truss rods

[Max Girouard]

If it's true that a straight rod deep in the neck is all thats needed, then what was Ted McHugh thinking when he went with the curved rod? My best explanation is to have the adjusting nut far into the peghead. About the only place you can put the adjuster with a straight rod is right where the rod enters the peghead. That seems to limit the leverage to me.

The reason the truss rod is bent, is because both ends are secured with a washer and nut. If the rod was straight, there would be nothing preventing the entire rod from spinning in the slot which would render it useless. Most builders who use a straight rod will either use an anchor or a simple bend of some sort at the end of the rod.

[Jim Hilburn]

To me part of that equation is the long curved heel which allows the deep set. To me the "straightening" force is spread along the entire rear downward curve and that is in the thickest most massive part of the neck. But at the same time the nut end which is the thinnest and weakest part will give long before the rod can straighten. The other thing to remember is we're only trying to counteract the upward pull of the strings. Your just trying to slightly move the fingerboard in the first 5 frets or so. The rod isn't intended to fix any other action problems other than relief.

[Jim Hilburn]

Not so sure Max. Seems only a slight bend would solve the spin problem but they seemed to go out of their way to sink it so deep in the back.
Still only conjecture on my part though.

[Max Girouard]

Very helpful to see that picture. So what happens when you tighten the curved rod (meaning increased tension)? It looks like it would try to straighten and also apply pressure to the bottom of the channel, which would add to forward bow. My brain hurts.

Check out post #8 in the thread linked below, Adrian does a great job of explaining it.

https://www.mandolincafe.com/forum/t...ss-Rod-Profile

[Wrnchbndr]

uhhh... I install my trussrods in a reverse orientation to the ones shown in the pictures. I do it like Gibson and Fender do it on their guitars. They work. ...totally different principles involved.

[Parker135]

Check out post #8 in the thread linked below, Adrian does a great job of explaining it.

https://www.mandolincafe.com/forum/t...ss-Rod-Profile

That was really helpful. I also read some comments there similar to mine about the action of the bent rod being counter-intuitive. It was fascinating to see the X-ray of the old Gibson neck as well.

[pops1]

Isn't the end in the heel welded to the rod so the rod can't spin. Most I have seen one anchor does not turn. The nut end only turns, or in the case of a guitar soundhole adjustment the opposite end.

[sunburst]

Isn't the end in the heel welded to the rod so the rod can't spin. Most I have seen one anchor does not turn. The nut end only turns, or in the case of a guitar soundhole adjustment the opposite end.

Nope. In the old Gibsons there is a nut and a washer at each end of the rod.

[John Bertotti]

Is there any reason not to use a rod adjusted through the sound hole on A and F oval hole models?

[pops1]

Is there any reason not to use a rod adjusted through the sound hole on A and F oval hole models?

Breedlove does it with a special tool thru the endpin.

[darylcrisp]

this may or may not add to the discussion, here's an xray i took of a Collings MT without strings, around 2015

[Parker135]

I assume with the rod set that deep in the neck, it's capped on top with a carefully fitted and glued wooden strip to restore some torsional stiffness to help guard against twisting. For those of us not set up to do this on a table saw, is it practical to use a router for such a deep narrow (and necessarily accurate) slot?

[Jim Hilburn]

https://www.mandolincafe.com/forum/t...ional-trussrod

I'm hoping this will link to a thread I posted in 2012.

[Parker135]

Jim: It does, and thank you! It looks like that's going to answer my questions, and it appears this can be done without a table saw.

[Jim Hilburn]

A straight rod slot like the Collings can be done by gluing an elevating board near the nut although I remember using a couple of screws into the part that would be cut off to get the shape of the neck. Then just do multiple passes although the last pass might make you pucker up. But you can move the neck along side the bit and see just how deep it's going to cut.

[Jim Hilburn]

I just looked up Ted (Thaddeus) McHugh's trussrod patent. While he didn't explain why the rod was curved he did say one reason for the invention was so you wouldn't have to be so selective as to what wood you were going to use. About saving the company $.

[Martian]

hey guys , I posted this, so as to learn more about the function of the truss rod, and really learned a lot, and a shout out to Jim H, on that post you did in 2012, , thanks. I also use double face tape alot

[Martian]

also mr Hilborn, where you end the curve, do you have a bend in the rod that goes down into the heel? and are you making your own rods, ? thanks

[Jim Hilburn]

Cumberland Acoustic sold and I assume still sells a straight rod with a welded stop and comes with a great small adjusting nut. I just shaped it to my needs. A little shorter than as shown on Adrians plan and required giving up a little on each end.

[darylcrisp]

excellent thread, and thanks to everyone chiming in. especially like the posting of the older threads since I had not read those. lots of experience and knowledge being given, for many of us to learn and digest.

d

[Phil Goodson]

Am I thinking wrongly or isn't the rod only important to allow you to shorten the distance between the end nuts of the truss rod.
Shortening this distance simply squeezes the wood between the two nuts.
If all this is happening in the back half of the neck, it should slightly shorten that part of the neck and cause the neck to curve backward. (Helped by the stiffness of the fingerboard.)

So any configuration of curves or bends in the rod itself is insignificant. It's not the up/down movement of the rod that does the job.

Am I wrong?

[Parker135]

I agree, assuming the rod is stiff. At the other extreme, a cable, tightening would seem to induce forward bend as it pulls over the curved slot bottom.

By the way, thank you for your post. It got me rethinkng the curved rod idea.

[Jim Hilburn]

These xrays, one Gibson and one Collings help to illustrate the leverage I've mentioned. Compare the adjusting nut location on both.

[HoGo]

These xrays, one Gibson and one Collings help to illustrate the leverage I've mentioned. Compare the adjusting nut location on both.

You can aslo see how the nut of Collings falls right into the weakest area of neck and on Gibson it is further up te headstock in the wide part.
I asked professor of physics about the curved rods and simplified answer is that basically the important is where the nuts are if the curve is not too radical. The force of rod against wall of the slot is negligible when compared to the compression force and resulting bending (since the whole rod is below neutral axis anyway - the axis is somewhere right at the fingerboard joint).
Here is what I'm doing. I cut straight slot and drill angled hole from heel. I use tiny U shaped gouge to and finger shaped scraper to deepen the transition at the bend adn at the nut end. Has all the advantages (not prone to twisting, deep position and adjusting nut high on headstock) and is easier to cut than curved without need for shaped templates or jigs.