HNB said:
Shimming the neck makes the neck more parallel to the strings. You are still tuning them to the same tension to have them in tune. Raise your action to max... Then lower it... Notice you don't need to adjust your truss rod? This is because the tension hasn't changed. Just the neck angle. Parallel is parallel. Now change the string gauge one way or another. Heavier gauge strings have more tension when tuning to a given note than thinner ones. This is why you often have to adjust the truss rod to work with the different tension. Changing bridges or shimming a neck doesn't impact tension. It does impact action.
I know what shimming the neck does, it also changes the angle from the bridge to the nut, i've done it to a few of my guitars, the overall tension doesnt change from say 180 lbs of force to 250 lbs of force it does however effect the tension on the string as far as tuning because when the neck is shimmed and angled, the string has farther to travel due to the downward angle, which is why when its tuned, you have less windings around the post, unless you have the neck that way to start and account the trimming of the string to that length.
