Truss rods
Truss rods or other neck reinforcement measures are absolutely necessary on electric guitars with their long necks sticking out from the body. To reduce the likelihood of the neck twisting, it can also be made from several pieces.
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Non-adjustable truss rods |
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One way of making the neck stiffer is by installing an ebony or |
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carbon fiber rod beneath the fingerboard. This rod is slightly |
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tensioned and firmly anchored at a plate on either end. It has to |
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be only slightly longer than the distance between the two anchor |
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plates and is glued in under pressure. |
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Owing to their great stiffness and low weight carbon fiber |
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strips are very popular these days. They are cut from blocks made |
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up of extremely thin layers of carbon fiber and epoxy resin. |
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Carbon fiber can be planed or sanded. Always wear gloves and |
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safety glasses when you machine this hard, brittle, celery-like |
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material. It will dull your cutting tools and it should not be |
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machined with power tools. For shortening the material better |
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use a hack-saw. The material can be sanded but the dust is |
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dangerous and you should always wear a dust mask. |
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If you want to use power tools to cut carbon fiber, use abrasive |
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blades or wheels. The carbon fiber manufacturers cut these |
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materials with lasers or water-jet cutters. |
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a |
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b |
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Because of their low weight even two or more 3mm x 10mm |
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(5/16" x 3/8") carbon fiber strips can be glued into the neck using |
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Superglue, epoxies, or wood glues. Figures a and b show typical |
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neck designs that use an adjustable truss rod and two reinforcing |
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c |
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rods for rigidy and stability. Figure c shows an approach for five |
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and six-string basses where two truss rods are used. The two |
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truss rods help to control warping and twisting of the neck on |
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this type of guitar. |
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Another way of making the neck stiffer is by installing a 10mm |
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x 10mm (3/8" x 3/8") square steel tube under the fingerboard. |
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Solid metal inlays would be far too heavy. |
Adjustable truss rods
Adjustable truss rods give more options for adjustments: if the neck bends too much under the pull of the strings, it can be straightened with the truss rod.
If you make your neck thick like a base ball bat you can do without one but with modern, thin necks its a necessity. You need the ability to adjust the neck relief when you change string gauges for instance. Let's say you had medium strings on your guitar and put on lighter ones: the string pull becomes less and the neck which was adjusted for the pull of medium strings will now bend back. Its good to have the possibility of adjusting a slight relief again which you can do by loosening the truss rod.
If you change from light to medium strings on the other hand the neck will bend more because of the increased string tension. You'll be happy if you have a truss rod which will straighten the neck if you tighten the truss rod nut.
Compression rods are the most common type of truss rod installed in electric guitars: a 5mm (3/16")-diameter steel rod is embedded into a slightly curved channel (1) in the neck. One of
the ends of the rod is firmly anchored, the other end is threaded. 1 The truss rod is covered with a wooden fillet which is also curved
on its bottom side, the curve matching that of the rod so that the latter has no room for movement. The neck can then be adjusted by means of an adjustment nut with washer: tightening the nut straightens the truss rod and with it also the neck (2). This type of truss rod only works in one direction and can only straighten the neck or bend it backwards. The more deeply it is embedded into the neck, the more effective it will be.
It is also possible to install a straight truss rod inside the neck, as long as it is embedded as deeply as possible and not much wood is left under it so that the neck is effectively bent backwards when compressed. Such straight truss rods are, however, less effective than curved ones because they require a lot of force.
Truss rod released
Truss rod tensioned |
2 |
1 |
Using twin rods (1) is another very effective method of neck |
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adjustment: the neck is bent strongly and in a very even curve by |
only slightly turning the nut. As the name implies, twin-rod systems consist of two metal rods; these are about 5mm (3/16") in diameter and are joined at one end and placed above each other in a channel inside the neck. At the other end the top rod is anchored in an anchoring block while the bottom rod is threaded and has an adjustment nut, which, when tightened, bends the rod downwards. Twin rods are fitted into a flat-bottom channel and can also be removed from the neck after installation. They can only bend the neck backwards, and are, because they need a deeper channel, less suited for access from the peghead. The top rod can also have a rectangular cross-section to better fill out the space inside the channel.
Another possibility is to glue a 12mm x 10mm (15/32" x 13/32") aluminium U-channel with internal 5mm (3/16") steel rod into a flat-bottomed channel (2). This is basically a twin-rod system, the only difference being that the top rod is replaced by an aluminium U-channel, which gives greater stiffness. When mounting such a system care has to be taken to ensure the opening of the U-channel faces the back of the neck so that the neck will bend backwards when the nut is tightened. This system is also effective in one direction only.
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Using a two-way adjustable compression truss rod makes the neck adjustable in both directions: at either end of a steel rod a nut is firmly anchored in the neck; the nuts are oppositelythreaded (one of them turns clockwise, the other counterclockwise). The neck is adjusted by turning the entire truss rod. Depending on the turning direction, the nuts are either pulled towards each other or pushed apart so that the neck will bend either forwards or backwards. Such truss rods are best installed slightly curved (see above). If you make such a rod yourself, make sure that the front nut is twisted on the thread before the adjusting head is firmly connected (e.g. welded on) to the rod.
Two-way twin rods have both ends of both rods firmly bolted into blocks. The rods have a clockwise thread at one end and a counterclockwise thread at the other. By turning the bottom rod the neck can be adjusted equally well in both directions and be made to bend forwards or backwards, depending on the direction the rod is turned. For installing this system both rods have to be simultaneously turned into the threaded blocks. To ensure the bottom rod can be turned the front block has to be mounted and an adjusting head has to be firmly connected (e.g. welded on) to the rod at one of its ends. This type of truss rod requires a flat-bottomed channel and can, if necessary, be pulled out and removed easily after installation.
The patented ABM two-way twin rod (3) has a threaded insert with a counterclockwise 8mm thread on its inside. The insert is welded to the end of the top, 6mm x 3mm, flat rod. The adjustment nut has a counterclockwise 8mm thread, too, but on its outside, and a clockwise 5mm thread on its inside. The bottom rod has a clockwise 5mm thread. The other ends of the rod are welded together. The truss rod is operated with a 5mm allen key. Turning the nut in a clockwise direction causes the bottom rod to be pulled towards the nut while the oppositely-threaded top rod is pushed away. This forces the rod to bend downwards, just like any other rod. When the nut is turned anticlockwise the rod will bend the other way, i.e. upwards. This is another system that is fitted into a flat-bottomed, 6mm-wide and 10mm-deep channel.
What are
two-way truss rods for?
If the the string gauge you've chosen is not able to bend the neck either because the strings are very light or the neck is quite thick or stiff it comes in handy to be able to adjust the truss rod the other way around so that it bends the neck a bit forward for a sufficient relief.
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