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String Stretch and Creep:
Stretch and creep are generally taken to mean pretty much the same thing, however stretch is really related more to the instant, short-term / immediate elastic or 'spring-like' properties of the string (think of it like it is a rubber band) and refers to it's ability to recover to its initial length after tension has been increased then decreased. Whereas creep refers to a more permanent deformation of the string (which usually occurs over a longer period of time).
With most materials, stretch and creep tend to go hand in hand, where each stretch is also accompanied by a tiny amount of creep - a permanent deformation, and the permanent deformation also leads to less stretch over time.
Example:
For instance, let us say we have a string that has a tension of 50lbs on it. Now let us say (for arguments sake) that when the tension has increased to 500lbs the string then increases in length by 10 millimetres - this 10 millimetres increase in length is the amount of STRETCH. Now if this particular string has absolutely no creep, then when the tension is allowed to decrease back to the original value of 50lbs the strings length will then decrease back to its original length.Now let us say that the 500lb tension has been applied for a lengthy period of time and when the tension is decreased to the original value of 50lbs we find that the string has returned to its original length PLUS one millimetre - this one millimetre is the amount of CREEP (or semi-permanent deformation) undergone by the string.
Next time we apply a 500lb tension on the string we should find the amount of stretch has decreased to 9 millimetres. If we then hold it under this tension for a lengthy period of time again and allow the tension to decrease back to 50lbs again, we will find it then returns to its original length PLUS (almost {it's exponential}) two millimetres.
If we keep repeating this process of increasing and decreasing tension, eventually we'll find that the amount of stretch is almost zero and the amount of creep is almost 10 millimetres.
Breaking Strings:
Now some may say that when there is no more stretch the string has deformed so much that it is ready to break, but this is not quite correct. What is correct is that the string still has the same breaking point whether it has been subjected to any creep or not, however, when the string still retains some stretch, this stretch provides a cushioning effect for any abrupt shock forces due to the deceleration of the limbs when shooting an arrow.
But when the string has crept out to its maximum value over time, it then has very little stretch and the cushioning effect is lessened, the limbs are then forced to decelerate much more quickly - now, depending on the critical failure point of the string material used, it is this increase in deceleration forces that allows the deceleration forces to approach and eventually exceed the force required to break the string. i.e. it is generally the increase in deceleration forces that causes the string to eventually break rather than any inherent 'weakening' of the string.
Ideal String Material:
So, ideally we want a string material that is very strong, very light, and will not creep at all, yet at the same time, a material that also has a very very small amount of inherent recoverable stretch that is sufficient to provide cushioning so that the deceleration forces do not increase to a point where they exceed the force required to break the string. i.e. a very strong, light / low-stretch / no creep string.
Commonly Used String Materials at This Time:
The two most commonly used string materials at this time are Spectra and Dyneema,
Spectra: Spectra is low stretch but tends to creep quite a lot (recurvers may find they have to twist their strings every week to maintain the correct brace height) and the amount of creep actually increases over time.
Dyneema: Like Spectra, Dyneema is also low stretch but after a small amount of initial creep, it thereafter creeps much less than Spectra, with the amount of creep decreasing over time.
Test Data:
Attached is some test data (in MS Word format) that was on the BCY site a few years ago but was later removed. It gives the amount of stretch and creep for Brownells Spectra FastFlight® strings and compares it against some of their own Dyneema string materials, it also gives the breaking strength per strand and the number of draw cycles before the various strings fail, you can click here to view and / or download the data (wait for it to load).
Pre-stretching Strings:
To reduce or eliminate the tuning and brace-height problems that creep can cause, it is best - if possible - to 'pre-stretch' all strings and compound cables that you make yourself before using them. Pre-stretching is accomplished by putting the string (or cable) under a 300lb or so load for an hour or so (overnight is best) and pre-stretching is also best done before being served - making sure each strand is under the same amount of tension.
Force-Draw Curves
Force-draw curves are of most interest to the longer draw archer using a shorter than recommended bow for their draw, it allows them to see exactly where the bow starts to "stack" and whether they are drawing into the region where it stacks and are thus perhaps over-stressing the limbs, it also allows them to see where the bow will feel 'smooth' to draw.
In addition, a force-draw curve also allows us, by looking at the area under the curve, to get an idea of the energy being stored by a given bow - which is a very good indicator of how fast an arrow shot from this bow may be - and to compare one bow (or one set of limbs) against another.
You can draw a force-draw curve of your own bow with the use of some scales and a long (30 to 32 inch or so) shaft marked at regular intervals.
Draw the bow (with the scale attached) to the first marking, write down the draw-force measured at this mark, then draw the bow to the second mark, write down the draw-force measured at this mark, then continue with this process until you have measurements for all the other markings.
Now transfer your measurements to some graph paper, putting the draw-length measurements on the horizontal X axis and the draw-force measurements on the vertical Y axis.
Of course if you want automatic and really professional looking markings you could also invest in Eastons 'bow-force mapper' and do it that way - but that approach is really quite a bit of overkill if you don't intend to draw force-draw curves for different bows regularly and on a professional or semi-professional basis.
Examples
The first sketch below left is that of a theoretical spring constant and the one on the right is the force-draw curve for a longbow. The longbow slope is the slope for a fairly good longbow - a typical longbow force-draw curve would more than likely be closer to the linear slope of a spring constant that has a region where the spring 'stacks'.
The next sketch below left is the (again, exaggerated) force-draw curve for a good recurve bow, the relatively larger area under the curve showing that the stored energy is greater than for a longbow. The 'hump' in the curve shows that a bow with a force-draw curve such as this would be a very smooth drawing recurve, because once over the hump and near the full draw position the curve doesn't rise as steeply, and this holds true right up to the point where it enters the 'stack' region.
The last sketch (below right) with the very large area under the curve simply shows how much more energy a compound bow will store for the same draw-force being held while aiming - note that in this case a more realistic compound force draw curve would actually be much more exaggerated, with an even greater amount of stored energy than that shown below (it was simply drawn that way so as to fit into the same chart area as the other sketches).
Now as we move from the sketches for longbow to recurve to compound we can see that the area under the curve become greater and this means there is more energy stored (the energy is equal to the area under the curve).
But, in addition, we should note that there is one thing in common here, and that is - the dip in the curve where the force-draw curve meets the linear slope. In compound terminology this dip is known as the 'valley' and the stack region is known as the 'wall' (this is mainly due to the fact that a compound bow usually 'stacks' at a much more aggressive rate than the other bows).
If we now look again we can see that the fundamental shape of the draw-force curves themselves also have a remarkable similarity that simply becomes more and more exaggerated as we move from longbow to recurve to compound.
Specifically, in the very early part of the draw the force-draw curve is steeper than the linear curve, it then momentarily reaches a point where it is the same angle as the linear slope and as the draw progresses further it then becomes less steep than the linear slope, and later descends to a point where the force-draw curve and the linear slopes are equal again - it then becomes much steeper than the linear slope in the stack region.
While this similarity can be seen to be clearly related to the area under the curve (i.e. the amount of stored energy) we could also make an educated guess that because all these bows vary similarly but to different degrees from a spring constant, there is some basic mechanical principle or principles related to energy storage that they all have in common - this idea shall be returned to later in a separate article, where we'll explore what part the limb shape plays in both energy storage and arrow speed.
What is Meant by an "Aggressive" Cam?
A "smooth" or "aggressive" cam has nothing to do with the way the poundage increases near brace height, it refers to the way the poundage decreases just before the archer reaches the "valley".
A smooth cams draw-force descends to the valley at a slower rate than an aggressive cams does. The slower rate of descent makes the archer feel more in control and this makes the bow feel more manageable, with the archer then being able to relax somewhat while aiming.
On the other hand, an aggressive cams draw-force drops off very abruptly and, once the archers in the valley and relaxes they may also unwittingly creep forward a little, the aggressive cams draw-force then suddenly becomes much greater, so there is a tendency for the archer to be under more tension in case they find they have to suddenly 'fight' the cam for control of the bow.
Compare the typical side-by-side force-draw curves of these two types of cams. While it is clear the aggressive cam stores more energy and should thus be faster, and with a greater (or more aggressive) acceleration just after release, the benefit of the resulting extra speed must always be tempered against the ability of the archer to manage the bow while aiming - this manageability thus has a direct effect on the archers accuracy. Note: while a few archers can manage an aggressive cam quite effectively this is not generally the case.
Holding the Bow up Till the Arrow Hits the Target
While "hold the bow up till the arrow hits the target" is an old mantra that may seem ridiculous to many simply because we all know that after the arrow leaves the bow there is absolutely nothing we can do to influence its behaviour. While in some sense it is ridiculous, there are sound reasons for giving this advice.
Release and Follow-through
Forces
Due to the forces involved, after release the vertical motion of the archers bow + arm can either “naturally”; fall, remain stationary, or even rise during and immediately after the shot without any ‘additional’ muscular force being required.
Force = mass times acceleration. In the equilibrium position the vertical acceleration acting downward on the bow (and arm) is due to gravity minus any upward acceleration due to a vector component of the draw force acting on the archers arm so as to help lift the bow (and arm) up - this has been discussed on the home page. The archers muscles adjust to these forces (and their respective accelerations) so they can hold the bow steady in the equilibrium position while aiming.
Zero Draw-force Vector Component
If the vertical upward muscular acceleration due to the vector component is zero (such as with an anchor around shoulder level), and the archers muscles have adjusted to this, then immediately after the string is released, the time for the archer to further adjust their muscle tension while the arrow is still on the string is quite small at around 20 ms. Now, a normal reaction period is about 10 times greater (~180-200 ms) than this, so there can be no 'additional' reaction by the archer during those 20 ms, hence muscle tension remains unaltered, the weight of the bow (and arm) remains balanced by the muscular force and these will not drop when the string is released.
Non-Zero Positive Draw-force Vector Component
If the vertical upward force due to the vector component is not zero (such as with a normal anchor around the chin region), the archer has to push ‘down’ a little to resist the upward pull and the muscles adjust to this. Immediately after the string is released all the forces become unbalanced and the bow will then start to move down due to the downward acceleration (a) by the archers ‘pushing down’ muscular effort at the equilibrium position (for the same reasons – too short a time to react).
Non-Zero Negative Draw-force Vector Component
We can also see that if we make the upward force negative (such as with an anchor below shoulder level), after release the muscular effort is not balanced by the strings tensional force and the bow (and arm) will move up.
Reaction Time
However, regardless of whether the bow will normally rise, fall, or remain stationary; due to the small time that the arrow is on the string after the instant of release, the archer simply does not have enough time to react and voluntarily change their muscle tension to cause the bow to drop or move at any different rate, that is, they have no choice in the matter unless they have anticipated the release and started reacting to it prior to the release.
This is where the ‘focus on keeping your arm up’ comes into play – to try to stop them from anticipating the release (and thus changing muscular effort a very short time before release - where they then relax and drop the bow while the arrow is still on the bow). The idea being to not focus on the time of the release (particularly when they're fatigued) by asking them to actively focus on some later point in time and thus ‘follow through’ with the shot.
That is, the idea of ‘holding the bow up till the arrow hits the target’ is more about mental preparation and focus than anything else.
Summary
In summary, when aiming, the force of gravity acts down (the bow + arms ‘weight’) and there is an equal and opposite muscular force acting up. There is also the tensional component of the draw-force pulling the bow up and an equal and opposite muscular force pushing it back 'down'. These forces are all balanced so there is no movement when aiming but - the instant you release the string the forces become unbalanced - the muscular component that was pushing down against the string tension is still acting, causing the bow (and arm) to accelerate downwards at a (generally) small rate. The much greater gravitational acceleration due to the weight of the bow only comes into it (usually later) when you relax the bow-shoulder.
The main point is that dropping the bow - or any other bad thing - is initiated prior to the instant of release, the archer just doesn't have time to react 'during' the shot.
For instance, most recurvers train themselves to release as soon as they hear the clicker (as a reflex action), but there is an inherent reaction period of around 0.2 secs between them hearing the clicker (t1) and initiating the release (t2), and there is another short period between them initiating the release and the string leaving the fingers (t3). So if the clicker goes off at the instant they realize their sight is not on the gold and think "oh shit" there is a very short window of opportunity (t3-t1) to maybe hesitate and/or initiate a panicked correction.
But as t3-t1 is very small, there is very little time to actually make the correction and move the bow before the arrow leaves the string, so any larger movement an observer may see as a poor 'follow through' action was due to the archers actions initiated just prior to the string leaving their fingers.
When they're fatigued they're essentially over-bowed for that day and time and more likely to make mistakes. In addition, their muscles are screaming out to them "Stop this immediately! - It hurts!", so the instant the clicker goes off they may think "Oh good, I can relax now" - and immediately relax and start to drop the bow, which an observer sees as a faster than "normal" (for that archer) movement of the bow-arm.
E.G. If they start to associate hearing the clicker (or deciding to trigger the release aid) with both releasing and relaxing their bow-shoulder and thus relax the bow-arm immediately on, or just after, hearing the clicker, in the ~200 ms reaction period plus the ~50 ms the arrow takes to leave the string, the bow (and arm) can drop up to 2 to 3 cms before the arrow even leaves the string - that is a huge error.
You have to try to get the archer thinking PAST the instant of the clicker going off (or deciding to trigger the release aid) to a ... "the shot's not over till the arrow hits the target" type mentality and focus on holding the bow up (i.e. by keeping the same muscular tensions) past the point in time where they either heard the clicker or made the decision to trigger the release aid.
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