Mechanics of truss rods

[richrips]

After toying with the idea of making my own bass, and having several weeks free this summer, i've been looking in to materials, shapes, pickup positions etc etc and had a thought about truss rods.

The truss rod is usually situated just beneath the fingerboard. It's job is to counter the bending effect of having strings under tension on one side of the neck. To do this it provides a tensile force of its own.

A couple of things strike me as strange in the standard arrangement.

1.The moment of the force the truss rod can exert is a function of the tensile force within the truss rod (how tight it is) and it's position in relation to the pivot about which that force operates. In the standard arrangement, it would appear that the pivot is the wood between the strings and the truss rod, which is essentially the fingerboard, hence the distance from the pivot is increased as the truss rod is set deeper in the neck (furthur from the strings, deeper below the fingerboard).

2. If the truss rod were routed in to the back of the neck, rather than just below the fingerboard, it could be between 1.5 and 2 times farther away from the strings than it currently is. The distance from the point at which the truss rod acts and the pivot (which would be the position within the neck at the mid point of the 2 opposing forces- string tension and truss rod tension) would then be increased. This would mean less tensile force would be needed in the truss rod to produce the same neck-straightening force (moment) as a truss rod located nearer to the strings (such as a traditional position directly beneath the fingerboard. Less tension in truss rod = less overall force acting on neck.

This would appear to have benefits such as:

-Less chance of truss rod being damaged since it is under less tension.

-potential to use lighter truss rod as its strength requirements would be lower = lighter neck

-Possibility that the truss rod could be just beneath the lacquer on the back of the neck, so replacement would not require removal/damage of fingerboard if it did break.


I'd be interested to hear from luthiers/builders as to why this is not common practice as i have never seen this, yet to me it seems logical???

Any responses much appreciated!

Rich

[Mr. Foxen]

[quote name='richrips' post='832179' date='May 9 2010, 01:46 PM']Less tension in truss rod = less overall force acting on neck.[/quote]

Since the tension of the strings is a constant, might be some flawed thinking here. The wood of the neck is acting as your fulcrum point, I'd guess and this would put equal pressure on the neck, but in a different place. The idea of the truss rod is to provide adjustability, double action truss rods are pretty much the norm on engineered instruments now, the single action is more of a traditionalist (i.e. Fender style) thing.

Also, consider the maple neck, the truss rod is inserted from the back, under the skunk stripe.

[ikay]

[quote name='richrips' post='832179' date='May 9 2010, 01:46 PM']The truss rod is usually situated just beneath the fingerboard. It's job is to counter the bending effect of having strings under tension on one side of the neck. To do this it provides a tensile force of its own.[/quote]

The attached technical paper from Fender exploring an improved neck and truss rod design may be of interest
[attachment=49310:Guitar_neck_design.pdf]

[richrips]

[quote name='Mr. Foxen' post='832298' date='May 9 2010, 04:00 PM']Since the tension of the strings is a constant, might be some flawed thinking here.[/quote]

Not really. The strings exert a constant force, but the overall force on the neck (which is compressive, end to end) is a sum of the tensile force in the neck and tensile force in the truss rod (there may be other forces at play, let me know). These forces needent be equal as each depends on the perpendicular distance of the force from the fulcrum. If the truss rod is set nearer the back of the neck, the distance of the force exerted by the truss rod to the fulcrum is increased, thus the force can be smaller whilst providing the same moment as a shallower truss rod. Hope that makes sense.

Just checked out the link for the technical paper. Fascinating stuff. I like the idea of putting a load bearing member beneath the fingerboard at the inertial cedntre of the forces acting on the neck, rather than rely on wood, with its weird respnses to temp/humidity. Not so sure about the modified truss rod slot, as it would imply a wide range of rod movement. if this were adjusted frequently, it seems there is potential for the rod to wear on the sides of the slot and develop the dreaded rattle.... I'd rather see what optimum flex characteristics could be acheived using a rod with a fixed position and fixed pivots which are effective (as a compromise over the 2 seperate tension/compression pivots in the moving rod suggested in the pdf). There should be an optimum curved shape, without seperate pivots, that is still an improvement on the current continuously curved rod placement which obviously is not ideal because as neck thickness varies along its length there are bound to be changes in flex characteristics.

Good observation that the guitar industry is able to maintain sales growth despite a distinct lack of concerted technical engineering innovation. I guess that's why a USA p bass still costs nearly a grand after 50 years of no technical advancement.

Rich

[ikay]

[quote name='richrips' post='832418' date='May 9 2010, 06:23 PM']Just checked out the link for the technical paper. Fascinating stuff. I like the idea of putting a load bearing member beneath the fingerboard at the inertial cedntre of the forces acting on the neck, rather than rely on wood, with its weird respnses to temp/humidity.[/quote]

Are you aware of the Bunker Tension Free neck? More info here [url="http://bunker-guitars.com/articles/tension-free.html"]http://bunker-guitars.com/articles/tension-free.html[/url] and patent doc attached.
[attachment=49335:Patent_US7326838.pdf]

[BigRedX]

Or have a look at the [url="http://borntorock.com/index.html"]Born To Rock instruments[/url] where the design means that the string tension is used to hold the neck in place rather than exerting a pulling force on it meaning that they don't need a truss rod at all.

Edited May 9, 2010 by BigRedX

[richrips]

interesting system. Good to see there are alternative approaches out there. Any more?

Cheers,
Rich

[Mr. Foxen]

[quote name='richrips' post='832418' date='May 9 2010, 06:23 PM']Not really. The strings exert a constant force, but the overall force on the neck (which is compressive, end to end) is a sum of the tensile force in the neck and tensile force in the truss rod (there may be other forces at play, let me know). These forces needent be equal as each depends on the perpendicular distance of the force from the fulcrum. If the truss rod is set nearer the back of the neck, the distance of the force exerted by the truss rod to the fulcrum is increased, thus the force can be smaller whilst providing the same moment as a shallower truss rod. Hope that makes sense.

Just checked out the link for the technical paper. Fascinating stuff. I like the idea of putting a load bearing member beneath the fingerboard at the inertial cedntre of the forces acting on the neck, rather than rely on wood, with its weird respnses to temp/humidity. Not so sure about the modified truss rod slot, as it would imply a wide range of rod movement. if this were adjusted frequently, it seems there is potential for the rod to wear on the sides of the slot and develop the dreaded rattle.... I'd rather see what optimum flex characteristics could be acheived using a rod with a fixed position and fixed pivots which are effective (as a compromise over the 2 seperate tension/compression pivots in the moving rod suggested in the pdf). There should be an optimum curved shape, without seperate pivots, that is still an improvement on the current continuously curved rod placement which obviously is not ideal because as neck thickness varies along its length there are bound to be changes in flex characteristics.

Good observation that the guitar industry is able to maintain sales growth despite a distinct lack of concerted technical engineering innovation. I guess that's why a USA p bass still costs nearly a grand after 50 years of no technical advancement.

Rich[/quote]

Technical advancement means change in sound though, which is the point of a musical instrument. Graphite necks have a different sound. Look at the whole debate over neo vs non-neo. The magnet doesn't affect the sound, but the better engineering accompanying the neo speakers does, generally for better mechanically (greater xmax etc.), but some people don't like the tone (corresponding different breakup modes).

There are a few forces at work. The main bulk of the tension from the strings is transferred to the neck via the machine heads, which are beyond the truss rod, with an additional witness point at the nut. A single action truss rod can only provide compressive force, and only to the section of neck between its witness points, generally just above the nut, and around the 15th fret. It doesn't necessarily run parallel to the strings. Without the action of the truss rod, some parts of the neck are under compression, and some are under tension, the part under compression I'd guess is what you consider your fulcrum, your truss rod acting on the part under tension. It is also a dynamic system, as the tension on the neck changes during playing, especially if you do gripping the headstock bends and such.

Alternate system: No trussrod.

Truss rods are for people who can't deal with big wood in their hands.

Edited May 9, 2010 by Mr. Foxen

[richrips]

[quote name='Mr. Foxen' post='832635' date='May 9 2010, 10:36 PM']Technical advancement means change in sound though, which is the point of a musical instrument. Graphite necks have a different sound. Look at the whole debate over neo vs non-neo. The magnet doesn't affect the sound, but the better engineering accompanying the neo speakers does, generally for better mechanically (greater xmax etc.), but some people don't like the tone (corresponding different breakup modes).

There are a few forces at work. The main bulk of the tension from the strings is transferred to the neck via the machine heads, which are beyond the truss rod, with an additional witness point at the nut. A single action truss rod can only provide compressive force, and only to the section of neck between its witness points, generally just above the nut, and around the 15th fret. It doesn't necessarily run parallel to the strings. Without the action of the truss rod, some parts of the neck are under compression, and some are under tension, the part under compression I'd guess is what you consider your fulcrum, your truss rod acting on the part under tension. It is also a dynamic system, as the tension on the neck changes during playing, especially if you do gripping the headstock bends and such.

Alternate system: No trussrod.

Truss rods are for people who can't deal with big wood in their hands.[/quote]

That seems to make sense. Hadn't considered the strings not ending at the nut in the context of tension. I guess any distance from nut to tuner is then going to add it's own warping effect to that end of the neck, although the short distances would make this effect quite small relative to what is going on over the neck as a whole.

I may have to start another thread on the more general lines of precision in bass guitar design with regards neck flex dynamics. I think extreme precision could be key in making the bass guitar an instrument suitable for producing the edgy, precise, electro sounds found in much modern music. After all, we don't want to give everything up to the keyboard players if we can help it!

Thanks for the interesting responses so far,

Rich

[Mr. Foxen]

[quote name='richrips' post='832648' date='May 9 2010, 10:56 PM']I think extreme precision could be key in making the bass guitar an instrument suitable for producing the edgy, precise, electro sounds found in much modern music.[/quote]

Gotta think of something that isn't covered by graphite necks. Also, MIDI pickup if you don't actually want any 'life' in your sound.

[decify123]

[quote name='ikay' timestamp='1273422307' post='832375']
The attached technical paper from Fender exploring an improved neck and truss rod design may be of interest
[attachment=49310:Guitar_neck_design.pdf]
[/quote] Hello! I know this is a very old thread but is there any chance anyone could direct me to the Fender pdf listed in Ikay's comment above? Im a final year engineering student and research into alternative neck reinforcement is my final year project. Thanks.

[ikay]

[quote name='decify123' timestamp='1351623464' post='1853428']
Hello! I know this is a very old thread but is there any chance anyone could direct me to the Fender pdf listed in Ikay's comment above? Im a final year engineering student and research into alternative neck reinforcement is my final year project. Thanks.
[/quote]

PM'd

[iiipopes]

[quote name='richrips' timestamp='1273409193' post='832179']
If the truss rod were routed in to the back of the neck, rather than just below the fingerboard, it could be between 1.5 and 2 times farther away from the strings than it currently is. The distance from the point at which the truss rod acts and the pivot (which would be the position within the neck at the mid point of the 2 opposing forces- string tension and truss rod tension) would then be increased. This would mean less tensile force would be needed in the truss rod to produce the same neck-straightening force (moment) as a truss rod located nearer to the strings (such as a traditional position directly beneath the fingerboard. Less tension in truss rod = less overall force acting on neck.[/quote]
This is how Leo Fender installed his first truss rods.

[Roland Rock]

Here's another one, the 'floating neck' used by Treker and Henman.

http://www.trekerguitars.com/html/necks.html

http://www.trekerguitars.com/html/neck_adjustment.html

I think it works by having the rod attached to each end firmly, and the headstock is a separate component to the neck, there is no tension of the neck itself, eliminating dead spots and increasing sustain. Apparently.

Edited October 31, 2012 by Roland Rock

[ikay]

[quote name='Roland Rock' timestamp='1351677942' post='1853970']
Here's another one, the 'floating neck' used by Treker and Henman.
[/quote]

This uses the Bunker tension free neck mentioned above. The old bunkerguitars.com website seems to be dead so I guess they've now teamed up withTreker.

[Roland Rock]

Ah, sorry, missed that

[BigRedX]

[quote name='ikay' timestamp='1351679441' post='1853993']
The old bunkerguitars.com website seems to be dead so I guess they've now teamed up withTreker.
[/quote]

[url=http://www.bunkerguitars.com]Still working for me[/url]

[ikay]

[quote name='BigRedX' timestamp='1351686710' post='1854115']
[url="http://www.bunkerguitars.com"]Still working for me[/url]
[/quote]

Hmm, so it is! Bit wierd, wasn't working earlier, maybe the server just dropped out for a bit.