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Wood Beneath Truss Rod.


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I seem to remember having seen it somewhere here, but I can't find it.

So, how much wood do I have to leave below the truss rod?

Yes, I can imagine that it is wood type dependant and what not, but like in general.

2mm - 3mm.. I'm using StewMac's Hot Rod and the last (first) I made was slightly

too thick to my taste, so I hope that I can make my next thinner. But I would very

much like to know how much I need so I don't break my neck... :D

Thanks!!

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I seem to remember having seen it somewhere here, but I can't find it.

So, how much wood do I have to leave below the truss rod?

Yes, I can imagine that it is wood type dependant and what not, but like in general.

2mm - 3mm.. I'm using StewMac's Hot Rod and the last (first) I made was slightly

too thick to my taste, so I hope that I can make my next thinner. But I would very

much like to know how much I need so I don't break my neck... :D

Thanks!!

I'm using the Hot Rod on my necks too and these are 16mm (sans fingerboard) deep at the first fret. The hot rod is about 11mm IIRC, so that would leave 5mm of wood behind the rod.

I think you can safely go down to maybe 3mm at the first fret. If you want a neck that's significantly shallower then you might need to use a different TR. I have read here that the Gotoh two-way rod is shallower than the hot-rod.

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3mm (1/8") MINIMUM, not much more than 5mm max. The closer it is to the back of the neck, the better it works (away from the neck's neutral axis).

I have a bunch of Allied Lutherie dual-action rods which are thinner and work at least as well as the hotrod.

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When you get to the headstock, leave a volute (bump) in the place where the neck bends down for the headstock. This will give you some extra strength there, as that's where most breakage will occur. It sounds like it will look weird but it's that bad really, and it adds a lot of strength if you're doing a really thin neck.

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The Hot Rod is a dual action truss, isn't it? Since it works by pushing against the wood in the neck, depth is not as critical as a single action rod. I would just route the channel to a depth that looks like your leaving enough wood around the headstock joint. I wishes' suggestion of the volute is a good one. Single action rods have to be mounted low in the neck for proper leverage, but double action rods just have to be mounted low enough (and with a strip of wood above them) that they don't crack the fretboard off under tension. I'd be worried that if I routed a double action slot too deep, it might pop out the back of the neck!

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Ok then, I guess I'll just bust out and say that I like single action truss rods better for two reasons. One is that they can be set deep in the slot with a filler strip above leaving lots of wood still in the neck. Secondly.....(deep breath)...I don't think that truss rods...work...by tension. Or, at least I don't think they work by pressing against the wood of the neck. I think a 'proper' truss rod works by balancing the pull of the strings on the other side of the thickest and most stiff portion of the neck, which is just below the fingerboard. A single action truss mounted low in the neck can compress that side of the neck because there is less resistance as the neck gets narrower into the C. Mounting single action rods under a curved wood insert is 'wrong' too, because it's still pressing against the wood. Pushing on the wood is bad because it changes the frequency response. It's always for the bad, because tension never lasts. Wood always 'relaxes' eventually. A single action truss rod that is only touching the neck at it's two ends is 'proper'. That way it is acting on the neck in exactly the same way the strings are, only it's balancing them out. The necks I've built this way have incredible sustain and even response. Of course, there's always the possibility that the sustain is due to some other thing, like wood. So, this last build, I made an electric with a CF reinforced poplar (!) neck and single action truss. Sustain? For days. If you think about balancing the pull of the strings, you can see that a single action rod puts only lengthwise compression on the wood of the neck, which leaves it more free to move as one piece with the guitar.

I know what I'm saying is controversial, but my limited experience so far is supporting it. Just the fact that so many people accept the double action theory on it's face should at least throw up some red flags for some.

Sorry for the rant, thanks for reading!

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Ok then, I guess I'll just bust out and say that I like single action truss rods better for two reasons.

I am not sure I understand your theory about the rod not bearing on wood. If it doesn't bear on any wood, it can't do anything.

The good thing about a double-action rod is that it can correct backbow if the strings can't pull the neck out of backbow. I built a bass and I guess the neck moved on me into a slight back bow. It would have been hopeless, but I had a double-action rod in it. The backbow was probably my fault (some flaw somewhere down the line), but the rod corrected it for me.

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Here's a bit from FRETS.COM by Frank Ford, he 'splains it better than I did;

The basic action of the standard adjustable truss rod is by compression of the back of the neck. When the nut is tightened, the back of the neck is compressed and causes the neck to bend backward. It is really quite predictable because the back of the neck has a much smaller cross-sectional area, so it compresses much more easily than the front. If the truss rod were mounted closer to the fingerboard it would have to pull harder to compress the back of the neck, but it could still do the job, because the fingerboard is very dense material and really resists compression.

Frank is a repair guy, not a guitar builder. On his site he explains all the types of truss rods, but doesn't weigh in heavily for one or the other...unless you read between the lines :D

It's really a moot point. Double action rods seem to work acceptably well, just that my ears (professional piano technician) say they dampen the woods natural response. Now that I've made a neck of poplar, I'm ready to say that I think single action rods make a 'live' neck with greater sustain and more even response.

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The liveliest necks I've made (= livlier than any single action rod necks I've heard) have CF rods and a dual action truss rod. Takes care of wood relaxation/memory to a great extent, and gives you very resonant, stable, musical necks. ALL guitars are an exercise in tension; an acoustic is the prime example, made of very, very thin and light woods with a buttload of tension and little in the way of trussing. The tension here is good, because otherwise there'd be no sound. Want lower tension on the wood of the top? Congrats, you've built a banjo, which has a tensioned head (otherwise it'd be floppy, see) but little torque or string force on the soundboard. All I'm saying is tension ain't always bad....

Your theory on bearing against wood or not all rather like voodoo to me; yes, rods apply compression. Yes, the other part of the neck goes in tensions. But you can't have one without the other, and you can't have it without bearing on the wood (something dual action rods do less than single action rods, that are Usually glued into a tight channel.)

That's why rods - if they need to work as little as possible/work as efficiently as possible, should be placed with their bearing surfaces towards the back of the neck if you're looking for compression of the back. That's where the force acts, the rod that's actually turning is irellevant to the argument. Ergo, dual action rod that's not sitting dead center, but goes quite deep can work just as efficiently as a single action rod at the same depth. Flipside: for the rod to work the other way (the point of a dual action rod) you want to compress the fingerboard, and don't want the top (bearing surface for this bit) to be too far away from the fingerboard.

Pretty much all rods bear on the two ends- that's where the compressive force is applied. There's pressure in the middle on the tension side of the rod, but that applies to all types unless you're mounting it in a yawning cavity, and my thinking is that you risk rattles if the thing's not snug; everything's vibrating in there.

Edited by Mattia
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Mattia, I used to think exactly what you're saying now. Then, I thought some more....

But, like I said, it's really a moot point. You say, "The livest necks I've ever heard have CF and dual action rods". With the CF in the mix, it's a bit hard to point to the rod as the cause of the 'liveness', isn't it? Also, I like you're chutzpa! If a was a 'normal musician', and a professional piano tuner told me that he heard something, I'd be inclined to trust his ears over mine :D

Any way, don't worry about it too much. Like I said, if it sounds ok to you, it's good enough. I've been working professionally with sounds that other people don't hear my whole life.....

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I'm not saying the truss rod defines the tone, but I am saying it doesn't impact the tone negatively. Could it maybe sound better with a single action rod? I guess. Plenty of people feel single action rods sound nicer than dual action rods. But with CF in the mix (an improvement in evenness I do hear - gluing them in raises the resonant frequency of most necks (higher stiffness and mass compared to without) and also increases the sustain/decreases the rate of decay) I'm not willing to compromise on functionality - I've only ever built two necks that needed more relief when strung up, but they did need it.

As for sounds - you hit the nail on the head. What matters to me is what I can hear; I spend stupid amounts of time listening, tapping, feeling, rubbing bits of wood (for acoustics more than electrics, easier to excite them into vibration), trying to 'hear' what bits of wood are telling me. What others can hear doesn't really matter to me if I can't reproduce it ;-)

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