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So what field of engineering did you graduate in?
For an Engineer you are always so disparaging of any attempt to apply basic engineering theory.

Classical action might be high but the bass string can be quite floppy. Give it to a strong player and there will be some buzzing on the first 5 frets - as a result many builders add about 0.01" of relief there, which then needs to gently slope away to the 19th. One can add about half of this for the A and D (I don't really do this myself) and no relief for the treble strings.

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Build log

You can take a horse to water...

But when the horse isn't thirsty, he'll question why you lead him to the water in the first place....

So what field of engineering did you graduate in?
For an Engineer you are always so disparaging of any attempt to apply basic engineering theory.

Power Engineering.

I only question the application of theory when it doesn't quite add up to real-world examples, and even without crunching the numbers, something didn't sit right for me, here.

I gave-in and went to Hurd's site and played around with the spreadsheet. Entering data typical to my steel string necks, it suggests I'll see .0071" deflection at the 12th fret with my steel reinforcement. Sorry, but that doesn't follow what I see in practice. Even if I doubled the width of the steel in the spreadsheet, it still suggests I'll see a deflection of .0056". Not even close.

It's -those- kinds of findings that lead me to question everything.

If you wish to double check the results(I could well have missed something) using your formula, here were the entered parameters:

Neck width:1.687"
Neck length: 14"
Neck depth: .625"
Fretboard depth: .225"
Insert width: .125"
Insert depth: .5"
Insert length: 14"
String to fret @ 12th fret: .095"
E of mild steel: 30,000,000 psi (enter value as: 3.0E+07)

EDIT: Don't forget to change the string tension to 180lbs.

Hi,

Mario, I would like to ask you a question regarding using steel square tube for non-adjustable truss rods...

I would like to experiment with future guitar projects by using non-adjustable truss rods, for as a player of 25 years and a builder for eight, I have only adjusted a truss rod once, and that was merely out of curiosity.

I would be grateful if you would let me know your opinion of aluminium (I'm from the UK so I spell it wrong! ) square tube - would it be suitable for a non-adjustable truss rod? It's a darnsight cheaper for this poor luthier.

Thank you for reading this.

Aluminum's way too weak, sorry.

Thanks for your reply.

I'm no engineer but I know enough to be pretty skeptical of a formula like that. It may be very useful for giving you relative info (what generally happens if I increase the depth of cf vs increasing neck depth) but it would have to make way to many assumptions to be accurate in absolute terms. For instance, how does it account for fret slot width, depth, and wood compression around the the fret barbs? The fret slots and compression around them are a huge part of the equation since they are on the stiffest part of the neck, farthest from the neutral axis. There's no way a formula can account for variation in real world working methods where a .001" difference in fret slot width will potentially make a huge difference in stiffness and creep. And if he assumes the FB is solid, that's a huge mistake.

Again, not questioning that there's value in the formula, just saying there's no way it can give absolute answers.

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http://www.chassonguitars.com

if you are talking about maple, you don't need a stiffening/"strengthening" rod at all....just go for it. you'll notice a difference in sustain and tone, and it will be much stronger than a neck with a truss rod- just not adjustable. mahogany on the other hand needs a little help in that department

I suspect that the deflection calculations do not take any account of the taper of the neck or the stiffening effect of the heel so I would not expect them to give an absolute deflection value which lined up with the real world.
That is why I looked at comparative crossectional stiffness and stopped at the EI values.
Applying theory is often about knowing when to stop

Exactly right.

Right again.

Knowing when to stop is what separates those who have read about it from those who have practised it.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I agree. Stiffness is proportional to the cube of the height. IMHO a 3/8" tall reinforcement from a material that is not as stiff as steel is not going to do much.
The only CF reinforcement I have used was a single bar measuring 0.30" wide by 0.47" tall. I didn't measure its stiffness, but it was made from UD CF/epoxy that was very difficult to flex at all by hand. The neck was dead straight with no string tension, and had almost no relief (~0.002") when strung with medium steel strings.
Classical fingerboards are not only thicker because of the desire to increase the neck stiffness. The traditional classical guitar neck has zero neck pitch (parallel with the soundboard), so that a thicker fingerboard is needed to achieve enough string height at the bridge.
Maybe I have been lucky, but I have witnessed few problems with my necks warping from moisture changes. Nearly all my guitars have had ebony fingerboards.

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John