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carbon truss rods - are the needed?   You are logged in as Guest
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paul.troutfisher

Posts: 161
Joined: Feb. 16 2011
From: Danville, CA

carbon truss rods - are the needed? 
Hope this isn't a rehash of an old topic (did some looking first).

What are peoples thoughts on the carbon truss rods in cedar necks?
Here's some specific questions I have:
- do they really help keep the neck straight over extended periods?
- do they reduce neck weight a noticeable amount?
- how large (width/thickness) would you make them?
- what kind of glue to install them (epoxy)?


paul
  REPORT THIS POST AS INAPPROPRIATE |  Date Feb. 28 2011 20:54:33
 
estebanana

Posts: 9370
Joined: Oct. 16 2009
 

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
If you have good wood you don't need a CF rod. A few guys are putting them in as a matter of course these days, but select your wood well and have no worries.

If you have to do a repair on a guitar that has a whippy neck or it has bowed wildly, one of the repairs is to use Carbon Fiber rods to discipline the neck. I've done it a couple of times and found it useful. It saved a few medium quality classical guitars from the dumpster.

As a repair process, my feeling is that I'd rather go though other forms of getting the neck back in order before I resort to a CF treatment. The method I have used is to rout a groove in the neck under the fingerboard and press the CF rod, quite tightly fit, with epoxy. Then clamp it in the position it needs to be in while the epoxy cures. You can do the same think in new construction and end the CF rod just before it gets to the nut, letting it run out of the neck bock on the sound hole side.

I've used the 1/4" by 3/8" rectangular rod. People have been using this for repair work on older instruments too like mandocellos etc. where neck resets are not practical course of action.
CF will not really make the neck lighter, in fact you should summon up the incomparable Anders E. for his thoughts on neck lightness. He has some good ideas and observations.

In this case I would say fear not to spare the rod, as the guitar will be unspoiled.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Feb. 28 2011 23:48:45
 
Tam DL

 

Posts: 21
Joined: Jan. 1 2011
 

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
Carbon seems normally to get miss used in lutherie, at least structurally. The most common place to put a truss rod is at a more or less neutral axis (under the fingerboard). While the rod may feel very stiff when bent in the hands, the neutral axis to compression axis position means that the wood would have to have bent and possibly failed well before the carbon rod is fully deployed. This is not a bad way to use carbon in conjunction with wood, it is just that in the case of the guitar, one hopes the neck will not flex deeply so the carbon is largely wasted. The place to put it would be much closer to the thumb side of the neck.

Carbon may also help with rotational distortions about the longitudinal axis of the finger board. But these should not normally occur with properly selected stock.
  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 11 2011 15:14:46
 
estebanana

Posts: 9370
Joined: Oct. 16 2009
 

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
A cabon fiber rod is really not a "truss rod", it is a stiffening stringer. And I disagree with your assement of where to place the stiffener and how it works.

A true truss rod is an adjustable rod that can tension or de-tension the neck to change the neck relief. It's operated by a threaed component in the truss rod.

A carbon fiber rod does not adjust, it stiffens and it is glued into the wood of the neck directly under the fingerboard. Forward movement in the neck toward the bridge will be counter tensioned directly by the carbon fiber because it is in contact with the fingerboard and is an integral part of the strength of the neck.

The problem with putting it on the bass side is that there may not be enough room in the neck to sink the carbon fiber rod and still have enough meat on the neck to carve the neck to its 'D' shaped section profile. And if it will fit, it's not in position to take full advantage of the wood behind it to create the stiffening effect. The stiffening depends not on the carbon fiber itself, but on the carbon fiber backed by the as much neck thickness as possible. another factor in how stiff it becomes is in how tightly it is glued in.

Another engineering principle comes into play with tightness and gluing. Two separate beams can be made stronger by laminating them together to make a beam that deflects less because the beams can't slip passed done another. The same principle works in the way fan braces hold up the arch in a solera constructed top.

I also know carbon fiber rods act as stiffening stringers because I've repaired flexible necks by installing them. It depends on which carbon fiber dimensions are used and how tight the jib is done. Carbon fiber can also be used to straighten a curve in a bowed neck by gluing it up with neck in a tensioned position, like putting the neck in traction. When the glue is dry the carbon fiber and the neck together create a laminated beam and the neck will stay straight.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 11 2011 16:18:07
 
Jeff Highland

 

Posts: 401
Joined: Mar. 5 2010
From: Caves Beach Australia

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
I think when Tam says that the CF rod should be installed towards the thumb side of the neck, he actually means towards the back of the neck, furthest away from the fingerboard, rather than towards the bass side of the neck.

I have done this on a steel string guitar with a 4mm dia CF rod epoxied in place with a timber spline over it up to the fingerboard. It worked.

The key aspect is to have it bonded in position so that the neck can act as a composite rather than individual pieces (as estebana rightly notes)
In the past, many steel bar reinforced necks had the bar at the neutral axis, not bonded well, and hence were fairly inefficient for the weight they added.
  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 11 2011 17:06:28
 
estebanana

Posts: 9370
Joined: Oct. 16 2009
 

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
Putting carbon fibre on the back of the neck is a terrible idea. It would make carving the neck a huge PITA- Having to deal with the differnce in density and hardness between wood and carbon fiber would be horrible to shape and to what aesthetic end? And if it ever needs to be messed with you have try to work with it in an area that has cosmetic and playing performance issues. No way, bad plan.

There's a reason why those who theorize about stuff and those make and fix things on a daily basis things have differing opinions. Things that don't work get shaken out of use over time. Just because older steel sting guitars had a certain design does not mean it was good.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 11 2011 21:21:49
 
Jeff Highland

 

Posts: 401
Joined: Mar. 5 2010
From: Caves Beach Australia

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
Not on the back of the neck, that would be stupid for all the reasons you list.
Towards the back of the neck with at least 3mm of wood remaining.
Noone is suggesting putting it on the back of the neck, certainly not me or Tam
And I was not suggesting that old steel string designs were good, quite the contrary.
I am not sure what you are actually responding to?
  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 11 2011 23:34:13
 
estebanana

Posts: 9370
Joined: Oct. 16 2009
 

RE: carbon truss rods - are the needed? (in reply to paul.troutfisher) 
I'm responding to Tams post because it seems to born out of theory rather than actual hands on practice.

And yes I'm admittedly grouchy with and suspicious of people writing things in an unclear way and making sweeping generalizations about repair work or guitar making technique. If this happens then guitarists come down the road and say I read such and such about carbon rods and I'm scared blah blah will happen. Then you have to spend a bunch of time unraveling a problem in some guys head because he read confusing or incorrect information on the Foro.

I've participated in other guitar making forums for many years and if someone says something vague and does not use guitar makers vernacular it gets questioned. That way the advice givers get vetted out by the whole population of the forum. It's just a process of parsing out which people know which things from actually having done them.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Mar. 13 2011 15:37:02
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