A question for the Gibson history experts!

[J.C. Bryant]

In what years did Gibson put silver truss rod covers on their mandolins and which mandolins had them?

thanks

[pops1]

My '22 has one. I think that is the era you are looking for. Not sure what other years had them, but that's where I usually see them.

[J.C. Bryant]

Thanks pops1, I have a '22, also, and it has one also.

[rcc56]

I think they only ran the metal covers starting in late 1921 through sometime in 1922 before they switched to celluloid. They're cheaper and more practical to make in plastic.
I've seen them mostly on various A models, but I've seen them on F's also.
Covers are easy to swap, so while you might encounter one on a later instrument, it probably came off a '21 or '22 model.

[siminoff]

On April 5, 1921, Gibson engineer Thaddeus "Ted" McHugh filed a patent for his design of a truss rod. The patent, 1,446,758, was awarded on February 27, 1923. While most manufacturers would wait until a patent were to issue before manufacturing parts that include the new design feature, it appears that Gibson jumped the gun and started preparing for the new truss rod as soon as the patent was filed, and began using the truss rod in all instrument models in late 1921 before the patent was issued. To herald the inclusion of a truss rod in its necks (neck straightness was a big deal back then - as it is now), Gibson covered the truss rod pocket with a nickel truss rod cover on all instrument models that had a truss rod. I don't know exactly how many instruments were actually made with the nickel truss rod cover, but around the middle of 1922, as rcc56 suggested, the nickel truss rod cover was replaced with a black celluloid cover. I think it had more to do with aesthetics than cost, but that's something we'll probably never know.

Gibson filed a few trademarks for its bell-shaped truss rod cover over the years (I think the first was Reg. No. 1,022,637, Oct. 14, 1975), and the truss rod cover design has been a bone of contention with Gibson issuing numerous law suites for folks who were/are making or selling instruments with a bell-shaped truss rod cover.

Photo below shows a Gibsonian band with Loar holding a black-faced H4 mandola with a nickel truss rod cover. Behind him is a 1921-period K5 mandocello with nickel truss rod cover. Notice the two F5s with black truss rod covers.

Roger

PS: Interesting to note that Ted McHugh's truss rod design featured an upside-down rod that was high in the center and low at both ends - his vision was that tightening the nut would pull the bottom of the peghead and bottom of the neck heel toward each other causing a "bow" (high center of fretboard). His was a failed design. In "post tensioning" the rod should be low in the center and high at both ends. So, in McHughs' design the rod worked to a point and then worked backward. 15 years later, the rod was inverted to a low-center design and has remained that way every since.

[William Smith]

1922-1924 is when I've seen them on plain A's to the beast F-5's. My Uncle's Feb. 18th 24 F-5 still has the original Nickle TRC that has the original owners name engraved on it. That's the latest one I've seen, there may be more however? I do remember seeing them on a few Loar era guitars as well.

*****GREAT NEW LOAR BOOK ROGER*****

[Hendrik Ahrend]

PS: Interesting to note that Ted McHugh's truss rod design featured an upside-down rod that was high in the center and low at both ends - his vision was that tightening the nut would pull the bottom of the peghead and bottom of the neck heel toward each other causing a "bow" (high center of fretboard). His was a failed design. In "post tensioning" the rod should be low in the center and high at both ends. So, in McHughs' design the rod worked to a point and then worked backward. 15 years later, the rod was inverted to a low-center design and has remained that way every since.

Dear Roger, I beg to differ. A good friend of mine, an accomplished banjo maker has done quite some r & d on that subject and concluded that McHugh's design works, indeed, very well (much like finger muscles) provided that the rod is installed quite low in the neck (below the "center line", if I under stand correctly). Gibson might have altered its course due to lower neck profiles; just guessing.
The old style rod in three F model mandolins from '23, '24 and '28 I own (or used to own) work very well.

Greetings,
Hendrik

[siminoff]

Hendrik…

Thanks for the rebuttal. For this discussion let’s set aside any of the double-acting rods, double-rods (one nut, two rods), square tubes, carbon-fiber rods, T-shaped bars, etc., and just focus on the single-rod system as McHugh describes in his patent. There are three basic positionings of a single-rod system: 1) rod inserted high in the center (as McHugh describes), 2) straight rods (no bend), and rods inserted low in the center.

1) The high-center rod (McHugh’s patent) presents two forces: a) It compresses the neck wood, which can be compressed more easily than the fretboard wood - sort of a bi-metal effect - and this results in the neck being bent in a direction that favors the neck wood causing a bow (raised area in the center of the fretboard) which is what we want. and b) However, as the truss rod nut is tightened the rod wants to straighten (post-tensioning) and this forces the neck to bend backward to create a hollow (low area in the center of the fretboard), which is what we DON’T want. These two forces counter each other and there is a point while tightening the nut where the neck first shows a slight bow and then with further tightening, the rod takes over, wants to straighten, and forces the neck and fretboard into a hollow. The two counter-forces also require excessive tightening of the nut. Not too efficient.

2) A straght-rod can work reasonably well because as in “a)” above, the neck wood can be compressed more easily than the fretboard wood and tightening the nut results in a bow. The lower the rod is installed in the neck wood, the greater the mechanical advantage for creating the neck bend. This also requires moderate tightening effort of the truss rod's nut. (Luthier Tom Morgan and I did some fun experiments with straight-rod systems years back - fond memories here.)

3) The low-center rod has the best mechanical advantage of either the high-center or straight-rod systems. Here the neck wood is compressed (which causes a bow), and more importantly, the rod wants to straghten when the nut is tightened and the two forces work together to efficiently create a bow in the fretboard. This system is the most efficient of the straight-rod systems and requires the least tightening effort of the nut. Gibson switched to the low-center rod around 1933 and has been using it on hundreds of thousands of instruments ever since.

Below is a photo of a dissected 1973-era Gibson electric guitar neck showing the low-center truss rod. Below it is a 1923 Gibson tenor banjo neck showing the high-center McHugh design.

Lastly, if the high-center truss rod system is so good, why did Gibson switch to the low-center system and is still using it 90 years later?

Roger

[j. condino]

Since we have officially derailed the truss rod cover conversation...

About ten years ago I was working on a mandolin with a broken truss rod, so I called around and asked for some help with design ideas. I asked 10 of whom I considered the best mandolin builders in the world their thoughts and designs about truss rods. Guess what- I received about 25 conflicting arguments!

So....I took what seemed to be the most reasonable ten designs- curve up, curve down, straight pull, et cetera. I made up ten finished and fretted necks with one of each of the designs. Guess what? As long as as they were used with a reasonable amount of discretion and not cranked to dangerous levels and the rod was below the neutral axis, they all worked identical. I showed those results and designs at a Guild of American Luthier's Convention workshop where you could check everything out, handle them, and measure exactly how well they worked. My least favorite is the one where there is a right angle bend at the heel because those are where I have seen the most broken and it puts an odd compression at the bend where I am not trying to effect the neck.

I was always under the impression that the main reason for the curved truss rod design was the early use of threaded nuts on each end. If it was straight with two unsecured nuts, when you turned one, it has the potential to unscrew the other. The bow tends to lock the rod in place slightly better under tension.

As for the plastic cover vs. metal, in 1922 metal was old school; plastic was new & exciting and exotic, the same way it would look if I used a fancy carbon fiber cover today.

[HoGo]

I was always under the impression that the main reason for the curved truss rod design was the early use of threaded nuts on each end. If it was straight with two unsecured nuts, when you turned one, it has the potential to unscrew the other. The bow tends to lock the rod in place slightly better under tension.

I agree here. The mandolin necks really need the truss rod pocket as far from string nut as possible to preveent weak spot there and the original tr patent helps with that. Banjos and later guitars have the thick "volute" or a heel at the headstock end that allows pocket closer to string nut. I also agree that the bent rod prevents turning in the neck.

Purely from physics view the anchor points are what matters most and the vectors od force applied at the anchor points are extremely large AND they cause bending by compression while any vector that results from the bend in the center is negligible and incapable of bending the neck. The neck that is shown in the photo above with reverse curve (low center) has the rod still quite deep at the ends and that is what matters.
A reverse example of a truss rod forces is a rope. Have you ever tried to pull a 20 foot rope tied to a tree hard enough to straigthen it perfectly? You won't succeed. Even with all your strength the gravity of the rope itself will make it bend inch or so out of straigth line at the center and you'll actually break the rope before it straightens. You could even measure how much further it will sink if a tiny hummingbird sits on it when tightened hard.

[KCNelson]

[Photo below shows a Gibsonian band with Loar holding a black-faced H4 mandola with a nickel truss rod cover. Behind him is a 1921-period K5 mandocello with nickel truss rod cover. Notice the two F5s with black truss rod covers.]

Wow! I've never seen a black H4 before. Are the whereabouts of this particular instrument known? I'd love to see more pictures.

[siminoff]

James…

Thanks for joining this post. You and I share similar experiences with truss rods, but my experience had a different outcome. In 1974, I licensed US Patent 3,901,119 to Gibson for a vertabrae truss rod system. My vertabrae truss rod was able to make several corrections to a neck at one time with one rod. It was able to put a bow in one end of the neck while at the same time able to remove a hollow from the other end. It could also develop a localized force to bend a neck sideways. Bruce Bolen, Gibson’s VP of Artist Relations and Product Development at the time, whom I worked for as a consultant, put me on a six-month project to evaluate every truss rod system I could lay my hands on, including Gibson’s failed McHugh truss rod, a few other truss rod systems that Gibson had come up with over the years but didn’t implement, as well as the current low-center rod (that is photographed in my previous response). We evaluated the Kaman bar (Jim Rickard’s patent), double rods, double-acting (two-way) rods, my vertabrae system, a few other patented systems, as well as static systems: non-adjustable square tubes, T-bars, and carbon fiber inserts. Our tests included independent truss rod systems as well as systems that depended on the neck wood to develop their bending moment.

Bruce is a very accomplished guitarist and was keenly focused on finding out exactly what was needed for Gibson to manufacture perfect necks, as well as reducing or eliminating Gibson’s warranty issues relating to neck and fretboard straightness.

I was working with someone in engineering, and we had at our disposal a stack of virtually identical standard Gibson necks for our guinea pigs, and we ran a bunch of tests, doing final assembly of the necks just as Gibson did with their in-process necks so we’d have consistency in our tests. After a lot of tightening, measuring inch pounds of torque against bending loads, checking squareness, and more, our findings were that the different truss rod systems absolutely did not work identically. Some systems were totally ineffective, some were mediocre, while others had the ability to shatter a neck. Some corrected only part of the neck, others were more global.

From this extensive testing, in a production environment where all other variables except the truss were the same, and from several similar consulting projects since, it's been my experience that not all truss rod systems are equally effective.

I agree that the L-bend rod you refer to is a problem because it is difficult to precisely lock the corner of the bend into the filler strips (and I think you alluded to that).

Regarding the idea that if the rod was not bent, turning one nut could turn the rod and loosen the other nut. That could only happen if the embedded nut had a left-hand thread. If the tightening nut locked and allowed the rod to turn, and the nut at the other end of the rod had a standard right-hand thread, the rod would tighten into the nut, not loosen from it.

Gibson was already using cellluloid for binding and for fingerests from the get-go (cellouse nitrate [celluloid] “plastic” was invented in the mid-1870s and was in limited use for consumer goods). I don’t think Gibson would have changed from the nickel truss rod cover to the black celluloid cover because of some new interest in plastics, especially because the black truss rod cover is virtually invisible, so there’s no cosmetic or marketing advantage there. (Aside from the critical need for certain plastics during World War II, the “new and exciting plastics” boon didn’t happen until after the War. The ‘50s and ’60s are considered to be the heyday of plastics.)

Earlier in the post, Will Smith spoke about engraved truss rod covers, and yes, for hand-engraved truss rod covers Gibson did use nickel- or gold-plated truss rod covers. I’ve also seen a few engraved black celluloid truss rod covers where the engraving was yellow-ed in.

Roger

PS: Just for the fun of it all, below is a photo of a neck I made in ’74 and brought to our truss-rod-wrap-up meeting in Kalamazoo. It was my way of showing everyone at the meeting what we went through to test a bunch of truss rods!!!!

[HoGo]

....evaluate every truss rod system I could lay my hands on, including Gibson’s failed McHugh truss rod, a few other truss rod systems that Gibson had come up with over the years but didn’t implement, as well as the current low-center rod .....

Please don't tell that to all the Loar owners or Gilchrist and whole lot of other known mandolin makers. AFAIK, Gibson has used that "failed" McHugh design for last few decades in their mandolin necks, at least since they "remodelled" the mandolin production under Charlie D.

I know electric guitar makers use whole bunch of inverse curved rods sometimes embedded from the rear side of neck but I still prefer deeply burried rod with even deeper burried ends.

[J.C. Bryant]

Even though I started the truss rod cover discussion, I have enjoyed the truss rod discussion and certainly take my hat off to all of you and your knowledge and experience. Thank you!

[KCNelson]

James…


PS: Just for the fun of it all, below is a photo of a neck I made in ’74 and brought to our truss-rod-wrap-up meeting in Kalamazoo. It was my way of showing everyone at the meeting what we went through to test a bunch of truss rods!!!!

This was supposed to go on the Salvador Dali signature model.

[siminoff]

Kc...
Perfect!