Official Luthiers Forum!

I was looking at diagrams on the web comparing profiles of necks with back bow and forward bow and noticed that there are two types of diagrams out there that differ in what they show about the how the neck bends forward. Some show the neck being pulled forward from the nut to produce a continuous upward curve betwen the nut and the body that results in increasingly high action as you move up the frets (first figure below). Others show the neck actually cupping between the nut and the neck/body joint similar to the way an archery bow bends when you pull on the string. This results in the middle of neck moving downward below the original plane creating a recess relative the plane of the fretboard past the neck/body joint (second figure below, "relief").

I've always thought that forward bow is best represented by the first diagram. What do you think?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yeah, the first one looks right to me. It may be possible to create the second using a double acting truss rod, but I don't think string tension would ever do that.

I most likely am blind....but, I don't see the any difference in how the neck bends.

The first just shows a red line depicting what would have been neck angle along the top of the frets with a straight neck.

The second set has no indication of how the neck has moved in relationship to that red line but it does show the relationships to the top of the frets when a neck bows either way.

_________________
Joe Beaver
Maker of Sawdust

In the first figure, top neck, Move the red line up so it hits the nut and imagine it represents the strings. If you do that, the height of the strings increases from the nut position all the way up the neck with the highest action at the body end of the neck.

In the second figure, middle neck, the action is low a the nut, is highest around the 8th fret, and then deceases again until its low again at the body end of the neck. This is because it's showing the neck bending the way a Go-bar bends: the ends remain stationary but it bends in the middle.

How the neck bends in the two diagrams is very different.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Actually, now that I look more closely, I'm not so sure. Notice the heel in the second one. It's rotated, which implies that the neck is bent just like the first picture, but then the whole image (which would include the body if it had one) has been rotated. And then presumably the strings are being held down between the neck and bridge to show the relief.

Looks like a perspective bias to me. Without getting out my scale and straight edge, it seems as though the two drawings show the same thing. The apparent conflict seems only that in the first the heel area remains static, and in the second the neck is rotated slightly as drawn, using the string as constant reference rather than the heel.

First drawing the body would be the same in both examples. Second drawing the body would be straight in the first, tilted upward in the second, and down in the third.

_________________
Eschew obfuscation, espouse elucidation.

Here's a clearer example with text stating that if the truss rod is too loose, "the neck will bow forward and cause a dip in the middle of the neck". The diagrams are intentionally exaggerated, but the point is the same.

The reason this came up at all was a discussion on another forum in which help was asked for a guitar that had string buzz from the 5th through 13th frets. Another poster advised the OP that he needed to tighten the truss rod to get rid of the buzz. My first reaction would be that the buzzing could be caused by back bow and that it might be addressable by loosening the truss rod. Not tightening it. The advice was based on the assumption (based on a diagram like below) that the neck bends like a Go-bar between the nut and the neck/body joint causing a dip in the middle of the neck that puts the middle frets far enough below the plane of the frets above the neck/body joint to cause buzzing when fretting frets 5 through 13. In this scenario, tightening the truss rod would take out the dip and eliminate the buzzing. This didn't make sense to me, so I did a search to see if the idea that the neck can bend that way has any general acceptance.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I believe the neck curvature is the same, it's just that the neck angle where it meets the body is different. If the neck angle is, say, 0 degrees, the action will increase as you get closer to the saddle. If the neck angle is, say, -2 degrees, you can have the situation where action increases over the first several frets then lowers again towards the end of the neck.

I'd say it's little more than the drawings and presentations not being that thoroughly thought out. The neck bends are still all the same, but the artists were just careless in wrongly shifting it's angle relative to the body.

Looks like simply some rough art, but of course no, the neck angle doesn't shift with changes in neck straightness, as I suppose some may infer if they take the drawings as too literal or accurate. My guess is that whoever made those drawings was trying to convey what relief and backbow mean in the most basic sense (for those who don't have a clue which term describes which direction). For this they work fine, but they're certainly no blueprint.

_________________
Eschew obfuscation, espouse elucidation.

As to the example you gave, the solution is neither (or at least neither on it's own).

If you're buzzing near the body joint, no truss rod adjustment will have the slightest impact here at all.

Sounds to me like your textbook dried out guitar - top is sunk, neck is pulled in to excess relief.

The saddle needs to be raised, which should happen naturally as the guitar is rehumidified, this raising the bridge and saddle as it swells back to a proper crown. The neck may straighten on it's own as well, but will likely need the truss rod tightened a bit as well in final setup.

_________________
Eschew obfuscation, espouse elucidation.

Since most of the replies are questioning the diagrams rather than the concept, it suggests that the idea that the neck bends the way the poster on the other forum suggested is not being considered as a real possibility. Which is fine with me.

Just to try to put this to rest, the poster who recommended that tightening the truss rod would eliminate buzzing from frets 5 through 13 based his recommendation on the diagram below. I traced it back to a post from 2010 by a professional builder on the AGF. The lower drawing clearly represents the middle of the neck bowing downward in the middle. He indicated that this is what would happen after adjusting the truss rod to increase relief.

Does anyone think that this is representative of the effect of the interaction between the truss rod and string tension on the neck? I don't think it is. Can this actually happen or is it just a bad drawing?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yeah, that's just a bad drawing. Fine to describe relief to someone if they didn't know what relief was, but totally inaccurate in terms of movement of the instrument as a whole.

Looks like the drawing may have been compromised by simplicity or restraints of the drawing software they were using. Easier to bend a few line sections in the center than curve an entire section and raise the headstock upward.

_________________
Eschew obfuscation, espouse elucidation.

Thanks, David. I couldn't see how a neck would be affected that way and had never read anything to that effect, but I continue to find out new aspects of how guitars work that I had not heard of before. So I wanted to be sure.

Drawings like this carry a risk. The guy on the other thread who saw this drawing interpreted it quite literally as The Way Things Work and gave advice based on it that's not likely to help solve the string buzz problem that the OP has with his guitar. Beware the internet.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yeah, I get it now - I think I initially missed how literal an interpretation of the drawing the other party was assuming.

I'd say you could unequivocally state that your appraisal was correct - a truss rod will not raise any frets in the center, but simply bring them back toward the end. The simplified drawing was an (entirely understandable) error resulting from restraints of the program it was drawn on, and should not be taken as gospel.

Neck may need straightening, but more importantly the saddle needs to come up.

*side note - I found the discussion in question on the AGF. I'd participate directly, but unfortunately they hold a strict ban against participation from professionals.

_________________
Eschew obfuscation, espouse elucidation.

I have seen where the dreaded 14th fret body joint hump would make it appear to be the case as in the exaggerated second image. It happens when a guitar is built with an over set neck so in a way it's like others have mentioned about perspective. An overset neck would be like the first image only rotated counter clockwise. Then you would have a point at the end of the fretboard and the point that makes the apex of the body hump make a straight line to the saddle and the middle of the fretboard (concave apex) would be below that. In situations like that you tighten the truss rod to make the neck dead flat and then raise the saddle.

http://theunofficialmartinguitarforum.yuku.com/reply/1942081/Understanding-neck-angle-relief-etc#reply-1942081


I'm far from completely confident about any rules for how necks behave with changes in truss rod and string tension, but I have had a large variety of instruments run through my hands.

I feel like truss rods vary tremendously in how much they either, "Bend the neck like a go bar between the nut and the heel" or pull the nut and neck forward above the plane of the top. It seems like headstock angle and differences in flexibility end to end in the neck, as well as truss rod design play a role.

I agree the diagrams are most likely just poor representations but I think they do represent two types of movement that do take place to varying degrees.

Not positive though.

Edit: Of course I said it backwards. The strings pull the neck up, the t rod counters the upward force to some extent.

This is exactly what I was wondering about. The possibility that what I understand about how a guitar works might be overly simplified and there might actually be more to it than I thought. Not stressing over someone being wrong on internet, btw. (I do like that cartoon though, Hesh.)

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

The problem is that most of these deflections are very small, and they are interrelated.
I'm assuming we're talking acoustic guitars, electrics are a bit different.
String tension does several things. It pulls the top (and the bridge) up just a little, it rotates the bridge (and the top) just a little, it rotates the heelblock (and the neck, and the top) just a little, and it causes the neck to bend just a little. All these effects raise the action, so the neck angle must compensate for everything except the curvature of the neck, which is compensated by the truss rod. I'll see if I can put together a better drawing.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

when the neck bows the headstock will rise the center doesn't dip.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Exactly, but because of that last drawing, there is at least one person out there who has it firmly embedded in his mind that it does, and is giving advice accordingly. Because he saw it on the internet....

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yeah, right.....

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

the most cringing line a guitar repairmen can hear . In fact I had a customer tell me that then proceed to tell me how to fix his guitar. I handed him my glue bottle and told him to take it home. At that point he was NO NO I need you to fix it. I said I will pass . If I do it your way it will fail and you will be mad at me .
I would rather you mad at me for not doing the work than doing it poorly

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it