There are many different styles and types of truss rods. Each has its advantages and disadvantages. I think it is important to understand what types of different truss do and how they work before we install one.
There are two main types of truss rods: one-way and two-way truss rods. There are many variations of truss rods within these two main types, such as twin truss rods and compression truss rods. For a brief explanation of how one-way and two-way truss rods work, please visit the one-way vs. two-way truss rod page.
A compression rod is a simplistic example of a one-way truss rod. Compression truss rods are typically made out of a piece of .1875" steel rod. One end of the rod is attached securely in either end of the neck where as the other end of the rod has a threaded washer or disk screwed on the rod and attached to the truss rod channel. This rod fits in a curved channel in the neck with a wooden shim or fillet covering the top of the rod. As the name suggests, this one-way, compression truss rod only adjusts the neck one-way. By tightening the nut on the truss rod, you compress and straighten the neck in one direction. This compression counter acts the tension that the string place on the neck and brings the neck back into a straight position. For more information about how one-way truss rods work, please visit the one-way vs. two-way truss rod page.
One-way truss rods are easy to make and somewhat easy to install. They get the job done for the most part. Most necks can be straightened with a one-way truss rod.
One-way truss can only be adjusted to straighten out relief in the neck. If you strings are not heavy enough to pull the neck back, your one-way truss rod will not be able to put back bow in the neck. It's not as versatile as a two-way truss rod.
Personally, I would not use a one-way rod in a new electric guitar. They are not as versatile as a two-way rod. The only real reason for a one-way rod, in my opinion, is if you are replacing a one-way rod or building a "vintage" styled guitar to match the original.
Not all compression truss rods are adjustable in only one direction. A two-way compression truss rod compresses the neck just like the way a one-way truss does, but a two-way compression truss rod has washers threaded on to each end of the rod in opposite directions and anchored to the neck. One of the washers or nuts is threaded clockwise and the other is threaded counter clockwise when the truss rod is tightened or loosened. Because the nuts are fixed in place in the neck, the nuts or washers are pulled together or pushed apart from the reverse threading. The rod will bow forward or backward when the rod is turned. Keep in mind; I said when the rod is turned. This style of truss rod requires that the entire rod rotate in the channel in order to adjust the neck. Most often this style of truss rod is installed in a curved slot or channel in the neck and a shim or fillet is glued in place to cover the rod and protect the under side of the fretboard.
Easy to make and install.
Requires full rotation of the rod for adjustment. If the rod is in its slot too tightly or if the neck is stiff, it will have a hard time turning.
Unlike the compression rods, the twin rods rely on the use of two rods to bend the neck--not physical neck compression. A twin rod truss rod is made of two parallel rods that are welded together on one end. On the other end, one of the rods is welded to a block and the other rod is threaded through the block. The rods are usually made out of .1875" steel rod. Then the rod is laid in the neck channel with the rods on top of each other. The block is fixed in place--usually at the heel of the neck. The thicker truss rod usually does not allow it to be adjustable at the nut side of the neck. When the truss rod is tightened, the completely fixed rod will bend, as the threaded rod will allow the block to be moved. Note that this truss rod only adjusts the neck in one direction.
Effectively adjusts the neck with little effort. Only a small amount of turns are required to adjust the neck.
One-way truss can only be adjusted to straighten out relief in the neck. If you strings are not heavy enough to pull the neck back, your one-way truss rod will not be able to put back bow in the neck. It's not as versatile as a two-way, twin rod truss rod and it is just as difficult to make.
Two-way twin rods consist of two rods that are bolted into blocks at either end. The rods are threaded on the blocks in opposite directions. A nut, screw-head, or adjusting head will need to be mounted or welded on the end of the bottom rod. When the rod is turned, the neck can be adjusted in both directions due to the bending of the rods. Two-way twin rods are installed in flat channels and can be removed relatively easily if needed. This is a common style of truss.
It can bend the neck in both directions in case the strings can't pull the neck back on their own. Can be removed without disassembling the neck if in need of repair.
Requires a large slot or channel in the neck. Can be involved or challenging to build.
A dual action truss rod consists of a flat rod that has a counterclockwise nut or sleeve welded on one end. The nut or sleeve is threaded onto a rod, usually about .315" in diameter. The rod and the flat rod are then welded together on the opposite end. The adjustment nut is threaded counterclockwise on the outside and clockwise on the inside. The adjustment nut threads onto the rod and thread inside of the flat rod sleeve. When the adjustment nut is turned clockwise, the rod is pulled toward the adjustment rod, thus shortening its length and causing the flat rod to bend outward. The opposite is true when the adjustment rod is turned counterclockwise.
This is a sophisticated system that works quite well. The rod easily bends the neck in both desired directions.
Difficult to build. You are better off buying one than trying to build one.