Truss Rod Physics

[bluesoverlord]

I'd like to get everyone's idea of different truss rod designs work. I'll tell you mine, then I'd like you to fill in any holes in my knowledge base.

When I first read about truss rods the explanation said that the bow in the rod was in the same direction as the force you want to exert. So, if the rod is bowed toward the back of the neck the neck will pull backward and, thus, against the pull of the strings. The reasoning was that the rod "wants to straighten out." I couldn't get my mind around that at all, especially when I started to work with this thin piece of steel. This explanation was useful for remembering how to put the rod in and which way to turn in, but it was just the plain wrong way to explain the physics of the rod. I find that if I don't understand the mechanism I wont be happy.

So......I read further. I found something that made more sense (for a single rod). The explanation was that the as the nuts are tightened the neck wood is put in compression between those two points. The neck yields where the wood goes in to compression. The real understanding comes into why the back of the neck yields. And the next question is why the rods have to be shaped the way they are in certain guitars (if they really do). And where on the axis of the neck should the rod be placed.

Now, the way I understand it is that when a rod is placed toward the back of the neck (away from the fretboard) it will compress the back wood more than the front and produce backbow. A single rod placed in this position can only do this one thing. So, if that is true, then a rod could theoretically be placed in the back of a neck for its whole length. Does this mean it can be straight? And then could make the access on the back of the headstock if you wanted? The problem here is that my mind then wants me to believe that bow would be created in the direction of the front of the neck. Actually if I think of it on a very small scale, I think of the first molecule of a turn that bends the rod EITHER way and the smaller amount of wood on the back of the neck would yield. And if we kept going and the rod never broke, would the neck backbow and thus take the rod with it into a backbow (assuming the fibers of wood had infinite compression without fracturing)?.

Now the other thing that got me thinking about this is the format of of the channel in other guitars like Les Pauls, or even a Benedetto. The LP has a straight channel (at least it does in Gil's build...this is where I got thinking about this stuff), but it is angled towards the back of the neck and puts the back of the neck in compression. Which way would the rod bow if it went out to infinity in compression? The LP proves you don't have to start out with a bow if the position of the rod is correct.

Benedetto likes a little curve at the upper one third of the neck, but straight towards the body. Those rods have their axis towards the back of the neck.

So....you can have a straight single rod behind the mid axis of the neck and get back compression, therefore, back bow. Not sure which way the rod would bend.

You can have a bowed rod with most of its mass behind the mid axis of the neck.

This brings us to dual rods. This to me CAN work on the same principle of compression of the neck, but can do it in either direction. If the rods are placed centered around the mid axis of the neck, the work they must do to compress the wood in either direction is equalized. If I move the rod toward the back the compression of the back wood (less mass) becomes easier and the power needed to backbow would become less. However, it seems to me that the power needed to frontbow would increase. If you moved the rod toward the front (fingerboard) the opposite would be true.

What do I get from all of this?

If I choose a single rod system, the rod will only apply backbow forces to counter the strings. The further back (deep) the rod is the less work it must do because it is compressing a smaller amount of wood. It doesn't need to start of with a bow (correct?). It could be accessed on the back of the headstock if you wanted (correct?).

If I choose a double rod system, the rod can bow either way, so its best to keep it as close to center as possible if you EXPECT to need backbow and possibly frontbow during the life of the guitar, e.g., heavy strings changed to light strings, humidity changes, age, etc. However, it seems to me that the double rod could still be placed deep because the power NEEDED to frontbow would be SHARED by the rod and the strings. Therefore they could more easily compress more wood on the front. The trick is to know where that balancing point is.

I hope all that made sense....OR.....that someone can at least set me straight.

Thanks,

 

[Bolide]

Think of a guy wire supporting a telephone pole or a sailboat mast. The greater the angle to the pole the more supporting force it can give, with less force compressing the pole. But this angle comes at a cost, and that cost in the case of guitars is neck thickness.

Fender starts out his production line thinking he can produce thin necked guitars without truss rods. Turns out he thought wrong. Needs good fix fast.

Somewhere, I don't know where, a brilliant idea comes: Curve the truss rod channel, curve the truss rod.

This allows Leo to continue putting out thin necks, which turn out to be a big selling point; by curving the truss rod the rod gives some resistance to neck twist, more than a straight pull rod or a double rod does, anywhay; and because he has another scathingly brilliant idea he puts more curvature toward the headstock end than the heel this gives stronger action where more action is needed, away from where the neck is anchored (Thanx, and a tip-0-the-hat to Jack Wells and Joe Desperado).

 

[guitarbuilder]

"Somewhere, I don't know where, a brilliant idea comes: Curve the truss rod channel, curve the truss rod."


I think Gibson may have had the Truss rod patent first....not totally sure but I'm leaning in that direction.

Yep from wiki
The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921 [1]

 

[Bolide]

"Somewhere, I don't know where, a brilliant idea comes: Curve the truss rod channel, curve the truss rod."


I think Gibson may have had the Truss rod patent first....not totally sure but I'm leaning in that direction.

Yep from wiki
The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921 [1]

I was hoping someone knew the answer
Pretty good bet on the TDPRI.

 

[bluesoverlord]

Ahhhh....the light bulb popped on with the idea of the rod helping with twist a little and also that a DIFFERENTIAL of force can be designed into the curve.

 

[Jack Wells]

I think your "compression of the wood" explanation of single action truss rods iis wrong.

 

[Joe Sailor]

I have wondered if a "T" shaped truss, with the vertical leg up towards the fretboard, would give the neck greater stiffness than a curved rod. Placed in the back of the neck

 

[Engraver-60]

I don't stiffness is as much an issue as adjustments. Thoughts?

 

[Jack Wells]

Here's an explanation of some commonly used truss rods.

Truss Rods - How They Work - Including The Bi-flex.

 

[Bolide]

I agree with mgdesigns, lower gauge strings have less tension but need more relief than higher gauge strings, a stiff reinforcing beam would limit the choices of string gauges for a particular neck.

 

[TRexF16]

I think your "compression of the wood" explanation of single action truss rods iis wrong.

Me too, but what do I know. "The rod wants to straighten out" is a perfectly understandable explantion to me.

I am by no means an expert on this subject, but I thought a lot of non-Fender style rods, whether single or dual action, are not really anchored into the neck at all (so they can't produce any longitutional compression), they just kind of lie there in their routed channels and the rods themselves bend in response to turning their adjustment nuts, bending the neck with them. I have not used these systems but it sure looks like that's happening from the many build threads I've perused.

I'm probably completely wrong but interested in being educated on this...

Rex

 

[TRexF16]

Oh, and BTW, does the Fender truss rod channel really have a different radius curve near the headstock than back at the body? I thought is was a even curve and as far as I know that's how I made my routing jig. But I just copied the radius on the .pdf diagram of the neck side view, I didn't actually measure the radius.

Thanks,
Rex

 

[Jack Wells]

I'll try to give a little better explanation of the single action truss rod than that given in the thread I linked to above.

A single-action truss rod sits in a curved channel. It is anchored at one end and has an adjusting nut at the other end. Tightening the nut on the rod would shorten the distance between the anchor and the nut if the rod were not installed in the neck.

With it installed in the neck this shortening of the distance between anchor and nut cannot occur. Since the distance between anchor and nut is fixed, tightening the nut shortens the amount of rod between anchor and nut thus straightening the rod. This results in an upward force being applied to the center portion of the neck. This corrects excessive relief.

......

Yes ........ the truss rod curve on Fender necks is made up of two different radii as shown on this Fender blueprint. Is it necessary ....... who knows? It probably determines how the neck bends when adjusted.

 

[Engraver-60]

Oh, and BTW, does the Fender truss rod channel really have a different radius curve near the headstock than back at the body? I thought is was a even curve and as far as I know that's how I made my routing jig. But I just copied the radius on the .pdf diagram of the neck side view, I didn't actually measure the radius.

Thanks,
Rex

Rex: I created a CAD drawing for my one-size-fits-all jig and upon inspection of both sides of the tele curve it was 148.0" radius; and the P-Bass is 248.0" - but consistent curve on both sides of the ramps.

As a tangent (I'm good at those) - Martins of old had non-adjustable steel rods in the necks, and the may have kept the necks stiff, but warpage was not rectified except for a neck reset (remove neck & reshape the dovetail joints).

 

[crazydave911]

Truss rod physics? , man, I need to get back to my builds

 

[bluesoverlord]

Jack, I just wrote a long post with my attempt at a proof. It went poof somewhere in the ether of the Internet. Bottom line is that I think the tensile force of the rod is what we are experiencing. I think we are just describing it in different ways. In your case the rod shortens between two fixed points and has a force to push the neck. That assumes the neck does have two fixed points. However the neck is not immovable. The rod shortens and the two points come closer together. The nut is moving down the threads after all. You are saying that in becoming shorter the rod is pushing the channel. And I say that the neck yields to the tensile forces along the rod and the rod straightens as a result.

It is that exact tensile and compression forces that would make the rod "stiffer" that would allow it to push the neck as you are alluding to, e.g., think how flimsy the rod is outside of the neck. You couldn't push someone over with that rod unless you put it under tension or compression. That tension and compression is a function of the strength of the thing holding it. If my arms are holding it then it probably won't get that stiff. If a piece of angle iron is holding it then it could be extremely stiff.

A neck is going to be like my arms, it's going to yield before the rod is made stiff enough to push on the side of the channels. That's my reasoning why i think the rods tensile force on the neck putting it into compression is more important than the rod's compression forces pushing the neck into tension. On the other hand, they are probably both working.

I know from studying bow physics that different woods have very different tension and compression properties. If the force is as Jack describes, this must be a compression on the rod which pushes on the neck from the side of the rod and forces the neck into tension on one side and compression on the other. A tensile force along the rod (think of trying to stretch the rod) is going to put the force on both ends of the neck and force the neck into tension and compression.

If the rod was inn the midline the neck would bow according to the properties of that wood, e.g., heartwood vs. Sapwood, orientation of grain, etc. Perhaps it is jack's compressive force that "pushes" the neck which is under tensile forces, in a direction away fromthe bow of the rod.

Again, I think these forces are working together. I'd just like to understand them. To simply say the rod is push in the neck can't be all theree is to the explanation.

 

[Jack Wells]

The rod shortens and the two points come closer together.

I do not accept your premise.

You seem to be saying that the curved channel is not necessary. If a single action truss rod was put in a straight channel, as you tightened the adjusting nut and put the wood under compression you would hear a sickening sound .......... a loud snap ...... as your truss rod broke.

 

[davmac]

Jack, I think you might be missing a key bit of information. The straight channel compression truss rods do exist and were in use long before the curved channel single action. This straight compression style is the one that was patented by a Gibson employee in the early 20s. The rod sits really deep into the neck and as it compresses it shortens the back side of the neck forcing the neck to bow against string tension.

Here's a picture of a cross section...

image removed

And a good explanation with more cross-section pictures at (link removed)

Edit: ...and I didn't mean to imply that this was an out-dated approach either, several manufacturers still do it this way. That picture is a modern Taylor.

 

[bluesoverlord]

Jack, I think you might be missing a key bit of information. The straight channel compression truss rods do exist and were in use long before the curved channel single action. This straight compression style is the one that was patented by a Gibson employee in the early 20s. The rod sits really deep into the neck and as it compresses it shortens the back side of the neck forcing the neck to bow against string tension.

Here's a picture of a cross section...

image removed

And a good explanation with more cross-section pictures at (link removed)

Edit: ...and I didn't mean to imply that this was an out-dated approach either, several manufacturers still do it this way. That picture is a modern Taylor.

In this case the rod is placed deep in the neck so that when compression is applied to both ends of the neck the thinner amount of wood on the back goes into compression and the front wood goes into tension. If the rod was closer to the fretboard then the opposite would happen. The position of the rod was chosen carefully and for a reason.

I would guess that with a curved rod you would have this same compressive force, but ALSO probably have the force of the rod trying to straighten and thus pushing the channel, as Jack describes. I'm just wondering the relative values of the two forces. I see them as two different axis, but working in concert to bend the neck. The curved rod would help cut down a third force, or axis, which can translate into twist or curve. Are there more dynamic forces? Just look at the shape of boards as they dry. Wood is a very dynamic thing.....that's one of the reasons I love it!

Thanks for getting me thinking about this Jack. It was your original post that made me interested in this topic. I appreciate your expertise on this forum. My disclaimer: I may or may not be wrong. Always question boldly!

 

[TRexF16]

What about all the truss rods that just lie in the channel and curve when you tighten the nut, BENDING the neck directly. They aren't anchored into the necks and thus don't put anything into compression except as it is isloated within their own mechanisms to create the curve. Seems to me if the objective is to bend the neck the best way to do so is to directly bend the neck against the force of the strings, as just described, not through longitutional compression. Like this one does:
(link removed)
or this one (I think, never used it):
(link removed)

enquiring minds want to know...

Thanks,
Rex