% or it goes from 300fps to 306fps. To double check that, another way to think about it is to reduce the arrow weight by a similar factor, this reduces our arrow weight to 342 grains. A 342 grain arrow with the same energy applied will fly.. (flips to arrow speed calculator) 302.66 fps or 2.66fps faster. Ok, the net result is somewhere between 2.6fps and 6fps faster *in a perfect world*.
The problem is that the world is not perfect. The upper end limiting factor will be the bows efficiency. The beads simply cannot add energy much beyond the bows design efficiency and certainly it hits the wall at somewhere just below 100% energy transfer. In reality the beads add aerodynamic drag, they add weight to the string, and they very well may create just a different harmonic resonance problem on the string. Then you have the fact that the effect is going to be somewhat canceled out by the string stop, and the force imparted is such a small percentage of the stored energy in the bow (the string is sitting there at some much larger amount of static force even when it is at rest) that it probably isn't going to add much. At peak weight that 2.14% just isn't going to be much of a factor.
Anyhow, fun thinking exercise and it could quite possibly be mistaken but I am still learning. What I didn't learn was how it was possible for a bow to go from 300fps to 320fps by adding a few beads to the string and that brings me back to the original hypothesis that the whoever it was that was in the linked video probably was more successful eliminating issues than they were adding new capabilities to the bow. What a giant friggin diversion from the original question.
What this does tell us is that the speed beads probably have little relationship to the arrow weight as long as the total of speed beads + arrow weight does not push the bow over the edge with respect to its overall efficiency. (becomes suddenly wildly inefficient). If the speed beads do anything positive, it will come down to finding the relationship between drag (negative effect) and velocity (potentially positive effect) but the arrow itself won't be much of a factor.
I believe it has more to do with string take up by the cams. As the limbs reflex and the cams roll back at such an high rate that added weight in the sweet spot allows the string to be taken up by the cams a tiny bit more efficiently. That's why you rarely see single cams with them. A perfectly round wheel or idler will not benefit from speed knocks where an egg shaped cam will benefit because of the oblong take up by the cam. The string feed from the top cam is even and round not oblong so no need for added weight for the string take up by the power cam. I'm sure there are laws at play with force, rotation and physics that I know nothing about but that's how I'd imagine in my brain why it works.
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Yeah, I think Nuts was just babbling. JMO.
I think it has to do with the harmonics of the string. I'm pretty sure the string has a wave effect when shot. Even adding a string stop will change the frequency and amplitude of the waves. I believe the nocks tune the frequency to get the peak amplitude of the wave to be colinear with the nock, thus propelling it faster. Just my theory, but I would like to find the correct answer that has been proven.
I think it has to do with the harmonics of the string. I'm pretty sure the string has a wave effect when shot. Even adding a string stop will change the frequency and amplitude of the waves. I believe the nocks tune the frequency to get the peak amplitude of the wave to be colinear with the nock, thus propelling it faster. Just my theory, but I would like to find the correct answer that has been proven.
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I bet John Dudley knows.......
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I shoot a G5/Quest Torrent, and they came on my bow from the factory. and from the manual. States must be placed 2-3/8 from tip of cam.
"SPEED STUD:
All Quest bows equipped with SPEED STUDS in the bowstring
must be placed between the colored bow string strands and
properly tied before shooting as shown below. DO NOT FIRE A
BOW WITHOUT THESE TIED SECURELY."
Rocky
"SPEED STUD:
All Quest bows equipped with SPEED STUDS in the bowstring
must be placed between the colored bow string strands and
properly tied before shooting as shown below. DO NOT FIRE A
BOW WITHOUT THESE TIED SECURELY."
Rocky
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Great. Now, just to find it.
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That could be a problem!!!!!!!!!!!
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Thanks
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