# CF rods position, parallel to the TR or the sides?

Discussion in 'Luthier's Corner' started by octaedro7, Jan 21, 2012.

1. ### octaedro7

I've been serching LC and doing some thinking about this. My former plan was to place them parallel to the sides of the neck. The line of thought is that they will better add reinforcement and stiffness if placed following the real direction that the strings run and closer to the edges, where especially the lower strings generate more tension.
On the other hand, placing them like that will cut the grain of a bigger area (and volume) of the neck than when they run parallel to the TR.

So I'm in doubt which way to go. In my case the neck wood is wengue, so making it weaker and then add the rods to reinforce it seems somewhat contradicting. But still I cannot help thinking about the the tension that the edge strings apply to the neck.

What are your thoughts on this?

Sep 3, 2010
3. ### roberthabraken

Jan 8, 2009
The Netherlands
There was a thread about this topic not long ago. I guess opinions were 50/50. I prefer parallel to the fingerboard edge, because it creates a triangle that way, which I think is stronger than two parallel rods. But that's entirely based on feeling, I do not have mathematical or scientific evidence.

4. ### harryboschBanned

Aug 18, 2008
Also to be considered is how you carve the neck. I like a neck with very little shoulder, placing the bars further away from the truss rod would effect how much shoulder can be removed. However, if the neck is wide like that of a 6 string, I do fan them slightly since there is more material with which to work.

5. ### Dirk DigglerSupporting Member

Mar 3, 2004
Anytown USA
How many strings on the neck? With Wenge I don't even think you'd need CF at all, even in a 6 string. I'm a bit puzzled by saying using Wenge is going to make it weaker, it's a very stable wood when cured properly. Unless you are talking about routing out for the CF, then I would agree.
Dirk

6. ### octaedro7

Yes Dirk, what I meant is that the cavities will run diagonal to the neck´s grain, hence there are more cut grain than when they run totally parallel to the TR. That´s why I said that Wengue is strong, but if I cut more grain by putting the rods parallel to the edges I sort of introduce a weakness to add reinforcements which seems kind of contradictory... Did I make any sense?
I love wengue precisely because it´s super strong (and beautiful may I add). I read on other posts your opinion that wengue doesn´t really need CF and I can totally see why it doesn´t. The thing is that one of my sources of inspiration is my Ibby 5006 which has an exquisite wengue/bubinga 5 piece laminate neck reinforced with KTS titanium bars. I wan´t to achive the same profile and stiffness, that´s the only reason I went for the CF rods.
...I don´t know, I just didn´t want to fall short.
BTW: It´s a 34" 6 string bass with .728" @ bridge and 2,16" @ nut.
Bajovaquero, I read somewhere different values per string but I don´t recall the source. I guess that the material and mass of the string and obviously the scale of the bass has a lot to do with the overall tension. I haven´t put any maths there anyway (and I don´t intend to ). Hopefully Pete, that is doing a PhD on string tension, can jump in .
Anyway the advantage I can see with the rods running parallel to the edges is that they seem to counteract the external strings tension better, but at the cost that I mentioned before.
Harry: indeed the effect on the neck profile is certainly something to consider. This is a 6 stringer so I probably wouldn´t have issues either way I take. I myself like less "shoulders" so if we were talking about a narrow neck width wise, then rods parallel to the edges could be a problem.

Nov 8, 2001
US-NY-NYC
8. ### octaedro7

Sort of a gauss distribution on a 6 string bass, isn´t it?

9. ### roberthabraken

Jan 8, 2009
The Netherlands
I do not think there is any penalty when cutting across the grain. The Wenge grain is pretty short and I do not see how cutting that makes it weaker. You're not cutting the sides of the neck parallel to the grain either, are you?

In general, if there's enough material left, it doesn't matter. A certain amount of wood taken out for the rod channel makes it less strong, the material placed back makes it stronger again (do not know if it'll become stronger than before), but the direction in which you do this doesn't influence the strength noticeable.

If it would, I personally still think the triangle would be more stiff, couteracting the disadvantage (if any) completely.

10. ### pilotjonesSupporting Member

Nov 8, 2001
US-NY-NYC
Angling the rods gets them closer to outside of the neck, which increases the resistance to torsion. This is desirable in a genera structural sense, but I won't speculate on whether this is desired musically.

I hadn't thought before about the fact that angled rods cut the longitudinal grain. If the rod is tight fitting and fully glued, you're essentially doing a very long, shallow scarf joint to the rod on both sides of the rod.

11. ### octaedro7

You got a point there.

I would think the same. If the cavities orientation have little to no counter effect to the neck strength then I also think that parallel to the edges should work better. Seems more natural to me...

12. ### octaedro7

So you´re saying that since with adding the rods and the glue you´re actually making it stronger, the effect of cutting longitudinal grain is despicable?

13. ### pilotjonesSupporting Member

Nov 8, 2001
US-NY-NYC
Despicable?? Deserving of hatred and contempt? Not what I was thinking.

What I had in mind is that is that a long, shallow, properly executed scarf joint should be only slightly weaker than undisturbed grain. But I don't know of any test of that, and there's also the issue of glue creep, not to mention the natural creep of undisturbed wood.

14. ### ehque

Jan 8, 2006
Singapore
You only can achieve a small angle by angling the rods in a normal bass neck. Since the torsional strength is proportional to that angle, I don't think it makes a difference either way.

15. ### pilotjonesSupporting Member

Nov 8, 2001
US-NY-NYC
Not quite. The torsional strength is increased not due to the angling, but due to the fact that the rods are (at the base of the neck) closer to the outside edges. Since the rod's contribution to torsional (twisting) rigidity is proportional to the square of the distance to the centerline, this is a significant measure.