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How do truss rods work?

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Adjusting an acoustic bass truss rod
An evaluation of different truss rod settings versus bridge height

A truss rod has a simple function, it counteracts the guitar's string's pull holding the neck in the desired place. Without a truss rod, many guitars would simply forward bend over time with the pounds of pressure exerted by the strings.

It's possible to make a guitar without a truss rod but the necks just have to be able to withhold enough tension. There are a few types of rod including traditional, 2 way and non-adjustable truss rods. I wont go into their workings but they all roughly do the same job.






I hope you find this page interesting as I explain (with the aid of painstakingly drawn Microsoft Paint diagrams!) the secret workings of your guitar's truss rod.

All drawings are hideously exaggerated for illustration purposes!

My neck should be bent??

I see guitars on eBay with the seller saying "Top notch guitar, yep, the neck's nice and straight too" - great. The neck should be in straight looking order but it indefinitely will have to be slightly bent to allow for lower action and greater playability.

Why?

Consider the pattern a string would make when plucked open.

The two fixed points are in this case the nut and the bridge saddle with the most string vibration in the centre of the string. When a note is fretted, the left-most fixed point would be the fret pressed.

There's always the trade off between the truss rod's relief setting and the bridge saddle's height. You cannot get a buzz free action from just setting your bridge saddles low. Exactly the same as you cannot get a super-low-non-buzzing-mega action by just bending the neck back.

Lets evaluate how the guitar's neck should correspond with the elliptical pattern of the string's vibration.

Truss rod settings:

Slack:

In this diagram the guitars truss is set far too slack resulting in an enormous action. This does mean that the string will be much less likely to hit against the frets resulting in fret buzz however this guitar would be VERY tiring to play!

Tight: Back-bow

Here we have an extremely over-tightened truss rod (if you tried to tighten this rod further you, without doubt, would risk damaging it). If you look closely you will see that I have raised the saddle in this diagram. Even with this saddle compensation the string nearly touches the tops of the frets when the string is stationary let alone when plucked.

Flat:

When you set the neck flat it looks great at first glance however when the string is plucked at any decent strength you will find that the string will hit against the frets which co-incide with most likely the centre point of the vibrating string - in this example 12th fret. you would have to raise the bridge height abnormally high in order to get the neck playing free of string buzzes thus sacrificing the action.

Optimum:

Here is roughly the perfect shape for your guitars neck to be in. Look closely and you'll see that the neck closely follows the elliptical profile of the string allowing for the most even feeling action down the neck.

Each guitar has it's own relief setting defined roughly by the players preference and playing style. I set up each of my customer's guitars according to what they prefer as well as making an assessment for myself after watching them play. For a really tough player I set the relief very loose (lots of bend) to allow for a wider vibrating string pattern.

See Why can I get my guitar's action lower when I play softer?


Common neck relief settings

These setting are roughly what I would set customers' guitars to if no preference was specified:

Caution - Be very careful when you adjust your guitars neck. I cannot be held responsible for your guitar unless I adjust it for you. Please take your time to slowly adjust the truss rod in small increments. Do not exert too much force onto it as you could cause irreparable damage! Feel free to call me for advice.

Electric guitar: .002 - .010"

Acoustic steel string: .002 - .010"

4 String Bass guitar: .010 - .014"

5 String Bass guitar: .010 - .014"

Acoustic Bass guitar: .014 - .016"

Spanish Guitar: .003 - .010" (if adjustable)

Note - These settings would be set the same even with different gauge strings with different overall pressures.

See How do I set my guitar's truss rod?