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Under Bowed Neck |
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"Thanks for the action adjustment- the guitar plays and sounds great!"
-A.D. Ithaca, NY
Why do necks become underbowed?
Necks will bow with changes in humidity. As the neck loses moisture it will become underbowed (or concave). Conversly, as a neck gains moisture it will become overbowed (convex). String tension also plays a role in bowing a neck. Throughout the course of a stringed instrument's life it is under constant string tension (180 plus pounds on a steel string acoustic guitar!). The steady load of pressure slowly bows the neck of the instrument. There are two major factors in the construction of an instrument that contribute to the neck's rigidity: the particular piece of wood used to build the neck and the type of glue used to bond the fretboard to the neck.
Neck Woods
The woods most commonly used to construct fretted stringed instrument necks are mahogany, maple and cypress. All of these materials can make good strong necks, however, different cuts of wood from different trees of the same species will vary in resistance to bowing. Some necks are prone to a higher degree of bowing when under tension whereas other stock will prove to be more stable under tension.
Adhesives
The types of glue used to bond the fretboard to the neck are: hot hide glue, aliphatic resin glue and polyvinyl glue. Hot hide glue is the traditional glue used for stringed instrument building. Used for hundreds of years (and well into the early 20th century by the major guitar manufacturers and still used by all reputable violin family instrument builders) hot hide glue creates a very strong bond. This glue holds up well to heat and heavy loads and will not creep. Aliphatic resin and polyvinyl glues (such as "original titebond") are synthetic glues. They are the most common type of glue used by modern factory fretted stringed instrument makers. These types of glue have a tendency to creep. That is to say, if the instrument becomes hot enough (spending some time in a parked car in the sun) the glue will soften and the glued parts will shift if they are load bearing. We commonly see instruments with bowed necks caused by a creeping fretboard/neck glue joint.
How do you repair an underbowed neck?
Depending upon the severity of the under bow we typically choose one of five ways to repair a bowed neck: adjusting the truss rod, leveling the frets, refretting the instrument in order to plane the fretboard straight, regluing the fretboard with hot hide glue (sometimes the neck must be planed straight while the fretboard is removed) and replacing the fretboard with a thicker one which allows us to plane the neck straight without drastically thining the neck. Adjusting the truss rod works only for concavity caused by climate changes and mild fretboard and neck joint creeping.