CF rods and computing neck stiffness

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Michael Thames

CF rods and computing neck stiffness

Post by Michael Thames » Tue Apr 09, 2013 12:05 am

I feel like "Jack Swigert" in the movie "Apollo 13 when he is going over the calculations and discovers they are coming in too shallow and must make a course correction, and says " I've been going over these numbers and they don't add up".

I too have been going over the numbers in this article on calculating neck stiffness, and they don't add up.
http://www.ukuleles.com/Technology/neck.html

While the calculations seem to be sound, the information used is not. For instance, the carbon fibre "Young's Modulus" is rated at 10,000,000. While http://www.goodfellowusa.com/ rates carbon fibre at 18,000,000, almost twice that given in the article. Also, the specific gravity of ebony is way off, while I don't have those specs handy, I do know one thing for sure...... ebony sinks like a rock in water, and the specs the article gives has it floating. I'm comparing the Specific G with the density as these are usually related, and given the other ratings in the article the ebony seems off.

This miscalculation for ebony is important because the stiffer the ebony is when glued onto a neck blank of cedro for instance it will draw the neutral plane closer to the ebony. If the ebony is less stiff it will push the neutral plane further away towards the opposite side. All of these things make a difference in the stiffness of necks when reinforced with CF rods....... however, the most important is the under rating of the Carbon Fibre in these calculations. With a course correction of the CF alone, it would make the neck almost twice as stiff as the author concludes, and imagine if the CF was actually placed deep into the neck off the neutral plane, I don't know but in the illustrations in the cross section showing the CF rod he has it right under the fingerboard...... pretty much on the neutral plane.

Reading information Trevor Gore provides he mentions a 0.7mm thicker neck is the equivalent to a CF rod, this always seemed counter intuitive, and now after a course correction I see why. This article was linked by Trevor as evidence to back up his findings. I just feel like setting the record straight because a lot of makers use CF reinforcement rods with very good results that are counter to what this article states as fact.

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Re: CF rods and computing neck stiffness

Post by jeffhigh » Tue Apr 09, 2013 7:56 am

Hi Michael and welcome to the forum

As for your questions on inputs to the calculations for neck stiffness I'll make these comments

The properties listed are stated as being given for convenience. If you have materials tested better or worse you should use those actual properties in the calculations.

The properties of CF rod are to a great extent dependant on the quality of manufacture and proportions of epoxy used
That article is probably close to 10 years old and commercially available rod may have been poorer quality.

I would agree with you on the density of ebony being suspect, that is far too low. The youngs modulus however looks to be in the right ballpark depending on species and that is what would have an effect on the calculations rather than the density

I agree with you on positioning the CF further to the back of the neck off the neutral axis. this will certainly have the most effect on stiffening, but it is not how suppliers like stewmac and others are promoting it's use and how most builders are using it.
I have only built one neck with Cf reinforcement, a steel string. I used a round bar set in epoxy at the back of the neck,no other adjustable rod, worked well.

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Re: CF rods and computing neck stiffness

Post by Trevor Gore » Tue Apr 09, 2013 9:27 am

Hello Michael,

Welcome to this side of the planet!

In response to your queries, What Jeff said!

I seem to recollect saying 0.9mm rather than 0.7mm, but I not going back to check! Suffice to say, the number is quite small, but 0.7-0.9mm is certainly a difference that can be felt by a player's fretting hand. If you're playing with Hurd's program (can't remember the name of the guy who actually wrote it for him) try playing with the Young's modulus numbers for the wood as well as the CF and see what that does for you. Also, try altering the thickness of the fretboard. Cedro, generally, is not particularly stiff as woods go, even though you might prefer it for other reasons. The other part of the equation, if you like, is the loading part, string loads and humidity "loading". In this part of the world at least, adjustable truss rods make a lot of sense as a way to compensate for humidity load variation in a way that non-adjustable rods of any kind cannot, which is why I prefer them.

I've built with CF right through the neck, similar to multilam wooden necks (also with an adjustable truss rod, BTW) and the minor difference in performance is just not worth the aggravation and damage to your tools. Even with a ~doubling of the Young's modulus for CF, if it is used the way it is conventionally used, the "depth equivalent effect" would still be quite small, probably less than 1.5mm, though I haven't done the calc. The same effect could be achieved using a stiffer neck wood without altering the overall neck thickness. Spruce neck shafts, anyone?!!

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Re: CF rods and computing neck stiffness

Post by Dominic » Tue Apr 09, 2013 11:11 am

Déjà vu. This is obviously just a math problem that can be solved with the right values for the actual materials used and some work in excel. Some homework for you Michael?

I stopped using black ebonies for fretboards in favour of more stable woods and stopped using cf and now just use allied truss rods and am happy with the results. Minimising the difference in the movement of the fretboard and neck wood as humidity changes seems like the best way to create a stable neck. Rather than trying to constrain those forces within the neck by using cf.

As with wood, CF rod seems to come in many varieties and epoxy/cf ratios so I would think actual numbers for the rods should be used rather than a potential or theoretical value.

Hard science has proved many intuitive ideas wrong over time. Its what keeps things interesting moving forward.

Cheers
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Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Tue Apr 09, 2013 2:05 pm

Why thanks Jeff, and Trevor for the warm welcome.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 12:05 am

Jeff, yes it does say "connivence" but, I took that at face value, meaning, here are the specs so you don't have to look them up. I didn't read into that...... here are the wrong specs!
The properties of CF rod are to a great extent dependant on the quality of manufacture and proportions of epoxy used
That article is probably close to 10 years old and commercially available rod may have been poorer quality.
Yea true, I don't know for the life of me why all of us who were debating this didn't realize that. Tervor kept using these specs to support his theories, in fact within the article itself there are two "case studies" used as an example, reaching conclusions as if this were fact.
I agree with you on positioning the CF further to the back of the neck off the neutral axis. this will certainly have the most effect on stiffening, but it is not how suppliers like stewmac and others are promoting it's use and how most builders are using it.
I have only built one neck with Cf reinforcement, a steel string. I used a round bar set in epoxy at the back of the neck,no other adjustable rod, worked well.
Jeff that is good to know! I'm building my 2nd steel string guitar........ 850 classical's and 2 steel strings. I was wondering what to do for the neck reinforcement, that's why I started going over these spec's again. As a result of being a classical maker I seem to have developed an aversion to adjustable truss rods for better or for worse. So to hear you used a CF rod and it works fine is good news.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 12:35 am

Hello Michael,

Welcome to this side of the planet!
Hello Tervor, I was just over in your neck of the woods (Thailand) a few weeks ago....... I do like that side of the planet!
I seem to recollect saying 0.9mm rather than 0.7mm, but I not going back to check! Suffice to say, the number is quite small, but 0.7-0.9mm is certainly a difference that can be felt by a player's fretting hand
It was 0.7 mm I had a look. Be that as it may, it now seems rather obvious that a well placed CF rod has more of an effect than you previously thought.
In this part of the world at least, adjustable truss rods make a lot of sense as a way to compensate for humidity load variation in a way that non-adjustable rods of any kind cannot, which is why I prefer them.


I agree, but, the debate was centered around the CF rod adding any significant stiffness. In the classical world there are many traditionalists who look down upon adjustable truss rods in in classical guitars, they want to uphold and preserve the traditions of the past, then there are the new generation of players who don't care. I am in the middle so I simply opt for a CF rod that will not detract from the traditional appearance of a classical guitar. I have zero interest in metal adjustable truss rods on classical guitars because I don't think the metal will contribute to what I consider a positive trend. Call me old fashioned if you like. Besides, adjustable truss rods don't do _ Jack _ to the fingerboard that floats on the soundboard above the 12th fret.











I

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 12:50 am

've built with CF right through the neck, similar to multilam wooden necks (also with an adjustable truss rod, BTW) and the minor difference in performance is just not worth the aggravation and damage to your tools. Even with a ~doubling of the Young's modulus for CF, if it is used the way it is conventionally used, the "depth equivalent effect" would still be quite small, probably less than 1.5mm, though I haven't done the calc. The same effect could be achieved using a stiffer neck wood without altering the overall neck thickness. Spruce neck shafts, anyone?!!
Trevor, what size CF rod are you using, are you using more than one? Adding on 1.5 to 2mm on a classical guitar would be the kiss of death, it's not an option, so the this doesn't really apply to our conversation. I have been using a high modulus CF rod 14mm by 5mm.... that gives me a 2mm safety zone before exposing it. I have also used two of these rods making it 10 mm X 14mm.

Spruce neck blanks? Twice as hydroscopic as cedar....... not a good idea!

I have a friend who is a brilliant physicist who as a hobby makes guitars with me on the weekends, he is writing up a new program to compute this stuff...... it think it will be far more detailed and with the correct specs on materials, as soon as he has it finished he promises me he will post it, should be interesting.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 1:24 am

Déjà vu. This is obviously just a math problem that can be solved with the right values for the actual materials used and some work in excel. Some homework for you Michael?
Dominic, as you can see I've been doing some homework, and is why I posted these wrong specs here, that everyone used as fact.
As I said, my friend is working on a new program with more details and we will post these as soon as he is finished.
As with wood, CF rod seems to come in many varieties and epoxy/cf ratios so I would think actual numbers for the rods should be used rather than a potential or theoretical value.
Yes yes, I agree lets get all the parties involved including Tervor, perhaps he too could use "actual numbers" instead of saying he used a carbon fiber rod once, with minimal results. But, don't worry ALL of these things will be considered and computed in my friend Jim's new program.
I stopped using black ebonies for fretboards in favour of more stable woods and stopped using cf and now just use allied truss rods and am happy with the results. Minimising the difference in the movement of the fretboard and neck wood as humidity changes seems like the best way to create a stable neck. Rather than trying to constrain those forces within the neck by using cf.
I don't know if you are referring to classical guitars, or steel string. If you are doing steel strings, then by all means set yourself free and use whatever material that pops into your mind. If on the other hand you are referring to classical guitars then the perimeters are a bit more confined. Using anything other than cedro and ebony would not be well accepted amongst the upper echelon of players........ there is a reason these woods have been used traditionally for the past 150 years or so in fine classical guitars, these woods also contribute to the accepted tonal qualities of a classical guitar, if you stray too far from the norm then you may run into problems.
Hard science has proved many intuitive ideas wrong over time. Its what keeps things interesting moving forward.
Yes, and Einstein once said, "All great achievements of science must start from intuitive knowledge. I believe in intuition and inspiration.... At times I feel certain I am right while not knowing the reason."

My friend, I think it's a big mistake to build guitars along the lines of pure science.

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Re: CF rods and computing neck stiffness

Post by jeffhigh » Wed Apr 10, 2013 7:09 am

A 10mm wide by 14mm deep rod will certainly make a difference. significantly more so than the 5mm by 9.5mm promoted by stewmac

With the very stiff neck Michael, how are you introducing relief, are you planing it into the fretboard?

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Re: CF rods and computing neck stiffness

Post by Trevor Gore » Wed Apr 10, 2013 10:10 am

Michael Thames wrote:It was 0.7 mm I had a look. Be that as it may, it now seems rather obvious that a well placed CF rod has more of an effect than you previously thought.
Thanks for taking the trouble to check. Maybe what I was recalling was a 9% increase in stiffness based on the material specs quoted by Hurd. The fact of the matter is that it doesn't really matter, the difference is bugger all, and if you want a specific answer rather than an indicative answer you have to measure the material properties of the specific pieces of wood and CF that you are going to use. The within species variation in Young's modulus for wood can easily be a factor of two, and, given that the surface of the wood is way off the neutral axis, that is going to have a larger effect on overall stiffness than introducing CF (which also seems to have a Young's modulus variation of a factor of two) close to the neutral axis. Choose your wood based on its material properties and you can increase neck stiffness by a factor of two compared to the low end of the scale, rather than 10-20% by introducing CF into random wood. Until Jim has built his model, I'd encourage you to mess with Hurd's, into which you can put material constants and dimensions to your heart's content and get an idea of the sensitivities, which is where the real learning is.

There is no doubt that CF can make a big difference if used in the right way, i.e. well off the neutral axis, but as Jeff pointed out, that's not the way it is promoted or generally used. One supplier is selling 'D' section CF tube which takes up most of the neck and clearly that would work very well in increasing neck stiffness (if that's what you want) once you'd figured out a good way of installing it.
Michael Thames wrote:Besides, adjustable truss rods don't do _ Jack _ to the fingerboard that floats on the soundboard above the 12th fret.
Depends how you design your neck joint. If you design them like this, you can run the adjustable truss rod almost to the end of the fretboard.
Michael Thames wrote:Spruce neck blanks? Twice as hydroscopic as cedar....... not a good idea!
There are a few reasons why spruce might not be a good idea for necks, but that's not one of them. The reason why necks bend with humidity change is because of the mismatch in the dimensional change with humidity of the component woods, just like a bi-metal strip bends with temperature change due to the differences in thermal expansion of the two metals. The ebonies have one of the largest long grain dimensional changes with humidity, cedro significantly smaller, so there's a large mismatch. On your assertion, spruce would be closer to ebony in its response to humidity, so a smaller mismatch, and is also typically getting on for twice as stiff as cedro. So from that perspective it would be a good choice. Get Jim to look at the humidity problem whilst he's at it.

Bi teh whey, mi mane is speld 'Trevor'.

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Re: CF rods and computing neck stiffness

Post by nnickusa » Wed Apr 10, 2013 10:33 am

picky...... :lol:
I wish I was half the man my dog thinks I am....

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Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 10:40 am

OK here are the preliminary findings, these are computed for a cedro neck depth of 23 mm total...... combining a 6mm ebony fingerboard, a 17 mm cedro neck contoured to shape, with a CF rod 13 X 6.35 mm low modulus (19.6 mpsi) it will increase the neck stiffness by 92%. Introducing a high modulus CF rod (24.6 mpsi) same dimensions will increase the stiffness by 109%.

The 10,000,000 Young's Modulus given in the article by Joshua H. Gordis is rating for CF unidirectional sheets.

So as we can see the CF rating is not for lower grade CF, it's for sheets........there is really not that much of a difference between the high modulus, and low modulus in the final calculations. I'm assuming most of us who use CF rods are getting the lower modulus stuff.

So as we can see contrary to Tervor's ideas of a CF rod in which he states....."A stable neck can be manufactured from a wide range of woods. Further, if reasonable wood is chosen, the author’s experience has been that it is unnecessary to complicate the manufacture of the neck by using multiple laminates of either wood or composites to add stiffness and stability to the neck. Adding carbon fibre to the neck does not seem to produce any audible advantage whilst adding significantly to the build complexity".

I don't know if Trevor is talking about classical's or steel strings here........... I will say that adding CF to the neck is a simple procedure and is in no way adding significantly to the build complexity, however, I think adding an adjustable truss rod is more complicated than an inlayed CF rod.

Also, just off the top of his head my friend said to get an increase of 92% to 109% he imagines the neck (shaped as a guitar neck) would have to be approx 40% thicker, not the 0.7 to 1.5 mm Trevor states, but he said not to hold him to it, he needs to do the calculations for that.

Jim will come up this weekend and publish his findings with all the correct specs, program etc.

So, I hope this is useful information for those who use or are considering using CF rods in the neck and have been slightly mislead by the info published by the website in question, and those who echo it's conclusions.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 10:42 am

Oh I just want to say an increase of 92% is a far cry from Trevor's claim of 9%.

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Re: CF rods and computing neck stiffness

Post by Trevor Gore » Wed Apr 10, 2013 11:00 am

I'll look forward to seeing Jim's program.

Hurd's was published as a part of his PhD thesis, so I would hope it got some reasonable scrutiny, as they passed him!
Michael Thames wrote:Also, just off the top of his head my friend said to get an increase of 92% to 109% he imagines the neck (shaped as a guitar neck) would have to be approx 40% thicker, not the 0.7 to 1.5 mm Trevor states,
No, Trevor didn't state that! He said 0.7mm (or there abouts) for ~9% increase in stiffness as per Hurd's program and material specs.

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Re: CF rods and computing neck stiffness

Post by nnickusa » Wed Apr 10, 2013 11:04 am

I'm not a great physicist, tho my father is, in fact, a world-renowned physicist, and has been recognised as such for the better part of a half century. Google Raymond Dingle, if you don't believe me...

Anyway, the point is....What is your point? We all are aware that adding CF to anything will make it stiffer, by what degree? Buggered if I know, but probably enough if you're using softer timbers....

As to your contention that installing a CF rod is less complicated than using an adjustable truss-rod, I'm not sure the degree of complexity needs analysis from a physicist. What I do for both is.....wait for it....

Rout a channel. Glue the bugger in. Not too complex.....


If you are taking exception with the mathematics and have a more refined approach, given that there are variables that need to be taken into account, such as the specifics of any individual bit of timber and the other issues as regards CF, then I'm sure others may have interest in such specific equations. I don't. I just build them to the way they sound to my ear.....I am not a disciple of the Gore/Gillet books, nor do I dispute them, but rather believe that everyone ought to do their own due diligence, and find their own course......I will, once I get up a bit of coin, buy the set, and derive whatever I can from them, you'd be an idiot not to, but I get the feeling you're really just shit-stirring.....
I wish I was half the man my dog thinks I am....

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Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 11:30 am

Trevor, I believe you said, that a CF truss rod adds 9% stiffness, and a 0.7 mm increase in the thickness of the neck would do the same thing as a truss rod.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 11:56 am

Nick, good for you, we are all free to approach guitar building as we see fit. I build by intuition too, and my intuition told me CF rods add significant stiffness to my guitar necks.... I was then informed by Trevor Gore that I was wrong about that, he used the UKE website to back up his science. Instinctively it never felt right to me, so recently I discussed this with my friend and we came up with these findings, and felt like setting the record straight.

I have no problem with science, in fact I love science, but when the science is worthless, and misleading, I feel it's our duty to correct it. So, I chose to correct it. If you choose to call it "shit-Stiring" so be it, that's up to you, my freind.

I too haven't read Trevor's books, but, if one were sitting on the coffee table while I was at the dentist office I might give it a look...... that of course depends on if there were a Playboy next to it, then I might not being that time is limited.

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Re: CF rods and computing neck stiffness

Post by Trevor Gore » Wed Apr 10, 2013 12:42 pm

Michael Thames wrote:Trevor, I believe you said, that a CF truss rod adds 9% stiffness, and a 0.7 mm increase in the thickness of the neck would do the same thing as a truss rod.
Yes, that's right. Thanks for clearing that up.

Looking forward to seeing Jim's stuff. Is he also going to show where Hurd erred, other than "negotiable" material properties?

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Re: CF rods and computing neck stiffness

Post by charangohabsburg » Wed Apr 10, 2013 12:57 pm

Michael Thames wrote:I have no problem with science, in fact I love science, but when the science is worthless, and misleading, I feel it's our duty to correct it. So, I chose to correct it. If you choose to call it "shit-Stiring" so be it, that's up to you, my freind.

I too haven't read Trevor's books, but, if one were sitting on the coffee table while I was at the dentist office I might give it a look...... that of course depends on if there were a Playboy next to it, then I might not being that time is limited.
By any means Michael, reach for the Playboy which apparently (according to the quote above) fits your understanding of science much better than the book.

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Re: CF rods and computing neck stiffness

Post by jeffhigh » Wed Apr 10, 2013 1:31 pm

Couple of questions for you Michael, serious not having a go.

Will your Friend's modelling take into account the effect of the fret slots?

Will the model be in imperial or metric? we have been all over the place with units (metric please)

With an ultra stiff neck you are unlikely to pull in any relief with nylon string tension. Are you dressing in relief to the fret board or frets? How much and at what RH?

Are you going to contribute in a more general way to this Forum? I am sure that you have a lot of experience to share.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 2:13 pm

jeffhigh wrote:Couple of questions for you Michael, serious not having a go.

Will your Friend's modelling take into account the effect of the fret slots?

Will the model be in imperial or metric? we have been all over the place with units (metric please)

With an ultra stiff neck you are unlikely to pull in any relief with nylon string tension. Are you dressing in relief to the fret board or frets? How much and at what RH?

Are you going to contribute in a more general way to this Forum? I am sure that you have a lot of experience to share.
Jeff, I'm not sure about the fret slots, he did in fact mention it, but, it might be one of those things that just can't be measured, so many variables, saw width, fret tang, material, etc. I think for practical purposes we should stick to the basics here.

Don't know if it's metric or not, I would also like metric but, he is an American so probably not.

I don't put any relief in my necks, with the CF re-inforcement the neck still bows forward a bit. I'm personally one of these guys who don't like much relief in the necks. I've heard Ruck for instance doesn't put relief on the treble side.

Yes, in fact I joined this forum because it seems made up of a lot of steel string makers, and rather than contributing much, I'd like to learn more about steel string guitars. Doesn't seems like too many classical makers here........ and that might be a good thing!

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 2:53 pm

trevtheshed wrote:
Michael Thames wrote:Trevor, I believe you said, that a CF truss rod adds 9% stiffness, and a 0.7 mm increase in the thickness of the neck would do the same thing as a truss rod.
Yes, that's right. Thanks for clearing that up.

Looking forward to seeing Jim's stuff. Is he also going to show where Hurd erred, other than "negotiable" material properties?
Trevor not sure if Jim even read Hurd's program. I think in fact it would be better if he didn't, so he has a fresh perspective. He did look over the UKE site with the calculations and such, and mentioned that there didn't appear to any problems in that regard. However, he did immediately notice the errors in the materials. Jim is quite capable of coming up with his own program and will perhaps add a few other things in the mix. Whatever Jim does you can rest assured it will be impeccable.

Trevor, the calculations you used and kept using in your argument gave the neck an increased stiffness of 9%, which supported the basic premise of your statements that CF rods are insignificant and a big bother to use. Jim's calculations gave it an increase of 92%. This what I was referring to. You would have to increase the neck thickness by 40% to get the same effect as a CF rod........ you stated, you would only need to increase the neck thickness by 9% to get the same effect as a CF rod according to the calculations you were expounding. So, I wasn't mis-quoting you.

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 3:02 pm

charangohabsburg wrote:
Michael Thames wrote:I have no problem with science, in fact I love science, but when the science is worthless, and misleading, I feel it's our duty to correct it. So, I chose to correct it. If you choose to call it "shit-Stiring" so be it, that's up to you, my freind.

I too haven't read Trevor's books, but, if one were sitting on the coffee table while I was at the dentist office I might give it a look...... that of course depends on if there were a Playboy next to it, then I might not being that time is limited.
By any means Michael, reach for the Playboy which apparently (according to the quote above) fits your understanding of science much better than the book.

Cheers,
Thank you! Then you do understand there are more important things in life than reading text books on guitar making. I think Stradivari would agree with my choice of reading materials too!

Michael Thames

Re: CF rods and computing neck stiffness

Post by Michael Thames » Wed Apr 10, 2013 3:11 pm

Trevor here is a little article about Jim Martin.
https://share.sandia.gov/news/resources ... te-a-stir/

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