"Exposed truss rod" design opinions

[gormsby]

Since @ZachM linked to Millimetric Instruments in a different thread last month, I keep coming back to how they expose the truss rod nut in some of their designs, like the MB4 (Millimetric instruments). See the link for a gallery, some pictures of which show what I'm referring to quite well. Basically, instead of burying the truss rod nut fully under the fretboard and string nut as with traditional headstock access, they instead allow the truss rod nut to protrude into a, I guess I'll call it a "bay" cut out of the headstock and connecting to the truss rod channel. The angled headstock also comes in to play as I'll explain.

We have a few threads here discussing the benefits of a straight truss rod route, meaning the ends of the rod fit within the same channel cross section as the main body of the rod, so that no widening is needed for either end of the rod once the main channel is cut. Such a fit means that theoretically the rod may be removed (when not in tension) for potential repair or replacement by pulling it out the access hole, without the need for removing the fretboard or skunk stripe.

The logic of this seems sound as far as it goes, but it still leaves the complication of how to get a good hold of the rod in order to pull it out. There are more options for destructive removal (i.e. destroying or damaging the rod), but I'd characterize the question of "how to grab the rod" as non-trivial, even if the straight pull makes it easy to remove once you have a good grip. Arguably, this is not a common need, and further optimization only addresses rare cases and occaisional need. But still...

What I love about the Millimetric Design is that it takes the "theoretically easier" benefit of the straight-route approach and turns it into a trivial practical operation. The angle of the headstock along with how the "bay" is cut out means that you could, if needed, slacken the rod, grab the nut with a good pair of pliers, pull it out, and slide a new one in without any of the hassle of "how to grab it" that traditional designs leave us with.

Now for my question: Setting aside the aesthetic issue -- This approach is certainly a "form follows function" design and many may dislike exposed hardware like this -- does anyone here see a technical or perfornance trade-off in exposing the truss rod nut this way? For the sake of argument, let's compare only with traditional headstock-access designs, setting aside (I think) the comparative neck-to-headstock-strengthening benefit of heel-access rods.

I'm thinking of trying this in my next build for myself, but would love to see if there's some trade-off or gotcha that I might be missing.

 

[gormsby]

I'll note too that this same easy-removal advantage accrues to heel-access designs that use a spoke-wheel nut, so it's not the only option in town in that regard. Nevertheless, it opens up options for headstock-access approaches.

 

[Gilmourisgod]

Isnt that already true of almost any DA rod? If the headstock has some downward angle, and you don't have a wood strip glued over the trussrod at the adjuster end, what's to stop you from pulling the rod out through the hedstock end?

 

[gormsby]

Isnt that already true of almost any DA rod? If the headstock has some downward angle, and you don't have a wood strip glued over the trussrod at the adjuster end, what's to stop you from pulling the rod out through the hedstock end?

I guess I'm thinking of the "getting hold of the rod" part -- needlenose pliers, I suppose, or jamming something into the presumably-not-working-anyway hex-key hole, but the nut is normally surrounded by wood on four sides, so some work is involved. And if it's under load, all the more difficult for a that tenuous purchase to lead to success.

 

[gormsby]

The analogy I forgot to include was that this moves it from a "shop operation" to a "field operation" -- At the gig - "Shoot, truss rod's broken. Once sec...have a spare in my bass case...[crunk]...[swoosh]...[tighten]...OK, we're good to go."

 

[Lo-E]

The analogy I forgot to include was that this moves it from a "shop operation" to a "field operation" -- At the gig - "Shoot, truss rod's broken. Once sec...have a spare in my bass case...[crunk]...[swoosh]...[tighten]...OK, we're good to go."

I’m a pretty prepared guy at gigs. I bring spares of practically everything.
…but even I see this as a solution in search of a problem.

 

[gormsby]

I've had just one truss rod nut ever shear off under virtually no load and I'm never going to forget it.

 

[Beej]

I've not read anywhere that he explains it, but I would hazard a guess MM's design allows him to use the same size rods across multiple designs or scale lengths.

 

[Murivan]

I am not sure if I fully understood the design and problems you are describing but I can tell you that I only needed one hand and a small amount of force to remove a truss rod from a Ken Smith bass.
Smith and Fodera use a Dual Rod-Single Action design, and the entire assembly sits in a straight rectangular channel. I believe that Warwick pre '96 had a very similar design. You can search this forum for more pics.

 

[Bruce Johnson]

I'll step up as the old grey-haired Luthier wagging his finger at the sky and ranting:

The overwhelming reason why truss rods break is because they have been overtightened beyond their capacity, while trying to straighten a warped neck. And if the neck has warped enough that you break the truss rod trying to straighten it, then it doesn't really matter if you can rapidly pull out the broken truss rod and slip in a new one. Because the new one will break at the same load point. The neck is still warped, and still requires the same amount of load to flatten it. If you don't fix the warp with some surgery or whatever, you'll just keep breaking truss rods.

If you put the effort into building your necks so they don't warp, then you'll never have problems with truss rods getting broken.

And, if you design your necks so that the truss rod can be easily removeable, the truss rod probably won't be as powerful, in mechanical ratio. So, the point at which it breaks is lower. And you'll have more breakage.

Rant over. Sorry.

 

[Bruce Johnson]

I've had just one truss rod nut ever shear off under virtually no load and I'm never going to forget it.

So, why did that one break? Bad manufacturing quality? Was this a bass that you built, or from a manufacturer?

 

[rudy4444]

I use a through channel on all my instruments. I add a slightly enlarged section at the head or heel of the neck depending on where the adjustable end will go. If the adjustment is at the headstock then I close off the other end with a small plug that can be easily removed, but DA rods are easily removed from the adjustment end provided the channel has been cut to extend outward to allow the rod to slip out easily.

I've never had a rod malfunction, but I do think it's a good idea not to build in a manner that makes future work extremely difficult should the need to replace a rod ever crop up. It's so easy to add that feature now that DA rods are so common. Back in the original days of true "truss" designs this wasn't really an easy detail to add.

Seems a bit of a "no brainer" to me.

Photo shows heel end adjustment, a plug is glued in place at the headstock end. I have a long brass rectangular rod that is used to "ram-rod" small slightly dampened wads of paper towel to remove ALL glue squeeze out when the fretboard is added. The channel is cut for a loose slip fit of the truss rod.

Headstock pocket cover:

 

[gormsby]

So, why did that one break? Bad manufacturing quality? Was this a bass that you built, or from a manufacturer?

This was one I had just built. I'm chalking it up to it being a bad build quality outlier. One of the last DA rods from LMII. Just starting in on the fret leveling process and when I started to put some load on the rod to get just the start of a backbow, starting from slack/neutral, the nut sheared off. Barely any pressure on the wrench. It was a heel-access rod so with the nut missing, the remaining end was a good inch or more inside the channel, and couldn't figure out a good way to get at it. I was already unhappy with the fretboard at that point for other reasons, so I ended up just cutting off the fretboard and getting the rod out that way. New rod, new fretboard, and now the neck is in active service with glowing feedback from the owner.

 

[gormsby]

Photo shows heel end adjustment, a plug is glued in place at the headstock end. I have a long brass rectangular rod that is used to "ram-rod" small slightly dampened wads of paper towel to remove ALL glue squeeze out when the fretboard is added. The channel is cut for a loose slip fit of the truss rod.

So the channel is accessible at both ends, and should the need arise, you can extract the rod out one end by pushing it out from the other. And it looks like your headstock channel allows the slot to run all the way out, so if you do have to push it out through the headstock end, the rod can exit straight (where the typical shorter channel allows a wrench to get in, but not so much a rod to come out.) I like it.

 

[gormsby]

And, if you design your necks so that the truss rod can be easily removeable, the truss rod probably won't be as powerful, in mechanical ratio. So, the point at which it breaks is lower. And you'll have more breakage.

Comparing these "slide in" approaches with your epoxy and carbon fiber backstrap method, yeah, I can see where there's some mechanical loss given the overall looser fit.

To compare apples to apples, assuming a sub-optimal looser channel design as a constant: In the Millimetric design, the full load bearing part of the rod is still enclosed fully within the fretboard and neck - only the adjustment nut is outside the loaded area. Generally, we don't want the nut bearing downward load (or upward, if inducing forward bow in a too-stiff neck) in any case, right?

 

[Bruce Johnson]

Comparing these "slide in" approaches with your epoxy and carbon fiber backstrap method, yeah, I can see where there's some mechanical loss given the overall looser fit.

To compare apples to apples, assuming a sub-optimal looser channel design as a constant: In the Millimetric design, the full load bearing part of the rod is still enclosed fully within the fretboard and neck - only the adjustment nut is outside the loaded area. Generally, we don't want the nut bearing downward load (or upward, if inducing forward bow in a too-stiff neck) in any case, right?

My point about the lower mechanical ratio is that, if you are going to make the neck's slot a straight pass-through, so that the truss rod can be pulled out, you are probably going to be using one of the low profile DADR rods. And they don't have the mechanical power of the high profile rods.

And yes, in any DADR rod installation, there should be some small clearance around the nut. In particular, it should not be pressing against the bottom of the slot.

 

[rudy4444]

My point about the lower mechanical ratio is that, if you are going to make the neck's slot a straight pass-through, so that the truss rod can be pulled out, you are probably going to be using one of the low profile DADR rods. And they don't have the mechanical power of the high profile rods.

And yes, in any DADR rod installation, there should be some small clearance around the nut. In particular, it should not be pressing against the bottom of the slot.

It's absolutely necessary to provide a small bit of clearance, otherwise the adjustment nut can contact the slot bottom as the rod "arches up" when the rod is tensioned. That will place an extreme amount of stress on the weld that joins the nut to the threaded rod.

I route the channel using a router table and form the additional clearance by leaving the fence in position and switching to the next larger bit. Adjust depth of cut 1/16" deeper and you end up with a nice pocket with 1/16" clearance for the nut in all directions.