“Two way” vs “dual action” truss rod - difference??

Hello, I'm a novice builder too. But to my knowledge dual action and two way truss rods are the same.

Your hyperlinks are not working for me.

 

They are the same thing and the terminology is just different. They are more modern as opposed to a vintage rod which is just a one way compression type rod. You should know that the blue import rods require more fiddling with the slot than a Stewmac hotrod which only requires one rectangular slot.

The end block of the import rods are wider than the rod and taper downward a bit. A chisel can do the work.

 

The ones you linked to--they are made slightly differently from each other, and the two-way requires a slightly smaller channel. But they both operate the same way: they can force the neck to flex either up or down.

 

Bitterroot defines the differences on their web site. You can download drawings of each.

1. Dual Action. This rod is threaded at each end and works be pulling the rod in different directions either toward the center of the rod or away from the center. It allows the top and bottom part of the rod to move away from each other and apply pressure up and down at the same time to adjust the neck

2. Two Way. This rod is welded together on one end and threaded on the other end with a metal cap welded over the threaded nut. This allows the nut to push against either the rod or the end of the cap causing the entire rod to flex in one direction to allow adjustment. The nut is covered completely by the welded cap and turns inside the cap. Do not remove this cap, it is part of the design.

FWIW, I have always used the style rods with welded fittings at each end (LMII are built that way). I believe with the Bitterrood rods that style has a slightly less deep route which might be significant in a really shallow neck. If in doubt, contact Bitterroot direction and ask.

 

A rod welded at one end is a compression rod and works the same as a Rickenbacker style rod which has two parallel rods and one nut threaded to compress.


https://hazeguitars.com/blog/rickenbacker-truss-rod-adjustment

If Bitterroot is saying that a "two way" compression rod works in two directions, I guess it kind of -sort of does, but only if you consider the decompression an action. It's a poor and misleading description of what is going on in my opinion and could cause confusion in what you get.

A two way rod (Dual Action named by Bitteroot) like a hotrod allows adjustment in both directions. Turning the adjustment nut clockwise forces the middle of the rod up and causing the block ends go with a downward force. Counterclockwise turning of the nut causes the opposite to occur.

A compression rod would only do that if some relief were sanded into the fretboard when it is adjusted to compress the neck. This practice was common in single rods built by custom builders.

https://www.guitarrepairbench.com/guitar-info/dual-vs-single-trussrod/

 

I recently heard a doctor talking about bilateral knee replacements.
It means "both".
Jeeze...just say BOTH.
Dual action...2 way...
Being that there are dual truss rods, I would avoid that word in the context of a single rod.
Less is more.

 

Here are a couple of LMII double acting rods. In the first picture they have been adjust neutral and placed against a straightedge. The normal orientation is with the shiny square rod against the bottom of the fretboard - think of the straightedge as the f/b in both cases.



In this picture I have given the adjuster one full turn, CCW in the upper rod, CW for the lower one



The normal situation is the bottom one, the neck has too much relief, the rod is adjusted to the center portion is pushing UP on the middle of the fretboard and the ends are pushing DOWN at the nut and heel - that would counter act the tension and force the neck to straighten out. The upper situation is pretty rare, in that case the ends would be pushing up to ADD relief to the neck - not unheard of but not common.

The other thing that is significant is that the rod bends a lot with only one turn of the adjuster. in reality when we adjust the relief in a neck we use maybe 1/8 of a turn at at time, even that can have a lot of effect. These are powerful mechanical devices.

Here is a StewMac hot rod t/r. Same idea and basic construction, the square bar is replaced by another round rod that is not threaded. First picture is adjusted neutral, second is one turn CW. Again, the straightedge represents the f/b, it is "upside down" in the picture

 

Traditional, single action trussrods need an arched channel to function. There's a nuance to that design in the Fender "biflex" trussrod which is still a single rod, but the nut bears against a stop if you back it off and keep turning so it will exert pressure in the opposite direction to combat back-bow. There's an extra piece of hardware to keep the rod from pressing out the skunk stripe. I really don't think any private builders are using that method.

As a practical difference, most two-way or double-action trussrods are meant to be dropped into a straight channel without the need for an arched jig for routing the channel.

 

A curved rod is helpful to concentrate the forces, but not all guitars had them in a curved slot. Many Gibson guitars had a rod in a straight channel. The anchor just needs to be lower in the heel of the neck and lower than the centerline of the neck shaft in order to work.

See the neck drawing in post 1 of this thread:

https://www.tdpri.com/threads/1959-les-paul-build.194271/

 

One of the key parts of those designs that I failed to mention is that the blocks at the ends are both threaded, but one is a standard thread and one is reversed. Just like a turnbuckle is threaded. When you turn the rod one direction BOTH ends move towards each other, turn the other way and they move apart. The second Bitterroot design appears to have one threaded block and one that the rod pushes against but doesn't actually have threads. I've never used one and so I can't say for sure.

Another truss rod design is the Martin single acting rod in a square tube (not to be confused with the non adjustable Martin rods. The square tube rod seems to work fine (I used them in a couple of early guitars) and has the advantage of requiring a fairly shallow flat bottomed channel.

 

I have used quite a few of both of the bitterroot rods. I even talked to the owner of Bitterroot years back about them & he gave me a full explanation. I can't remember a word of it today! Wait, what was I saying?

What I do remember about the discussion was, I decided to used the dual-action for one specific reason. The adjuster head is smaller and requires a slightly shallower route. Yep, I'm that shallow! Truthfully, the last time I ordered a stack of them I accidentally ordered the two-way and use them just the same. They do function slightly different but the reality is they accomplish the same end result. Moving the neck in 2 directions.

Now if I could get them to quit sending me a stinking hex wrench for each rod! Holy cow, I buy them in bulk and I can only throw so many of those away before I start feeling guilty!

Eric

 

quite simply.. a single action rod only bends the neck back to counter the natural tendency of the string's tension to make the neck bow...

a dual or double acting truss rod can bend the neck both directions so, should the neck ever develop a severe back bend that the strings cannot "pull" out.. the truss rod can bend it in that direction to a degree the neck becomes playable..

the weight difference is relatively insignificant...

rk

 

I guess the significant thing for me is how the truss rod applies its bending force to the neck. Single acting compression rods (Fender, Gibson) has the rod on the back side of the neutral plane and tries to "pull" the nut towards the heel (same thing the strings are doing on the other side of the plane). Dual acting rods apply a force perpendicular to the fretboard that is somewhat like trying to bend the neck across your knee. Both work, just different systems.

There was an article in American Lutherie a while back evaluating different kinds of truss rods (it was written about bass rods but applies to all instruments). One of the things that stood out to me was single acting compression rods moved the neck more for a given amount of turn on the adjuster than other types of rods. A significant problem is that they work better the farther they are from the neutral plane, which means a thicker neck or closer to the back side of the neck.

https://luth.org/journal/american-lutherie-131-fall-2017/

I'll add that while double acting rods can introduce relief into a neck that doesn't have any ("correct back bow") I can't recall ever using one for this. All the necks that I make are flat with the truss rods neutral and I can't recall ever needing it on a commercial neck. I know it happens, but I think it is rare.

 

Be leery and make sure you know what you're buying before you build a neck around it!

Warmoth has something they call a double expanding truss rod that does not go both ways. It only adds backbow and does not allow relief. Learned that on the hard way.

 

there ya go.. that's the truth.....

a double acting is like you're anticipating your neck "going bad" and requiring a "reverse correction". personally I do not give it a thought... In a correctly built neck it's over kill, it's simply not required.... I only use them if it's specifically requested....

rk

 

When you work with wood that moves, a dual action rod can make a difference in your life. Many don't have a climate controlled situation is which to store their wood, the time to store the wood, or the patience to store the wood if they had a place. This is especially handy to a new builder of which many here are. Also a new builder who doesn't have their clamping skills up to snuff can save a project neck with a little tweak here and there.

The hotrod is also the easiest ( the stewmac hotrod) to install of all the most used rod styles. If I were starting out and somebody recommended it, I'd consider it. . I'll admit I balked when I saw the price of 20 dollars or so compared to what you can buy if you wait 3 weeks from China, but heck I ordered two of them from stewmac today.

http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=123981851883