Truss Rod Effect on "Action"
I've known there's a common but mistaken belief that the truss rod exists to raise or lower the "action" at the 12th fret rather than just providing neck relief. My new experimental-subject dreadnought has taught me where that belief comes from. I've never seen a guitar where a tweak of the truss rod that changes the 7th fret relief by, let's say, 0.005" will produce a change in the 12fth-fret action of two to three times that much. Seriously!
My other guitars would have needed a in the middle of the neck visible from the Space Shuttle to raise or lower the action by 1/64". So from that perspective of course it's foolish to crank on that truss rod when you really need to raise or lower the saddle. But if I make this neck dead flat (i.e. less than 0.002" of relief at the 7th fret) the 6th string sits about 0.115" above the 12th fret. That is with the truss rod about 1/8th turn clockwise from neutral. If I put it 1/8th turn anticlockwise from neutral the 7th fret relief goes up to about 0.008" on the bass side and 0.006" on the treble (???) and the 6th string action rises to 0.140" at the 12th fret.
That's a 0.025" increase in action for less than a 0.008" change in relief and barely a 1/4 turn of the truss rod. Have you ever seen that kind of sensitivity? The only thing I can think of is the neck itself must be either soft or warped or maybe the truss rod is mounted too shallow or deep. This is BTW on an inexpensive Chinese dred that came with a pretty poor neck angle from the factory. I had to lower the saddle as far as practical to get even these action numbers, when I got it the 6th string was 0.145" above the 12th fret with the truss rod in neutral position.
P.S. As should be obvious, this is a double-acting truss rod.
My other guitars would have needed a in the middle of the neck visible from the Space Shuttle to raise or lower the action by 1/64". So from that perspective of course it's foolish to crank on that truss rod when you really need to raise or lower the saddle. But if I make this neck dead flat (i.e. less than 0.002" of relief at the 7th fret) the 6th string sits about 0.115" above the 12th fret. That is with the truss rod about 1/8th turn clockwise from neutral. If I put it 1/8th turn anticlockwise from neutral the 7th fret relief goes up to about 0.008" on the bass side and 0.006" on the treble (???) and the 6th string action rises to 0.140" at the 12th fret.
That's a 0.025" increase in action for less than a 0.008" change in relief and barely a 1/4 turn of the truss rod. Have you ever seen that kind of sensitivity? The only thing I can think of is the neck itself must be either soft or warped or maybe the truss rod is mounted too shallow or deep. This is BTW on an inexpensive Chinese dred that came with a pretty poor neck angle from the factory. I had to lower the saddle as far as practical to get even these action numbers, when I got it the 6th string was 0.145" above the 12th fret with the truss rod in neutral position.
P.S. As should be obvious, this is a double-acting truss rod.
Views: 90
Replies to This Discussion
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Permalink Reply by Rick Gentry on
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The phrase "quarter turn" on a truss rod is quite relative.
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Permalink Reply by Brent Hutto on
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Well, if flat zero-relief is at 1:30pm the 0.008" relief is probably 11:00am. Would that be considered a big tweak?
If so, maybe this isn't a case of the 12th-fret action being sensitive to truss-rod movement but rather the 7th-fret relief being insensitive...
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