About this site: The intent of this site is to provide a collection of "plain english" archery articles relating to the 'hard' physics of archery but including some biomechanics. The articles are firmly based on very well known physical principles and presented with the absolute minimum of mathematics, formulas, tables of data, formal proofs, and such-like so as to try to make it readable by almost anyone.
Formal proofs: It is felt that formal proofs are not needed as such proofs should be almost obvious to anyone with a basic (high-school) physics education that has read the article(s), and they would only make the article(s) obscure and unreadable to those without that education. However, should you feel that more proofs are needed please leave a message in the Guestbook and that policy may be reconsidered.
Quick Links For This Site
Article Title & Link
Category
The draw-arm helps to hold up the bow ? Biomechanics Forces acting on the bow shoulder due to the weight of the bow. Biomechanics Stabilizer Setup Physics/Mechanics Arrow Length and Arrow Flight: Physics/Vibrations and Waves The Pressure Button Physics String Stretch and Creep General Force-draw Curves Physics/Mechanics Aggressive Cams Physics/Mechanics How Fast Does Your Arrow Spin? Physics What does 'True Helical' Mean? Physics/Maths True Helical Fletching General Directional Stability General Resources Links/Downloads
The draw-arm helps to hold up the bow ?
Yes, it is not often realized that there is a vector component of the draw force acting so as to actually "lift" the bow and bow hand while aiming.
Introduction:
If you have had a great deal of experience with beginners you'll note that there are an occasional few that dont just drop the bow hand after release, they actually appear to be throwing the bow downwards as soon as the string is released. This is particularly noticeable when they may present to you with a very light bow with a heavy draw-force, and it becomes even more of a problem with a high (corner of mouth) field anchor. (this particular problem may be all the more confusing - even paradoxical - for a coach that thinks the person is doing this because they are "weak" as the person concerned can often be quite strong and has no problem holding their arm and undrawn bow out at arms length).
This problem arises because the upward force on the bow (due to the vector component of the draw-force) may be so large that they feel that it's "floating" and they have to actually "push the bow down" to line it up with the target. Now, because there may then be a pre-existing muscular force acting downwards while holding/aiming this quite naturally leads to a downwards "follow-through" with the bow-arm the instant the string is released.
Apart from using a bow with a much lower draw-force, the cure for this is generally to load the bow up with weights so the upward force is negated, or even to load it up even more so they have to actively "lift the bow up" while aiming, this gives a muscular pre-tension in the "up" direction that helps to counter the downward force (due to the weight of the bow) that appears the instant the string is released.
Measuring The Upward Force:
Rather than setting up any sort of complicated apparatus to measure this force, it is far easier to calculate some good approximate values for it, the approximation (given below without formal proof) can be used for this - use the picture below to see whats being referred to.
The vector component of the draw force acting vertically on the bow hand = draw force * height / length {note: height / length is the sine of the angle between the red and blue lines}.
where height is measured perpendicular from the horizontal red line to the blue line (either at the nock or at the draw-elbow), and length is the distance from the pivot point to the nock or the draw-elbow (the blue line). (Note that this approximation ignores the fact that the archer in the picture (Marco Galiazzo) is seen here shooting at long-range and the bow-arm is thus actually raised above the true horizontal - but this makes no real difference for the purposes of this particular calculation).
Example:
As archers conventionally use Imperial units in preference to SI units - for a draw force of 40 lbs, height of 3 inches, and length of 28 inches, the vertical force is...
F(vertical) = 40*{3/28} ~ 4.3 lbs, so a bow weighing 4.3 lbs would negate the upward force nicely.
Caveat:
There is also a slightly different issue that is related to this - the torque exerted on the shoulder due to the weight of the bow is also proportional to the product of
1) the weight of the bow and
2) the distance from the centre of its mass to the shoulder (the bow is basically exerting leverage on the shoulder muscle).
The further forward you move the centre of gravity (COG - aka the centre of mass or COM) the greater the torque exerted on the shoulder, and because of this torque the bow may then appear to the archer to be heavier than it actually is (but it is not). Now it is quite possible there may be some that have moved the COG forward like this to prevent the bow from floating, if so, this was a VERY ill-advised move and they would be better off investing in a few more stabilizer weights instead.
Moving the COM / COG forward like that also goes against the basic physical principles of good stabilization where, for maximum inertial resistance to movement, you need the actual COM / COG to be as close to the pivot point as possible (with any added stabilizer weights being as far away from the pivot point as possible) - this is discussed below under the heading "Stabilizer Setup".
Application and Implications:
To take full advantage of this lifting effect to help hold up the bow and thus reduce tiredness and soreness of the bow-arm shoulder we should see that it is very important that the archer:
1) develops a form where keeping the bow-shoulder down is a priority, and does not allow the bow-shoulder to raise up in the first place (as will sometimes happen near the end of a long tiring day - thereby guaranteeing a rapid increase in tiredness as this also then leads to them having great difficulty in getting through a clicker), and
2) "stands tall" and "thinks tall" and doesn't slouch, that is, they should focus on lifting the head up and keeping it erect - this increases the distance between the anchor point and the shoulder, which then increases the amount of "lift" by the draw-arm and also helps to keep the bow-shoulder down. As an added bonus - lifting the head up also lifts the string away from the chest and gives better string to chest clearance.
Bearing in mind the twin criteria of keeping the bow-shoulder down and the head up, I'll now present some pictures of leading archers that meet both, or only one of these criteria - I will let you be the judge...
So which do you think meet both criteria? ... Which look to be the most relaxed and comfortable and would be expected to be still shooting strongly after a long day in the office, and which almost look as if they're overdrawn and appear to be already struggling to get the arrow through the clicker? (Hint: to get a better idea of who is doing both, look at where the line of the arrow is with respect to the top of their bow-shoulder).
Note that there's more than just some basic physical principles involved here, it is also a matter of the archer being able to take full command and control of the bow, to make it bend easily to their will rather than letting the bows forces take command, dictating what the archer does and forcing them to bend and buckle under those forces.
Competitive archery is very much a mental game and there are great psychological advantages to be had by always feeling strong and in command no matter how tired you may be. When you feel tired it is a fairly good bet that you will also start to slouch a little, your concentration may slip and your bow-shoulder starts to rise and then you become even more tired, that is when you need to really focus on keeping the bow-shoulder down and avoid developing a tired slouching stance, this is where a thinking "tall" - in fact even a "think arrogant" attitude needs to kick in to help with this.
Forces acting on the bow shoulder due to the weight of the bow.
We can treat an out-stretched arm holding a bow as a simple lever (with the pivot being at the shoulder joint). Now, the downward acceleration due to gravity acts on the mass of the bow to create a downward force that acts on one end of this lever and, to hold the bow at arms length this force has to be countered by the shoulder muscles (primarily the deltoideus - shown below) and we can note that the deltoid attaches to the humerus (upper-arm bone) at the deltoid tuberosity which is a small distance ℓ from the point where the arm is pivoting from.
The leverage exerted by the the forces of gravity acting on the bow is simply the ratio of L to ℓ, i.e. if L is five times greater than ℓ, then the downward gravitational force acting on the bow has a mechanical advantage of 5 to 1 over the upward force exerted by the deltoid muscle, so a small addition in the weight of the bow can cause a marked increase in the force needed to be exerted by the deltoid just to hold up the bow. But note that the downward force is countered somewhat with the draw arms contribution to the effort as mentioned above.
For a given (constant) bow mass, the longer the length of the lever L is, the greater the force that is required to combat the gravitational forces acting on your shoulder. So we can see that the trend for some compound archers to bend their bow-arm shortens the length of L and makes it easier to hold the bow "up" – but definitely not "out" (at full draw) and we need to note that this practice increases the risk of elbow injury.
Note:
One point need to be carefully noted here: L must be measured from the shoulder to the centre of gravity of the bow in the horizontal plane.
Implications:
What the previous note means in practice is that if you add stabilizers in such a way that the bows centre of gravity is well in front of its pivot point, the bow will "feel" much heavier than one whose centre of gravity is at or very near the pivot point. However, note that in the first case some may well consider the bow to be better stabilized simply because it "feels heavier" - but the truth is that the extra effort required from the deltoid acting in the vertical plane does not act to increase the bows inertial resistance to movement in the horizontal plane, it actually decreases the effect of the stabilizers (because the bow always pivots about its centre of gravity {provided you’re not choking it to death}).
For the exact same effort you can actually put extra weights on and increase the weight and inertia of the bow (and thus its stabilization) simply by juggling things around so that the centre of gravity’s very near (+1 cm, - 0 cm) the pivot point.
Stabilizer Setup
There are simply way too many variables to permit an exact formula approach to stabilizer setup, there are only some "general principles" that need to be followed, in summary they are:
To provide the maximum amount of stabilization,
- the bows centre of gravity (COG) needs to be kept in very close proximity to the centre of the bow grip where the archers hand meets the bow (i.e. the bows pivot point) - this is the very first principle of bow stabilization. To narrow it down further, the stabilizer weights need to be juggled around until the COG is about one to two centimetres in front of the bows pivot point (which puts it around about the centre of that part of the riser - where we could describe it as being at or very near the "true" pivot point of the bow) and about one, two, (three or four at most) centimetres below the pivot point.
- all the stabilizer weights need to be concentrated at points, front and back, top and bottom, and placed so they are as far away from the bow-hand as is physically possible - this is the second principle of bow stabilization. (To help understand the reasoning behind the use of "long" stabilisers that are 'placed so they are as far away from the bow-hand as is physically possible', it may assist if you read my 'A short history of Stabilizers (ST 101) - downloadable here)
- every single point of connection between the bow and the stabilizer weights needs to be as firm and rigid as is physically possible.
**We are talking about a theoretical ideal here that simply doesn't exist in the real world. If your interest lies more in performance than appearances, then for all practical purposes the nearest you could get to a theoretical ideal stabilizer rod would most probably be large diameter thin-walled carbon-fibre rods. (The current 'multi-rod' stabilizer trend may possibly lead to more rigid stabilizers that may perhaps stabilize better than a larger single rod stabilizer, but this is only provided there are no supporting apparatus for the rods between the bow and the stabilizer weights (i.e. like the smaller V-bar rods). Which means - and I really am not singling out any specific brand here - a very tentative yes to stabilizer types such as this >
but a most definite NO to types such as these >
) - while the last type shown may perhaps be very convenient for moving the bows COG back and forth to satisfy the first principle of bow stabilization, they completely ignore the second principle - it's best to use a number of small screw-on weights on the ends of more conventional stabilizer rods and move/juggle the weights around until both principles can be satisfied.
For an explanation and more info see here (wait for it to load) or download the attachment.
Important Postscript (Stabilizer Damping)
Since posting this I've noticed from remarks on several forums that there is a growing body of people that indicate the main reason they use stabilization is to suppress or dampen bow shock and vibration that occurs after the shot, and that if stabilizers don't fulfill this damping function very well then they are obviously 'poor quality' stabilizers. This situation is not helped by the number of stabilizer manufacturers that - in the race to sell their product - place undure emphasis on the superior damping properties of their own product. Which leads me into making these comments:
The function of stabilizers - as the name implies - is to actually "stabilize" a bow by adding mass to increase the bows inertia, 'stability' being used here in the sense that they provided a more stable shooting platform for the archer while aiming and during the execution of the shot. We should thus consider stability as being the primary function of stabilizers with (as mentioned before) ideally, any connection between the bow and the stabilizer weights being completely weightless and infinitely rigid.
Damping takes place due to the fact that real stabilizer rods are neither weightless nor infinitely rigid and it is this small inherent non-rigidity or 'floppiness' that gives some degree of stabilizer damping, and (as many are aware) decreasing rigidity even further by using loose couplings such as 'torque flight converter' or 'doinkers' increases the after-shot vibration damping properties of the stabilizers.
Now stability can be increased (but not infinitely) by using stabilizers set up with stability as the goal, and shock and vibration can be decreased (but not completely) by using stabilizers set up with vibration suppression as the goal. But whatever the end goal for using stabilizers, either way, the end result is at its very best only a partial solution that can have only a relatively small effect on either 1) stability or 2) shock and vibration suppression, so we need to be aware that the goals of "stability" before the arrow leaves the string on the one hand, and "shock and vibration suppression" after the arrow has left the string on the other, are not only completely separate goals, they are also mutually exclusive goals - i.e. you sacrifice one for the other - you can't have your cake and eat it too.
The only way you could obtain the full 'stabilization' and 'shock and vibration suppression' potentials from stabilizers would be to use some form of very exotic material(s) that abruptly changed state from 'completely rigid' before the arrow left the string to some sort of energy-absorbing 'plasticine' consistency the instant the arrow left the string, and then became completely rigid (and the correct shape) again as soon as the vibrations ceased - as far as I know, there is no such material.
Arrow Length and Arrow Flight:
Why are arrows not just cut to the minimum length so as to save weight and gain speed?
There may be several good reasons given for this, but as someone who has tried shooting arrows that were cut down to the absolute minimum length, my own experience was that I just could not get good arrow flight, yet when using arrows that were 'sticking out the front' I found I then got perfect flight (it was Hans Wright that first gave me this tip {click here to read background info}).
The technical reason for this can be found in the underlying physics of the matter - which is fairly complex (as is the motion of the arrow). However, even though it's complex we can summarize the basics of it as follows:
When the tip of the drawn arrow is very near to the plunger button and the string is released the arrow is forced to flex and oscillate in single wave C / reverse C manner about two nodes that travel in a straight line - because the tip and nock are not on these nodes they 'waggle' (in unison) from side to side about that straight line. However, when the tip of the drawn arrow is well in front of the plunger button this superposes a secondary wave that allows the arrow to also flex and oscillate in a snaking S / reverse S manner about three nodes that are located at the tip, the centre of mass, and the nock - because the tip and nock are now on the nodes they travel in a straight line without any waggling motion.
Here are some head-on stills taken from a high speed Beiter video that show this snaking S motion...
You can view the entire high-speed video clip these stills came from by clicking here (wait for it to load).
There are a number of other free high-speed Beiter videos on the Beiter site - click here to view the Beiter site.
Here are some further stills from a video clip from another source, this clip is of an archer using a compound with a finger release (the same principles apply) which show the S / reverse S motion as the arrow heads towards the target.
First, here is the archer at full draw an instant before release - this shows clearly how much of the arrow is sticking out in front of the bow.
These next shots show the snaking motion, unfortunately they were not taken from the best angle (which would be from overhead) to view this, but nevertheless the snaking motion is still visible...
I really must apologise for the lack of clarity for all the images shown here, but I simply have to work with what is available - the video clip these came from can be viewed by clicking here.
Here (click here and wait for it to load) is another very interesting high-speed video clip that is best watched in its entirety rather than from stills. It shows an arrow shot from a longbow with the arrow coming towards the camera.
The interesting part of this clip is that in addition to showing a good deal of yawing (fish-tailing) - which is really only to be expected of an arrow shot from a non-centreshot bow - it shows clearly the superposition of the two waves, that is, the fundamental wave from the rear that's induced by the string leaving the fingers and the fractionally time-delayed harmonic wave from the front that's induced by the fore-part of the arrow pressing against the bow as it slides past.
You can see the arrow variously assumes both a 'C' and an 'S' shape as these two waves travelling in opposite directions interfere with each other. But watch carefully and you'll see there is also one very interesting shot just before the arrow passes over the top of the camera where the larger wave that emanates from the rear is at a maxima while the smaller wave that emanates from the front is also at a maxima - but their respective maximas are in opposite directions!!! (i.e. the waves are in completely opposite phases). The shape of this is hard to describe, as it's neither C nor S, but I suppose it's best described as a 'combined C and reverse C' shape.
This phase/opposite phase behaviour can be observed just after the point shown in the picture below when the two waves are both in phase with each other (and where the maximas are both in the same direction) giving the shape of the arrow a sharply bent appearance near the centre at this point (so bent you almost wonder why it doesn't break
) - anyway, just watch the video...
Fundamental and Harmonic Waves in Phase - Sharp Bend
Discussion
Most archers have been led by the available literature to believe that arrows only oscillate in a C / reverse C motion and most video clips they see appear to confirm this, however, the above stills show this to be quite incorrect. In fact, you don't even need this visual confirmation given by the stills, if you carefully analyze all of the physics involved, the physics alone will tell you that this S / reverse S motion must always be there to some degree or other. But we need to note that there are some circumstances where the motion may appear to be more of a 'C' motion than an 'S' motion ...
From the first sketch below, you will see that a C / reverse C motion must lead to both the tip and the nock waggling from side to side as the arrow flies through the air, with the tip more than likely to have some sideways motion when it reaches the target - thus losing some of its direct (head-on) penetrative energy.
When we look at the next sketch (below) you can see that an S / reverse S motion leads to the tip, the centre of mass, and the nock all travelling in a straight line as the arrow flies through the air, with the tip unlikely to have any sidewards motion when it reaches the target - its direct penetrative energy is thus more likely to be maximized.
So why hasn't this 'S' motion been documented as much as the 'C' motion has?
Who knows? It is possible that if previous researchers/authors have developed a somewhat simplistic belief or model (or have simply not used due diligence to fully analyze all of the physics involved) their conclusions may be quite contrary to the facts and this may perhaps lead to them also tending to be selective with their testing, documentation, and examples - but perhaps I'm being unfair there...
As already mentioned, a careful analysis of all the forces acting on the shaft will tell us that the S motion of the arrow must always be there, but the amplitude of the secondary wave (which originates from near the tip) may, in some circumstances, be so very small as to be not readily observable - e.g. if the arrow has been cut down to its absolute minimum length or if the pressure button tension is too light or if the tips internal shank is too stiff, long, or heavy, then the amplitude of the secondary wave may well be quite unobservable to the casual observer. (Another way in which this manifests itself lies in the well known fact that the much smaller, denser tungsten tips fly so much better than arrows using their larger stainless steel counterparts - effectively, the shorter shanked tungsten tip leaves more shaft between itself and the pressure button).
An example where the arrow is cut short so that the tip is 'inside' the bow instead of hanging out the front is given below to demonstrate this (note that the S shape is still there, but it's very difficult to see it now, you really need to copy these pictures to your PC and blow them up to 200 or 400 percent to see it properly). The video from which these stills were taken is on the Variable bow website - click here to visit the Variable site to download or view these videos.
So what are the physics involved in this?
Essentially, there are two points where the bow makes contact with the arrow shaft, these two points are
1) at the nock (the bowstring is considered as being a part of the bow), and
2) where the pressure-button makes contact with the shaft.
Looking from above, when the string leaves the fingers it is displaced sideways to the left and this sideways displacement initiates a tranverse wave that starts to travel along the shaft from the nock to the tip and the shaft is bent into a reverse C shape by this - the amplitude of the reverse C is also increased by the inertia of the tip which opposes the longitudinal thrust of the string being applied to the nock.
When this transverse wave reaches the tip, the shaft then pivots about the pressure button and this pivoting motion then causes the tip to also move sideways to the left.
However the inertia of the tip and the pressure then being exerted on the shaft behind the tip by the pressure button creates a force couple that initiates a secondary (and time-delayed) transverse wave that is of opposite phase and direction (i.e. travelling from the tip to the nock) to the primary wave and whose magnitude is dependent on a number of things - the primary ones being:
a) the size of the tip and the distribution of the majority of its mass (i.e. whether it's essentially 'ball' or 'rod' shaped),
b) how much shaft was initially in front of the pressure button, and
c) how 'stiff' the pressure button spring is.
Note that all the above assumes that there is a normal (i.e. good) release and shot execution, when there are errors introduced, such as the string making contact with the face and/or body after release, it is then possible for a third (or even more) wave to be induced by this contact - in that case the contact would upset the basic tuning and getting good clearance between bow and arrow would become very difficult indeed.
As can be seen, the arrows motion is really quite complex, and while I have not seen them myself, I've been told (by Hans Wright) about very high speed (in the order of tens of thousands of frames per second) film that shows the motion of the arrow may be even more complex than what was described above.
On an historical note, it is also quite possible that the greater than normal length of arrows used in times long gone was not only because archers of the day drew them further back than we do now, but also because they found (from very long experience) that the arrows actually flew better and penetrated deeper when a fair length of the shaft was left sticking out the front of the bow while at full draw ...
The Pressure Button / Plunger Button
However you like to refer to it - pressure button, plunger button, cushion button, compensator button, or even cushion plunger button - Vic Berger manufactured the very first of these buttons. They were originally sold as 'Berger Buttons' hence pressure / plunger buttons were commonly known as Berger buttons (although I've since read that Vic Berger did not actually 'invent' the pressure button per se) however, please note that during the course of this article I will refer to them as either a i) Pressure Button, or ii) Cushion Plunger - the two terms being interchangeable.
The most obvious advantage of a pressure button is that it allows for very quick and easy adjustment of centre-shot for different diameter arrows. The pressure spring mechanism also provides another advantage, but the reason for this spring loaded part of the mechanism is often shrouded in mystery, confusion, and misunderstanding.
So why are pressure buttons used - why are they needed? The short answer to this is that prior to the invention of the pressure button archers concerned with accuracy faced the long, arduous - and often quite expensive - task of obtaining a set of arrows that were spined exactly right for their bow.
The invention of the pressure button allowed an archer to quickly 'tune-in' a wider range of arrow shafts so they could be made useable. There was no longer a need to buy a number of sets of shafts of slightly different spine ratings and experiment until the exact shaft, tip, and arrow length combination for their bow was found, all that was needed was to purchase a set of shafts that were maybe just a little stiff but of 'around-about' the right spine and use the pressure button to do the fine tuning required to get them to shoot well.
Correct Spine
How can we define 'correct spine'?
First requirement: we need shafts that give good arrow to bow clearance and,
Second requirement: most of us prefer that the arrow travels in a straight line and for it to be pointing in the same direction that we are pointing the bow - i.e. we don't want the arrow to go to the left or right of where we are aiming, and similarly - we also want the arrow to travel 'point-first' and not 'sideways'.
Third Requirement: there is another, but "technical" requirement that the amplitude of the shafts bend be small enough that its motion can still be considered to be simple harmonic motion.
First Requirement - Arrow to Bow Clearance
If you have read the article above about arrow length and arrow flight, you will see that the fletch area of a well spined arrow will move away from the bow - you should also know from that article that in order to do this, the entire shaft needs to bend and oscillate.
For a given draw-force, shaft type, shaft-length, and tip-weight, the period of this oscillation is fixed and needs to be such that (in an ideal world) the fletched part of the arrow is at its furthest distance from the bow as the arrow goes past.
However, we don't live in an ideal world and we should be able to see that this ideal situation is not really necessary to obtain good clearance - in fact the fletches could be at this extremity either a fraction before, or a fraction after they go past the riser and we would still get good clearance - absolute perfection is certainly not an essential for good arrow to bow clearance, but we certainly need to be somewhere close to this ideal.
So, for the first requirement (clearance) to be met we need the arrow to oscillate in a somewhat narrow frequency range such that the fletches move away from the bow as the arrow moves past it, and, because of this frequency requirement, this means that the shafts need to have spine ratings such that that they are neither too 'stiff' nor too 'soft' otherwise the result will be:
Too stiff: If the shaft is much stiffer than required it doesn't bend very much and it oscillates at a higher frequency than required, hence if it's too stiff we may find that
a) it doesn't bend far enough to give clearance for the fletches, and / or
b) we could also find that it has completed its first cycle too soon and the fletch area is actually bending back towards the bow as it goes past, with maybe some bow contact - the worst-case scenario here is that the shaft undergoes one and a half cycles when the fletched area passes the bow.
Too soft: If the shaft is much softer than required it bends too much and oscillates at a lower frequency than is required, hence if it's too soft we may find that it's taking too long to complete its cycle and the fletch area is still bent back into the bow as it goes past and slaps it on the way through - the worst-case scenario here is that the shaft has only undergone a half cycle when the fletched area passes the bow.
Second Requirement - Straight-line Arrow Flight
Now, for the second requirement (straight-line arrow flight), what we are really saying is that we would like the arrow nodes to be aligned with the direction that we want the arrow to travel - we thus need to have "the nodes aligned".
Third Requirement - Shaft doesn't bend "too far"
A shaft may possibly meet the first and second requirements, but if it bends too much it can exhibit non-linear (anharmonic) motion that can cause the shaft to bend in ways that are not easily predictable and it can then behave rather erratically.
'Correct Spine' Definition
We are now in the position to define correctly spined arrows as being: arrows that - for a given archer/bow configuration - don't bend "too" far, oscillates at a frequency that gives good arrow to bow clearance and whose nodes are also aligned with the direction of travel.
Considerations for Aligning the Nodes
But just how do we do align the nodes? To align the nodes we need to look at the separate behaviour of all the component parts and the contact that is made between bow, string, and arrow in some more detail and then put it all together...
i) Arrow/String contact
When the string leaves the fingers it is deflected by the fingers so that it first moves from the central position and swings over to the left, it then swings back to the right - overshooting the 'central' position - then starts to swing back towards the left again just before it reaches the brace height position... (Aside: After the nock leaves the string the nocking point then moves in a decaying spiral motion before air damping brings it to rest).
At the point where the nock leaves the string, the string is just past the brace height position, has passed the 'central' position, is still travelling forward and to the left but it is also starting to decelerate. At this point (provided the arrow is properly spined) the rear end of the shaft is bent so that it, and the nock, are in line with the direction of the strings travel. This is when the nock leaves the string, hence the nock is (also) to the left of any centrally drawn line when it clears the string and has a small sideward momentum to the left imparted to it.
ii) Arrow/Riser contact - rigid contact between shaft and riser
When the string leaves the fingers, its initial motion to the left causes the arrow shaft to first assume a C shape, with the arrow being pushed forcibly against the bow-riser by this bending.
As the arrow moves further forward, there is a point where the amplitude of the bend in the shaft reaches its maximum value and the shafts internal forces are then acting to stop the shaft from bending any further - the shaft is under the greatest tension at this point.
After the shaft passes this point, its internal restoring forces then act so as to straighten it out and it's then pushing against the bow-riser and string - and because there are no real constraints to prevent the fore-part of the shaft from moving to the left - the shaft effectively pushes itself away from the riser ... i.e. The end result is that the restoring forces in the shaft act so as to cause the fore-part of the shaft to effectively 'bounce' or 'spring' away from the riser, thus giving the fore-part of the shaft a small amount of sideward momentum away from the riser.
iii) Arrow/Riser contact when a cushion plunger is used
From the previous discussion regarding rigid contact, it should now become almost obvious that if we use a spring loaded button to prevent direct contact between the shaft and riser, the spring will cushion the shaft to riser impact to a degree that is dependent on the spring tension, the amount of plunger travel, and the mass of the moving plunger mechanism (a small amount of spring tension in conjunction with a large amount of plunger travel and a light plunger mechanism giving the greatest cushioning effect) - the effect of this cushioning is to prevent the shaft from springing back as far or as fast as it does when there is firm or rigid contact between shaft and riser.
Node Alignment
From the previous we can see that after springing away from the riser, the fore-part of the shaft continues to move - albeit relatively slowly - to the left, and (for a separate reason) after clearing the string the nock end of the shaft also moves slowly to the left - but this in no way means that the the fore and aft parts of the arrow are both moving to the left by the same amount!
In simple terms, "aligning the nodes" just means that we need to adjust the spring tension in the cushion plunger until the front and rear nodes move out (left) by exactly the same amount, i.e. if we find the arrows are shooting to the left we need to reduce spring tension so the fore-part of the shafts don't bounce quite as far - and if the arrows are shooting to the right we need to increase spring tension so that they bounce out a little further.
Conclusion
That's it - it's really quite simple isn't it? The general principles that follow from this are:
A) if the arrows shot out of a bow with a completely rigid button go straight down the middle, then they are what is commonly referred to as being 'perfectly' spined and you may as well just throw the button away for all the use it is to you at this time.
B) if the arrows shot out of a bow with a hard or rigid button go to the left, then they are over spined to some degree or another and you need to decrease the button tension to bring the nodes back into line so the arrows go straight down the middle. But if the button tension has been decreased to the point where there is very little tension and the arrows are still going to the left then they are of much too high a spine rating and are useless to you in the current bow to arrow configuration. You need to either buy a set of shafts with a lower (softer) spine or to look at making some other drastic configuration changes - such as increasing the draw force of the bow or the tip weight.
C) if the arrows shot out of a bow with a medium or soft button go to the right, they may not necessarily be under spined but you do need to increase the button tension to bring the nodes back into line so the arrows go straight down the middle. But if the button tension has been increased to the point where the button is completely rigid and the arrows are still going to the right then they most definitely are under spined and are useless to you in the current bow to arrow configuration. You need to either buy a set of shafts with a higher (harder) spine or to make some other drastic configuration changes - such as decreasing the draw force of the bow or the arrow length.
D) arrows either need to be perfectly spined, or, a little (but not too much) over-spined.
Note that aligning the nodes should be a once-off operation that is done in still conditions and, once done correctly, the button should then be locked in place and left alone - you should most definitely not be later routinely using spring tension to correct for wind-drift - that's what the windage adjustment on the sight is for.
Sponsors
For All Your Archery Needs Visit:
For the best quality, price, advice... www.abbeyarchery.com.au
Subscriptions
To keep informed of new articles as they are posted why don't you subscribe to this site?
If you want to send a link to a friend >
Page Views:
