Single action or dual action truss rod?

Why is it you are thinking about a single action rod?
I personally don't see why anyone might use them.

 

Tom;

Yes, the shallower depth at the headstock is one of the reasons that I like single-rod truss rods. Less metal, less weight, and with the right design, they are stronger.

Remember that single-rod truss rods can be built as either single-acting or double-acting. You've seen my single-rod double-acting truss rods, right?

It gets confusing, but there are four main types of truss rods:

Single-rod single-acting: The classic Fender, Gibson, etc. design

Single-rod double acting: Not as common

Double-rod single acting: Rickenbackers and other production instruments

Double-rod double acting: Most of the aftermarket rods, like the LMI, Grizzly & Stew-Mac.

They all have their advantages and disadvantages.

 

I have been using the single action U shape and really like them. They are strong and offer in my opinion extra strength, like having carbon fiber rods in a neck but all in a single unit. The only thing about using a single action rod is, you cannot get rid of the pressure bow the frets can cause. So now I make sure to turn the truss rod some before doing the radius on the fingerboards, to sand in a little back bow. So I am not sure how deep the rods you are using are compared to the one I am using. I get mine from Stew Mac.

 

I think I have never seen your SRDA rods Bruce, what is their height?

 

Yes, please share about the SRDA.

 

Basically, to make a single rod truss rod become double-acting, you add a rotating head assembly that can apply force in either direction. Here's the standard head assembly that I make up, although I make variations on it for different applications.

That first picture shows the parts and routing for a neck with the adjustment at the heel. It will get trimmed off at the line when finished. The same assembly is used up at the headstock for necks that have the adjustment up there.

The two brass barrels are connected together by the sawed-off socket head cap screw. That little stainless cross pin locks the screw shank to the rear barrel. When you turn the socket head with an Allen wrench, everything rotates together except for the aluminum cross bar. The faces of the two brass barrels form thrust bearings against the front and back faces of the aluminum cross bar. The rear brass barrel has the threads that thread onto the truss rod itself.

So, when you tighten it in the normal direction, the front brass barrel bears on the aluminum cross bar, while the threads in the rear barrel pull on the rod. The rod itself doesn't rotate. It's set down into a curved channel with a fixed anchor at the other end. Tightening the head pulls on the rod, trying to pull it straight between the aluminum cross bar at one end, and the anchor at the other end. That bends the neck backwards, just like a normal single rod single-acting truss rod. Turning the head the other direction works it all backwards. The threads on the rod try to expand the length of the rod between the two ends, with the inner barrel pushing on the back side of the aluminum cross bar. Because of the curved shape of the rod, it makes the neck bend forward.

That's how it works. There are a few other configurations that I've seen builders use, but they all use the same push/pull idea. I introduced this design in my truss rods in 2005, and I've put them into about 450 necks so far; my own basses and in necks for other Luthiers. In most of the installations, it will bend the neck back 1/8" and forward 1/16", which is plenty, without having to apply much torque. As far as I've heard, no one has broken one of these yet.

You can see the tradeoffs involved. These truss rods are more complicated to install, with more routing and fitting. In my installations, I cast them into the neck in a solid bed of hard epoxy. Most manufacturers aren't going to bother with all that. Double rod designs are most popular these days because they are the least labor to install, both for hobbyists and manufacturers.

Of course, Fender and Gibson stay with their SRSA rod designs because, well, they've been doing it that way for 60 years!

 

Bruce, what do you mean SRDA can be stronger than DRDA rods? Are you referring to the wrist of the neck, or just overall?

 

What makes a truss rod strong is the geometry that it uses to bend the neck. It's a little hard to explain in words, but it's the effective leverage ratio that the rod has against the wood. If it's installed correctly, a single-rod truss rod will have a larger effective leverage ratio within the same depth, than you can get with a double-rod truss rod. What that means is that, for the same amount of bending of the neck, the single-rod design will need less torque on the nut. That puts less load on the threads and less tension on the rod. Less chance of anything breaking.

It's like using a crow bar to pry up a heavy object. With a longer bar, you don't have to push down as hard.

A single-rod truss rod is set into the neck in a curved groove. That is, the rod "droops" in the center. If you draw a straight line from one end to the other, the vertical distance that the center droops down from that straight line is the important dimension. That dimension is what determines the leverage ratio that the rod has. The greater the droop, the greater the leverage ratio, and the more force the rod is able to exert on the neck, for the same amount of load on the threads.

From my many years of building necks and experimenting with truss rods, I've found that 0.180" is the magic number for the droop in single-rod truss rod installations. That amount gives the rod plenty of power to bend the neck backwards 1/8" without overloading the threads. If you install the rod with 0.120" of droop, you will probably strip it or snap it trying to bend the neck. If you make the droop much larger than 0.180", the rod gets more powerful quickly. But that makes the adjustment more sensitive. Tiny movements of the nut cause larger movement of the neck.

As I mentioned before, there are trade-offs with the different types of truss rods. Single-rod designs have some significant advantages, when they are properly installed. But they are trickier and fussier to install. That droop dimension has to be in the right range, or it isn't going to work right.

In comparison, double-rod truss rods are very simple to install. The mechanical geometry is all built into the metal parts. You lay it into a straight slot in the neck and it's done. That's why they are so popular for the aftermarket of hobbyist builders. Many manufacturers have switched over to them too, for the same reason. Less labor to install, and relatively foolproof.

But, one of the trade-offs is that, for the same depth of installation, the double-rod design has a smaller effective leverage ratio than a single-rod design. It's complicated to explain why that is, but it means that when bending the neck, the double-rod design has higher loads on the threads and the rod. It reaches the point of stripping or breaking sooner.

On double-rod truss rods, look at the distance between the rods. That's what determines the effective leverage ratio. The farther the rods are apart, the greater the leverage ratio is, and the more power it has. It's capable of bending the neck farther before it strips or snaps. The compact ones, with the rods right against each other, are weaker and the threads are more highly stressed. Things to consider when you choose. A shallow double-rod truss rod assembly is going to be easier to break than a deep one, no matter who makes it. It's simple geometry.

Someone is going to ask:
Well, if a single-rod design is stronger, why do so many Fender truss rods break? The classic single-rod single-acting truss rod design used in all Fenders has the right amount of droop. The rod itself is actually pretty strong. The problem with the Fender design is that the anchor points at both ends are weak. There isn't much surface area between the metal and the wood at the anchor end, or under the nut. When you tighten the truss rod hard, trying to correct a bowed neck, the wood at either end crushes under the metal. This limits the amount of tension that you can put on the rod, and how much it can bend the neck. Keep turning the nut, and the wood keeps crushing, and the neck isn't getting any flatter. Eventually, the nut runs out of threads and jams up on the rod. Keep twisting at that point, and you snap off the rod. Fender rods rarely break from tension; they get twisted off because the threads get locked up.

A common trick with bowed Fender necks is to add washers under the truss rod nut, when you feel the threads lock up. Okay, that will prevent twisting off the rod, which is a good thing. But it doesn't solve the overall problem. The reason that the rod ran out of thread is because the rod is smashing the maple inside the neck. And the reason that it's smashing the maple is because the rod simply doesn't have enough mechanical power to straighten out that much of a bow in the neck. Adding washers to give it some more thread doesn't make the rod more powerful. You may be able to coax the neck a little bit flatter in the short term, but the maple inside is going to continue slowly crushing. A bowed neck is a bowed neck, and needs to be fixed.

That's the most important thing to remember about truss rods: They are not capable of fixing seriously bowed necks. They are only able to adjust the curvature of the neck over a small range. Some percentage of all production necks are going to bow or twist, due to drying of the wood and poor grain alignment choice. Those necks can't be fixed by cranking on the truss rod. That's why truss rods get broken.

End of sermon....All rise and sing with me.....

 

Great post, Bruce!

Lutherie is such a tradition-driven practice that many luthiers I've worked with are very set in their opinions and are unwilling to even entertain other options. Conversely, you have some pretty radical ideas about construction, and you have the ability to explain and reason why instead of just chalking it up to luthier magic.

When you say single rod truss rods can have many advantages, what specifically are you referring to? Just the leverage ratio and decreased likelihood of breaking one from over-torquing? Where is the lowest point of the droop dimensions? Would you be willing to sell SRDA rods? They sound very intriguing, and I'd certainly like to try one.