Bare shaft tuning is a method frequently used to adjust the nocking point and pressure button settings to 'tune' a bow. Enough has been written about bare shaft tuning to fill a large warehouse so I am not going to describe the methodology but rather describe the principle of the approach.
Invention of the bare shaft tuning method is credited to US Archer Max Hamilton in 1963 (Third Edition of the National Archery Association Instructor's Manual published in 1982).
Suppose we shoot three arrows exactly the same at some distance, e.g.
90 metres, with identical arrows apart from the fletching size. One arrow
has a 'large' fletching, one arrow has a fletching half the size of the
large fletching and one arrow with no fletching. The fletchings are assumed
to be of the plastic variety. The trajectories of the three arrows would
be something like that below.
The arrow with the largest fletching hits lowest. The medium fletched arrow hits a bit higher and the bareshaft arrow hits quite a lot higher. The separation in height between the arrows happens in second part of the flight, on the way down.
The height difference between the arrows is often explained as being caused by the larger fletching having more drag slowing the arrow down. This is a bit of a myth. If it were true there would be a much larger difference in hit height between the medium fletched arrow and the large (twice the area) fletching. The amount of drag depends on the drag area. The drag area of a fletching which slows the arrow down is the 'edge' profile, say around 0.3 square cms for plastic vanes. Compare this with the area of the shaft at around 40 square cms. The drag on the shaft swamps the fletching drag.
Suppose we shoot two arrows, again identically. The arrows are exactly
the same (weight, fletchings etc.) apart from having different FOC values.
The (last part) of the trajectory is illustrated below.
What you get is that the higher the FOC of the arrow, everything else being the same, the lower it hits on the target. In this case it is obviously not 'fletching drag' causing the difference in height.
What dominates how an arrow files is the magnitude and direction of the total drag force on the arrow which moves it about of which a significant component is the drag force on the arrow shaft. The magnitude and direction of the shaft drag force depends on the angle between the direction the arrow is pointing and the direction the arrow is travelling.(ref) When you shoot an arrow at say 90 metres the arrow starts off at an angle around 9 degrees (the bow angle). The arrow hits the target at an angle around minus 9 degrees i.e. the arrow rotates in the vertical plane during its flight through an angle of around 18 degrees. What rotates the arrow is the drag force on the fletchings and a section of the shaft so how fast the arrow rotates depends on the area of the fletchings and the FOC as well as the angle between the direction of flight and the direction the arrow is pointing. There is always a lag between the direction the arrow is travelling and the direction the arrow is pointing, the larger the fletching/FOC the smaller this lag is. As the shaft drag depends on this lag it is this lag, which depends on how fast the arrow rotates, which causes the difference in hit heights shown above.
The following graph illustrates how the angle of where the arrow is
pointing varies over its flight. (the medium and bareshaft arrows as per
the first diagram are used).
Both arrows start off at the same angle, which is also the initial direction of flight. During the first part of the flight the bare shaft 'rolls over' much more slowly then the fletched arrow. The fletched arrow becomes horizontal at around 49 metres, not long after the arrow starts to fall down. The bareshaft arrow does not become horizontal till about the 64 metres distance. During the last part of the flight the bareshaft arrow is rotating a lot faster then the fletched arrow so the two arrows end up hitting the target at fairly similar angles.
The following diagram represents the drag forces on the fletched/bareshaft
arrow shafts on the way down at some specific distance to illustrate why the
slower rotating arrow ends up hitting higher on the target. (the direction of the drag force from the pile and edges of the fletchings
always runs along the shaft axis).
The bareshaft arrow because it has a much larger offset angle then the fletched shaft has a much larger shaft drag. The drag direction is mainly upwards so the rate of fall of the bareshaft arrow is slower than the fletched arrow. This is why the bareshaft arrow ends up higher on the target. The larger the offset angle the more drag there is acting to rotate the arrow which is why the bareshaft arrow ends up rotating faster than the fletched shaft.
Arrow roll-over also relates to how wind affects the hit height of the arrow (ref).
What has all the above to do with bareshaft tuning? With bareshaft tuning you look at the relative hit positions of a fletched and bareshaft arrow vertically (nocking point) and horizontally (button). The reason the two arrows hit at different points is for reason just described, the bareshaft arrow rotates slower than the fletched shaft.
Nocking point tuning is aimed at getting the arrow out of the bow with
no arrow rotation in the vertical plane. If the arrow comes out with rotation
then what the fletching is doing is braking this rotation. The less fletching
you have then the less brakes you have so getting rid of the rotation takes
longer with the bareshaft arrow than with the fletched arrow. The following
diagram illustrates the different braking effect you have between a fletched
and a bareshaft arrow and the consequent effect on how shaft drag affects
the arrow flight.
In this case the nocking point is low so the arrow rotation is in an anticlockwise direction. Because the fletched arrow rotates faster than the bareshaft arrow it accumulates much less shaft drag in the upwards direction and so ends up hitting lower on the target.
Because, as indicated in the introduction, the bareshaft arrow will end up hitting above the fletched arrow because of the effect of the 'gravitational trajectory' you can only nocking point tune at short distances. It is found by experience that with a tuned set up the fletched and bare shaft arrows will hit the target about level with each other at 30m. This is why 30m is the distance used for bare shaft tuning (nocking point in conjunction with button spring)
Button tuning is aimed at getting the arrow out of the bow without any rotation in the horizontal plane. The effect is exactly the same as for nocking point tuning described above. (just rotate the diagram through 90 degrees). Button tuning with a bareshaft approach at 70/90 metres sounds likely to be a very sensitive approach. Hovever as other factors come into play regarding bareshaft to fletched arrow behavior at longer distances (variation in speed, drag lift and the effect of arrow spin) sticking to 30 meters for bare shaft tuning is the recommended option.
The recommendation when bareshaft tuning is first get the nocking point sorted and then tune the button. There is a reason for this. The settings for nocking point and pressure button are not independant of each other. If you change the nocking point then the effective button setting will be changed and vice versa. The reason the nocking point is adjusted first is because, for basic tuning, it's the less important. As mentioned in the section on Tuning Principles the optimum nocking point position depends on the target distance so you only ever have a 'thereabouts' setting. Ignoring wind effects, there is an optimum button setting and group sizes are more sensitive to the button setting than the nocking point. If you determined the button setting first and then adjusted the nocking point then the result would be the more critical button setting being 'off'.
Last Revision 1 July 2009