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Carvin truss rod design: what's going on here?!

rojo412

rojo412

Sit down, Danny...
Supporting Member
Feb 26, 2000
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15,078
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Cleveland, OH.
#1
I happened across a deal on a Carvin LB75 (ca. '92-95), due to a "maxed truss rod."
Now I'm pretty well versed on doing work with just about anything, but this one... well, it has me a bit confused.

The truss adjustment requires a 1/4" socket, no problem there.
(Some refer to it as a "hash pipe wrench" or whatever, but I happen to be using a normal socket for the job with no issues.)

IMG_1112.jpg


But I'm completely lost as to how this is functioning, so maybe someone can shed light on it for me.

- The nut on the end of the rod is welded in place.
- When you adjust it, the whole rod turns.
- Looking at the rod, it's reverse threaded (lefty tighty, righty loosey). BUT it works intuitively: clockwise introduces backbow, counter clockwise allows forward bow.
- Turning the rod nut either way doesn't move the rod in or out, so something internal is being manipulated... somehow.

What is this actually doing in there?
I get what a normal dual action does, I get what a single action does. This one is some kind of odd system that has me scratching my head. Backwards threading? Full turning rod?

On the plus side, this rod is not broken. It works the neck forward and backward. Granted, the neck needs to be heat treated and lined back up properly, which I'm currently doing. I'm just wondering if insight into what is going on inside of the neck itself may help me with anything down the road.

Also, did they ever change this out for a more normal system or is this still their chosen method?
 
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#2
rojo412 said:
I happened across a deal on a Carvin LB75 (ca. '92-95), due to a "maxed truss rod."
Now I'm pretty well versed on doing work with just about anything, but this one... well, it has me a bit confused.

The truss adjustment requires a 1/4" socket, no problem there.
(Some refer to it as a "hash pipe wrench" or whatever, but I happen to be using a normal socket for the job with no issues.)

View attachment 4558218

But I'm completely lost as to how this is functioning, so maybe someone can shed light on it for me.

- The nut on the end of the rod is welded in place.
- When you adjust it, the whole rod turns.
- Looking at the rod, it's reverse threaded (lefty tighty, righty loosey). BUT it works intuitively: clockwise introduces backbow, counter clockwise allows forward bow.
- Turning the rod nut either way doesn't move the rod in or out, so something internal is being manipulated... somehow.

What is this actually doing in there?
I get what a normal dual action does, I get what a single action does. This one is some kind of odd system that has me scratching my head. Backwards threading? Full turning rod?

On the plus side, this rod is not broken. It works the neck forward and backward. Granted, the neck needs to be heat treated and lined back up properly, which I'm currently doing. I'm just wondering if insight into what is going on inside of the neck itself may help me with anything down the road.

Also, did they ever change this out for a more normal system or is this still their chosen method?
Click to expand...

That's an older-style Double-Acting Double-Rod truss rod design. The upper rod is round, and the whole rod rotates when you turn the adjusting nut. It's threaded left-hand on the adjusting end and threaded right-hand on the anchor end. The lower rod may be round or it may be a flat rectangular strip. Steel blocks with the threaded holes are welded on to the two ends.

When you rotate the upper rod (tightening direction), the left and right hand threads try to push the two steel blocks apart from each other. This makes the pair of rods bend, lifting up in the middle, bending the neck back.

Rotating the upper rod in the loosening direction does the opposite. It pulls the blocks toward each other, bending the assembly, pushing down in the middle, bending the neck forward.

A classic design, strong and reliable in most cases.
 
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#3
As I remember, the operational orientation is NOT reversed...."righty tighty, lefty loosey". I used a 1/4" combo wrench for better purchase. IIRC and as you stated, the entire rod turns although I'm not sure of how things are anchored. I will say this: you must manually flex the neck backwards when tightening the TR or you'll snap that sonofabitch right off.

Riis
 
#4
Bruce Johnson said:
That's an older-style Double-Acting Double-Rod truss rod design. The upper rod is round, and the whole rod rotates when you turn the adjusting nut. It's threaded left-hand on the adjusting end and threaded right-hand on the anchor end. The lower rod may be round or it may be a flat rectangular strip. Steel blocks with the threaded holes are welded on to the two ends.

When you rotate the upper rod (tightening direction), the left and right hand threads try to push the two steel blocks apart from each other. This makes the pair of rods bend, lifting up in the middle, bending the neck back.

Rotating the upper rod in the loosening direction does the opposite. It pulls the blocks toward each other, bending the assembly, pushing down in the middle, bending the neck forward.

A classic design, strong and reliable in most cases.
Click to expand...

This is much better than I could do. Having bought a new one from them during that period, I do remember that there were supposedly TWO truss rods...and that the one nut was controlling them both.
 
#5
Pedulla was one of the early basses to introduce this style of truss rod, in the '80's. The rod assemblies were made in-house by Mike & crew. But the design was reversed, so the rotation of the adjusting nut bent the truss rod in the opposite direction from "normal". This caused a lot of problems for Pedulla. Many fine Pedullas got broken truss rods because owners didn't understand which direction to turn them. I've repaired a few.

By the '90's, almost everyone (well except for Fender, Gibson and Rickenbacker) had switched to DADR truss rods, designed to work in the normal direction. That's what's in the Carvin.
 
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#6
Zooberwerx said:
As I remember, the operational orientation is NOT reversed...."righty tighty, lefty loosey". I used a 1/4" combo wrench for better purchase. IIRC and as you stated, the entire rod turns although I'm not sure of how things are anchored. I will say this: you must manually flex the neck backwards when tightening the TR or you'll snap that sonofabitch right off.

Riis
Click to expand...

Yes, these old Carvin truss rods will snap like a twig. Take a wild guess at how I know?
 
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#7
Well I do appreciate all the info, thank you! This helps.

The system is still working and that is good. I do think that this design would break if you looked at it wrong and I'm certainly sorry that you (@mmbongo) had to find out the hard way. Though I will say, I've torqued on this one pretty hard and it's still okay (phew!).

The guy who sold it to me said that he consulted Carvin about it (under his assumption that it was a 2002, and it isn't) and they said the rods from that era were weak and troublesome. Not sure if the 92-95 were the same thing though.
He also took it to 2 different local luthiers and they basically told him it was toast and needed a new rod installed. And while I'm sure that removing the fingerboard and putting a better truss rod in here would make it more reliable, the fact is, THIS ONE WORKS. And yeah, if you're dealing with a neck that bows forward in an extreme way, taking off the board, levelling the neck, adding support rods, then reinstalling the board would be a proper fix. And while you're at all that, you may as well replace the truss rod system with something better.

BUT... I'm kind of surprised these guys would write it off so fast. I've had some good luck "torturing" necks back to straightness in the past, without all of the removal efforts involved. So I'm hoping this is no different.
 
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#11
I passed on a six string Carvin because of truss rod issues. While it worked, the bottom threaded block must have slipped because the neck could only be adjusted between being a ski ramp and a bunny slope - no back-bow at all.

I'm confused as to why dual-action TR's are used because you already have string tension pulling up on the neck - why the need to ensure you can force a neck to bow in that direction?
 
#12
Razman said:
I'm confused as to why dual-action TR's are used because you already have string tension pulling up on the neck - why the need to ensure you can force a neck to bow in that direction?
Click to expand...

sometimes it's just not enough. especially on a short scale with extra light strings. i've ran into this on several Ibanez Mikros.
 
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#13
@two fingers Here's a couple pics. I'll have better ones when the bass isn't being tortured.

IMG_1110.jpg

IMG_1125.jpg

IMG_1126.jpg


@Razman It's funny you mention that because I was just working on a Martin yesterday. BRAND NEW and the neck was bowed back really badly. I loosened the rod to where it was wiggling and it was still backbowed. It's very fortunate that they used a DA rod in this because I was able to bow it forward and make it feel good.
Granted, time may move the wood forward enough to reintroduce backbow tension, but until then, the DA rod is the only thing making it playable.

I honestly don't understand why anyone would build a neck with a single action rod in this day and age. If the wood pulls back too hard against string tension, the neck is unuseable. The DA rod is like an insurance plan.

Now granted, this bass is an example where it didn't fully help the bass remain true for a lifetime. That would be a job for carbon supports, if you ask me. And again, in this day and age, why wouldn't you? But... 1992 was a different time.
 
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#18
rojo412 said:
@two fingers Here's a couple pics. I'll have better ones when the bass isn't being tortured.

View attachment 4558615
View attachment 4558607
View attachment 4558608

@Razman It's funny you mention that because I was just working on a Martin yesterday. BRAND NEW and the neck was bowed back really badly. I loosened the rod to where it was wiggling and it was still backbowed. It's very fortunate that they used a DA rod in this because I was able to bow it forward and make it feel good.
Granted, time may move the wood forward enough to reintroduce backbow tension, but until then, the DA rod is the only thing making it playable.

I honestly don't understand why anyone would build a neck with a single action rod in this day and age. If the wood pulls back too hard against string tension, the neck is unuseable. The DA rod is like an insurance plan.

Now granted, this bass is an example where it didn't fully help the bass remain true for a lifetime. That would be a job for carbon supports, if you ask me. And again, in this day and age, why wouldn't you? But... 1992 was a different time.
Click to expand...
Gotcha. Nice looking bass btw! :thumbsup:
 
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#19
So far, the operation was a success. I actually overcorrected the bow, but managed to fix that too. And now the truss adjusts both ways. I'm gonna give it a week to make sure it stays that way, but it's good so far.

Here's a (somewhat unrelated) question for the Carvin folks who know... Are the pickups reversed on the blend knob from factory or was that a mistake? I know some companies (Sadowsky for one) do that on purpose. And I know it's easy to change. But while I have some of you on the line, can't hurt to ask.
 
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#20
rojo412 said:
So far, the operation was a success. I actually overcorrected the bow, but managed to fix that too. And now the truss adjusts both ways. I'm gonna give it a week to make sure it stays that way, but it's good so far.

Here's a (somewhat unrelated) question for the Carvin folks who know... Are the pickups reversed on the blend knob from factory or was that a mistake? I know some companies (Sadowsky for one) do that on purpose. And I know it's easy to change. But while I have some of you on the line, can't hurt to ask.
Click to expand...

The active carving I had in the past had a typically wired blend, but I'm not sure if that is standard for them or not.
Glad you had luck with the correction. How did you do it? And did yours turn out to be reverse turning? I just got a b4 from this time period and I'm having the same issue.

Thanks!
 

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