Bow tuning was a lot different in the olden days when risers weren't cut past center. Risers were made of wood and later aluminum or magnesium. The design and materials were not strong enough for a thin cross section of riser. As a result, the center of the riser was about the edge of the sight window. The arrow for a right handed shooter would be pointing to the left by half the diameter.
There was obviously no left or right adjustment. The only adjustment was arrow spine and moving the nocking point. The arrow had to be spined correctly so that it flexed the proper amount to clear the riser. The archer's paradox, that series of bends the arrow makes when shot, was necessary to have good arrow flight.
The arrow would first flex bowing out away from the riser. Then as the tail end of the arrow approached the riser the arrow would flex the other way moving the tail out away from the riser. The arrow wouldn't hit the riser if the spine was correct and that gave you good arrow flight. Spine was critical. Tuning was really just timing the bends of the arrow to clear the riser.
Tuning is so much easier today with risers cut way past center and shoot through or fall away rest. Plus most people shoot release aids for a better release. However, you still read and hear lots of talk about tuning your bow. There is paper tuning, tiller tuning, bare shaft tuning, and the list goes on.
I think that the best statement I ever heard about tuning was by a well know archery coach that said people spend too much time worrying about tuning their bow and equipment and not enough time practicing their shooting. Test show that an out of tune bow will shoot arrows in the same hole from a shooting machine.
A well tuned bow may make it more forgiving but it is still up to you to have the proper form and consistency to shoot good groups. So why do you read and hear so much about tuning? People have to talk about something besides the weather. Writers have to sell articles and experts have to keep being experts.
This article is going to be a practical way to set up and tune your bow, which will be plenty good enough for 99% of the bowhunters. Then you can spend your time practicing so you can be a good shot.
THE PURPOSE OF BOW TUNING
There is only one reason to tune your bow. That is your arrows fly straight and group where you are aiming. Nothing else matters. It doesn't matter if your sight pins line up over your sight. It doesn't matter if you tear perfect holes in paper, unless that is your goal rather than good flight and groups. It doesn't matter if your arrow is in perfect center shot or nock is perfectly at 90 degrees. Your only goal is for good flight and groups. Keep that in mind and the tuning job will be a lot easier.
ADJUSTING FOR CENTER SHOT
Finding True Center Shot.
All bows are different. The string is not always in the center of the bow. It is to the left of center (right hand shooter) on most bows. In addition, the string runs at an angle on most single cam bows. There are devices you can buy that are supposed to help you set your bow up for center shot. I haven't found one that works. Some work on the assumption that your riser is straight, which a lot aren't. The riser on a lot of bows flexes and limbs twist when you draw them. Do you want the arrow at center shot when the bow is relaxed or drawn? The biggest thing is your bow may not shoot the best at perfect center shot because of your form or spine of the arrows. So adjusting for center shot is only a starting place. It doesn't make much sense to spend a lot of money on a center shot device when you may be moving your arrow to the left or right of center.
The string travel on any well designed bow should be parallel to the sight window. That means your arrow theoretically should also be parallel to the sight window so it travels the same plane as the string. Put your arrow on the string with the lower limb of the bow on the floor. Now look down at it and see if it is parallel to the sight window. Move your rest either in or out until it is. It is not important that it be perfect because you may change it later. Just eye ball it until it looks parallel. You can use a straight edge to draw a parallel pencil line on the bow shelf if the window is too far away to tell. Move your arrow parallel with the pencil mark.
SETTING THE NOCKING POINT
Adjusting your nocking point.
There are several theories on setting the nocking point. Some say the arrow should be level and some say the nock should be slightly high. The particular rest, arrow, bow, whether you use a string loop, and how you grip the bow will determine the best place to set the nocking point.
The center of a bow is anywhere from the low point of the grip to the hole in the riser where you attach the rest. Most are somewhere in between. Take a tape measure and measure the distance from axle to axle. Then divide that in half to find the center of the bow. This will tell you something about the design of the bow. The grip and arrow can't be in the center because you would have to shoot through a hole in your hand.
Something is going to be out of balance. If the arrow is in the center your grip is below center. That makes the distance from the grip to the upper axle greater than the lower axle. More leverage is then applied to the upper limb. The bow will tend to rock in your hand as you draw. The upper limb will come back towards you and then move back out as the system balances at full draw.
The bow won't rock in your hand if the center is at the grip, but then the arrow is above center and you have unequal forces with a greater distance from the arrow to the bottom axle. It makes you wonder how you can ever get an arrow to fly right with all the unequal forces. The forces are consistent so you tune your bow to the forces and the arrow flies straight. That is why there is no set rule for where to set the nocking point.
I mentioned that in the olden days the bow was set up so the arrow flexes out away from the riser to clear it. The same holds for an arrow rest. The arrow will rise up off the rest if the nocking point is slightly above 90 degrees. That gives better clearance and the arrow doesn't slam down into the rest. Somewhere between 1/16" - 3/16" seems to work best for most people. Fall away rest work so different that you need to experiment or go by the recommendation of the manufacture. Most set the arrow at 90 degrees on fall away rest.
Here is something to get you thinking. The fall of gravity is 32 feet per second squared. Let's say your bow is shooting at 250 F.P.S. and the rest is falling at 32 F.P.S. Does the rest fall before the arrow gets there? High speed video made by a manufacturer of prong type rest showed the arrow was clear of the rest before the rest came back up. Both these concepts show that there is no hard core rule on setting the nocking point. In addition, some bows, especially single cam, have nock movement. The nock does not move in a straight line. None of this makes any difference because you will tune to your bow.
Now you should have your bow set up in a static condition. Shooting the bow is a dynamic condition and things change. Easton did high speed photography that shows what happens when you release the string. It is very impressive. You can see the series of bends the arrow makes and it looks like a snake trying to get away. The string is also making a series of S curves on 2 axis. That is why setting up the bow in the static condition is just a start. The arrow probably doesn't leave the string in the same relationship it started. How it leaves the string is at least as important as how it started.
Use only new or arrows you know are good for tuning your bow. Arrows that are shot a lot will lose spine. Arrows that have been hit by other arrows or hit objects may also change spine or straightness. It does no good to try and tune a bow with less than perfect arrows. The same is true when you sight in your bow.
Most bows with a reasonably high brace height are pretty forgiving. Generally they will shoot an arrow pretty straight when set up close to center shot and the nocking point even to a little high. Striking the rest is what causes most of the problems with arrow flight. A powder test tells you exactly what the arrow is doing as it leaves the bow. A paper test tells you what it is doing several feet from the bow. Knowing how the arrow leaves the bow is more important.
Buy the cheapest spray foot powder you can find. Spray the fletch end of your arrow and shoot it using your best form. Then look at the powder on your arrow and your rest. You should see anyplace the arrow struck the rest or bow. There will be powder missing from the arrow and powder on what it hit. It is like going hunting after a fresh snow. You can see everything that has happened.
The fletching should have past through the center of a prong rest. If you see it hit on the left side then move the rest to the left and shoot again. You should be able to see either no powder missing or just 2 lines on the shaft where the arrow skimmed over the rest with the fletch passing between the prongs. Moving the nocking point or changing the spring tension on a prong rest will fix problems with the arrow hitting too hard on the rest. You will see how moving the nocking point or rest will make changes. Move them until you get the changes you want, which should be low to no contact.
The real test is shooting your bow. Sight your bow in close enough so you won't miss the target and then shoot at the longest distance you can shoot comfortably. You should have good arrow flight if your arrows are spined correctly and not contacting the rest. You should be shooting good groups. Number your arrows to make sure an arrow doesn't shoot off. Shoot a 5 arrow group. Disregard any arrow that is way out of the group because that was probably you or the arrow.
You will know it was the arrow if it is always out of the group. Now the important part is record your group. Don't depend on your memory. Record the horizontal and vertical distance of the group. You might want to shoot a couple of groups before you make any changes. Start with the biggest group. If your horizontal group was the biggest you want to make slight changes moving the arrow rest left or right. First mark where it is and record or mark how far you move it. Then shoot again and see if the changes make any improvement. Do the same thing with the nocking point for the vertical group.
I have found that on a forgiving hunting bow I can move the rest as much as 3/8" and still get good arrow flight and can't really tell a lot of difference in the group size. There might a 1/8" area that I can't tell any difference. I have found the distance on the nocking point to be a little more critical, usually about 1/8" for good flight and 1/16" for no difference. Your results may be different, depending on the bow, your form, and how well your arrows are spined for your bow.
Maybe you could tell a difference if you were good enough to shoot inch groups at 40 yards, but the average bowhunter can't tell a difference. Now record the measurements where your bow shoots the best and spend your time practicing your shooting rather than messing with your equipment. Remember, a shooting machine can shoot arrows in the same hole from an out of tune bow because it shoots exactly the same every time. You need to make your form consistent enough that you can shoot good groups with a well tuned bow.
SPECIAL CONSIDERATIONS AND PROBLEMS
It is probably a spine problem if you still have poor arrow flight. Try a stiffer arrow or you can reduce the weight of your bow to see if under spine is a problem. That is a lot cheaper than running out and buying a new set of arrows. It is the arrow spine if reducing bow weight helps.
It is probably a nocking point problem if the arrow porpoises. It could be the nocking point does not move in a straight line because of cam design. You may need to move your nocking point as much as ½" high. The powder test should show the arrow is slamming into the rest.
Worn cam bearings can cause poor arrow flight. Check your cam if you had good flight and now it changes. Your cam will lean a lot at full draw with bad bearings.
Equipment problems are consistent. Some problems may make your group spread horizontally and others vertically. Equipment will almost never change your groups in different directions. Only the shooter can do that.
Line your arrow up parallel with the sight window.
Set your nocking point 1/16"-3/16" above square.
Do a powder test to check for arrow contact.
Shoot your bow for good flight and groups. In most cases it will shoot as good as you can shoot it.