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Building Your Own Custom Truss Rods
- Thread starter Bruce Johnson
- Start date
I usually go with Plutonium. It’s expensive, but it’s worth it for the weight gain.That's what AnkleShot Tungsten Tuning Machines are designed to prevent. Order a set today, with matching Low-Mass Carbon-Fiber Bridge!
One of my standard DASR embedded truss rods is about 120 grams, for comparison. DADR style truss rods, like the one I made up for this Pedulla, are the heaviest design. That's one reason why I don't use them in my own new construction necks. But they are the simplest to install, which is why they are so popular. Most Luthiers are too lazy to do a good installation. They just want something that they can buy for cheap and drop in a slot. And is strong enough to straighten their unstable necks.
The lightest truss rods I've ever made were simple SASR design, made from 7075 aluminum. Lighter than titanium.
The lightest truss rods I've ever made were simple SASR design, made from 7075 aluminum. Lighter than titanium.
While not exactly 100% on-topic, it will become more so when you start discussing your DASR design in earnest:
You’ve discussed what you have found to be the ideal deflection or “droop” for your single-rod truss rods a number of times and that it doesn’t matter what the rod length is, particularly. In light of that, what sort of jig are you using to cut your curved-bottom truss rod slots? Is it a universal (adjustable) jig that adjusts the curve to match the slot length or do you use separate jigs that are bespoke for each length? And is it a router jig (I assume that it is)?
Hey Ethan;
A very reasonable question. Yes, DASR and SASR style truss rods need to be installed into a slot which is curved in the vertical plane. I call the amount of curvature the Droop; how much the middle of the slot is deeper than the two ends. The amount of Droop is very important to the function of the truss rod, because it determines the mechanical ratio that the rod will have, to be able to bend the neck.
From my experience, I've found that most electric bass and guitar necks need around 0.180" of droop. That seems to be optimal. And it doesn't seem to matter what the scale length of the neck is. Less than 0.180", and the truss rod has less muscle to bend the neck. More likely to get snapped off straightening a warp. Much more than 0.180", and it can become too powerful and too sensitive to adjust.
Anyway, my standard way to cut these slots is with....a routing fixture. I'm sure you guessed that.
The fixture clamps down onto the neck blank. There's a gap down the center of the fixture. There's a collar on the router base which slides down that gap. That's what keeps the router in a straight line down the neck. Then there are two MDF rails on the outside which have the curvature of the Droop cut into them. The router base rides on those rails, which controls the depth of cut, and the curvature of the slot.
This is a simple router fixture that cuts in 3-D. The X and Y axes are controlled by the collar riding in the gap, and the Z axis is controlled by the base riding on the contoured side rails.
These fixtures are easy to make up. I have different ones for Bass and Guitar necks, but they will usually work for different scale lengths.
If you look close at this fixture, the side rails curve down from the headstock end to the middle, but then stay level out to the end of the heel. This neck is getting a DASR truss rod with the carbon fiber backstrap. I'm cutting the slot to curve down into the Droop on the outboard half, from the nut to about the 5th fret. The rod gets epoxied in, pressed down tight against that outboard half of the curved slot.
But, from the 5th fret to the heel, the slot stays straight, parallel to the top surface. The rod curves back up to the surface at the anchor. It has the whole Droop curvature, 0.180" deep at the 5th. The long narrow wedge gap under the rod, from the 5th to the heel, is filled with epoxy and the carbon fiber TOW strands. That's the Backstrap. Cast in place, filling that space. The Backstrap prevents the back of the neck, from the 7th to the heel, from stretching over time. As in 12th fret Kink aka Ski Jump.
This is more advanced stuff that I'll show in more detail in the third chapter of this thread, about DASR Embedded truss rods.
But that's how the curved slots are cut with a router. Not hard at all to do.
Sometime soon I'm going to be making up a new fixture to cut these truss rod slots on my pin router. Same general idea, but upside-down. The pin in the pin router's table will be replaced with a blade with a curved top. The fixture will slide over the table with a curved slot in the bottom of the fixture sliding on the blade.
I'd be interested in building your DASR truss rod. I also hope to start working on a brucified bass later this year. Brucified truss rod would be great addition to the project.
Thank you for sharing your knowledge. It's tremendously inspiring!
As a follow-up:
Jeremy finished up the rebuild of that Pedulla neck, and tested out the DADR truss rod. It worked beautifully. Very smooth strong action bending the neck forward or backward. Almost no dead play in the middle between the two directions. He didn't push it anywhere near the limit, but was easily able to bend it 0.060" back at maybe 60% tight. And that's a neck with steel reinforcing bars. It definitely has the mechanical ratio and power. I'm happy with the design, and I don't think it needs any more improvements. I'm going to lock that down as the standard DADR design.
Per our usual process, he tightened the truss rod partway in the forward direction and kept it there while he routed the fingerboard surface radiused and straight. That's good general practice when building a neck. We've talked about that on other threads.
Jeremy finished up the rebuild of that Pedulla neck, and tested out the DADR truss rod. It worked beautifully. Very smooth strong action bending the neck forward or backward. Almost no dead play in the middle between the two directions. He didn't push it anywhere near the limit, but was easily able to bend it 0.060" back at maybe 60% tight. And that's a neck with steel reinforcing bars. It definitely has the mechanical ratio and power. I'm happy with the design, and I don't think it needs any more improvements. I'm going to lock that down as the standard DADR design.
Per our usual process, he tightened the truss rod partway in the forward direction and kept it there while he routed the fingerboard surface radiused and straight. That's good general practice when building a neck. We've talked about that on other threads.
I tend to lurk reading stuff in this bit of basschat, I’m a bodger who enjoys making stuff, and the next project in the future will be trying to build my own neck… reading about you writing your DASR Embedded truss rod a while ago it struck me that the tooling would be within my ability and workshop range - and would let me try something that you literally couldn’t get in a shop. (I don’t really care if there is a tonal advantage or not, I love the engineering beauty of it)...
Hello Luke;
Well, good! That's why I write these threads. I want to encourage all of you to try more advanced processes and build your own special fixtures and tools. Once you understand how they need to work, they really aren't very difficult to build. And yes, the R & D engineering is most of the fun of building basses! The challenge of solving problems and pushing farther.
Has anyone else watched Ken Parker's Archtoppery series on truss rods? He experimented with wire cable truss rods, as well as rods made from 2mm piano wire. They end up around 16 grams and he tested them to be suitable for guitar building! Interesting stuff.
I haven't seen that series, but I've seen some pictures and articles about those truss rods that Ken experimented with.
About 15 years ago, I did a project for another Luthier with that same goal: The lightest reasonable truss rod to go in some Strat-style guitar necks that were made from some very expensive super-lightweight woods. I looked at a lot of different ideas; wire cable, piano wire, carbon fiber cable, titanium, and 7075 aluminum. The cables and piano wire had problems attaching them to the anchor and adjusting head.
I ended up going with the 7075 aluminum. A fairly standard SASR rod design, 3/16" rod, 10-32 threads, 0.180" droop. All the parts were made from 7075 aluminum except the stainless socket head cap screw that was the head of the adjusting nut. It was about equal in strength to my standard 304 stainless rod, but 1/3 the weight. 7075 aluminum is an amazing material. But it's about 4 times the price of stainless.
Aluminum truss rods are a workable idea, if you are really into saving weight.
So... aluminum on aluminum threads. Is 7075 more resistant to galling than other grades as well? Or just a matter of making sure they get anti-seize at assembly time?
Yes, that's one tradeoff to all-aluminum truss rods. Aluminum-on-aluminum threads can gall and seize if they aren't lubed or get grit in them. So, you have to be careful. 7075 aluminum is less likely to gall than 6061, I think.
I assume you mean clamp a cable? Yes, you could install a pre-tensioned cable in a neck as a fixed-tension stiffener. You can also do that with carbon fiber TOW. If you specifically want to stiffen the neck.
But a truss rod is an adjustable stiffener. You need to adjust the tension in it to change how much it bends the neck. And that's the tricky part about working out a cable-type truss rod: Anchoring one end of the cable into some kind of adjusting mechanism.
I had never really thought of using a cable instead of a truss rod, but it is an interesting idea. I suppose the easiest way would be to use a folded cable, with the ends anchored say at the headstock end, and then a simple threaded j-hook type of adjuster at the heel, pulling on the fold of the cable. One of these days I should give it a try; I would think bicycle brake cable would be strong enough, and it is light and easy to work with.
Some sort of damping would be required, as the cables would act as a pair of sympathetic strings that would vibrate at a certain pitch, but that should be easy enough to control with some foam rubber or something similar.
Some sort of damping would be required, as the cables would act as a pair of sympathetic strings that would vibrate at a certain pitch, but that should be easy enough to control with some foam rubber or something similar.
There’s no reason they couldn’t be epoxy bedded in a curved channel beneath a strip of wood - sort of a cross between Bruce’s CF tow method and a standard SASR truss rod. It would be an interesting thing to experiment with. Cable stretches, though. You’d have a hard time ensuring that the cables wouldn’t continue to stretch over time, even if you pre-stretched them. It’s an interesting concept but I think it would take a lot of refining to get it to work.
