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One way truss rod issues
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10123&t=44714		 Page 1 of 1
Author:  absrec [ Tue Dec 09, 2014 8:24 pm ]
Post subject:  One way truss rod issues
I've been struggling with this quite a bit. I've been trying to build a neck with a fender style one way rod. I'm not going to even say how many of these failures I've built because it's just embarrassing. When I get to the leveling portion of the build the neck doesn't have enough adjustment range. Most of them have somewhere in the neighborhood of .015".

I've tried to help myself through Internet searches and asking the few people I know for a little advice. I've even gone as far as taking an aftermarket neck with a one-way truss rod that worked very well and sawing it down the middle to see what I was doing wrong. This neck had at least .035" of adjustment range. Probably more.

For a while I was thinking it was the curve of the channel. However, the channel wasn't all that different from mine. If anything my channel had a little more curvature to it. Then I started thinking it was the angle that was drilled in the headstock since the neck I cut in half had a 5° angle and I was using a 3° angle. I built one more neck and painstakingly copied the curve, headstock drilling angle as well as the angle drilled into the heel. This one only has a little over .020" of adjustment range. I am very meticulous when it comes to making the channel, inlaying the rod and gluing the filler strip. I even make my filler strips wider than the rod's channel and inlay the strip into an appropriate sized channel above the rod in order to prevent it from getting pinched. I spray dry lubricant on it prior to installation as well. The knurled anchor nut is fit and the rod is peened. I also put a drop of Loctite red on the threads before attaching the anchor nut to prevent it from ever backing out. Many of you were probably thinking this is overkill. Maybe so, but I don't mind taking extra measures to prevent future heartache. I've been very patient with this but it's getting ridiculous.

I know this is a touchy subject on this forum. Please try to limit your responses to ones that concern my issue. I understand that many of the forum members here prefer 2 way truss rods. I have used these before and they do work well. My question is about one-way truss rods.

Thanks in advance for your responses.


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Author:  dzsmith [ Tue Dec 09, 2014 9:53 pm ]
Post subject:  Re: One way truss rod issues
Hi Aaron,
I wonder how stiff your neck is compared to a Fender.
I made a Maple neck that was so stiff the rod would not move the neck.
I ended up bending the neck by hand while tightening the rod.
This neck was flat sawn and must be some kind of super stiff Maple.
Just a thought.
Dan
Author:  absrec [ Tue Dec 09, 2014 10:12 pm ]
Post subject:  Re: One way truss rod issues
dzsmith wrote:
Hi Aaron,
I wonder how stiff your neck is compared to a Fender.
I made a Maple neck that was so stiff the rod would not move the neck.
I ended up bending the neck by hand while tightening the rod.
This neck was flat sawn and must be some kind of super stiff Maple.
Just a thought.
Dan

Which of the 7 necks that I built (did I just divulge that?) are you referring to?

I was thinking about wood movement being a possibility but I think at least one of them would have worked correctly by now as they were taken from 3 different billets. It has something to do with the implementation I'm pretty sure. I've remade my TR channel templates twice trying to perfect the curve. Very little difference if any.


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Author:  David Collins [ Wed Dec 10, 2014 12:10 am ]
Post subject:  Re: One way truss rod issues
The curve matters very little. The position (depth) matters a lot.

More details though - what woods have you used - maple, mahogany, rosewood, ebony, wenge, etc? What style of neck exactly is this - one piece construction with skunk stripe, separate fingerboard/neck? And perhaps more importantly, how deep in the rod, the anchor, and the nut/washer? Pictures could be helpful if you have any.
Author:  absrec [ Wed Dec 10, 2014 7:04 am ]
Post subject:  Re: One way truss rod issues
David Collins wrote:
The curve matters very little. The position (depth) matters a lot.

More details though - what woods have you used - maple, mahogany, rosewood, ebony, wenge, etc? What style of neck exactly is this - one piece construction with skunk stripe, separate fingerboard/neck? And perhaps more importantly, how deep in the rod, the anchor, and the nut/washer? Pictures could be helpful if you have any.

Strat style Neck. Hard maple core - 0.75" Rosewood fingerboard - 0.25"

With this style of neck you don't have much wiggle room as far as where the anchor lies in relationship to the string nut. I tried to make it to where the koa plug sits on the face of the headstock about 1/4" to 3/8" in front of the string nut. That way, with an angle it will fall roughly 1/16 to 3/32 underneath the not slot which is 1/8" to 3/32" deep. The neck will be roughly .900" thick around the 7th fret after carving. Minus .25" for the fretboard leaves around .650" of maple. The channel ends up being around .500"-.550" at that point leaving .100"-.150" of buffer. The channel ends up around .350" deep about 2" from the heel/access point. I drill a 3/16" hole somewhere between 2.5° and 3° into the heel and it lands perfectly in the channel.

I have this all calculated in a spreadsheet. I do my carves with special sled that moves below an overhead router. It requires the use of templates so all of these calculations are critical to make sure I don't carve too deep and hit truss rod.


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Author:  absrec [ Wed Dec 10, 2014 7:32 am ]
Post subject:  Re: One way truss rod issues
Isn't the definition of insanity doing the same thing over and over and expecting a different result?

The neck that's cut in half is a Squier bullet I bought for $20 years ago. The truss rod worked (and still works) quite well.
Author:  David Collins [ Wed Dec 10, 2014 9:18 am ]
Post subject:  Re: One way truss rod issues
I would suggest deemphasizing the curve, using only enough to raise the nut to a reasonable access point at the adjustment end. If your adjustment nut is at the headstock, drop the heel anchor much further toward the back.

Curvature of the rod is of relatively small influence. Straight rods work exceptionally well (I've never had any problem getting over .1" range with straight rods), and even the old reverse curved Gibson rods yield more adjustment room than you seem to be achieving there. The important point is keeping the rod as far back below the neutral plane of the neck as possible.

With both the anchor points so high toward the face you are relying primarily on perpendicular forces placed on the neck by the rod trying to straighten itself, and very little of the more influential longitudinal forces of the rod pulling along the back half of the neck.

I've never tried building with that much curvature and such high anchor points, so I don't have a first hand comparison. Still, my gut feeling is that you would be better served by moving at least one I your anchor points much further back.
Author:  absrec [ Wed Dec 10, 2014 2:42 pm ]
Post subject:  Re: One way truss rod issues
David Collins wrote:
I would suggest deemphasizing the curve, using only enough to raise the nut to a reasonable access point at the adjustment end. If your adjustment nut is at the headstock, drop the heel anchor much further toward the back.

Curvature of the rod is of relatively small influence. Straight rods work exceptionally well (I've never had any problem getting over .1" range with straight rods), and even the old reverse curved Gibson rods yield more adjustment room than you seem to be achieving there. The important point is keeping the rod as far back below the neutral plane of the neck as possible.

With both the anchor points so high toward the face you are relying primarily on perpendicular forces placed on the neck by the rod trying to straighten itself, and very little of the more influential longitudinal forces of the rod pulling along the back half of the neck.

I've never tried building with that much curvature and such high anchor points, so I don't have a first hand comparison. Still, my gut feeling is that you would be better served by moving at least one I your anchor points much further back.

I think I understand what you are saying. Kinda like the LMI compression rod - it goes in a straight channel, has a welded adjustment nut on one end and a 90° bend on the opposite end.

This sounds easier than everything I'm doing. :)

However, Fender's truss rods work. What I don't get is how mine could be inlayed the same way with practically indentical specs and not work the same.


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Author:  absrec [ Wed Dec 10, 2014 9:40 pm ]
Post subject:  Re: One way truss rod issues
I re-drew the curve and made sure to keep the rod around .125" from the back of the neck between the 9th & 3rd frets This brought the anchor down a bit at the headstock end. Overall this template will place the rod much deeper in the channel than the last attempt. We'll see.


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Author:  David Collins [ Wed Dec 10, 2014 11:16 pm ]
Post subject:  Re: One way truss rod issues
I'd bring it down further. Curve it up as necessary to bring the nut up to the surface at the headstock, but leave the heel anchor as far toward the back as practical. The curve has very, very little need or benefit to a well functioning truss rod. How far beneath the neutral plane of the neck's stiffness the truss rod lies is far and above the dominant factor. The further it is toward the back, the more effective it will be.

As to why the neck which was cut worked better than the others, that's tougher to say. If it's useful range was anywhere near the .035" range or even twice that, I would say it wasn't working very well either, so it could be teetering on the verge of not functioning sufficiently as well. A rod .100" from the back of the neck, rising only as much as necessary for access to the nut, there should be no problem getting .100-.150" range of adjustment.
Author:  absrec [ Thu Dec 11, 2014 5:06 am ]
Post subject:  Re: One way truss rod issues
You've mentioned the "neutral plane" of the neck a couple times. Would you mind explaining a little more what you're talking about there?


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Author:  absrec [ Thu Dec 11, 2014 6:26 pm ]
Post subject:  Re: One way truss rod issues
I mocked up a neck out of a 2x4 today. The rod is buried a lot deeper in the neck. Looks like it's maxing out around .040". A whole lot better than I was getting. Of course, I don't know how much different it's going to be once I make it out of some maple and rosewood. One thought is that a harder wood is going to offer more resistance to the rods ability to straighten the neck out. The other thought is that range is range regardless of what it's counteracting. I don't know which theory is correct or if both are correct to some extent. Regardless, I couldn't bring myself to waste some more good hardwood.
Author:  David Collins [ Thu Dec 11, 2014 10:24 pm ]
Post subject:  Re: One way truss rod issues
The "neutral plane" is simply the center of stiffness in the neck. Above the plane compresses with string tension, below it expands.

Image

The curve of the rod is often miscredited as a primary influence on it's function, but it actually has very little impact in the way often described. Yes, as you pull on a curved rod it wants to straighten, but the perpendicular force it exerts on the neck is tiny, relative to the force applied along its length (like trying to pull a suspended rope straight, massive force can be applied and barely make a change in the rope's curve under nothing more than its own weight.

The truss rod functions more like a guy line on a telephone pole. The strings are pulling on one side of the neutral axis, and the truss rod counters this by pulling on the other. The further you can get behind this neutral axis, the more leverage and effectiveness it will have.

For single action compression rods, the most effective designs are the farthest back along their length and anchor points, rising toward the face only as much as necessary at one end for access to adjustment. The headstock end on yours of course has to be brought up to the surface, but everything else (including the heel anchor) should be kept as far back as possible. The curve is not so much a design feature as it is a necessary compromise to be incorporated as little as possible.

*Edit - I was mistaken in thinking you were making headstock adjustable necks. I missed in the picture that it's clearly heel adjustment. So keeping the heel up of course may be necessary to some degree, but I personally see this as a significant functional compromise. I believe this to be a design flaw in Fender necks, part of why they don't work nearly as well or reliably as a Gibson style rod, and a contributing factor to why 99% of Fenders require the upper frets to be dressed down after a few years under tension. This is a very predictable and consistent failure on Fender necks, which I believe is quite fairly attributable to this truss rod design.
Author:  absrec [ Fri Dec 12, 2014 6:48 am ]
Post subject:  Re: One way truss rod issues
David Collins wrote:
*Edit - I was mistaken in thinking you were making headstock adjustable necks. I missed in the picture that it's clearly heel adjustment. So keeping the heel up of course may be necessary to some degree, but I personally see this as a significant functional compromise. I believe this to be a design flaw in Fender necks, part of why they don't work nearly as well or reliably as a Gibson style rod, and a contributing factor to why 99% of Fenders require the upper frets to be dressed down after a few years under tension. This is a very predictable and consistent failure on Fender necks, which I believe is quite fairly attributable to this truss rod design.

David, thank you so much for all of your help and information. I was starting to go a little nuts I'll admit. :)

A friend of mine once said "It is a poor craftsman that blames the tools". My thinking is that the same philosophy applies to classic designs. For they are classics for a reason and all of their good as well as not so good attributes contribute to their greatness. I guess I was wrong about this one.

I suppose at this point the only thing to do is to proceed with the build and make whatever modifications I can to improve functionality without aesthetically changing the design. Somehow we are able to get Fenders playing great in the end. I guess I just never noticed how little adjustment range their truss rods offer.


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Author:  David Collins [ Fri Dec 12, 2014 8:46 am ]
Post subject:  Re: One way truss rod issues
I'll admit I never really answered your question though, did I. idunno Why does their rod work noticeable different from yours when the designs are so similar? I had a few ideas at first, but you seem to have all your work in very good order, clean and accurate, so it really seems odd. So I really don't know, and from afar all I can do is make suggestions to address and improve, but can't find a smoking gun to explain the differences you've found.

My guess though is that their highly curved rod design is just less robust than the Gibson style which remains further toward the back throughout. Perhaps a little change in neck shape, or thickness of the board will move the effective neutral plane just enough to affect a difference. Maybe the rod averages position so close to the neutral plane that if it gets shifted a millimeter it's enough to make a difference. Your work looks pretty careful and clean, no indicators at all of sloppy fit or bad anchors, so that hyper-sensitivity of the Fender design to minor changes is about all I can think of.

Good luck!
Author:  absrec [ Fri Dec 12, 2014 12:03 pm ]
Post subject:  Re: One way truss rod issues
In light of the whole "neutral plane" concept, I thought about thinning one of the necks which is a one piece maple with a skunk stripe as an experiment to see if it makes a difference in the functionality of the rod. It would be easy enough to pop it out, take a few measurements, glue it back in and grab the microplane.


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Author:  absrec [ Mon Dec 15, 2014 6:26 am ]
Post subject:  Re: One way truss rod issues
David, thanks again. You were completely right.

I pulled the filler strip on that maple neck last night. And there wasn't nearly enough depth on the truss rod. Since it was rear loaded, I made a shim using my routing template and just angled it down to zero at the adjustment end which was at the headstock. Basically when I was done it propped the truss rod up so that it was sitting around .100" from the back of the neck all the way to the heel. I was able to get about .050" worth of range out of it. Unfortunately I was using a cheap allen wrench and it snapped off inside the adjustment nut as it was maxing out. Oh well, at least now I know.


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Author:  absrec [ Sat Feb 07, 2015 12:09 pm ]
Post subject:  Re: One way truss rod issues
David - Hopefully you notice this. I've been able to finally wrap my head around the concept of the neutral plane.

Question...

I was installing a Martin style U-Channel rod recently and wondered how it can get away with being installed directly under the fretboard. With a Hot Rod, I imagine it's because there are 2 rods working together so you get twice the torque. With the Martin, it is only one rod and strictly pulls the neck in the backward direction. I've known for some time how the various truss rod styles are implemented but (apparently) not so much how/why they work.


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Author:  Rodger Knox [ Mon Feb 09, 2015 12:40 pm ]
Post subject:  Re: One way truss rod issues
Single action and double action truss rods are fundamentally different in the way they work.

A single action rod compresses the material on the back of the neck(below the neutral axis) to offset the curvature caused by string tension. There is no force perpendicular to the fretboard with a single action rod.

A double action rod changes curvature with adjustment, and takes the neck with it. All of the force exerted by a double action rod is perpendicular to the fretboard.
Author:  absrec [ Mon Feb 09, 2015 3:30 pm ]
Post subject:  Re: One way truss rod issues
Rodger Knox wrote:
A double action rod changes curvature with adjustment, and takes the neck with it. All of the force exerted by a double action rod is perpendicular to the fretboard.

Thanks. I was more asking about the Martin style "U-channel " which is The same as a one-way truss rod in its operation. It's implementation is like the double action rod in that it sits above the neutral axis. This is slightly confusing for me because I've seen the effects (or lack of) firsthand of not having the rod installed deep enough in the neck.



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Author:  Rodger Knox [ Tue Feb 10, 2015 2:51 pm ]
Post subject:  Re: One way truss rod issues
absrec wrote:
Rodger Knox wrote:
A double action rod changes curvature with adjustment, and takes the neck with it. All of the force exerted by a double action rod is perpendicular to the fretboard.

Thanks. I was more asking about the Martin style "U-channel " which is The same as a one-way truss rod in its operation. It's implementation is like the double action rod in that it sits above the neutral axis. This is slightly confusing for me because I've seen the effects (or lack of) firsthand of not having the rod installed deep enough in the neck.


I'm not familiar with how that rod works, I've never used one. What happens when you adjust it, does it bend? On the StewMac website, it looks like it operates like a double action rod, but is only adjustable to decrease relief.
Author:  absrec [ Wed Feb 11, 2015 10:33 am ]
Post subject:  Re: One way truss rod issues
Rodger Knox wrote:
absrec wrote:
Rodger Knox wrote:
A double action rod changes curvature with adjustment, and takes the neck with it. All of the force exerted by a double action rod is perpendicular to the fretboard.

Thanks. I was more asking about the Martin style "U-channel " which is The same as a one-way truss rod in its operation. It's implementation is like the double action rod in that it sits above the neutral axis. This is slightly confusing for me because I've seen the effects (or lack of) firsthand of not having the rod installed deep enough in the neck.


I'm not familiar with how that rod works, I've never used one. What happens when you adjust it, does it bend? On the StewMac website, it looks like it operates like a double action rod, but is only adjustable to decrease relief.

It's a rod inside an extruded aluminum channel.


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Author:  Rodger Knox [ Wed Feb 11, 2015 12:35 pm ]
Post subject:  Re: One way truss rod issues
A rod in an extruded aluminum channel, with the rod attached to the channel at one end, and the adjustment end tightens the rod? That kind of arrangement would work by bending the rod, but it would only bend in one direction.
That type of operation exerts force perpendicular to the fretboard, and it's operation is not dependent on it's position relative to the neutral axis.
Author:  absrec [ Wed Feb 11, 2015 2:43 pm ]
Post subject:  Re: One way truss rod issues
Rodger Knox wrote:
That type of operation exerts force perpendicular to the fretboard, and it's operation is not dependent on it's position relative to the neutral axis.

Okay… So if I am understanding you correctly it a essentially works like a hot rod but it only torques the neck one way. Is that what you're saying?



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Author:  Rodger Knox [ Wed Feb 11, 2015 3:21 pm ]
Post subject:  Re: One way truss rod issues
Yes
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