Trussrod Theory.....

[Davo]

Hi People,

I'm getting the necessary gear for a first build and spending many hours (weeks) researching all the variables involved in guitar building but there is one small bit of theory that I don't understand to do with the trussrod;

Assuming you have a "standard" dual action trussrod mounted right under the fingerboard, when adjusted how does that tiny 1/4 inch of ebony or rosewood resist the bending pressure of that relatively big lump of metal being bent into a bow underneath it that forces the whole neck to bend???

My brain can accept the older "compression" trussrods or even the fender style rods mouned in the back of the neck - I'm just interested in some theory to explain how large the forces are that the truss rods are exerting?

Cheers....

[dh7892]

I've always been a bit confused about truss rods too. I've never had any problems with any guitars' necks bending so I've never had to make any adjustments to the truss rods on my comercially made guitars. I guess that the fact that I rarely take them out of the spare bedroom where I play them help keep them stable.

As you say, if the small bits of wood where the truss rods concentrate their forces can resisit enough force when why can't those same bits of wood be used instead to stop the neck bending in the first place?

I'm sure that the folks on here that have way more experience than me will explain how important they are but I've some real doubts as to how much point there is to them on a well made (perhaps CF reinforced and/or laminated) neck for an electric guitar where the string tension and action (hence bending force) of the strings is so low. I can see it for a metal string acoustic where the string tension and action are much higher.

You don't see many truss rods in Classical guitars. I know that their necks are shorter (before they meet the body) and thicker than electrics and the tension is lower but the action is way higher.

[Mattia]

The 1/4" of rosewood or ebony doesn't resist the rod, it bends. That's sort of the point. HotRod's are probably the most uneven distribution (most of the force on the blocks, instead of on a flat bar, like with some designs), but remember there's less wood, and weaker wood, actually under the fretboard; usually not much more than 1/8" or so. Glue's as strong as the wood itself, so that's not an issue either.

You don't need huge amounts of force to add a bit of bend to a neck; you just need enough force applied in the right place. The actual forces applied will depend on the truss rod design as well as the compliance/stiffness inherent in the neck wood itself (ie, how much does the wood 'push back' when pushed).

dh: no offense, but if you've never adjusted your truss rod, you've never even looked at relief, and you've never set up a guitar to play at its very best. I've never seen a guitar in a store that couldn't be improved with some tweaking of saddles, truss, etc. It has little to do with stability, or maybe you're the most absurdly lucky person in the world.

The issue is plasticity: wood is a plastic material, which means it deforms under tension. Over time, it actually takes a set, and stays there. Lamination, CF, anything that increases stiffness can help, but the issue remains, at least with natural, non-engineerd materials, that you cannot predict precisely what a neck will do under tension. It'll bend (yes, even with 9's, usually), sometimes not enough, sometimes too much (ergo dual action in stiff necks; I've only built one neck that was 'too stiff', even with CF, all the others required 'traditional' style truss rod adjustment). The only way to stop a neck bending is not make it out of wood, and never put strings on it. Really. Also, carbon fibre on its own is rarely enough to stop necks bending at all. If you want a solid neck that won't move, put a steel tube in it, a la Martin's old guitars. And even then, no guarantees. CF is only stronger than steel for its weight. For its size, it's much, much, weaker and floppier. Even a steel rectangular tube will be stiffer than three CF beams in a neck, all else being equal. And you're stuck with the fact that different guitars, different guitarists, different playing styles all require different setups (some people want really low action with no relief, some medium with a little, and all else in between), and the easy way to correct that is to install a truss rod.

Classicals are a different beast: short, wide necks, still often reinforced with an ebony fillet, and they do still bend and deform over time, sometimes. Solution then is to re-plane the fingerboard. In terms of action, it's usually higher because of the playing style, but it's also personal preference (and cheap student classicals are not a fair representation of what the instrument should be like).

[Davo]

Hey thanks Mattia - your explanation makes alot of sense, as you say I need to view the trussrod as adding a small force in just the right place within a structure that is really more flexible than it seems!

Many thanks