Classical neck reinforcement. Yes or no?

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RodneyStedall
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Classical neck reinforcement. Yes or no?

Post by RodneyStedall » Wed Jul 18, 2018 5:09 pm

I am a classical guitar maker and generally install a 3x10 mm carbon fibre rod just under the fingerboard with the purpose of counteracting any neck bend caused by string tension. Never had any problems until my 57th guitar where just prior to attaching the fingerboard I have noticed a linear crack in the neck running from the headstock to the heel.. MY question is: what do other classical makers do. 1. No neck reinforcement or 2. A hardwood insert or 3. Carbon fibre. OR 4. Truss rod?

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kiwigeo
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Re: Classical neck reinforcement. Yes or no?

Post by kiwigeo » Wed Jul 18, 2018 7:13 pm

RodneyStedall wrote:
Wed Jul 18, 2018 5:09 pm
I am a classical guitar maker and generally install a 3x10 mm carbon fibre rod just under the fingerboard with the purpose of counteracting any neck bend caused by string tension. Never had any problems until my 57th guitar where just prior to attaching the fingerboard I have noticed a linear crack in the neck running from the headstock to the heel.. MY question is: what do other classical makers do. 1. No neck reinforcement or 2. A hardwood insert or 3. Carbon fibre. OR 4. Truss rod?
What sort of wood is the neck made from? How tight a fit was the CF rod in the slot?

My first few classicals didn't have any sort of neck reinforcement...my last few builds have been Gore style builds with a truss rod and bolt on/bolt down neck/body joint.
Martin

RodneyStedall
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Re: Classical neck reinforcement. Yes or no?

Post by RodneyStedall » Wed Jul 18, 2018 8:20 pm

Spanish cedar neck . The carbon fibre is an easy fit and never has to be forced in. Likewise, the wooden shim which locks the rod into place is not a tight fit. All glued in with epoxy. I am thinking that the very thing which is supposed to give strength to the neck might actually be weakening the whole system. Hence my question? I am wondering if luthiers who build classicals without any neck reinforcement have had any problems with necks bowing. String tension being much less in classicals than steel string flat tops?

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kiwigeo
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Re: Classical neck reinforcement. Yes or no?

Post by kiwigeo » Wed Jul 18, 2018 9:31 pm

You mention a shim....do you have a photo of the CF rod and shim?

Yes, a lot less tension exerted by nylon strings. A truss rod can be considered "overkill" but fitting one has other benefits (better sustain with a neck under tension etc).

The only time I've had a crack develop along a neck was when I clamped the neck in a vice without sticking a couple of temporary wedges in the rod slot to stop the slot collapsing.
RodneyStedall wrote:
Wed Jul 18, 2018 8:20 pm
Spanish cedar neck . The carbon fibre is an easy fit and never has to be forced in. Likewise, the wooden shim which locks the rod into place is not a tight fit. All glued in with epoxy. I am thinking that the very thing which is supposed to give strength to the neck might actually be weakening the whole system. Hence my question? I am wondering if luthiers who build classicals without any neck reinforcement have had any problems with necks bowing. String tension being much less in classicals than steel string flat tops?
Martin

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Trevor Gore
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Re: Classical neck reinforcement. Yes or no?

Post by Trevor Gore » Wed Jul 18, 2018 9:36 pm

It's worth looking at what might cause the various deformations of a neck over time.

Broadly speaking, the direct loading is not much of an issue. Steel string guitars have smaller neck dimensions and higher string tensions and so the stress on a steel string neck is considerably higher. What is an issue is the cold creep over time due to the constant stress imparted by the strings and separately the "bi-metal" effect of gluing two pieces of wood together (e.g. cedro and ebony) which have very different stiffnesses and very different unit response to humidity change.

Then it's worth looking at what luthiers have done to counteract this movement and keep a guitar optimally playable.

1) Do nothing, and the neck moves a lot necessitating a high action to keep the guitar barely playable when the weather is not amenable, whilst all the time the action is slowly rising
2) Reinforce with an ebony insert in the back of the neck, to counter the effect of the ebony fretboard. If done well, this works in keeping weather related deformations under control, but does little for the cold creep situation
3) Add CF inserts. If 10 x 3mm bars just under the fretboard are used, this will add no more than 9% immediate stiffness to the neck as the CF is pretty well on the neutral axis and so takes little load. An additional 1mm of neck depth would result in the same increase in stiffness. The CF will take more load as the wood sheds it due to cold creep, but by then the neck deflections compromising playability have already taken place. The ~9% increase in stiffness will reduce the movement due to weather related issues by that same amount, i.e. not much.
4) Use an adjustable truss rod. If done well, no matter the weather or the cold creep the neck can be jacked as straight as you want it, any time you want it. The oft (spuriously) quoted downside is "too much weight". Well, if weight is an issue, why use a thick ebony fretboard? Realistically, the weight is not an issue (a Smallman classical weighs in at ~3.5kg), and having an adjustable truss rod lends a playability that those without can only dream of.

Regarding your cracked neck. Yep, it's wood. It does that. Occasionally. You got away with it 56 times, so celebrate! Probably nothing you did other than not spot a potential crack in the blank. And that can be hard. That's assuming, of course, it didn't meet with any other mischief along the way (humidity swings, over clamping, accidental drops, stress/vibration/over heating whilst machining, etc. etc.)

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Steve.Toscano
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Re: Classical neck reinforcement. Yes or no?

Post by Steve.Toscano » Wed Jul 18, 2018 11:29 pm

RodneyStedall wrote:
Wed Jul 18, 2018 5:09 pm
MY question is: what do other classical makers do. 1. No neck reinforcement or 2. A hardwood insert or 3. Carbon fibre. OR 4. Truss rod?
I put a truss rod in most my nylon string instruments nowadays with spanish heel body joins. The exception to this is flamenco blancas where im trying to keep weight down, and then i have used 2 6x4 CF rods. As Trevor stated though, CF rod on the neutral axis doesnt do a great deal.
I have used the light weight truss rod that ALS sells on one build which seems pretty good and is damn light.

As to your crack, sounds like its time to make a new neck :( :( :(

RodneyStedall
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Re: Classical neck reinforcement. Yes or no?

Post by RodneyStedall » Thu Jul 19, 2018 4:08 pm

Thanks to Steve and Trevor or your comprehensive replies. Unfortunately my construction method is traditional Spanish so making a new neck is basically out of the question.
I have to rethink my future neck support or not?

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kiwigeo
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Re: Classical neck reinforcement. Yes or no?

Post by kiwigeo » Thu Jul 19, 2018 4:30 pm

Like Trevor said...you've made 56 instruments without any problem so I wouldn't view one cracked neck as an indication you have a serious issue. I've built a few steel strings with truss rods and CF rods in the neck and had no issues.

One advantage of building with neck and body as separate units is you have the option to re-do a neck if you have issues.....this has happened to me on two instruments. It all boils down to weighing up the pros and cons of each building method.

RodneyStedall wrote:
Thu Jul 19, 2018 4:08 pm
Thanks to Steve and Trevor or your comprehensive replies. Unfortunately my construction method is traditional Spanish so making a new neck is basically out of the question.
I have to rethink my future neck support or not?
Martin

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