The stronger truss rod

[Paul Rhoney]

I've been wondering lately, which style of truss rod is stronger. What I mean specifically is between a traditional compression rod in a curved channel, a dual-rod with double-action, or perhaps others, what kind is capable of creating the greatest amount of pull against string tension? Assuming all other elements are identical, such as neck dimensions and wood, fretboard dimensions and wood, fret type, scale length, truss rod steel type, truss rod shaft diameter, etc., if the strings were pulling too much relief into the neck, what kind of truss rod would require the least amount of adjustment to pull the neck back to a more reasonable amount of relief?

[David King]

The answers really depend on your criteria. "The least amount of adjustment" is different from the easiest adjustment. Coarse threads move the neck faster but finer threads are easier to turn and are much stronger.
Titanium is very strong, almost impossible to break but it stretches like a rubber band so it's not particularly effective as a truss rod material. Some steels are very resistant to stretching but they can also be brittle so not much help if the threaded section snaps off.
To answer you question, I don't think anyone has thoroughly tested truss rod types to determine how they compare. I was thinking of doing some comprehensive tests for a GAL presentation but most folks seem to be so set in their ways I doubt if a clear set of data would actually persuade anyone to change much. If it works, why "fix" it?
You didn't even bring up the issues of weight or the effects on sound.

[Mark Swanson]

The thing is, a well made neck should not need a ton of tension from the rod to maintain it. Most of the time a plain old cold rolled steel rod is plenty strong enough to provide the needed counter tension. I think a double acting rod with the different threads on each end is the best design going, plenty strong and it gives adjustment both ways with a minimum of routing needed to install it. I'm talking about the type of rod that Allied sells.

[David King]

I've actually wondered about a truss string running down the back of the neck instead of a rod. You could simply adjust it with a bass tuner. If it snapped you could easily replace it.

[Paul Rhoney]

You didn't even bring up the issues of weight or the effects on sound.

No, I didn't, and I don't intend to. I remember there being a million some-odd threads about those topics on the old forum. I personally tend to prefer traditional single-action rods made from O1 tool steel in a curved channel, for weight and tonal reasons. But I wasn't wanting to get into any kind of debate about that sort of thing here. I have nothing against dual-action rods, they can be heavier sure, but they have their benefits. Really I was just trying to get at force/strength. Let's say you were building some kind of crazy instrument with a lot of really heavy strings, a long scale, and a skinny neck. You might use a laminated neck blank to increase rigidity, but maybe you don't. Either way, the tension from the strings is going to really pull on that neck right? So let's pretend like weight and tone don't matter (obviously we all know that they do, but this is a hypothetical question), you just want to make sure that that neck can be made straight. So what's the stronger truss rod on this case?

[Bob Gramann]

I've seen a couple of traditional rods where the wood at either end failed making the rod useless. A lot of force can be concentrated on a small area. For years, I used the Allied (Blanchard) rod. I really like the way it works. Then I got a batch where the threads galled. Allied stood behind them, but I still had to take apart of couple of necks to replace the failed rods. I have changed to the Martin two-way rod. It is strong and inexpensive. I buy them directly from Martin when they have a free-shipping sale. As a test, I attempted to remove one from the neck after it was installed. I was able to remove and replace it without removing the fingerboard. However, I don't expect that it will fail. The other advantage over the traditional and even over the Allied rod is that the force is distributed over a large area along the neck.

[Mark Swanson]

Paul, if I was going to use a neck with multiple strings- I have built a bunch of 12 strings and nine string guitars- I'd rely on the neck strength and not the trussrod. I build the necks with CF in them in this case, so the strings have a harder time pulling the neck into relief, and that means I can use a conventional trussrod because the force needed to counter the larger string pull is less, because of the stronger neck, so a regular rod works just fine.
I don't know if this answers your question, but that is how I see it.

[David King]

Paul,
The strongest/ most efficient rod is the one that can get the most mechanical advantage over the string tension i.e. the one that can be closest to the back of the neck and for the sake of argument then, one that can't move it's anchors. I agree with you that a single curved rod is the one I've migrated to.
I used a Martin rod in an early bass and it worked but it was so heavy I swore it off there after. Obviously they are fine for guitars that don't suffer the extreme imbalance issues most basses face from the get go.
I used double "Gurian" style rods for many years and found that it was difficult to get even pressure over the length. They weren't terribly efficient and they also had an occasional habit of popping out the back of thinner necks at their free end.

I also build very stiff necks and I've made the determination that every neck eventually succumbs to constant string tension though it might take 10 or 20 years before it does.

[Mark Swanson]

David, do you have any input on the use of Cf in necks, and how that affects the trussrod? Many bass builders use Cf in the necks and I am sure you must have some experience and thoughts on it.
When you use a double-acting rod, or a martin type, there is no need to place it near the back of the neck and you don't need to route it out as much. I like that aspect of those rods. The Stew-Mac Hot Rod is too big in my opinion, and is heavy and takes a lot of wood out to use it. The Allied rods are the best.

[David King]

Mark, CF will stiffen a neck if implemented correctly. Most of the CF sold for this purpose is not actually doing much because it isn't tall enough to have much structural effect. It wasn't until I migrated from two 1/4" x 3/8" spars to two 1/8" x 1/2" ones that I really got to see the benefits. These two spars are nominally the same stiffness but once they get locked into the surrounding wood with glue the 1/2" tall spar seems to have much more effect. I'm sure part of this is due to the fact that the taller spar has to be partially embedded onto the fingerboard and that somehow seems to compound it's effectiveness. What I noticed is that the single straight rods were not working well in these stiffer necks. (In my design the ends of the rod actually bear directly on the ends of the CF so that anchors can't compress wood without compressing CF first which seems unlikely.) The curved rods do seem to have an easier time of it but in my most recent implementation I decided to try a deeper curve just to see if if that helps at all. The rod has a deflection that's 1.5x it's diameter. This puts the anchors also up into the fingerboard just to keep the rod from coming too close to the back of the neck. It's a 6 string bass neck that's quite thin front to back, about .7" at the first fret or just under 18mm. So yes a neck can be too stiff for a given truss rod to operate effectively. I see it all too often in higher end basses that should know better. Of course I only see the basses that have some problem so perhaps the problems are rarer than they seem.

[Dave Higham]

It wasn't until I migrated from two 1/4" x 3/8" spars to two 1/8" x 1/2" ones that I really got to see the benefits. These two spars are nominally the same stiffness

Just a small (pedantic) point. The ratio of stiffness of your 1/8" x 1/2" spars to the 1/4 x 3/8" ones is 64 to 54. So they are 18.5% stiffer.
For anyone who doesn't know, the comparative stiffness of 2 beams of rectangular section can be calculated by multiplying width x depth x depth x depth of the cross section. So, if you double the width of the cross section, you double the stiffness. But if you double the depth, you make it 8 times stiffer.

But you all probably knew that. I'll get me coat...

[David King]

Oh, I thought it was only 4x stiffer so that explains a lot. Thanks Dave!