Tag Archives: archery science

Central Plane ? of the Bow?

QandA logoI 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 ;ines 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.

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).

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Anchor Positions (Finger Shooters)

John William's "center anchor."

John William’s “center anchor.”

I was struck by a photo of an archer from the recent past who used a center anchor to some effect, namely John Williams of the USA, the 1972 men’s Olympic gold medalist (see photo at right). Currently, there has been an almost universal adoption of the “side anchor” specifically the variant in which the string touches the corner of the chin, but not farther back (see photo below). This post is about why these changes have been made.

Most beginners think that these things are about string positioning but really they are about head positioning. By having the string touch certain parts of the head, at multiple points if possible, this guarantees that the positions of the head and the all-important aiming eye relative to the bow are made more repeatable. The archer must still try to endeavor to hold his/her bow in the correct orientation and hold their head in the correct orientation besides, but consistency is much improved by monitoring these other “touches” tactilely.

Brady Ellison's "side anchor."

Brady Ellison’s “side anchor.”

Very few people use a center of the chin anchor position any more because it often requires a substantial tilt of the head to line everything up. This we consider to be suboptimal because when we tilt our heads, there are physiological repercussions. When our eyes aren’t level, we lose some of the benefits of binocular vision, for example our depth perception and ability to estimate distance are degraded. For another thing, we actually lose physical strength when we tilt our heads because this is a submissive posture. Recall your posture when your Mom caught you with your hand in the cookie jar (or whatever). Dogs will actually drop their heads to the side exposing their jugular veins to convey helplessness to another dog. This is why people of power want you to bow your head; it puts you into a weak, almost powerless posture and your body and mind accept those positions.

Now look at the two photos; which of the archers has eyes that are level/horizontal?

The idea of the string touching your nose is also a head positioning element. If the nose and string do not touch, what should you do (or ask your student to do)? Certainly you do not want to tilt your head to make this happen for the reasons mentioned above. You also don’t want to move your anchor position back along your jaw. Since your fingers are curled around the string toward you, when the string it loosed, the act of the string pushing your fingers out of its way causes the fingers to push the string back (Newton’s third law). This means that as the string moves forward it also moves toward the archer. High speed video has shown that when the string is held against the face behind the corner of the chin, the string drags along the archer’s face as it leaves, causing ripples in the skin! This drag lowers arrow speed and is a source of variation we do not need.

Positioning the string at anchor at the corner of the archer’s jaw allows the string to leave its position without drag on the archer’s skin and allows the archer’s eyes to remain level.

If there is no nose touch or too much touch and some is desirable, it is possible to create what you want by changing the length of a recurve bow, for example. The Korean women, who have short noses, tend to use longer bows, which provide less acute string angles at anchor. Compound archers who use a nose touch have learned this the hard way as bows being manufactured now are much shorter axle-to-axle than they were in the past, making vastly more acute string angles at anchor, making a nose touch harder to achieve. Be aware, though, that nose touches are less useful to compound archers because they have peep sights, acting as an additional alignment point to the face touches. So, going to great lengths to get a nose touch on a short ATA compound bow is probably not worth doing.

Throughout archery’s history we have learned through trial and error and by emulating the more successful, a process I describe often as: monkey see, monkey do. We are now teasing out why the things that work actually work, which means we are reaching a level of understanding at which we will have more control than ever over what we do to maximize our own and our student’s effectiveness.

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Point Weight Woes

QandA logoI got an email from a friend regarding a problem you may have encountered in your coaching. Here it is.

Hi, Steve!
I wanted to bounce something off of you if you have a moment. I had some unexpected issues with arrows over this past weekend when I was shooting my first FITA. I was struggling much more than anyone else with the light wind we were having. Right before the competition, I had been trying to tune in my new arrows that were acting too weak and I ended up reducing the point weight to 90 grains, which had them finally performing well in practices. I put up some personal best practice scores with my adjusted arrows right before the FITA round and was feeling good about my performance. I suspect, though, that the light point weight was my downfall and part of what was giving me such problems. My longer distance scores were the worst I’ve ever done in my life, however my shortest distance score (30m) was right on the money with my normal practice scores, even though the wind was the same and I should have been the most fatigued and dropping points at the end.
            I bought my arrows slightly long and am thinking about cutting off a 1/2˝ and putting 110 grain points back in the shortened arrows. My theory is that that will have a similar effect as having the longer arrows with lower point weight, but will give me more ability to cut through the wind. However, I can’t find any literature online or in my many archery books about point weight vs. arrow length in trying to make adjustments to arrow spine. Which is a better adjustment to make, and is there any such equation such as “each 1/2˝ of length = 20 grains of point weight” or whatever? I’m not keen on cutting down my arrows if that might not give me the results I’m looking for.
           
Thanks in advance for any help or advice you can give!

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I doubt your point weight made all that much difference so it may or may not have been the source of your woes.

First I have to ask: when you were tuning in these arrows, did you have a reasonable centershot, plunger button resistance, nocking point height, and was your aperture centered above your arrow? If not, you were tuning to a less than optimal setup. It is always important to have a bow in a proper setup when trying to tune. If your aperture is off center, for example, you are then trying to tune your arrows to a dynamic spine that will compensate for a mis-set sight!

Most Olympic Recurve archers have a FOC balance point of 13-15%, so that is something you might want to check. (FOC guidelines are the equivalent of the “equation” you desire.) The vast majority of OR archers have 100 gr points (110 gr being in second place, I think), unless … you are shooting a very lightweight all-carbon arrow such as McKinney IIs, then 90 gr and even 80 gr come into play.

As far as wind stabilization goes, there are two strategies: use a heavier arrow (like Easton X10s) or a lighter arrow (like Carbon Tech McKinney IIs or Carbon Express Nano Pros or Medallions). The heavier arrows have more mass and therefore require more wind force to move them (inertial stabilization). The lighter shafts are faster and hence spend less time in the wind for the wind’s forces to act on them (speed stabilization). When an arrow is shot long distance, a higher FOC is generally desired to keep the arrow on track during those longer flight times. When I was shooting field archery a lot I was using 60, 70, and 82 gr points in very long arrows with little downside. But shooting FITA rounds, I was using 100 gr or even 120 gr points (again, in very long shafts).

So, research Front-of-Center (FOC) balance and how to measure it (it is easy) and check your current arrows. If you are close to 13-15%, then it was not your point weight that was a problem. If it is 6-9%, then maybe so.

If it was not your point weight, I suggest you go back to a basic setup and retune (nocking point height 1/2˝ above square, centershot has inside edge of arrow point visually lined up with outside edge of bowstring with string centered on the riser (visually), plunger pressure mediumish, aperture centered above arrow when bow is vertical (I just run the aperture down to the bottom of the sight bar and eye-ball it)). Also, you need to take off all vibration absorption devices (Doinkers, et. al.); they can only mask the feel of good shots.

You may find that the tune you had wasn’t all that good.

The reason the tune is so important is the tune establishes the launch angle of the arrows (at what ever angle the bow is being held), so if the centershot is way outboard, for example, the arrows are launched point left. Then the fletching has to correct for that, but if the wind is blowing more than a bit, that “sideways” launched shaft is going to be blown in unpredictable ways (the shaft itself is a bigger source of drag than the fletches) and you are going to have very large groups as a consequence.

I hope this helps!

Steve

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