Truss rods

**mikeyb2** · Feb-11-2021, 12:27pm

Hi everyone, I've see the posts relating to truss rods vs carbon fibre rods , but I have a slightly different question.
I've build 3 mandolins so far and installed carbon rods instead of adjustable truss rods and so far no issues. I am now in the process of building a guitar shape octave mandolin, based loosely on the Pono M20 hybrid.
As this will have a slightly longer neck at 10 5/8 inches, I am considering an adjustable truss rod. The problem seems to be, at least here in the UK, that I cannot buy a two way rod of suitable length, and would be forced into making a traditional compression truss rod.( a 2way rod would be better as I could make the adjustment access from inside the sound hole at the other end of the neck) Making a compression rod itself isn't much of a problem, but I am hoping to have a slim guitar type neck with a slight volute behind the nut area, as opposed to a thick mandolin neck. The nut width will be around 34mm, and now I don't really like the idea of having a deep hole dug out of the end of the neck to enable adjustment nut access. This must surely weaken the neck, or does it? I know there are many mandolins out there with these type of truss rods but how reliable are they?
Sorry for the long winded question, any thoughts or do I go down the CF route again?

**poul hansen** · Feb-11-2021, 12:52pm

It definitely weakens the neck.
I bought a Kentucky mandolin with a broken neck, right through the trussrod cavity, where it widens for the adjustment nut.

I glued it and tested it with a very slight pressure and it broke again parallel to the original break, landing in the cavity again. I have now epoxied the rod and filled the cavity. Now it it even stays in tune.

**rcc56** · Feb-11-2021, 1:17pm

Traditional single rods work well if engineered and installed correctly.

I am not personally fond of double rod systems. I prefer methods that leave more solid material in the neck. In the decades I have spent repairing instruments, I have seen only a couple of necks that developed back-bows after assembly, except for instruments that had truss rods that were over-tightened. The laws of nature are against an instrument developing a back-bow, except possibly in a neck built out of unseasoned wood.

Carbon fiber works well. You can make your life easier by installing the rod slightly below the neck surface and covering it with a hardwood strip. It's easier to level wood than it is to level carbon fiber.

Solid neck reinforcements are probably the strongest systems, but you do lose the option of adjustment. At this point in my life, I probably prefer solid reinforcements. I've seen to much trouble caused by amateur rod adjustments and pegheads that were weakened because of the adjustment cavity.

**Richard500** · Feb-11-2021, 5:50pm

Offhand, I can’t see why the nut end of a truss rod can’t be at the other end, especially in instruments with an oval sound hole for access. Then the fixed end could involve far less weakening and much more location options. Might even help strengthen the neck joint by transfixing it.
I don’t build, nor have done this type of repair, and none of my instruments, even 100 plus years old and abused, have any reinforcement, let alone adjustment. I’m leaning toward the idea that a mandolin really has a short and potentially strong neck, or that it would be just as easy to repair a minor bow another way.
My latest toy is a saz, that has 19” of neck from the nut to the heel, no fretboard, and an approximate width of 1 1/16”, and steel strings — primitive thing, unknown age, and holding together just fine. Like its bowl, hollowed from solid with no apparent cracking - how could that be possible?

**rcc56** · Feb-11-2021, 7:35pm

Many instruments have been built with single action rods that adjust at the heel end of the neck. Some of them work well, some don't. I believe that their effectiveness depends on how well they were designed.

**MrMoe** · Feb-11-2021, 8:09pm

Hello Mikeyb2 , I have mostly used the TRST from LMI and often shorten them by welding and cutting . Single acting. They are slim. only 3/16 x 3/8 inch with a stop and nut only 1/4 wide. The only time I have needed to correct a back bow is after a re-fret and recently lamented not having a double acting rod. I have some silm double acting rods from eBay (china direct, the ones with blue shrink wrap not red) that could be shortened as well. Brazing with MAPP gas also works if you do not have a welder. I do not know how an adjusting nut inside the body would work without slacking or removing the strings, and having a special tool. Do you have any images of your instruments? We love to see what folks are doing. Best regards, Maurice

**Graham McDonald** · Feb-12-2021, 1:44am

For anything with a longer neck than a mandolin I use a truss rod, and the modern double rods are most effective. LMI will make a rod to whatever length you want and they fit into a 1/4" x 3/8" slot. The slim adjusting nut means the slot needs to be around 1mm deeper into the head itself. If it is a neck is in line with soundboard, it is quite common to have the adjustment from the sound hole, but you might want to get a slightly longer truss-rod so the allen key does not have to be so long. A OM neck can be a little over 1/2" deep under the nut (14mm in my measurements) though could go to 5/8" (15mm) if you are nervous and with a 34mm nut, that will make a comfortable neck profile.

**HoGo** · Feb-12-2021, 2:59am

There are many guitars with access from soundhole with both types of rods.
You can use standard compression rod in neck ending in the heel and corresponding hole in the neck block for nut - they tend to use the long barrel nuts with hex socket at the end for easy access. The nut of course is installed after the neck is attached to body. You can also let the rod protrude from the heel end but will need to cut corresponding slot into neck block so neck can be inserted into body - this is typically done with the two-way rods where nut removal is not possible.
The whole theory that trussrod weakens neck is BS. Many Chinese brands completely ignore their proper function and excavate half of the neck's wood volume in the most vulnerable place (under nut) from necks made of completely unsuitable wood (looks like Lauan with grain going steeply up towards the headstock veneer in the picture posted above, REALLY BAD choice of wood) and that is not fault of the rod but the producer. But if that neck was not hit hard it wouldn't break in the first place. ANY neck with angled headstock will break/split at the heel when hit hard no matter if it has TR or not. The original design of compression rod removed just the volume of the rod itself from all vulnerable areas and the nut cavity is 1" above nut where the headstock is wide and strong. I use 4mm rod with heat shrink tubing on it and rout 4.5 mm slot for it then fill with maple stick so I'm missing just 4.5mm square from wood. Since the rod is near back of neck and nut way up the headstock the most vulnerable area where all the breaks start (under nut) is held under compression and that even hypothetically helps preventing start of the crack. I don't have proof of this but wood breaks when the bending force (of impact or fall) creates tension on the surface of wood that the grain (necessarily compromised by the angle of headstock or bad cut of wood) cannot withstand the stress and ruptures so tensioned rod actually holds it together a bit similarly to layers of sinew or glassfiber on back of bows (archery).

**mikeyb2** · Feb-12-2021, 4:22am

Thanks everyone, lots of opinions and plenty to think about. To those that think I want a 2way rod because I am concerned about backbow, I must say I am not. I like the 2 way rods because I have fitted them to guitars and I'm familiar with the process, and also they are slim and do not require any wood removing around the nut area for adjustment nut pocket. My issue is that I've had trouble sourcing one of suitable length in the UK. Buying from the US is something I try to avoid because of the shipping costs. I've ordered one from China but when it comes is anyone's guess, so I've been thinking of alternatives. After reading all your posts, I think I'll rule out the single compression rod, and either wait for the Chinese rod or use cf rods again.
Cheers Mike

**j. condino** · Feb-12-2021, 11:36am

A traditional single action truss rod IS a two way adjustable rod: install it like normal, tighten the rod a half turn or so, then install the fingerboard. That way you have the ability to backoff a bit (rarely needed) or to tighten it. After several decades of using most of the commonly available two way rods, I've gone back to using traditional compression rods. Simpler, lighter, and less complicated. The exception is that I make all of my own out of titanium...

**pops1** · Feb-12-2021, 12:29pm

A few years ago a customer brought in a guitar with a broken truss rod, I took the fingerboard off and removed the truss rod. I wanted a duplicate for ease of reassembly, but couldn't find one. I took it to a local machine shop and had them make one and it was cheaper than any I could buy. You may be able to have one made locally.

**j. condino** · Feb-12-2021, 2:38pm

It takes about 15 minutes, $3 in materials, & $25 in tools to make a truss rod....the next one is $3, and the next....

Don't be held hostage by the truss rod unicorns and fairy dust that the retailers try to push on you. They are not difficult to make in a home shop and the design does not have to be very accurate to work well.

A number of years ago I asked 10 of the best builders in the world their perspectives on truss rod design and position. I received about 25 different and contradicting answers. Sooooooo....

I made up ten identical necks with ten different truss rods and stress tested them to failure. Guess what??? They all functioned almost identical. I picked the one that best fit my build process and never looked back.

Here are some of the titanium single action from the last batch of guitars: