Eric Reid wrote:
And Why They're Not:
I don't have any issue with the concept of double-acting truss-rods, but every thread needs a Devil's Advocate, so with the court's permission, may I approach the bench?
Let's start with the obvious (and most important): double-acting rods can create problems that don't exist in guitars with single-element rods, non-adjustable rods, or no neck reinforcement.
It's been a while, but I used to do quality evaluation for double acting truss-rods for a small manufacturer of high-end guitars. Here's a list of problems I encountered on a regular basis with the double-acting truss-rods:
#1: neck buzzing--I saw hundreds of these. To be fair, most of these didn't buzz when played. (They rattled like the devil when you rapped on the back of the neck with your knuckles.) Some that buzzed when playing were cured with a little tension on the truss-rod (and a little loss of playablity because now the neck relief wasn't optimum). Some of those that didn't buzz initially would buzz at random moments of humidity change--lots of fun for the repairman.
#2: Weld-fouled threads--I came into guitar building from a background in racing bicycle manufacture. All of the welded truss-rods I've seen have had amateurish welds. Let me say that again: ALL OF THE WELDED TRUSS RODS I'VE SEEN HAVE BAD WELDS. I'm not as much in the loop as I used to be, but every time I see a discussion here with pictures of a welded truss-rod, it takes me back. Fouled threads will limit adjustment.
#3 Cracked welds--ALL OF THE WELDED TRUSS RODS I'VE SEEN HAVE BAD WELDS. I would say that at least 70% of the welds I looked at had substantial cracks. In a very small percentage of the welds, the crack had propagated far enough to render the rod useless. Cracks propagate over time.
#4: Limited adjustability--Some designs rely on a difference in thread pitch between the two ends to achieve the relief adjustment. This results in a very precise adjustment that bottoms out very quickly. When you're done, you're done--at that point, you might as well not have a truss-rod
#5: Corrosion--Over time, the threads on any truss-rod may corrode. When that happens, you may be back to the repair options of a no-truss-rod neck. Multiple element truss-rods (single-acting, or double-acting) have more places for corrosion to occur. If the protocol for installing the truss-rod involves some sort of silicone sealant, it's important to note that most silicone sealants are corrosive. (Again, time is your enemy.) Non-corrosive silicone is your friend.
#6: Stripped hex socket. Many of the truss-rods I saw had poorly formed Allen wrench sockets. It was not uncommon for an accurately sized Allen wrench to spin in the socket of a truss-rod head that had been formed with an over-sized hole, and a hex-broach that just touched the corners.
#7: Truss-rod blows out the back of the neck. Rare, but this has been mentioned several times on the forum.
#8: Weight. For some playing styles (flamenco), neck weight is critical. Maybe Stuart's designs would solve that problem, but most truss-rods are a good deal heavier than Spanish cedar.
#9: Canary in the coal mine. There are several guitar problems that serve as a warning for something more serious. Low action and buzzing strings? Fret ends sticking out? Excessive relief? All of these are symptoms that precede cracks.
Much different experience with DA truss rods here:
1). Buzzing truss rods because they are double action is not been something that we've seen much of. On rare occasion an unengaged rod can rattle but simply just barely engaging it does the trick. Barely engaging need not fundamentally change that perfect relief. I would disagree with your contentions here. Buzzing rods do exist but the percentage of guitars doing this because of the truss rod is pretty minimal AND not exclusive to DA rods, SA rods can do it too.
I'll add that some of the rods that are actually more well known for rattling are SA rods. Fender for example can do this and that's why there is a repair person's fix of removing the dots, installing set screws that engage the rod mid length to support it and covering up the set screws with the dots reinstalled. We've done this on these rods and have never had to do it on a DA rod.
2). The Allied or Blanchard rod had better welds and I've not experienced fouled welds. Sloppy welds yes, welds that need to be filed, yes but not fouled welds. LMI rods from say ten years ago did have some bad welds and some welds that failed but this has not been my experience with the Blanchard rod or the Martin DA rod.
3). Never saw a cracked weld but again did read about some here and it was always the older LMI rod ten years ago or so. Lots of threads here advising testing the weld prior to installation.
4). I've never had a rod bottom out because it's a DA rod. Instead the Blanchard rod with different threads has the advantage of very easy adjustments making any damage to the adjuster from the wrong wrench less likely because it requires very little engagement force to turn the adjuster.
5). I've not seen or experienced any corrosion with the Blanchard "stainless....." rod or Martin DA rod. Corrosion is not limited to DA rods, SA rods do it too. Step one with G*bson SA rods is removing the adjuster and lubing it up because they corrode and commonly do so, again this is a SA rod.
6). Using the wrong wrench is not exclusive to DA rods either, it happens with SA rods and I would again argue that rods that require more force to use are more likely to eventually have damaged adjusters from improper wrench sizes.
Fender SA rods have a very shallow engagement depth and are known for breaking off the tip of the hex wrench (and getting it stuck in the adjuster...). I am unaware of any DA rod known for a shallow adjuster.
7). The only blow outs that we've seen and we've seen a few was the SM Hot Rod DA rod and that one has extra depth making for a thinner back of the neck. SM has since offered a shallower rod. I've never seen a Martin DA rod blow our the back of the neck or any other for that matter. I'm sure it can happen but this is not an epidemic by any measure....
8). Truss rod weight need not be a problem for instruments not equipped with truss rods at all. Classicals and Flamenco guitars rarely have rods with the exception of some low end, student Cordobas that are currently on the market and a few other examples. But the norm for these instruments is no rod at all.
9). Your symptoms are of a dry guitar not a defective truss rod or the existence of any rod at all. Proud fret ends are from a shrinking neck and frets that don't shrink, etc. None of this is related to the Truss rod EXCEPT that a dry, back bowed neck can be saved.... and often is saved.... by a DA rod that can bring it back to flat or some relief when the neck becomes permanently back bowed from exposure to an overly dry environment.
I may be misunderstanding your meaning here but a DA rod does not preclude this canary from going teats up and being that warning sign. Even with a DA rod these tell tales still occur and can signal other issues. A DA rod gives us options to address some issues that we don't have with a SA rod or no rod at all.
Most of what you are citing here is not what we experience in the repair world from DA rods. I'll reiterate that instead DA rods can save an instrument and often does in our climate.
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John my friend can I borrow your excellent term from time to time?
