Curved truss rod?

[putnamm]

I've devised a jig to cut a curved truss rod channel in my necks, and I plan to cut the same curve in to the filet/cap that fills in the neck over the rod. I'm using welding rods for my truss rods.

Does the rod itself also curve before installation to mirror the curve in the cavity/filet? Or is it installed straight? I've found conflicting information on this. In the end, I guess it doesn't matter since when the rod is "sandwiched" between the neck and the filet it should be bent anyway...

[sunburst]

In the end, I guess it doesn't matter since when the rod is "sandwiched" between the neck and the filet it should be bent anyway...

Yep.
Best I can tell, the old Gibson curved rods were pre-bent, but how would we really know and what difference wold it make whether that bent them before installing or bent them while installing? Sure would be easier to pre-bend, so I vote for that.

[putnamm]

Thanks, John. My only thought is that maybe pre-bending takes some of the spring out of the rod? But I'm not a physicist...

[Richard500]

Thanks, John. My only thought is that maybe pre-bending takes some of the spring out of the rod? But I'm not a physicist...

I am. Also familiar with welding rods. Most steel rods are quite soft, so it wouldn’t matter, but don’t you want something with the most tensile strength for its diameter? In general, soft metals have less strength than hardened/tempered ones. But yes, severe bending can work harden some steels; can’t imagine any sharp curves in a truss rod.

[putnamm]

Richard,

No sharp curves except the 90 degree bend to insert the truss rod in to the "fat" part of the neck near the body. Otherwise it's a pretty gentle curve.

Re: "tensile strength for its diameter." This is only my second build (and actually third and fourth as I'm planning to do three at a time). I'm not using fancy materials like torrified spruce or figured maple, and I'm trying to keep things economical. Really focusing on my process to determine if my results are replicable. So while I'd love to drop the money on something like three carbon fiber rods or dual action rods, I'm still going to have to buy tuners and other hardware when I'm done with these...

Also, welding rods and the like seem to have worked for more experienced makers for many years. They are readily available. (Ace Hardware is less than a mile from my house.) And I already have experience working with them.

[sunburst]

...don’t you want something with the most tensile strength for its diameter? In general, soft metals have less strength than hardened/tempered ones...

I switched from drill rod to welding rod for making truss rods. Welding rod has plenty of tensile strength for a truss rod, and harder steel can break if it is bent sharply or welded (DAMHIKT), unless it is annealed thoroughly. Welding rod does not harden so much as to become brittle from bending or from welding.

As for spring in the rod, there isn't any (to speak of) in the old ones, and there is none needed. The rod works by tension.

[HoGo]

BTW, what curve are you using? From the 90 degree bend at the end I suppose the "inverse" Siminoff curve?

[Doug Freeman]

On the very day you pose this question I notice this new build-in-progress photo from the site of a fairly well-known Australian builder. Guess that's a +1 for pre-bending!

185089

[putnamm]

BTW, what curve are you using? From the 90 degree bend at the end I suppose the "inverse" Siminoff curve?

I did get the 90 degree bend from the Siminoff book. But my jig (and thus the resulting curve in the rod) is based on plans for early 1920s Loar F5s--the origins of which I can't remember, actually... Those plans called for a thread and nut sitting in a depression on the heel end of the neck as well. But I think I'm just going to stick with the Siminoff "bend."

[putnamm]

On the very day you pose this question I notice this new build-in-progress photo from the site of a fairly well-known Australian builder. Guess that's a +1 for pre-bending!

185089

Doug, can you share who builder is or the URL of their website?

[luthier88]

Doug, can you share who builder is or the URL of their website?


fairly well-known Australian builder...

[Doug Freeman]

Doug, can you share who builder is or the URL of their website?

http://www.gilchristmandolins.com/current

[Nick Gellie]

Pretty sure it is Steve Gilchrist. He is a purist at heart.

[j. condino]

About eight years ago, I asked ten of the best builders in the world some very specific questions about truss rod materials, design, and shapes. The response surprised me- about 27 different answers, everyone the opposite of the other. Curved up, curved down, straight, single, double....so much that I was even more confused. The answer was simple: I made up ten different necks with ten different truss rods and built them up complete to be able to test every one and record the results.

Guess what? They all worked almost identical, so I chose to use a straight rod primarily because of its ease of manufacture and installation. The one major difference is that I make my own out of titanium, for the weight difference and there is something a bit difficult to describe, but it tends to have more of a responsive snap and spring rate that seems more musical to me than a dead thump of a piece of steel.

[putnamm]

About eight years ago, I asked ten of the best builders in the world some very specific questions about truss rod materials, design, and shapes. The response surprised me- about 27 different answers, everyone the opposite of the other. Curved up, curved down, straight, single, double....so much that I was even more confused. The answer was simple: I made up ten different necks with ten different truss rods and built them up complete to be able to test every one and record the results.

Guess what? They all worked almost identical, so I chose to use a straight rod primarily because of its ease of manufacture and installation. The one major difference is that I make my own out of titanium, for the weight difference and there is something a bit difficult to describe, but it tends to have more of a responsive snap and spring rate that seems more musical to me than a dead thump of a piece of steel.

Thanks. It seems like the answers to most of my questions end up being similar--i.e. there's lots of different ways to do it, and they all work about the same. That's good to know.

As for manufacturing my own titanium rods... I don't think I'm quite set up for that yet!

[pops1]

Straight rod, threaded ends, all you need is a die to be able to thread the titanium to the nut thread size you choose. Not that big of an investment. I like the titanium rod idea, thanks for the suggestion grandcanyonminstrel.

[sunburst]

Straight rod, threaded ends...

Simply threaded ends work just fine for curved rods because the rod cannot spin in the channel (or at least it takes an undue amount of torque to get it to spin). A straight rod can spin in the channel, so the captive nut at the heel can potentially unscrew, or the whole rod can spin rather than tighten. Locking the captive nut somehow is good insurance that it will not unscrew. Not sure how to do that with a titanium rod.
James, what do your titanium rods weigh?

[putnamm]

Simply threaded ends work just fine for curved rods because the rod cannot spin in the channel (or at least it takes an undue amount of torque to get it to spin). A straight rod can spin in the channel, so the captive nut at the heel can potentially unscrew, or the whole rod can spin rather than tighten. Locking the captive nut somehow is good insurance that it will not unscrew. Not sure how to do that with a titanium rod.
James, what do your titanium rods weigh?

Siminoff's book recommends inserting the welding rod truss rod through a drinking straw before placing in the slot and gluing. Would the same need to be done with a curved truss rod, since it can't spin anyway?

[Richard500]

Now we’re getting somewhere! In case anyone would need to experiment, McMaster-Carr sells small lengths and diameters of titanium materials and fasteners. A rod of 1/8” comes in different grades and ranges below about ten bucks. You always pay a premium for small amounts of materials. You could certainly buy a great deal more as wire coils from other metals suppliers. The grades refer to tensile strength, just like steel bolts. Threading isn’t an issue, and locking the fixed end is exactly the same as any rod; anything to keep that end from turning.
I suppose somebody has also experimented with interesting materials for the acoustic parts too. Forget the HHG and buy a TIG.

[amowry]

I did a bunch of tests with various rods too, and the straight compression rods seemed to work as well as curved ones, so that's what I used for about ten years until I switched to double-action several years ago. With the straight rods I just threaded the heel end and screwed it tightly into a barrel nut with some permanent Loctite for good measure, and then peened over the end. I haven't heard of any problems with those and I like the ease of installation of a straight vs. curved rod.

[sunburst]

Siminoff's book recommends inserting the welding rod truss rod through a drinking straw before placing in the slot and gluing. Would the same need to be done with a curved truss rod, since it can't spin anyway?

The straw is to keep glue off of the rod. If the rod is glued in place it will not work, or will not work correctly. We simply need to keep glue off of the rod, and we can do that without using a straw so straight or curved, the straw is not necessary.

FWIW, I use straight steel rods that weigh 23 grams. The material has a tensile strength of 70,000 PSI. I'm not enough of a math geek to figure out how much tension the rod can withstand, but I feel confident that I cannot apply enough tension to cause damage to the rod with a 1/4" nut driver on a brass nut.

[Dale Ludewig]

Well, John may not use a straw but I always do. On the truss rod. Almost never with a glass.

[j. condino]

James, what do your titanium rods weigh?

I'm not sure what the actual weight is, but in hand they seem to be about half the weight of a similar steel model- noticeable, especially when you get into longer lengths like octave mandolins and guitars. Probably 20 years ago, Lawrence Smart gave me a couple of solid titanium beams to use as neck stiffeners I liked the sound of those mandolins so much that I continued to experiment with the material.

Now, I make my own anchor nuts out of larger 3/8" titanium rod then mill and tap them to fit the smaller rods before they go out to a custom weld shop. It is more work, but I've been doing it for about ten years & will likely keep doing it. Most of the time I'll just make up a batch for the next six months work that will have mandolin and guitar family sizes so I can send out a batch to the other folks who help me. That said, on my personal mandolins, I almost always go with a simple fixed solid carbon fiber rod....

The main thing I got out of all the curved rod research was that it was primarily an issue of not having the rod slip. Even Gibson sometimes used curved up on one batch and then curved down on the next. I used to just bend a 90 degree angle in the rod and anchor it in the heel, but testing showed a lot of tension and stress load in that bent end and I've repaired several other builder's instruments where the bend actually broke over time. I like the clean straight pull better; 'bonus that it is easier to manufacture.

When you build and restore several dozen upright bass necks a year, mandolin necks are pretty easy and affordable. I could build about six complete mandolins just from one double bass neck blank!

Ebay is a great source for 3/2 Ti rods; often around 1/10 the price of similar rods from Stew Mac or other suppliers. I bought these for $3 each. 'Same thing when you buy carbon fiber direct from JB Allred for $7 a stick....

Here are a couple of photos I posted last year of the basic design. Note that the longer middle two have not yet been threaded, but the one in the dovetail has. I like the smaller profile LMI allen head truss rod nuts rather than the much larger Gibson style nut that needs twice as much material removed from the neck. Note the heatshrink over the inserted rod in the second photo (same application as a straw):