Rick McKinney's book "The Simple Art of Winning" published in 1996 is the earliest reference I know of
which made the correct connection between the alignment and rotation the arrow has when leaving the bow and the consequent
effect on it's subsequent flight direction (called in the book "nodal planing"). The following pages attempt to put
some technical substance to this topic hopefully leading on to a basic understanding of how the various tuning methods
work and why the optimum "tuning" setup for a given archer/arrow/bow combination is dependant on target distance and wind
conditions.
There is no "how to tune" guide contained here. There are lots of these around - some good, some not
so good and some just plain silly. For what it's worth, in my view, the two best bow setup and tuning guides around
currently are contained in "The Simple Art of Winning" - Rick McKinney and "The Heretic Archer" - Vittorio Frangilli.
As of 2008 now also have the Fita Recurve Tuning Manual- has to be currently the best overall bow set up and tuning reference.
All archers indulge in some form of bow tuning (or setup). The bow and
arrows we buy have some degree of tuning built into the design and the
archer usually carries out additional modifications to the setup to improve
the tuning. This discussion presents an overview of what tuning is, why
it makes a difference and looks in particular at what archers generally
mean by 'bow tuning', the control of the rotation the arrow has when leaving
the bow.
The following diagram is a schematic representing the elements resulting
in an arrow hit on the target.
ARCHER ---> BOW/ARROW---> INITIAL FLIGHT PROPERTIES----->ARROW
HIT
What determines where the arrow hits the target are the initial arrow
flight properties (direction of travel, speed etc.) and the physical properties
of the arrow (mass, length etc). If we replace the archer with a shooting
machine which operates the bow/arrow system exactly the same way on each
shot and the bow/arrow system responds exactly the same way on each shot
then the initial arrow flight properties would be exactly the same on each
shot. If all the arrows had exactly the same physical properties then every
arrow would hit in exacly the same spot. In this case there would be no benefit
to be gained from bow tuning.You adjust the shooting machine so the bow
is pointing in the right direction to hit the target centre and then you
get a perfect score.
Archers are not shooting machines (though some seem to get pretty close
to it). The way the archer operates the bow each time will in some way
be slightly different. As a consequence the operation of the bow/arrow
system will be slightly different. The resulting initial arrow flight characteristics
will be slightly different and as a result where the arrow hits will be
slightly different. The following diagrams summarise what happens. In each
graph the vertical axis represents the proportion of arrows shot.
The archer tries to make each shot exactly the same (like the shooting
machine represented by the vertical orange line). The archer will vary
round this perfect shot represented by the blue line in the 'Archers Shot'
graph. Most arrows will be shot at or near the peak of the blue curve.
The further away you get from the perfect shot than the lower proportion
of shot arrows (you hope) you will get. The variation in the archers shot
will feed through to a variation in the initial flight characteristics
the arrow has leaving the bow (the middle graph) and a consequent variation
in by how much the arrow misses the centre of the target (represented by
the third graph). Strictly speaking the shape of the curve in the third
graph should be like a letter 'M' rather than the bell shape illustrated.
The probability of making a perfect shot and hitting the 'x' is actually
very small (otherwise 'Robin Hoods' would be a daily ocurrence).
At the end of the day what we want is to produce the lowest variation in
where the arrow hits (minimise group size). There are four mechanisms utilised
to do this.
The first mechanism is designing the bow/arrow system to minimise archer's
variability in the first place.(It is this area which many of the archery
regulations are aimed to cover). Installing a mechanism to check draw length
(a clicker or a draw stop) will reduce variation in draw length and hence
variation in arrow speed. Using a back (peep) sight or a bubble level
will reduce variablity in aiming. Installing a set of pressure sensors
in the bow grip so that an LED lights up when you have the correct bowhand
pressure would reduce bowhand torque.
The second mechanism is designing the bow/arrow system so that the variations
in the archer's shot produce the minimum variation in the initial arrow
flight characteristics. (This is why there are two curves in the middle
graph above. The archer's shot variation can produce different distributions
of the initial flight characteristics). What is usually termed 'arrow matching'
is a prime example of this. Selecting the right spine/pile weight for a
particular arrow length/draw force will result in less initial arrow rotation
with a poor shot then selecting the 'wrong' arrow. Another example is using
a long sight sidebar. The longer the sidebar then the less error there is
in the arrow direction for a given archer misalignment of anchor point, head
position etc.
The third mechanism, which relates to the arrow design only, is minimising
the spread of arrow hits on the target for a given distribution in the
intial arrow flight characteristics. (This is why there are two curves on
the right hand graph. A given distribution of the initial flight characteristics
can produce different arrow hit distributions (groups) on the target depending
on the arrow physical properties). The obvious example of this was the
change over from aluminium to carbon arrows with their smaller diameter,
higher FOC etc. Everybodies scores went up (unfortunately not due to everybody
suddenly shooting better). Playing about with the physical properties of
the arrow to reduce groups is a form of tuning though that is not what
is generally understood by the term.
The fourth mechanism is what is generally understood to be 'bow tuning'.
The archer's perfect shot will result in a specific set of initial arrow
flight characteristics and variations from the perfect shot will result
in variations in the values of these flight characteristics. The question
is 'are there specific values of the initial flight characteristics associated
with the perfect shot that minimise the arrow hit distribution (group size)
resulting from the archer's shot variability'. Let's take each of the initial
flight characteristics in turn:
Direction
Divorcing direction from arrow speed, there is clearly an optimum direction
in which the arrow should be travelling when it leaves the bow to hit the
centre of the target. This direction will depend on target distance, an
uphill or downhill target and the effect of wind. Direction however is
defined by the archer at the time of the shot. Anything one can do to minimise
the effects of variation in direction are covered by mechanisms 1 and 2
above.
Vibration
I don't see any mechanism (other then the physical bending of the arrow
at short distances) by which the amount of vibration the arrow has will
significantly effect the arrow hit variation with archer's variability outside what
is covered by the arrow matching process of mechanism 2 above. The optimum is presumed
to be the minimum amount of vibration amplitude in flight (hence minimising vibrational
drag effects) compatible with good arrow clearance during the arrow launch.
There have been a number of reports that on completion of a fine (group) tuning process at 70m the
bare shaft ends up hitting to the left of the fletched shafts (RH archer). Arrow vibration amplitude/associated
drag effects possibly coupled with the arrow rotation characteristics is put forward as a totally speculative suggestion
as to the cause of this phenomenon and therefore vibration may have some small influence on group sizes.
Speed
Don't know the answer to this one. The problem here is that as you change
the arrow speed you have to change the properties of the bow and the arrow
and other factors like the archer's physique enter into it. At short
distances I don't think speed matters. At longer distances my guess is
that a 'higher' arrow speed will reduce group sizes for a given archer's
shot variability compared with a 'lower' arrow speed. Whether there is
an 'optimum' speed I have no idea.
Alignment
By alignment I mean the angle between the direction the arrow is travelling
and the direction the arrow is pointing (sometimes called Nodal Alignment).
As discussed elswhere the arrow misalignment can only be a small value and
it does not have a significant effect on where the arrow hits. Having said
that, the arrow leaving the bow straight is preferable.
Rotation
Arrow rotation is usually split between horizontal rotation (pressure button
tuning) and vertical rotation (nocking point tuning) so I will do the same.
In practice an arrow only has rotation (the combination of the horizontal
and vertical components) and as a bow is always shot at angle the horizontal
component is not horizontal but lets keep things simple.
Horizontal Rotation (Pressure Button Tuning)
Suppose we have an archer who shoots lots of arrows with a specific
bow/arrow combination. We mount a camera above him and record how much
horizontal rotation each arrow he shoots has when it leaves the bow. If
we then plot a graph of the number of arrows that came out against the
amount of rotation the arrow has then we get something like the following.
The arrow rotation with the highest number of arrows (the high point
of the graph) represents the archers average (perfect) shot and the spread
around it represents how much the archer varies around his average.
The shape of the distribution indicates how good, i.e. how consistent
on the shot, the archer is. For our imaginary archer the amount of rotation
of the average shot (maximum number of arrows counted) is at value 'A' and
the archer's variation in the shot goes from values A-2 to A+2.
The variation in arrow rotation is dependant on the archer, the bow and
the arrow. For a given archer/bow combination different arrows will result
in differing variations in rotation e.g. one arrow might give a variation
of A+5, A-5 while another might give a variation of A+2, A-2. A part
of the tuning process is the selection of the best arrow match giving
the lowest variation in arrow rotation (mechanism 2 above). This is usually
done by using an arrow selection chart.
If an arrow leaves the bow with horizontal rotation it ends up hitting
the target horizontally displaced by some amount. This displacement mainly
results from the initial arrow rotation (angular momentum) and offset
angle with a small contribution from the arrow fishtailing about. We do
another test, this time measuring the amount the arrow is displaced sideways
for a particular amount of initial rotation. The thing we would notice
is the amount the arrow is displaced increases faster than the increase
in the value of the arrow rotation. In other words the distance between
the arrows shot with rotations of 3 and 2 is bigger then the distance between
the arrows shot with rotations 2 and 1, and this is bigger than the distance
between arrows with rotations 0 and 1 (The vaules 0-3 are just numbers
to represent the relative amounts of rotation)
Suppose we give the archer a bow and some arrows and he shoots them.
By accident the average rotation value "A" for this setup is 2. The rotations
of the archer's arrows will vary between 2 + 2 = 4 and 2 - 2 = 0 (using
the variations from the average that were measured in the first test).
We then give the archer another bow/arrow combination to shoot with.
This time by coincidence the average rotation value 'A' is 0. The arrow
rotations for this setup vary from 0 + 2 = 2 to 0 - 2 = -2. The attached
diagram illustrates the arrow spreads obtained with the two setups. As can
be seen the smallest spread, or group size, is obtained when the average
arrow rotation value 'A' is zero.
The minimum arrow spread (group size) is obtained when the average arrow
leaves the bow with zero offset angle and no rotation. The purpose of
basic bow tuning is to get the setup so that the archers most frequent
(i.e. perfect) shot meets this criteria. The fact that it is a statistical
process does impact on the merits of the various tuning approaches that
have been proposed. You can speculatively take the argument one step further.
If the distribution of arrow hits produced by the archer is consistent
and has a bias in one direction or another then tuning to match the arrow
hit distribution, i.e. tuning to fit the archer's variability, would
in theory produce higher scores then the conventional zero rotation approach.
(Otherwise known as group tuning).
The fact that tuning relates to the archer's average and variation from
it in shooting means that a statistical approach has to be applied to
any tuning system. If you shoot one arrow though a paper sheet and it happens
to be dead straight you can't just go off to celebrate. That arrow might
be your 'A-4' arrow. A tuning approach either has to be based on looking
at the same time at a lot of shot arrows like the test archer above or if
the approach practically can only use a few arrows at a time, e.g. bare
shaft tuning, it needs to be repeated lots of times and a composite picture
built up.
A second consideration is the properties of the arrow itself. Sideways
arrow movement type tuning approaches like bare shaft and walk-back relate
to the variation of sideways drag force on the arrow with offset angle.
The arrow weight, speed, rotational characteristics and diameter will affect
how sensitive the tuning process is to the offset angle. For example compare
bare shaft tuning with an ACC carbon arrow and an aluminium arrow. If we
shoot the bareshaft carbon and aluminium arrow with the same initial offset
angle and rotation, all other things being equal, the gap between the fletched
and bare shaft arrows will be much larger for the aluminium then for the
carbon arrow. The carbon arrow will probably have a higher FOC then the
aluminium arrow resulting in faster arrow rotation. The sideways drag on
the aluminium arrow will probably be greater than for carbon arrow because
of the larger arrow diameter. It will be much easier to fine tune the
aluminium arrow than the carbon.
There are two additional points to be made with respect to pressure button
tuning which to some extent invalidate what has just been said.
The first point is arrow stabilisation. When an arrow is shot with horizontal
rotation it initially 'swerves' until the effects of ths initial rotation
are taken out by the fletching action. It obviously 'depends' but for your
typical arrow (unless you are one of those archers who fit windmill sails
on the back) it takes somwhere around 15-20 metres for the arrow flight
to stabilize. At longer distances this doesn't matter but if you are shooting
only 18 metres indoor the arrow is basically 'swerving' right up to hitting
the target. The optimum tuning set up for short distances may be found
to be different from the optimum tuning set up at longer distances.
The second point is that the assertion that the perfect shot should have
zero rotation is based on the assumption that there is no external influence
which affects arrow rotation between leaving the bow and hitting the target.
Suppose as per the following diagram there is some 'magic gizzmo' located
in front of the bow which adds angular momentum (rotation) to the arrow.
If the arrow leaves the bow with no rotation then it leaves the gizzmo
with rotation and hence the group size increases. If instead we adjust the
bow so that the arrow leaves with just the right amount and direction of
rotation to balance the gizzmo then it leaves the gizzmo with zero rotation
and we get the minimum group size again. The general principle is that if
there is something acting to change the arrow rotation between leaving the
bow and hitting the target then the optimum tuning setup for the bow will
not be zero arrow rotation. The optimum rotation of the arrow from the bow
will be that which best counteracts the external influence to provide minimum
group size. As any 'gizzmo' will in reality act over the whole arrow flight
it is not possible to completely compensate for it, only minimise the
increase in the sizes of groups.
Any wind acts like a gizzmo changing the arrow rotation. When shooting
in a wind therefore it follows that the optimum initial arrow rotation
(the button tuning) in terms of group size becomes dependant on wind strength,
wind direction and the distance to the target.
The diagram illustrates the affect of a cross wind on arrow group
sizes. The red lines indicate the flight paths and resultant group size
for a representative archer with a perfectly tuned bow with no wind present.
The blue lines indicates what happens in a cross wind to the same conventionally
'zero rotation tuned' archer/bow setup. The group size increases significantly.
Most of the increase comes from the arrow (the zig-zag blue line) that
comes off the bow with the nock end rotating into the wind. In this case
the sideways drag force from the arrow's forward motion reinforces
the sideways drag from the wind at a number of points during the arrow's
flight and as a consequence the arrow ends up with a high downwind lateral
displacement. The green lines show the flight paths and group size for
a bow optimally tuned for the wind conditions and target distance. In
practice you cannot optimally tune the bow for the current conditions.
As a general guideline a good setup when shooting in a cross wind will
require the arrow to leave the bow with the nock end rotating in the
downwind direction. The key to having good groups in a crosswind is to
avoid the sideways drag on the arrow from its velocity reinforcing the
sideways drag on the arrow from wind. This topic is discussed in more detail
in the section on Variable Tuning
Vertical Rotation (Nocking Point Tuning)
Much of the discussion relating to horizontal rotation applies equally
to vertical rotation. There is one key difference in that in the vertical
plane there is always an external force affecting arrow rotation - Gravity.
Because the gravitational acceleration results in the rotatation of the arrow there is no optimum value for the initial
vertical rotation of the arrow, it becomes a function of distance (time of flight and launch angle) as well
as the arrow rotational properties. The basic tuning setting of zero
vertical rotation for the arrow leaving the bow, although it works well
for short distances will probably become increasingly unstuck as the target distances
increase. In the case of wind the bow tuning is adjusted so that the arrow leaves the bow rotating in the same sense
as will result from the action of the wind on the arrow.If you regard gravity as acting like a wind that always comes
from one direction then gravity always rotates the arrow in the sense of pile down and fletching up so that should be the
rotation direction of the arrow at launch resulting from the nocking point position. (Effectively a paper tune should show the
fletching going through the paper slightly higher than the point). This makes the relative positions of the bare shaft
and the fletched shaft somewhat unclear as far as nocking point tuning using the bare shaft method goes. At longer distances
the bare shaft will hit higher than the fletched shaft (higher launch speed and higher initial lift force). At short
distances as the arrows are launched rotating pile down the bare shaft will get less lift than the fletched shaft so it's
maybe possible for the bare shaft arrow to hit lower than the fletched shaft. It will depend on a combination of factors.
It is not possible, short of having a height adjustable button/arrow rest
assembly, to adjust vertical arrow rotation on the fly and any adjustment
is limited by the arrow interaction with the arrow rest. So the general
nocking point tuning guidlines (assuming you don't group tune) runs something
like a) at short distances go for a zero rotation tune (bare shaft impacts
same height as fletched shaft) and b) at long distances have the bare shaft
impacting slightly above the fletched shaft. This of course assumes that
there is no headwind/tailwind component which will also effect arrow rotation
in the vertical plane. Short of having a set of strings with different nocking
points for different distances and wind conditions then you end up with a
some compromise nocking point setting. I'm told (before my time) that it was once fairly
common practice for archers to use different strings with different nocking points at
different distances. The practice seems to have died out.I suppose this is because
arrows these days are faster/more forgiving and competition distances have essentially reduced
to 70m and 18m.
The sections on Variable Tuning - Distance and Flight Instability relate
to the nocking point tuning subject.The following graphs illustrate how the effects
of gravity affect the basic principle of tuning as described above.
The graph represents the vertical displacement from the target centre
(vertical axis) as a function of the amount and direction of rotation in the vertical plane
of the arrow leaving the bow (the horizontal axis) at a target distance
of 18 metres.. The data is produced using the arrow flight simulator for
a specific set of arrow properties. A negative rotation represents a high
nocking point (bareshaft impacts below fletched shaft) and a positive rotation
a low nocking point. The red and turquoise horizontal bars represent the
arrow rotation for the 'perfect' shot (rotation 0 and 6) and the width
of the bar represents the variation in rotation from how much the archer
varies from the perfect shot. The corresponding vertical coloured bars
indicate the vertical spread (group size) of where the arrows hit resulting from the
archer's variation. I have ignored the fact that the turquoise archer will
adjust his bowsight so that his perfect shot has zero displacement from
the target centre which will have some effect on his group size.
The overall shape of the curve through the zero displacement point is 'S'
shaped. If the curve was a straight line then whatever the arrow rotation
was for the perfect shot the group size would always be the same. In fact
the group size for the archer with zero rotation for the the perfect shot
is smaller than for the archer with perfect shot rotation of 6.
If there was no influence on arrow rotation after the arrow had left the
bow then the curve would be symmetric about the zero displacement point
and there would be a clear optimum setting for the nocking point position.
The effect of gravity results in the curve being assymetric. At 18 metres
the effect of gravity is small but you can see from the simulation that
the curve risers faster with a positive rotation (low nocking point) than
it does with a high one. The arrow (vertical) groups will for an average
archer (wide-ish bar) be smaller if the nocking point is slightly high (bare
shaft maybe impacts below fletched shaft).
The 70 metres distance graph is generated in similar way to the 18m graph except it uses the Drift simulator.
The curve still shows the arrow displacement rising faster with a positive arrow launch rotation. In this
case the bare shaft arrow will invariably hit higher than the fletched arrow.
The model produces an interesting dog leg (in this case at a -1.6 rads/sec launch angular velocity) which occurs at a
very clear "magic" point regarding the arrow flight parameters. Any suggestions
about this point any whether it might relate to the "optimum" tuning setup would be welcome.
When it comes to setting a nocking point where you are going to be shooting
different distances, e.g. a Fita round, then the only option is to look
at the group sizes at the different distances and come up with the best
overall compromise setting.
Tuning Principles - The Bow Aspects
It has been pretty much an 'open season' in recent years about inventing explanations for
bow tuning. We've had arrow alignment, nodes, purple elephants, arrow vibration etc. None of these
'explanations' have agreed with known factual data or complied with Newtonian mechanics.
The above discussion shows that bow tuning is all about controlling the
angular momentum (rotation) that the arrow has when it leaves the bow.
For 'basic tuning' the bow is adjusted so that for the average shot the
arrow leaves the bow with zero angular momentum. For 'group tuning' the
bow is adjusted so that the amount and direction of arrow rotation for the
average shot is that which results in the tightest groups. Arrow angular momentum
on leaving the bow results from the overall torque generated on the arrow during the
power stroke. What is termed 'bow tuning' is therefore all about adjusting the torque input
into the arrow from the bow.
The archer has two basic arrow torque controls at his disposal the nocking point position (torque
in the vertical plane) and the plunger button settings (torque in the horizontal plane).There are
alternative methods of torque adjustment e.g changing draw weight but nock and button are the
conventionally used controls.
How the nocking point position affects torque input into the arrow is fairly obvious,
the higher the nocking point the more torque is generated by the string re rotation in the
vertical plane in the point going down direction and vice versa. Torque adjustment is made by moving
the nocking point up and down in response to the arrow behaviour in whatever tuning method you are
using.
How the button effects arrow torque is more complex (and as yet not undersood in detail). Looking
down on the bow and assuming a right handed archer then the following torques act on an arrow during
the power stroke;
The finger release and the string force on the arrow generate a clockwise torque on the arrow
in the initial part of the power stroke.(the button centreshot position varies the magnitude of this initial torque).
The force between the arrow and the plunger button generates an anticlockwise torque on the arrow.
(This force also generates a bending moment on the arrow which complicates things)
The torsion spring effect of the limbs generates an anticlockwise torque on the arrow throughout
virtually the whole power stroke
The string force on the arrow will generate an anticlockwise torque on the arrow in the latter
part of the power stroke
The string separation of the arrow from the nock can generate either a clockwise or anticlockwise
torque on the arrow depending the how the arrow behaves. Minimising nock - string torque is part of the
tuning process.
The stress/strain behaviour of the arrow itself can generate torque on the arrow
These torques are interdependant but fortunately the archer doesn't have to be concerned with the
detail only the end result of having zero arrow rotation on bow exit. Centreshot position is a coarse
adjuster of arrow torque. Increasing centreshot increases the overall anticlockwise torque on the
arrow decreasing centreshot decreases overall anticlockwise torque. The required centreshot position
is defined as reducing overall torque on the arrow to the point where arrow torque control becomes
within the control of the pressure button spring adjustment. Centreshot position has nothing to do with
'arrow alignment' as is often quoted. Alignment of a bent stick doesn't mean anything anyway. The pressure
button spring is the fine arrow torque controller. Increasing spring pressure increases anticlockwise
torque on the arrow and vice versa. The final test that a correct setup has been attained whatever
tuning methodology is used is that the button spring is in control of the arrow torque. e.g. with
bareshaft tuning you must be able to move the bareshaft hit position from left to right of the
fletched arrow with button spring adustment. If you cannot do this then the centreshot position is incorrect
Summary
The sources of angular momentum into the arrow comprise:
Horizontal rotation
Movement of arrow nock by action of the fingers on release
Bow hand position (torque)
String force on the arrow nock (buckling)
Force on the shaft from the pressure button
Torsional spring effect from the bow
Separation of nock from the string
Vertical rotation
Pressure distribution of the string fingers
Bow hand position (torque)
Reaction between arrow shaft and rest
Nocking point position
Vertical string forces on the arrow nock (limb movement variablity)
Separation of nock from string (maybe?)
The items above for the effects of the bow and string hands really relate
to the archer's technique and the bow parameters are adjusted to include
the effects of 'form'. The normal controls used to vary the net arrow angular
momentum are the nocking point position and the plunger button spring tension.
These are adjusted using some systematic process (the tuning method) to obtain
zero angular momentum or minimum arrow groups. By selecting the correct arrow
specification for the bow in the first place the arrow will leave with
near zero angular momentum and only fine adjustment is required.
The archer tries to make each shot exactly the same (like the shooting
machine represented by the vertical orange line). The archer will vary
round this perfect shot represented by the blue line in the 'Archers Shot'
graph. Most arrows will be shot at or near the peak of the blue curve.
The further away you get from the perfect shot than the lower proportion
of shot arrows (you hope) you will get. The variation in the archers shot
will feed through to a variation in the initial flight characteristics
the arrow has leaving the bow (the middle graph) and a consequent variation
in by how much the arrow misses the centre of the target (represented by
the third graph). Strictly speaking the shape of the curve in the third
graph should be like a letter 'M' rather than the bell shape illustrated.
The probability of making a perfect shot and hitting the 'x' is actually
very small (otherwise 'Robin Hoods' would be a daily ocurrence).
Suppose we have an archer who shoots lots of arrows with a specific
bow/arrow combination. We mount a camera above him and record how much
horizontal rotation each arrow he shoots has when it leaves the bow. If
we then plot a graph of the number of arrows that came out against the
amount of rotation the arrow has then we get something like the following.
If an arrow leaves the bow with horizontal rotation it ends up hitting
the target horizontally displaced by some amount. This displacement mainly
results from the initial arrow rotation (angular momentum) and offset
angle with a small contribution from the arrow fishtailing about. We do
another test, this time measuring the amount the arrow is displaced sideways
for a particular amount of initial rotation. The thing we would notice
is the amount the arrow is displaced increases faster than the increase
in the value of the arrow rotation. In other words the distance between
the arrows shot with rotations of 3 and 2 is bigger then the distance between
the arrows shot with rotations 2 and 1, and this is bigger than the distance
between arrows with rotations 0 and 1 (The vaules 0-3 are just numbers
to represent the relative amounts of rotation)
If the arrow leaves the bow with no rotation then it leaves the gizzmo
with rotation and hence the group size increases. If instead we adjust the
bow so that the arrow leaves with just the right amount and direction of
rotation to balance the gizzmo then it leaves the gizzmo with zero rotation
and we get the minimum group size again. The general principle is that if
there is something acting to change the arrow rotation between leaving the
bow and hitting the target then the optimum tuning setup for the bow will
not be zero arrow rotation. The optimum rotation of the arrow from the bow
will be that which best counteracts the external influence to provide minimum
group size. As any 'gizzmo' will in reality act over the whole arrow flight
it is not possible to completely compensate for it, only minimise the
increase in the sizes of groups.
The diagram illustrates the affect of a cross wind on arrow group
sizes. The red lines indicate the flight paths and resultant group size
for a representative archer with a perfectly tuned bow with no wind present.
The blue lines indicates what happens in a cross wind to the same conventionally
'zero rotation tuned' archer/bow setup. The group size increases significantly.
Most of the increase comes from the arrow (the zig-zag blue line) that
comes off the bow with the nock end rotating into the wind. In this case
the sideways drag force from the arrow's forward motion reinforces
the sideways drag from the wind at a number of points during the arrow's
flight and as a consequence the arrow ends up with a high downwind lateral
displacement. The green lines show the flight paths and group size for
a bow optimally tuned for the wind conditions and target distance. In
practice you cannot optimally tune the bow for the current conditions.
As a general guideline a good setup when shooting in a cross wind will
require the arrow to leave the bow with the nock end rotating in the
downwind direction. The key to having good groups in a crosswind is to
avoid the sideways drag on the arrow from its velocity reinforcing the
sideways drag on the arrow from wind. This topic is discussed in more detail
in the section on Variable Tuning
The graph represents the vertical displacement from the target centre
(vertical axis) as a function of the amount and direction of rotation in the vertical plane
of the arrow leaving the bow (the horizontal axis) at a target distance
of 18 metres.. The data is produced using the arrow flight simulator for
a specific set of arrow properties. A negative rotation represents a high
nocking point (bareshaft impacts below fletched shaft) and a positive rotation
a low nocking point. The red and turquoise horizontal bars represent the
arrow rotation for the 'perfect' shot (rotation 0 and 6) and the width
of the bar represents the variation in rotation from how much the archer
varies from the perfect shot. The corresponding vertical coloured bars
indicate the vertical spread (group size) of where the arrows hit resulting from the
archer's variation. I have ignored the fact that the turquoise archer will
adjust his bowsight so that his perfect shot has zero displacement from
the target centre which will have some effect on his group size.
The 70 metres distance graph is generated in similar way to the 18m graph except it uses the Drift simulator.
The curve still shows the arrow displacement rising faster with a positive arrow launch rotation. In this
case the bare shaft arrow will invariably hit higher than the fletched arrow.