The physics of paper tuning

[Mighty Mouse]

Do you have a favorite article on group tuning basics? I understand bare shaft tuning with nock position, at least I think I do. Interested in group tuning vs paper tuning again, but too many "experts" these days with a YouTube to ID the true experts.

So jot to hijack, a PM would be great

Paper vs. bareshaft vs. broadhead tuning in a nutshell below. In theory, all 3 methods should lead you to the same result. I use paper for initial ballpark tune, bareshaft to refine, broadhead as final verification.

 

[dcopher]

The first picture Mighty Mouse showed explains what is happening very well. The best way I have explained tail left/right tears is that the light end of the arrow (the tail) is trying to align with the heavy end of the arrow (the point) at the shot. That is why the movement of the string (shimming, yoke tuning, etc.) should follow the tail of the arrow, and movement of the rest should follow the point of the arrow. Sometimes it seems counter intuitive, but that is the physics behind what is happening.

 

[MattB]

Yeah, it’s difficult to articulate the concept I’m trying to understand. It’s a Mathews, so yoke tuning is done via top hat shims.

What effect do yoke tuning and shimming have on the string and arrow?

At full draw, how I hold my bow changes the riser angle relative to the string. Therefore, the string travel (and force) may not be in a perfect line parallel to the riser.

If your bow has a cable guard, it will cause torque. It may have nothing to do with your grip.

 

[jmez]

Rest at center shot for the bow. Cam lean causes the nick travel issue. Cam lean because of unequal tension on the axles induced by the cable guard pulling everything to one side. This is exacerbated by split limbs. You can't have equal tension across the axles when the cables are pulled tone side, all bows have cam lean.

With shimming or yoke twisting you are simply adding equalizing the force across the axles. Top cam is more important than the bottom. Grip is closer to bottom cam so more string plays off the top.

If you have a tail left tear your top cam will be leaning like this /. Causes left lateral nock travel ( string path to left).
To counteract that you would twist the left yoke and untwist the right. That is going to pull the left side of the axle down and raise the right side, leveling the axis and the cam. With shimming you would put a bigger shim on the right side of the cam and smaller on the left, this will shim it left, or simply make it stand up straighter.

If your rest is at factory center shot you are manipulating the cams to have the straightest string path toward the rest as possible. This is more efficient and forgiving than simply moving the rest to compensate for poor string/nock travel.

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[jimh406]

Assuming you are right handed, left tear could be a spine issue. I'd probably have someone else shoot it. (Just guessing that you aren't that experienced.)

That being said, how are the arrows grouping, and how o they shoot broad heads (if you hunt with this bow)?

You might be overthinking it.

 

[TheCougar]

You are correct that you want the rest in line with the path traveled by the nocking point from full draw to brace. The manifestation of left/right misalignment between the rest and string path and the adjustments needed to correct it seemed counterintuitive to me until I watched a video in the Bow Shop Bible app that shows what's going on in slow motion. I attempted to explain what the video shows in the sketch below, but the video itself is worth a million words (and the price of the app).
View attachment 540213

The conditions shown in the sketch would produce a nock left paper tear, bareshaft right of fletched shaft, and broadhead right of field point, which would all be fixed by moving the rest to the right (or by moving the cams to the left via shimming or yoke tuning). The seemingly counterintuitive left/right tuning adjustments now make perfect sense to me after seeing what's actually happening in slow motion.

Dynamic bending of the arrow shaft also plays a role, which I didn't attempt to include in my illustration. Rest type is also a factor: the longer the arrow and rest are in contact, the more pronounced the effect of misalignment between the rest and string path.

This is perfect, thank you. It is counterintuitive, for sure.

 

[Billy Goat]

Here's a good video on different tuning methods. At 11:45 he talks about group tuning. Explains it exactly how it was explained to me, you are actually taking the bow slightly out of tune to ensure the arrows react the same at launch every time.

It's a target thing, not for hunting bows.

 

[PeteB]

You are correct that you want the rest in line with the path traveled by the nocking point from full draw to brace. The manifestation of left/right misalignment between the rest and string path and the adjustments needed to correct it seemed counterintuitive to me until I watched a video in the Bow Shop Bible app that shows what's going on in slow motion. I attempted to explain what the video shows in the sketch below, but the video itself is worth a million words (and the price of the app).
View attachment 540213

The conditions shown in the sketch would produce a nock left paper tear, bareshaft right of fletched shaft, and broadhead right of field point, which would all be fixed by moving the rest to the right (or by moving the cams to the left via shimming or yoke tuning). The seemingly counterintuitive left/right tuning adjustments now make perfect sense to me after seeing what's actually happening in slow motion.

Dynamic bending of the arrow shaft also plays a role, which I didn't attempt to include in my illustration. Rest type is also a factor: the longer the arrow and rest are in contact, the more pronounced the effect of misalignment between the rest and string path.

Any chance you'd have a link to that slow-motion footage? Be fascinating to watch.

 

[dtrkyman]

Shooter influence on the bow is the most common tuning issue, too long of a draw length is so common it is ridiculous!

You would be shocked at the change in tune from slight draw length adjustments.

When a tune does not respond to moving the rest it is a very common problem.

When I managed a shop I would say 80% of people were shooting too long of draw and too much draw weight.

If you have a rotating module type cam you can play with it really easily.

Otherwise a tune that does not respond to moving the rest is likely a cam issue, yoke tune or adjusting cam spacers is the answer.

 

[MallardSX2]

I switched my top hats around on my matthews and it made a WORLD of difference.

 

[dkime]

Yup, I know what to do, but I’m trying to understand the “why” behind it. I’m not clear why moving the cams to the left is supposed to fix a left tear.

You are shifting the load ever so slightly to the left limb. Think of it this way, by shifting the load slightly left either by shimming the cam left or by twisting a yoke or by shimming a limb pocket on the right side. You are putting more cam lean into the system at full. Cam lean takes the left side of the axle and makes it slightly lower than the right side at full draw. Upon release the lean of the axle corrects back to somewhat of a level standpoint. The axle has to essentially twist itself up and slightly back to the right As does the cam. As the cam reals the string back onto itself it corrects the nock of the shaft to become aligned with the front end of the shaft (assuming you got your tune nailed)

You can also do none of this and simply torque your bow to the right at full draw.

Target is at 12 o’clock, shooter is the center of the dial. Stabilizer is the arm of the clock. Point the stabilizer at 1 o’clock at full draw while aiming at 12 o’clock. Take the torque out of the system that’s caused by the cable guard or by too long of DL, or by shitty form. I’ve shot bullet holes with 400 spine shafts at 75# with 200 up front using this method. Doesn’t make it right but it will work


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[50fps]

On one of my Vertix, I had a slight left-hand tear. I switched the bottom top hats around and it cleaned it up. I then bare shaft to confirm and if it's still very close a slight adjustment to the rest is in order. I then do a walk back and, in the end, a broadhead tune.