Truss rod -- very minor nudge has big effect on playability?

pdg

Has anyone noticed, or have any explanation for, the following: Sometimes it seems that the very slightest nudge of the truss rod nut -- almost immeasurable -- greatly affects the feel of string tension and the tone, even when it's impossible to measure any change to the neck relief or the action. For example, before adjusting the truss rod the strings feel too "sloppy" and the first string particularly doesn't have any "oomph." After an almost imperceptable nudge to the truss rod in the direction of increasing the neck relief, the strings have more "rebound," require a little more fretting force, the guitar is way easier to play properly, the volume is greater, and the first string now excites the body of the guitar. I can't measure any effect on neck relief or action resulting from such a miniscule nudge of the truss rod.

I was wondering whether the effect is not so much any effect on neck relief and action, but rather on how, with the rod barely turned, the neck responds to pressing down on the strings and/or how the truss rod internally interacts with the neck.

ChrisMartin

Well my guess is that you are just effectively 'pre-loading' the neck assembly.  From my old days working on race cars we used to 'pre-load' the suspension by winding a threaded collar up against the coil springs, often not enough to affect the ride height but just enough to take the initial bounce out of it so when a load is applied through the suspension the response is instant..  So my guess here, is that in its neutral position the truss rod is just a metal rod lying in a cavity inside the neck without any significant tension applied. It may even be a loose enough fit in the groove such that it is not in contact with the wood all the way along.  So then if you add a little turn it may be just enough for it to tension itself against the surrounding wood without going so far as to bend it, but enough to make the neck assembly locked into one solid piece and therefore more responsive.  If I am right this effect would apply more to some guitars than others, and probably not at all with some as it would require a certain amount of tolerance within the truss rod assembly and the routed groove it sits in, and if there is already enough tension built in it may not have that same change in resonance by applying a small turn.  Anyway, just my theory as a retired greasemonkey.

Bones

Is the neck not very stiff.

Truss rod adjustment should not really effect string tension.

AndrewUlle

@Chris Martin ’s comment makes a lot of sense, in more ways than one. Taking up slack in the truss rod could also (besides simply stiffening the neck with no change in geometry) in effect ‘unify’ the two parts of the neck assembly, improving the transmission of vibration. Lastly, since different materials have different frequency responses, the amount of contact between the rod and the neck, and the dampening affect of tightening the truss rod could affect the response also.

mac63000

I recently went through a guitar setup experience that I think backs up what @Chris Martin said about a truss rod sitting under no tension and its impact on playability. My guitar came with a really, really straight neck, which never bothered me until recently. I've had it for about a year and around the start of September it developed this rattle that would occur on certain notes. We had weird weather over the year I've owned it, but I discovered through process of elimination that the truss rod vibrating in the neck. Naturally, I panicked a bit (okay a bit more than a bit) and thought the rod had come loose and that the guitar would need a major overhaul to fix it. It turns out all it needed was some relief put in the neck and no more rattle. I had the guitar professionally set up (thank you @Michael Horowitz for sending me to John Saba) and now the guitar plays and sounds better than ever! I did a bit of research and found that I have a dual action rod, so my guess is that it was sitting their under no tension, given how straight the neck was.

It definitely resonates more freely and is easier to play. But I also had another bridge fitted to bring down the string height, which has more of an effect on string tension/action than truss rod, I would think. But if your neck is too straight, or even going in the wrong direction, that could make your instrument very difficult to play. Not to mention, it probably wouldn't resonate properly. Either way, it kinda makes sense that a metal rod under no tension vs one under correct tension would resonate differently. The guy who did the work on my guitar said that these guitars are made to resonate almost to a default, so anything that's slightly off could have a big impact on the system as a whole, the neck included.

Bob Holo

Chris' point can be true if your truss rod is very loose, depending on the type of rod. Never loosen a compression rod entirely - particularly if it doesn't have a captive nut. It'll affect sound and playability for the reasons he & Andrew stated and also the nut might fall off inside the guitar which is a PITA ;-) The nut should always be at least 'lightly snug'.

Given what you described and the time of year, it's likely that your zero-fret is a little low for your current setup and increasing the relief raises it relative to the other frets just enough. Zero fret height is the equivalent of nut height on Western guitars. The old European GJ setups had big zero frets and nearly flat fingerboards and higher actions. This makes it a bit harder to play, particularly in the lower frets, but it covers little warbles in the frets well, which is good because the tools and methods for measuring setup and the fret-plane were a bit more primitive back in the day. Modern setups tend to have lower zero frets and a more gradual bend in the neck that increases as you approach the zero fret and the tolerances tend to be finer, allowing for better playability, intonation and action in the lower frets. But setups change as the weather/humidity changes and strings change and player technique changes, so it could be any one or more of a few things.

Anyway, if a small truss loosening yields a lot of apparent change in the way you stated, you're probably right on the edge of having a zero fret that is a little low for your current setup. Before altering the zero fret, check whether your humidity is too low, because low humidity shrinks the top, and in the case of GJ guitars, that shrinkage lowers the top arch a little which lowers the bridge and hence lowers the action which can tip you into an edge case like that. The top moves a lot more than the neck when temperature and humidity change so if there is a humidity problem, address it first, because top movement is more likely to tip you into an edge case like this than neck movement (unless the guitar is old and the zero fret is heavily worn or the truss was completely loose to begin with) Because if you raise your zero fret to accommodate a problem that's caused by low humidity, you'll have a beast of a time dialing in a comfortable setup when humidity gets back to normal.

Buco

Ha I was looking for a time to jump in with my two cents when I read @Bob Holo comment. Zero fret is what I was going to mention. Because what you described @pdg is exactly the kind of change in the sound and feel on my guitar when I raised a zero fret. I did it with a shim made out of stainless steel. I think it's a 0.02", I need to double check. It was supposed to be a temporary solution until I replace the zero fret but I've had it more than 2 years now. And I did it after reading Bob's comments on zero fret effects. The change it made was exactly what I was looking for which the original post describes perfectly. The biggest change is in the feel on the first fret. Prior to the raised zero fret, when playing F6/9 chord it just didn't have enough oomph which it does now. Kinda like a difference between relaxed and flexed muscle. Then there could be occasionally some hint of buzz on the high E which is gone. And the entire guitar seems more excitable now. By the way my guitar doesn't have a truss rod at all and the neck has barely any relief.

Bones

Mac, I always put a little bit of preload on the truss rod before I level the frets. That way the truss rod can still be adjusted (tightened or loosened) and you still won't end up with a totally loose truss rod. That way the truss rod effectively is double acting and will never rattle or buzz.