Anyone want any LP info?

[Mark Wybierala]

I'm replacing a neck on a Gibson Les Paul. It has a broken headstock beyond repair -- the third break and this one goes through the trussrod channel. I have the neck steamed off and I've removed the fretboard. I believe the guitar is from the late 80s or early 90s but I don't know the model. Three pickup carved top, bound neck with celluloid trapezoid pearl inlays -- I'm certain they are celluloid from the way one of them ignited from my heat gun. The body has a carved top but the finish is a very dark translucent plum front and back including the neck. The headstock overlay is black with the Golden Gibson logo and "Les Paul Model script -- its actually silver under the ambered clear lacquer. I can't see a ser# on the back of the headstock. If anyone wants any measurements or other information just ask.

[Henrique Schneiter]

Some pics would be nice! Also the weight of the body without the neck and the hardware.

[David King]

I'd be curious to know how deep the truss rod is from the fingerboard gluing surface at various points to determine the shape of the curve but this info may be readily available elsewhere.

[Mark Wybierala]

Sure, I'll do some pics and map out the trussrod curve. I came across a post on another forum from a guy who titled himself as a Gibson Engineer saying that the rod was not curved. I doubt that this is true but I gotta check anyway. Years ago, I built a neck with no curve in the rod thinking compression was the only function and it didn't work. The neck is a write off anyway so I'll find a way to be accurate with the trussrod measurement.

[David King]

Gibson did plenty of straight compression rods and quite a few with the curve inverted and most of them worked just fine. All the Taylor guitars have straight compression rods as well. I tend to think that it doesn't matter.

[Mark Wybierala]

David, This is something that I did not know. Quite some time ago I did a 36" scale dual trussrod bass neck and the instrument was a failure because the trussrods were totally ineffective so I assumed that all compression rods were curved. The neck was mahogany and the fretboard was ebony. The failure also resulted in the trussrods pushing up the fretboard right in front of the nut. Possibly I didn't get a good enough purchase under the trussrod nut washers -- a glitch that made me arrive at a false conclusion.

I'll see what I can do to get some real info on how this trussrod is employed. After I document everything, I can actually just open up the neck with no reservation on method. I'm going shopping for a neck blank today. I've just been real busy and haven't taken the time to stop and take photos on this project -- I will. Business has picked up quite significantly in the past month and that is a good thing.

[David King]

Mark.
Since the rods are there to counteract the string tension ideally a rod would be the same distance behind the neck's neutral plane as the strings are in front of it. The location of the neutral plane is dependent on many things. Most notably on the compressibility of the fingerboard wood vs the neck wood and the depth of the neck along with it's cross-sectional profile. If you stacked a maple fingerboard on top of a purpleheart and wenge neck with a very squarish back you had better put that rod as far back in the neck as possible. Meanwhile if you have a snakewood fingerboard on a mahogany neck with a vintage triangular shape you could probably put the truss rod right behind the fingerboard and still expect it to work.

[Mark Wybierala]

I think that I like the "idea" of the added reaction of the curve but if you simplify what the trussrod is doing, a straight rod should work just fine. Hmmm.

Its just when you think that you have established a rule for luthery that you have it proven to you that there are no firm rules at all.

[Barry Daniels]

Creating a curved truss rod slot is pretty easy so it takes almost no extra work. And it really enhances the effect of a single action rod.

[David King]

Barry,

How do we come to the conclusion that the effectiveness of the curved rod is enhanced? That seems to be the global consensus but I'm questioning it now because the actual math and physics look dubious to me.
There's at most room in a neck for 2º of induced angle in a guitar rod and half of that for a bass rod. If you apply the mass-hanging-off-horizontal-cable model the additional upwards force from a curved rod at those small angles seems infinitesimally small compared to the compression force being applied to the back of the neck which is doing most of the work. Granted every little bit helps if your rod isn't far enough back but that's a lame argument.

However easy it is for you to do a curved rod I would argue it is even easier to do a straight rod. I haven't found curved rods to be particularly easy.

[Barry Daniels]

I base my opinion on previous experience with both straight and curved rods, not any mathmatical calculations. In fact, I think trying to model this would be fairly difficult. But if you think about a curved rod in a captured slot...when you tighten the nut there is a direct straightening force to the rod. However, a straight rod when tightened has to use its position in the back of the neck to achieve its bending force which seems to be an entirely different reaction.

By the way, I think my truss rod curves more than 2 degrees. In fact, it is straight except for the portion between the 5th fret and the nut which curves up a bit over 1/16".

[David King]

Barry,

Thanks for the reassurance. I just did some tests on the two identical neck that have these two rods and in fact the curved rod seemed to act a little quicker and with somewhat less torque applied. (I'll know better when my beam torque wrench arrives.) It also appears to have achieved a straighter neck when over-tightened. The straight rod got me to an "S" bend with a bit of a hump at the 5th fret. Funny how empirical evidence always trumps speculation...

[Jason Rodgers]

Woa, more truss rod models for the GAL presentation, David? I'm really excited about that, by the way. I just put a HotRod in my first neck, and the depth required (7/16"), plus a safe bit behind the slot, gave me just under 7/8" at the 1st fret. For what I'm doing with this guitar, that might be ok, but the next neck needs a different truss rod solution to get the depth down to 3/4" at the 1st fret.

Barry, so you're saying that your curved-slot compression rods only curve up under the lower frets? For a 14-fret to the body acoustic-type neck, that's almost half the extension of the neck from the body; for an electric neck that attaches higher up, the 5th fret is only about 1/3rd of the neck's reach. I've only seen curved slots that hammock from both ends of the rod (in books and other online instructional resources), but if that's all the curvature that's needed, then cool. If you're saying the rod is about 1/16"+ shallower at the nut, then how deep do you cut the slot through the straight portion in the upper end of the neck? Does the slot make a gentle bow up, or straight?

[David King]

Jason,

Lots of rod choices to get you there and no doubt that the hot rod is the deepest setting of the collection.

The desired curvature of a curved rod is a complex creature to analyze. I'm about to give up on thinking about it and build an acrylic neck with a movable curved rod the depth of which can be fine tuned from above and below. Of course acrylic isn't wood but wood isn't see-through. Unfortunately once an acrylic neck is carved the curvature of the back will distort the image of the rod. Back to the drawing board.

[Jason Rodgers]

Sounds like you need a friend who is an x-ray tech with keys and after-hours privileges.

[Barry Daniels]

My truss rod has a gentle curve. They are installed about 3/16" from the back of the neck.

[Mario Proulx]

To quickly visualize how a curved, even slightly curved, truss rod works, just hold a string between your hands, leaving it slightly slack, like a curved truss rod. Now, move your hands apart, tightening the string. What happened? Right! The string straightened right up. Works the same exact way at taking-out the bow of a neck.

Much, much more efficient than a straight compression rod....

[Barry Daniels]

The curved rod also combines the straightening force Mario described along with the standard force that a straight rod has (i.e; using its position offset from the neutral axis). So it has two straightening forces combined.

I may be repeating myself.

[Jason Rodgers]

Barry, do you have a picture of your truss slot cutting setup? Is there a little ramp jig for a router pass?

[David King]

Maybe we should split this into a separate topic and put it under tools and jigs?
If Barry can do that I would welcome it. Otherwise we should start a new thread there.