Dupont crack Repair Help!

Bob Holo

Well, the guy could be right, though... and just using language that isn't familiar. In English in America it's usually called a "body hump" or a 14th fret hump or something to that effect. Usually you want a neck that is relatively flat but that tapers off over the body a few thousandths of an inch. When you put string tension on, it pulls "relief" into the neck. What this means is that the area between about the 1st and 10th frets forms a bow. So what you wind up with is the neck sticking straight out from the body for about half a dozen frets and then slowly forming a slight ski-jump up toward the headstock.... and behind the body join it tapers off slowly. The relief, when you measure it, would seem to be a bow between the body and the first fret, but it's not. It's really just happening because the neck rises from about the 5th or 6th fret... and so... relatively speaking... I guess that does form an (upward sloping) curved area roughly between the body and the nut (or zero fret in the case of GJ guitars) But if he's saying the neck is S shaped, he probably means you have a hump over the 14th fret. It's common. Often you don't notice slight body humps if the action is high, but when the humidity drops and the action lowers, you notice them. Now... all guitars have some body hump if they are eased over the body... think about this and you'll see what I mean... if the end of the fretboard is eased over the body, then that area of the fretboard from the 7th to the 21st fret is back-bowed (relatively speaking). The important part is whether the area from the body join to about mid neck is relatively parallel to the body.... because if it is, then the string sees the eased area as increased room over those final frets where the string vibrates so much on open chords. Because the vibration of a string isn't purely ovoid as so many say. The first three harmonics figure prevalently in the string's movement - and the string doesn't just vibrate open... but at every fret. So, the vibration pattern is the integration of all the primary and harmonic content when fretted at all frets. So... what does that look like? well... sort of like a 'Torpedo' cigar with one end at the bridge and the other end at the nut/zero-fret.... and it follows that for maximum string room to vibrate that the shape of the fretboard should look like the outline of that cigar... well... a funny looking torpedo cigar about 25" long and 20/1000" wide at the center, but you get the point. Anyway, if the body join to the 7th fret is slightly downward and then rises back up toward the zero fret/nut... then the string sees only the bump around the body join as a decrease in room to vibrate. That is a body hump... or I suppose you could call it an S shaped neck.

Can't really put it into words clearly - I just googled it and found a good picture... I disagree with what this picture calls perfect & ski-jump, but just focus on his illustration of a body hump... that one is spot-on. That's probably what the Busan guy is calling "S shaped"... look down your neck like you're sighting down a rifle barrel and see if you agree. Look from both ends... sight down from the neck and body ends.

http://www.coloradoluthiers.org/tipsAnd ... /setup.jpg

marcos81

I couldn't really tell it wasn't very obvious. i'm not sure, but there is fret buzz from 1-3, 6-8, 13-14th frets. Bob you think you could look at my guitar? I'll just cut to the chase.

Craig Bumgarner

Bob Holo wrote:

. I disagree with what this picture calls perfect & ski-jump.....

Bob, I know this is a drift away from the thread but could you elaborate on how you disagree with the drawing you linked to (good drawing BTW) and what your idea of ideal is, especially in the 12- 21 range?

Thanks,

CB

Bob Holo

Hi Marcos,

It's not the type of thing you want or need to ship intercontinental to fix and any good guitar tech can do a fret dress so it's not wise for your pocketbook or my schedule to look at it. If the hump is not severe, you can just shim up the guitar till you get somewhere that you trust a guitar tech. I mean, it played well before the action dropped, so shims will get you back to that till you find a tech to work with. But really, it's not a huge problem. It's what guitar techs do literally every day - multiple times per day in a busy shop. Any good honest repairman/tech can take care of it.

Craig,

What I mean is the guy is showing a dip for standard relief but then the body easing slants down from there... which means that the place where the relief stops and the easing starts is in effect a body hump... or acts like it when you're playing where the center of that "hump" is the harmonic.... probably the 3rd or 4th fret? Anyway, what you really want is the neck to drop down from the zero fret to mid neck... go straight to about the ... oh... 16th or 17th and then gently ease down a few thousandths. So, when a good tech goes to put relief in your neck, he's not creating a "hollow" in the center of the neck, but a rise at its end so that when you fret the zero and the body fret that there is indeed a relief between those two points... but it's not because the neck is curved, more that it rises at the end so that relatively speaking, there is a curve between those two points. So, his "ski ramp" picture with a bit of body relief would actually be closer to ideal, though his ski ramp picture is really exaggerated... so obviously much less than that... you know... like 8/1000 to 15/1000 relief depending on how much of a monster hand the player has... flatter necks tend to have better intonation because the string stretches less & more linearly, but for a guy with a hard hand, you need more relief. Anyway, I don't mean to nitpick - the picture comes from a hobbyist site and god knows most books seem to describe the concept of relief incorrectly and encourage this "dip-ease" type of concept, but if you look at a really well leveled neck you'll see it's more of a ramp/ease.

Jeff Moore

I'm gonna be guilty of trying to highjack this guys thread, and of trying to draw the discussions to the mysteries of a perfect neck, but the illustrations Bob gave a thread to are compelling for raising a question about relieving necks.
http://www.coloradoluthiers.org/tipsAnd ... /setup.jpg

If you look at the last illustration, you see a straight neck and a open (unfretted) vibrating string. The greatest extent of string wobble or vibration shown is at the 12th fret, Yet the first illustration of the "ideal" neck shows that the highest fret on the "ideal" setup is the 12th fret. This does not seem ideal.

I've pondered this for decades, and once I was capable of making "ideal" setups, I instantly tried the easiest setup as well, (a perfectly straight neck) and found that my action could be lower and setups could be easier to do but most conveniently: checking necks for best playability becomes as simple as having an 18" straightedge and a light bulb, or even just sighting down them without any tools. All this preference for slight curves (relief) seems odd, yet has held as a "truth" for as long as I've played guitar.

Using the diagrams Bob provided, can anyone tell me why "we" want "relief" in our necks?

richter4208

This is actually really timely for me because I just bought a guitar that has the ski jump neck profile. It was nice to read thru this thread and discover these bits of wisdom and the informed analysis of Bob Holo. It's good to know my guitar isn't the only one and is quite common. Hopefully I'll be able to make my guitar great with the right set up. Thanks!

Jesse
Portland, Oregon

Bob Holo

My suggestion is to get it plec'd. Michael knows a gent in Seattle whom I would trust.

Jeff Moore

I've always imaged two string movements. Spiral, which is what is shown on the vid (thanks again for a great reference point); spirals (hard to see with the eye but there anyway) and what you see with the naked eye, ovoid - or if looking from the end of the string something roughly appearing as circles alternating into elipses. The spirals (looking like sine waves) moving along down the string will require the pointed hot dog shape to best accomodate them, except that as you pointed out; all string movement as you play up the neck (shorten the string) is smaller. That is the ovoids, spirals, and other craziness are all reduced in size as the length of the string is shorter. The spirals are also reduced in size at the ends of the vibrating string. So the spiral like the ovoid is greater (though not in a simple arithmetic way like the ovoid) also at the middle of the length of the vibrating string.
The open low E string has the biggest movements and as you play up the neck, all the movements get smaller and the maximum of those movements move towards the bridge.
So the center of the greatest, if simplest and most obvious, movement (especially to the naked eye) is right over the 12th fret or at the half point of the length of string on the open low E string.
On the "ideal" set up this is where the greatest choke is also placed. Not only that but when fretting at the 6th fret the strings are now at the valley of the ski jump or "relief" looking "up" at the 12th fret.
As you play up each fret one by one: 1. The center of maximum movement moves towards the bridge and eventually off the fretboard entirely. 2. The extent of all movements are reduced.
On a straight neck (that is; straight with the strings on and up to pitch), the string buzz is equal everywhere. I've tested this over and over. On an "ideal" neck, the buzzing will increase and decrease as you move up and down the neck.
On a relieved neck, the lesser choke points are several and change in degree with each fret but the 2nd-3rd, 10th, and 12 - 20th frets are the better areas to play on a relieved neck, though not equal.
Clearly you want your most string freedom on the open low E where movement is maximum. This is exactly where the most string freedom is on a perfectly straight neck. On a straight neck also, the node of maximum string movement moves more parallel to the frets as you play up the frets: The hypotenuse (moving part of the fretted string) also increases in angle (angle of neck and string), yet the extent of string movement is decreasing as you play up the neck. If the fretboard extended all the way to the bridge, at some point up that neck the string, fretboard and bridge would form an equilateral triangle! This increasing angle, and the decreasing string movement mean that all our string buzz problems decrease some as you move up the neck. The problems mostly occur in the 0 - 12th frets where the "ideal" places a curve.
It could be true that the "relieved neck" is mathematically the most even distribution of string freedom (particularly relating to spiral movements traveling lengthwise up and down the string), but it still provides the least advantage for open strings and again at the 6th fret and this is noticeable on "ideal" necks. I suspect that string movement is the issue that is least understood (by me at least) because the rest of the geometry (frets - bridge) are static.
A spiral movement where the node of movement is moving down the string, is the best case for some kind of relief in a neck. These spiral movements if equal all over the string would create a barrel shape in a plucked string. But its clearly not a barrel shape at ends of the vibrating string. Also fretting up the neck will squish the overall diameter of this barrel, and the spirals (and the resulting barrel shaped movement) are squeezed at the ends of the moving string to zero. So even the spirals are taking on another (if dynamic) ovoid shape giving us a curved barrel (football) where it decreases in diameter towards the ends where the strings are still (not moving at all). So overall the strings vibrate like a elongated football. A very busy football that shrinks as you play up the neck and is biggest on the open string.

I know that my argument is against the overwhelming consensus over a long time, but its less painful than surgery to talk about and central to everyone's set up.
I began to notice this 6th fret - open string choking decades ago with "store bought" setups. I began doing my own set ups. I started to read about string movement and also began reducing and then eliminating the relief altogether about 20 years ago. There are plenty of other areas of thought where I've persistently been very wrong over longer periods of time, but my experience has kept me confident about perfectly straight necks. I want to be humble in regards to the luthier craft on which I depend, yet contrary now after years of my own experiments and experience.

On a side note, a straight neck is a lot simpler to create, but that alone wouldn't make them better. Yet more people can do their own set ups as you eliminate variables. I'm always struck at the lack of set up on most guitars and the disadvantage to young players be pushed away from the mid and upper neck by rash sounds, high strings, from all kinds of set up issues.
I can do a straight neck in twenty minutes on any stiff guitar sized table with $10 of hardware store purchase, a tapered wedge, two bar clamps and a fret file or preferably a small rotary tool to "dress" frets. It'll play fabulously even if the frets aren't shiny finished.
I'm ok being wrong too, but that 6th and open fret problem seems real to me on most "ideally" set up necks. I can remember too, when doctors told mothers for a decade or so, that breast feeding is bad for your baby. There are lots of points where our consensus is quite wrong and led off course by a big advertising budget. Not saying this is one of them, but I think something is amiss. It seems that the miscalculation that "proves" the "ideal" neck is in how the vibration of the string is accounted for. This would be tricky without accounting for the several different, and of course simultaneous, kinds of movement. I'm not claiming to understand string movement. If strings moved simply, it wouldn't be hard to visualize or calculate. I've got my images and the little I've read, but I'm not comfortable with the current thinking either when put to actual use in set ups on guitars.

richter4208

Hey Bob,
You aren't by chance talking about Mike Lull are you? I've been meaning to ask if anyone has had a full gypsy set up done by him? My current guitar I bought on this forum was pleked by Mike after a setup by Josh Hegg. It is an awesome playing guitar even tho it is the lowly gitane d500. So I was thinking of having this new guitar plekked by him as well. It still would be nice to have a custom Josh Hegg Bridge even if I had this plekked.

Bob Holo

Mike Lull is a great setup guy - that's who I was talking about - for sure.

Or Mike Burdette in Portland... must be something about the name "Mike"... probably translates into "Fret God" in Sinti.

Tough sell on a budget guitar though... it's half the price of the axe! ;-)