Physics of tuning problems

[rpjazzguitar]

I'm just trying to understand the physics here.

My Comins GCS-1 had to be retuned several times a set. The low E was especially hard to tune, with the tuner display wobbling.

One issue was that I had strung it very light. 11 13 16 22 32 42. Fender strings. (9's, toss the 9 and buy a 13). This was an accommodation for arthritis.

So, I decided to go back to 10 13 17 26 36 46.

Tuning is now stable.

Why exactly is that?

[sgosnell]

Possibly the nut slots are too wide for the lighter strings, and the strings move in the slots, making tuning unstable. This is more likely if the slots were originally cut for heavier strings, maybe a .012 or .013 set. Impossible to know without examining the guitar in person, though.

[AllanAllen]

Could be the nut slots, also you can strike the string so hard it goes sharp.

I would say lighten your touch and see if that helps, but you already changed the strings.

Could also fret the guitar so hard you pull the strings sharp.

[Al Haig]

The first set has the tension distributed extremely unevenly, that could be it. 11 e is way higher than 42 E.

[sgosnell]

The lower tension of the lighter strings is certainly a possibility.

[rpjazzguitar]

I was thinking that it has to do with the string tension, but I can't quite understand how that works.

With the lighter strings I'd get them in tune using an electronic tuner, play for a little while and then checking tuning. The G and low E would usually be out of tune and the rest might be out of tune.

The low E would look unstable on the tuner whenever I checked it -- I always felt its tuning was approximate, but with the heavier E it tunes well.

With the heavier strings, I played two sets of big band and barely touched a tuner.

So, it appears that playing the guitar (or just the passage of time) made the lighter strings go out of tune. But when I thought about it I couldn't see why. Pitch is dependent on mass, length and tension. Once I tensioned them properly, what changed, and why? Is the lighter string elastic?

The nut slots were probably cut for the strings it came with, which were 11s. But wouldn't that be a tuning problem for strings that are too wide (rubbing too much in the slots), not strings that are too narrow?

When I changed to the heavier strings, I removed Fenders and installed D'Addarios. Unwound G's.

How's this for a hypothesis? The heavier strings demand higher tension. The higher tension exerts more force on the bridge and stops it from wobbling on the studs. The tunamatic slides easily onto the posts. The ease of installation means it's not a super tight fit. I shimmed it and it seemed okay, but maybe it was wobbling enough to make the guitar out of tune.

[starjasmine]

Could the D'Addarios just be better strings? Could you have gotten a bad pack of strings?

Could lower tension from the lighter strings make them get near enough to the pickups for the magnetic field of the pickup to interfere with the vibration of the string? (i.e. less tension on the neck resulting in slightly lower action/nearness to neck pickup.) I've heard that strings too close to the pickups can cause wobbly tuning.

I guess the apples to apples test would require swapping in a new set of Fenders that are the same gauge as the D'Addarios that worked well. Or trying to reproduce the problem with a lighter-guage set of D'Addarios.

[Al Haig]

The low E was lower tension compared to the rest of the set so it would tune more shoddily.

[rpjazzguitar]

The low E was lower tension compared to the rest of the set so it would tune more shoddily.

I think that’s right, but what is it about a thinner string that causes it to lose tune?

[Al Haig]

I would say it interacts worse with the tuner and the friction points in the set up, maybe tuning up inaccurately in the first place, getting bound or something and then slipping. Plus the average tension of the set or higher tension of the strings next to it could make the neck react funny and make it more likely to go out.

If the whole set was balanced to the tension of the 42 it would probably tune up better in my estimation.

[aquin43]

One plausible explanation:
The string is fixed at the tuner end by the capstan effect where the tension of the string is gradually absorbed along the length wrapped round the post by friction with the post. The details of this depend on the friction between the post and the string, the stiffness of the string and any friction between the turns of the string round the post. As long as these add up to the string tension, the string can be brought into tune. Unfortunately, there are many detailed different configurations of this sort that look identical and can support the tension but the only really stable one is the one where the string has maximum contact with the post. All of the others can slip from one to the next, detuning in the process. After first tuning, if you pull hard on the string while holding the guitar head, you can pull the turns tighter, requiring retuning. Two or three repetitions of this usually achieve the optimal state where pulling will not detune.
With lighter strings, the tension is smaller but the line of contact with the post is more or less the same so a lighter string is less likely to fall naturally into the optimum state.

[icr]

The 'physics' of it is that it will never be in tune. Not only is the guitar tuned to equal temperament, each string vibrates with a different effective length for the same fretting distance. On top of that the strings are stretched various amounts when they are pressed down. Add in potential for variability in string mass along its length and one may as well just give up on trying to tune the darn thing...

[sgosnell]

Higher tension holds the string in place more firmly, regardless of the width of the slots, either bridge or nut, preventing it from moving sideways. Wider slots allow the string to move from side to side, which affects tuning. It's probably a combination of slot width and tension. It's also possible to have tuning problems if the slots aren't cut correctly, and this can apply to both bridge and nut slots. The takeoff point for the string should be the inside edge of the slot (inside meaning toward the fretboard), but sometimes it may be just a tiny bit (or even a lot) behind, which causes multiple problems. Without playing the guitar, I can't say for sure what the exact combination of causes is.

[AllanAllen]

I still think you were hitting it too hard and pulling it out of tune. Light strings are hard to tune.

[rpjazzguitar]

I still think you were hitting it too hard and pulling it out of tune. Light strings are hard to tune.

How exactly does that work? Is it that the string is elastic? Stretches, and maybe bounces back, unevenly? Or is it the anchoring at the tuner? When the strings went out of tune, I think they were usually flat, not sharp.

Maybe the lesser tension on the neck allowed it to wobble more?

[Al Haig]

I still think you were hitting it too hard and pulling it out of tune. Light strings are hard to tune.

Nah, it's the wonky different tensions of the string set. Looser strings are less stable and harder to get in good tune, tighter strings are more stable and easier to tune. Even a balanced set of 9s would tune up better than a 9 E and an 11 e.

[sgosnell]

One other thing that causes tuning problems...

When the string is tuned from above, lowering a sharp pitch to the correct pitch, the string, because of its stiffness, doesn't completely unwind immediately. As it is played, or pulled, it tightens up on the capstan and goes flat. This is more likely with lower tension. So I recommend doing as Johnny Smith recommended, and give the string a good pull near the 12th fret before tuning from below the desired pitch. This helps insure that the string is tight on the capstan, and remains tight when brought to pitch. I always tune from below the proper pitch after pulling on the string, then pull again after tuning, just to make sure. Strings stay in tune much better for me if I do that.

[rpjazzguitar]

One other thing that causes tuning problems...

When the string is tuned from above, lowering a sharp pitch to the correct pitch, the string, because of its stiffness, doesn't completely unwind immediately. As it is played, or pulled, it tightens up on the capstan and goes flat. This is more likely with lower tension. So I recommend doing as Johnny Smith recommended, and give the string a good pull near the 12th fret before tuning from below the desired pitch. This helps insure that the string is tight on the capstan, and remains tight when brought to pitch. I always tune from below the proper pitch after pulling on the string, then pull again after tuning, just to make sure. Strings stay in tune much better for me if I do that.

I knew that you should tune "up", but this is the first I've heard about the underlying mechanics.

But why is it worse with lighter strings?

[aquin43]

Perhaps it is because the contact areas promoting friction fall in proportion to the string diameter and the tension falls in proportion to the square of the string diameter. Therefore the ratio of friction to tension rises as the string diameter falls, at a fixed tuning frequency.

[Cunamara]

I knew that you should tune "up", but this is the first I've heard about the underlying mechanics.

But why is it worse with lighter strings?

Lower tension so the string doesn't pull or release fully over the friction at the nut, I think.

[sgosnell]

I think the loosening on the capstan when the tension is lowered is the result of two factors - tension and string stiffness. They aren't the same, nor mutually exclusive. Lighter strings can actually show less of this because of less stiffness, but only if the tension is high enough to keep them tight. Heavier strings are stiffer, but the tension is usually higher. But tune heavy strings to a pitch which has low tension and they will require lots of pulling to get the wraps really tight. Lighter strings tuned to a higher pitch, with high tension, should be less affected, but the effect doesn't go away completely. There is always some friction from the slots, at both ends, and friction has a greater percentage of effect on lower tension strings. It doesn't take much difference in overall string tension to cause an audible change in pitch. At least that's my opinion, and it's worth every penny you paid for it.