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I recently came into possession of several neck blank sized pieces of thermally treated maple. Since I've been dying to see if the tap tone resonance I've observed could make this a holy grail tonewood, I'm planning to do some building in the near future. I have a ton of interesting test data on it as a material that should be ready to share in the next week or two.

To keep this simple, I will probably build it to fit one of my Strat bodies. My question for the rest of you relates to truss rods. While I understand the logic behind a curved channel, it seems to me that there is no reason a straight channel could work just as well provided the rod is installed below the neutral axis. I may end up being lazy and put together a two way rod... But this is always a nice group to bounce thought experiments around with. Thoughts/opinions/better ideas?

Since I don't have a curved rod channel jig ready to go, I might also consider a trade of services for some of this new exotic material. It's all from the northern latitudes with nice tight grain, and several pieces contain birdseyes.

Read why they are curved here. It's an older post but has some great information in it.

Reading through LC100's link I see no reference to what I think is a fundamental aspect of a single curved truss rod...as opposed to an off-axis straight rod.

Seems to me that by design or chance one might actually build a neck that requires little or no adjustment. In that case the curved rod will still apply side pressure to the channel and that will eliminate any chance of rattle. Now....one could argue that a curved rod could be adjusted with just the right amount of compression so it could flop around in the channel a bit but I think the amount of compression required to produce that effect is a MUCH narrower range than a straight rod. I think a straight rod could smack the sides even under small amounts of tension and compression.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Really? The very first post outlines a couple of the key aspects of a curved vs. straight rod.

Thanks for the link LC100, I actually came across that thread earlier this week. While interesting, particularly from a historical standpoint, it left me with more questions. This is where having an engineering background leads me to over analysis. A lot of the time instrument builders in the past were pretty sharp and along with trial and error may have already arrived at the best solution. Truss rods have me less than convinced.

From what I can see a properly positioned straight rod should provide a more appropriate stress on the neck to counteract string tension. My aim in this proposed design would be to machine the slot to perfectly surround the rod. If rattle is a much greater possibility, could it not be counteracted by cooling the rod to a much lower temperature and waxing it just prior to installation? Or even just waxing it up and filling the channel with epoxy?

The material I will be working with is so stable, I would have no reservations about building without any type of finish. It's dimensional stability in response to long term water soaks is staggering.

I may ultimately try a couple of different truss rod types.

Hopefully this discussion will continue. I suspect it could be interesting.

Hugh,

If you want to do the curved rod, rout the channel full depth, mill a filler piece to fit, cut the curve in the filler piece with your bandsaw, install your two piece filler strip and truss rod sanwich.
The second way is to send me your neck blank already pattern routed and I'll rout the appropriate curved slot for you.

Steve

I didn't say the post didn't give reasons to use a curved truss rod. I said it didin't speak to an obvious (fundamental) reason to use a curved rod.

A single, curved rod will have less propensity to rattle because it is bent, and is generally in tension along the channel....which pulls the rod tight to the side of the channel as it acts on the wood.....nothing to rattle.

I think the most important thing the linked post mentioned was the fact that with a curved channel one can control to a great degree where the truss rod will act along the neck...by shaping the curve of the channel accordingly. This is impossible on a straight rod. One would have to experiment but with cnc equipment the shape and path of the channel is entirely controllable. But you don't need a cnc machine to do this. It isn't difficult to conceive of a conventional method of creating a curved channel...and controlling the curved path of the channel. Assuming back tension...if you know you're going to need a little help in a certain area....that's where to decrease the radius of the arc.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I have a lot of experience releasing epoxy from metal. On green molds (fresh, polished metal...never treated before with any release agent) you CAN use wax but you want to be sure it's wax made for molding and releasing epoxy. Be carfeful...and ask the manufacturer specifically if it works with epoxy. Some do some and some don't. If you put only one coat of mystery wax on the rod and glue it....chances are good that's the end of your neck.

In any case, using a wax based release...you need to wax and polish roughly five times prior to molding.

Personally, I'd not use wax but rather, a polymer based release like Chemlease, Axel, Zyvax...ect. There are lots of brands. You still have to treat the rod as if it were a green mold but polymers go on much thinner and some are specifically meant for epoxy....which is pretty aggressive on surfaces. Given that the rod will be essentially surrounded by cured epoxy and that the release will have to be acheived by pulling the rod through that cured tube to effectively release it....the release agent needs to be as effective as possible....hence modern polymers.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

i am no pro, but i have been around/played/tinkered with electric guitars and basses for a good 30 years
i feel a (maple)neck with no truss rod void is considerably less likely to warp than a neck that has been routed for a bar/rod. the truss rod channel accounts for probably 20-25% of the neck volume, and probably contributes to nearly that much seasonal and situational bending.....i feel that for MAPLE neck electric 6 strings, that are kept tuned responsibly, there is no little to no need for a truss rod in the first place. granted i am no pro but i feel if good solid seasoned maple is used, then your average electric guitar, treated properly, is not going to exhibit warp/bend on the neck....the truss rod is like anti lock brakes, it compensates for lack of skill....
remember, my comments apply to MAPLE only here. my main axe is a late '80s american strat, that i have owned since '87.....it has been under constant string tension since then. plays like a dream. i havent screwed with the truss rod since the late 80s......initially i played with it "just because"
to sum it up: i think builders should re evaluate the necessity of an adjustable truss rod in maple necked axes......

I would have to disagree here. The reason to put truss rods in necks is not just for structural support but so you can adjust the bow to be exactly how you want it. If you decide you want a different gauge string one day, your neck bow is gonna be off and the only way to change that is with a truss rod. So even if you used your "skill" to make your neck perfect to never need adjustments, it will only be "perfect" with one gauge set of strings. And unless you make a very thick neck, you're probably gonna need some adjustment from the getgo.

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http://www.tinyhouseandland.com

You don't need a jig to cut a curved channel.
Just draw it up and route it in steps at increasing depth towards the centre of the neck then blend them together with a chisel.
I would have to agree that a curved channel is vastly superior for this application.

A curved rod only has to be tightened a smaller amount than a straight rod would have to be. With a curved rod the wood of the neck and the rod itself can both be more relaxed and vibrate easier. With a straight rod. if it is going to do anything it is going to have to squeeze that wood hard in the long axis.
A nicely made curved truss rod lets you adjust the neck with fingertip turning of the nut. Most experimental rods need to be reefed on hard with a long-handled wrench, and then the neck feels funny, less like wood and more like metal.
For the engineer: by keeping stress out of both the metal rod and the wooden neck you lessen the wave reflections caused by wave impedance mismatching. These reflections are within the neck as well as at the heel and peghead where you transition to simple wood.
Banjos sound best when their assembly nuts are just tight enough, not overtightened.

Thanks to everyone for your input. I'm still curious enough about the topic that I'm considering building one of each style and putting them through their paces in an Instron. This is likely a case of, curved rods work well enough to get the job done. Taking classes such as strength of materials, statics/dynamics, and vibration analysis has a way of warping the mind... which is what you've seen here. Due to varying thickness along the neck profile, its deflection pattern probably wasn't examined much when the first curved truss rods were installed. The next step I might examine would be creating an optimized truss rod curve just to see it adjustment control would be any better.

I love the interplay of art and science in musical instruments, and having access to fancy/expensive engineering tools tends to encourage my drive to understand the science behind the art.

I can't reiterate enough what a great community this is. Dare I say it is unlikely that a better collection of experts in working with guitars exists anywhere outside this forum. You guys rock!

Not the main topic, but I've always wondered why rod rattle is such a big issue for so many people. You can tighten the rod just enough to get rid of the rattle without putting enough tension on the neck to effect any change in geometry. It's like stopping your car from falling backwards down a hill at a stop light using just the gas.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services