truss rods/carbon fiber

[Clark_Kent]

I once again have the itch to build a new instrument.
The last one was a 10 string, baritone melody stick tuning, rear (exposed) truss rod.
For the new instrument I want more of a Warr or Megatar style (body, neck, can hang sideways), but this time I want 12 strings, still using the standard stick tuning.
I have no idea if two truss rods would work better than one, or if carbon fiber would be a good idea with one truss rod. I would prefer one truss rod simply to help keep the weight of the instrument down.
The neck will be one-piece quatersawn maple, as that worked very well for the last instrument, and it will be 34 inch scale.
Any thoughts from those of you with 12-string neck building experience?
Thanks,
Clark

[rjgoos]

To my knowledge, a 12-string Warr or Megatar uses two steel truss rods. This would be the "safest", but perhaps not the lightest, option.

To the engineers here....the standard measure of stiffness (Young's Modulus?)...is it based on cross-sectional area of the material being tested for stiffness, or its mass? I have read that carbon fiber is as stiff as steel....but is that on an equal volume or mass basis? The devil is always in the denominator.


Jay

[BigDaddyPoo]

Young's Modulus is the amount of deflection caused by an amount of force. E=stress/strain. Stress itself is pressure devided by cross-sectional area. So it does depend on cross section. The young's mod. of carbon fiber is about 20% greater than steel. So, I guess that strength compared to steel is based on cross-sectional area. Mass is no where in most strenght equations.

This is only based on a couple of charts in the back of my Mechanics of Material textbook, which is a class I just started taking, so there may be something else that comes into play that I'm not looking at.

I wish someone would make some truss rods out of micro-tube carbon fiber which is as much as 280% greater than steel. You could build hollow necks out of this material and it would be stronger than anything on the market now. It's probably 10 times more expensive than regular carbon fiber, though.

[BigDaddyPoo]

I agree with Jay though, you should probably use two trussrods on a 12 stringer. Maybe you could reduce the weight another way like using a headless design.

[RocknDrTom]

A headless design would help, and would be cool looking. I'd definitely go for two truss rods. The benefit is that you can correct "twist" which is more common with wider necked instruments than just a bow.

[BrendaEM]

For 12 strings, I would want 2 truss rods. I think that one rod could handle the tension, but I think that a 12 string neck neck is wide enough to twist too much with only one rod.

If you are worried about weight, perhaps the rods can be made thinner, and out of a stronger material. Perhaps you can use single-rod double-acting rods instead of double rods, but they will require more skill to install.

I thought about doing a carbon-fiber adjustable trussrod in my instrument. The thing that worries me, is bonding it together in a way that will not fail.

Certain types chrome-moly steel are very strong for their weight too, as is the T34 steel they use in British recumbant trikes. Titanium is strong, but it is almost impossible to solder, but it can be wielded.

[Clark_Kent]

The more I look at the design (and after reading your comments), the more I like the two truss rod idea. I think to control the weight issue I may make the body semi-hollow. I'd like 2-way adjustable rods, but I'll have to see about the weight with those (probably weigh as much as 4 rods).
A headless design would really look good, but I'd prefer to not deal with the engineering problems of tuners behind the bridge.

Clark