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Up until recently, I’ve always used a traditional single action truss rod in my guitars. That is, until I had a problem with a reverse bow on a batch of necks that I made all at the same time. Not sure what caused the problem, but it lead me to consider using a two way truss rod in my next guitar. The problem is the weight of these things. I know that on acoustic guitars it’s important to keep the neck weight to a minimum while keeping it as stiff as possible. I was curious to hear everyone’s opinion and preferences regarding truss rods specifically steel string acoustic guitars.

there are a few ways to handle this. What style of rod are you using? I used the compression rod ( threaded rod gibson style ) and the flex style ( Martin ) I used to flex the single action rods a bit to get about a .010 back bow and level under tension. That way I had a relief built into the single rod. Today the 2 way rods are more efficient but the designs are all over the place. I will not use any 2 way rod with a welding nut. I had 2 snap and as an engineer the process of welding will affect the properties. I went with the Martin style of the captured cylinder method. The threaded rod is not welded on so there is no stress riser.
Choose the one that best fits your style. I have not had any failures on the martin style.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Serviceability of an instrument has to be very important on a builder's mind because stuff happens when it leaves your shops.

Always use a quality double action rod or you may regret it. The one exception is building historically correct pieces that either had no rod or a single action rod but if these were supposed to be players I still would use a double action rod. Sure folks get way with single action rods all of the time but I also see guitars that are unplayable without major servicing to correct what resulted from not having a double action rod. I make my living from builders who do not use double action rods you might say....

In the winter I service hundreds of guitars that have dried out. One of the characteristics of a dried out guitar is a neck that may go into back bow. For the guitars that don't have double action rods and that's not very many these days string tension may not be enough to pull a neck into a bit of relief. If a double action rod is not present the next escalation in remedies starts to get complicated and expensive because it's a fret dress to once again impart some relief in the neck.

I was a weight freak likely beyond what most folks have heard of here before. Every part that went on my acoustics was vetted for weight but not the truss rods I went with The Blanchard double action stainless rod that IIRC was around 119 grams but don't quote me it's been a decade since I built my last guitar.

This notion that an acoustic guitar neck should be uber light weight I would not agree with. Heavy and light necks both have their place and do different but not undesirable things. Some downright heavy guitars can be very decent sounding guitars too such as a Gallagher steel string or the Australian builder Greg Smallman who's classical guitars can be 7 pounds or so yet Smallman's are famous for their excellent volume and projection. Two examples of heavy instruments that kick ass.

My goal with my OMs was 3.6 lbs hence the weighing of all components including the tone wood selection. I even weighted finishes.... So as you can see weight was important to me but I would not be looking to save weight on something as important as a truss rod.

Although I never built with it the Martin two way rod is excellent and having serviced hundreds and hundreds of Martins with this rod I've never encountered a failure.

Just saw my friend John's post and I completely agree. When I used the Allied Blanchard rod all nut welds were tested in a ice first and inspected too and there is a lot written on this forum about checking the welds on these. As John says the Martin rod takes the weld issue off the table.

I don't consider the weight as much as the balance of the neck and body. For a large bodied Jumbo the weight of a double action rod may not affect the balance of the guitar in a negative way. I often build small bodied parlor type guitars and prefer to use a light weight single action rod. As John mentioned you can finesse it to give a little bit of "double action". Classical guitars that use wood of thinner dimensions and are lighter in weight use no rod at all (sometimes an ebony or other stiff hard wood is inlaid in the neck).

When I started building I did what John mentioned above, build back bow capability into the neck system. But now I use double action rods and I also make so that the rod can be easily removed too. The Martin rods are great. They are thin and reasonably light weight. If you really want to go light weight then look into Titanium rods.

I use a single action rod, LMI’s TRST and TRSTA. My process uses highest quality air dried wood and I create my playing surface with the rod slightly loaded so that back bend is so unlikely it’s been 25 yers since I’ve seen it. I have tried double action rods but found all versions too finicky in adjustment, requiring ongoing messing about. My necks rarely need further adjustment as time passes, but if they do the rod response is extremely predictable.

I also make necks with carbon reinforcement and no adjustability. Despite considerable experience, the guess work involved means that about a third of these have to re-engineered and refretted before leaving the shop. But once they are right, and assuming no one decides they want medium strings when I have set up for lights, these necks appear to be stable for eternity. I use 2 pieces 1/8” x 3/8” on edge about 1/2” apart.

In either system I warranty the fret path.

_________________
Bruce Sexauer, luthier
http://mojoluthier.com

Now that I think about it, why does no one make truss rods out of aluminum? Maybe 1/4" diameter to compensate for the lower strength (especially on the threads). Still less than half the weight of 3/16" steel when you account for the larger volume of wood removed, and much cheaper than titanium.

For me, weight is about the feel of the guitar, not tone (added mass in the right place is an improvement if anything). And certainly balance is more important than total weight. But with a light neck, you can have good balance and low total weight at the same time. Especially important on harp guitars, which have a second set of tuners out on the end as well.

It was quite distressing to find that six necks I had built were all suffering from a reverse bend. As I said, I’m not sure the exact reason for it, but it was at least 9 months between me gluing on the fret boards and me going the attach them to a guitar. Possibly the change in humidity, or perhaps the tension of the truss rod in the curved groove without any string tension counteracting the bend of the truss rod. Either way it has made me consider the two way truss rod for all future guitars. I do have to agree though, I can’t abide neck heavy guitars. They are just unpleasant to play.

If you had them under tension, then that's probably the reason. That is the main pitfall of the single compression rod. It's basically just a giant string, so it needs to be loosened with the rest of them if you're going to leave the guitar unstrung for an extended period. But then you lose the exact setting you had, so even the few players who are aware of the issue (and have the necessary wrench to adjust it) may be reluctant to do it. Two-way rods do still exert force on the neck wood, but it's a smaller lateral force rather than using a large axial force to induce curvature, so it's less likely to become permanent.

Aside from bad wood, this is the only reason I'm aware of that necks ever develop backbow. Thus, I don't like the idea of tensioning the rod before leveling the fretboard to allow forward adjustment, because then you need even higher tension under normal strung condition. Might cause even worse backbow when left unstrung, compared to leveling the board with the truss rod loose.

Dennis, no, the truss rods weren’t tightened with a truss rod nut, but they did have a small amount of tension created by the bending of the rod as it lay in the curved groove of the channel. I’m not sure if this tension was enough to cause the problem but maybe over a long period. It could also be that the fret slots were slightly too tight . That can also put a back bow in the neck when the frets are driven in.

Nah, that shouldn't be enough force to cause permanent deformation. Frets certainly can, but that should have been visible immediately. Or were the necks un-fretted for the 9 month period, and then you installed frets in all of them without checking the board level first? If that is the case, then try pulling the frets out of one and see if it straightens out.

What neck and fingerboard woods did you use? And have the necks been kept in humidity control or not? All the cases of humidity-induced back bow I've heard of involved ebony fretboards, and I suspect it's specifically curly ebony that's such a problem, because some of the side grain is aligned with the long axis. If you have a way to dry them out, you can see if it's a humidity issue. You'll probably need to get them dryer than the RH% where you glued them up, due to hysteresis.

3 were curly maple with a celluloid bound rosewood fingerboard and 3 were Honduras mahogany with the same celluloid bound rosewood fingerboard. They were fretted at the time I made them but the frets weren’t leveled at that time. They were in storage in the house but the climate was not monitored during this period. It was only when I came back to them to work on them that I noticed the problem.

I’ve tried a number of double actions rods, and liked them all just fine. I’ve never had a failure, but I know failures happen. So, it makes sense to buy rods with the lowest risk of failure.

My current favorite is the double action rod sold by Martin. It works fine, it is not crazy expensive, and nobody hears of any failures with these rods. Those are good attributes.

I thought I would share some photos, because I think there are some things that are useful to know about the Martin rod.

Here is the neck for the guitar I am currently building (not carved yet, dovetail not cut yet), with the Martin rod in place:



Here is the Martin truss rod and its adjustment wrench:



Here is the adjustment end of the rod:



Here is the other end of the rod:



As you can probably tell, this rod is round on the bottom, and flat on the top. So, it fits best in a channel cut by a round nose router bit. I used a ¼” diameter, 1/8” radius round nose bit for most of the length. At the adjustment end, the rod is a bit larger, so I used a 3/8” diameter, 3/16” radius round nose bit for the part of the channel that houses the adjustment section of the rod.

Some of the double action rods I have used have rectangular blocks at both ends, so they need a square channel. I like the fact that the Martin rod is a good snug fit in a round bottom channel. More wood, less air inside the neck. But, the square channel is easier to cut, for sure.

I have used a filler strip for most double action rods, in order to trap the rod in the channel as snugly as possible. However, I am going to go without a filler strip on this neck. The Martin rod really doesn’t need it.

actually the amount of tension to pull a back bow is not that much. When we are talking less than 10 thousandths of an inch. Many had done this and it had worked so that is a proven technique . Aluminum won't make a good truss rod as you would need a pretty large one to work. With today's carbon fiber and other methods you can make a pretty stout neck with little weight. The old compression fretted necks are very stable when fretted correctly.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Six necks? Ok I was not paying attention. If it was one neck that's one thing but six indicates to me something in the process, a systematic error if you will. I have no idea what that would be but that's what it sounds like.

I have a project car going on right now that I have not attended to in probably 6 months. A 1994 Mercedes E420. When she was running it was pure joy to drive. I had some spare time today to go have a look in the garage and both tires on the left side were flat.... Hmmmmm What could that be? If it was just one then I'd chalk that up to something I caught out on the road or something but since it's two on the same side then maybe there was something on the garage floor?

That's what I mean.

Using water based glues to attach fingerboards to necks is an over looked and very common cause of back bowed necks. This is why I use epoxy for this joint. It also will release with heat if necessary in the future.

Yeah, 6 necks with 2 wood combos pretty much rules out problematic wood. Glue expansion is a possibility, but I would expect the amount of backbow to differ between the maple and mahogany necks. Fret compression seems the most likely culprit, so I'll repeat my advice to peel the frets out of one and see if it springs back. Always check board level before installing frets.


I think 1/4" would be enough. Mild steel and aluminum have about the same yield strength, and 1/4" is about 78% larger cross section than 3/16", so that should be significantly stronger. Young's modulus of aluminum is around 1/3 that of steel, so it should take around 60% more stretch to get the same effect, but that's arguably a good thing since it gives finer adjustment. And at least according to this chart, 1/4-20 threads in aluminum can handle higher torque than 10-32 in steel.

The main issue I can think of for a compression rod is needing a larger cavity for the adjustment wrench, making the headstock even easier to break off. So I'll either need to come up with a two-way design I can manufacture myself that adjusts with an allen wrench, or go with titanium for ultra-light compression rods.

How exactly are those Martin double action rods structured underneath that wrapping? Is it something that can be made without special tools?

If using a water based glue caused the neck to back bow then wouldn't the neck come back to shape once the water leaves it? Or are you suggesting that as soon as the water touched the surfaces they start to expand and then get locked in that position?

I've used nothing but Titebond or HHG on almost 70 instruments now and have never seen that problem.

I'm wondering how the oates clamps them up. What kind of cauls and so on.

I posted the photos so I guess I can add some info.

There is a flat bar on top and a round bar on bottom. On the adjustment end, the round bar feeds into a larger diameter tube, which is 1 3/4" long. That larger diameter tube is welded to the flat bar on that end, and the round bar is welded to the flat bar on the other end. Neither of the bars (flat or round) rotates. All movement happens inside the larger diameter tube. Inside, there is a hex nut that is captured inside the larger diameter tube with some type of collar, so that it rotates but is otherwise stuck in that spot. That hex nut seems to thread onto the round bar, so that, when you turn the hex nut clockwise, it shortens the round bar, which forces the truss rod to bend the direction of the round bar. When you turn the hex nut counterclockwise, it lengthens the round bar, which forces the truss rod to bend the direction of the flat bar.

You can buy the thing for $8 USD and a matching wrench for $6 USD. I wouldn't want to try to build one when I can buy one for such a modest price.

Thanks! And yes, it would be stupid to duplicate it exactly. I was wondering if it could give me any ideas for a weld-less dual action aluminum design, but apparently not.

I've never tried the Martin rod, although it looks good. I can say that I successfully used the LMI double-acting rod for a long time. It works well and I didn't find it objectionably heavy, but I found the manufacturing slightly sloppy and I was slightly annoyed by having to finesse each rod with a file to fit a 1/4" slot. Not a huge issue at all, just a minor niggle.

At the moment I'm trying the current double-acting stainless rod from Allied and am very happy with the manufacturing tolerances, fit and finish. It looks so slick it's almost a shame to hide it in a neck.

To come to the point though, since weight is an issue it's probably worth mentioning that Allied also has an "ultralight" model, which indeed uses aluminium. It might be new? I haven't tried it but they claim a 13-inch rod weighs under 1.5oz.

Looks like an unusual design - single action but designed to slide out of the neck, flip and re-insert so you can have that single-action in whatever direction is required. Interesting.

That ultralight rod is indeed new. Clever way to make backbow recoverable on the off chance that it should happen. The weight is even a bit less than a 3/16" steel compression rod. Heavier than a titanium compression rod, and a lot more expensive, but should make the lightest neck overall when combined with a wood like Spanish cedar or butternut which I'd worry about overloading with a compression rod. Thanks for the heads-up!

Wow, those ultra lite Allied rods look like they’re just the ticket. I think I’ll be getting me one of those to try out.