I always assume I am being perfectly clear, but I get help from readers who write to me and tell me when I am not. This is something for which I am grateful as it helps me do a better job of explaining things. Here is a recent request for a clarification that I thought I should share.
“On page 64 of The Principles of Coaching Archery, Vol. 1, you say that the sight aperture [should be] in the central plane of the bow (along with the bow string). I’m not sure what you mean by ‘sight aperture,’ and not sure about ‘central plane.’
“I’m assuming you don’t mean that the pin (or whatever) that is part of the sight should be obscured by the bow string. I shoot a bare longbow for practice, but I hunt with a compound bow that has a sight, and it wouldn’t do me much good if the pin were hidden by the bow string.”
Here’s my response:
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I usually ask whether you want the long answer or the short one, but …
If your bow is set up right (any of them), the bow string should share a plane with the riser. The riser, were it to be split in two slicing down its middle from top to bottom (from the archer’s viewpoint), that is what the “central plane” is. (If you have a metal riser, the screw holes on the back are in that plane so you can use them to visually check whether the bowstring is “in plane” in that it should line up with both screw holes. On recurve bows, before the advent of “adjustable limb pockets,” the string could be no other place. If you bought a bow and the string wasn’t aligned on the center of the riser, you sent it back. (If later, you acquired a twisted limb, then there is more than one problem involved.) Now that we can “adjust limbs in their pockets” I have seen bows with bow limbs tilted in the same direction, creating a situation that the bowstring was quite far from centrally located. These bows don’t shoot worth a darn if left that way.)
See how the bowstring lines up with the central plane of the riser, how the archer holds the bow vertically? All of these are needed criteria for repetitive accurate shooting.
Ideally when the string is pulled back and let go it moves toward the riser in or near to that plane. The arrow needs to be set up so that it sits in or very near to that plane so that the string pushes it along the axis of the arrow. If the string pushes on the back of the arrow and the arrow is sideways to that plane the arrow will spin like a helicopter blade! So, a basic bow setup requires the string and arrow to sit in this same plane. The arrow should, if it is spined right, then fly in this same plane toward the target, which means the sight’s aperture (pin, ring, scope with dot, scope with a ring, whatever) must also be in that plane (dead center, please).
When I first work with an archer, one of the first things I check is whether his/her aperture is “in plane.” If it is not, they do not have a good tune. The equivalent, if you are shooting Barebow and using a point of aim aiming technique, is that your POAs need to be in a vertical plane with the target center (a 12 o’clock to 6 o’clock line through the X-ring is part of this plane which, interestingly, is the exact same plane we were just talking about). If your POA is to the left of that line to hit the center, then your arrows are too weak (assuming a RHed archer). If the POA is to the right of that line, your arrows are too stiff.
All of this is determined by bow design and by the fact that when an arrow flies the only force remaining on it is gravity, so the arrow moves up and down only (absent wind) after it is launched. If that arrow doesn’t start in the central plane as described, it will not end up in it and will not hit the center of the target.
If the bowstring were off-center on the bow, it would tend to twist the bow in your hand and also end up pushing your arrows in a direction other than down the length of your arrow shafts and so your arrows would be hippety-hopping all day long (fishtailing primarily).
I am in the process of pushing a “principles-based archery coaching” approach in which coaches can learn a few of these basic design/physical principles which then allow them to figure out what is going wrong with bow setups, no matter the situation. Ain’t there yet, but working on it.
As to hiding the sight aperture (pin, ring, scope with dot, scope with a ring, whatever) compound bows allow the use of a peep sight which allows you to look through the string and for other bows, it is important that the pupil of your aiming eye (the hole that lets the light in) is lined up along side the bowstring, tangent to that string, meaning as close to the “plane” as possible without having the string block your vision.
And, of course, when you shoot Barebow, there is no sight aperture to place correctly or incorrectly.
An Added Note Now that you have some idea of this central plane of a bow, can you now see why a bow sight’s sight bar (the vertical part when being used) has to be parallel to the central plane? If it is not, then when you move the aperture up and down to adjust for shots of different distances, you will also be moving the aperture left and right relative to that plane. This will create left and/or right misses depending on the angle of the sight bar (the amount of the miss will vary with the distance).