Implications of a High Saddle/Shallow Neck Angle

[Brent Hutto]

My main steel-string guitar is a cedar/mahogany OM, built very lightly and setup for light-touch fingerstyle play. I've previously posted here about the back edge of the bridge starting to lift slightly, that was repaired by injecting a little hide glue under the lifted corners and clamping. That's holding fine without needing the bridge to be removed and replaced.

I'm pretty sure that the reason putting "Light-Medium" gauge strings on it for a month or two caused the bridge to start lifting is the high string height above the soundboard. A straightedge on the fretboard lands about 0.100" (2.5mm) above the top of the bridge and the low-E string is about 0.550" (14mm) above the soundboard at the bridge. So the result is plenty of saddle showing (from 0.125" to 0.160" depending on the string) with quite low action of 0.085" (2.2mm) on the low-E and 0.070" (1.8mm) on the high-E side at the 12th fret. WIth that big of a lever moment and cedar wood on the top, no surprise it needs light strings to protect the bridge, right?

What are the other implications of having the neck angle so shallow and the strings so far above the top? This guitar has the fullest, sweetest tone at very soft dynamics of any I've played. And it's just generally a very sustaining and resonant acoustic guitar, perhaps a little on the bright side. If that neck were at a more conventional angle, placing the strings maybe 1/8" lower with a little higher action and less saddle showing what changes would you expect from the sound quality and response?

The reason I ask this is that strumming or flatpicking can easily bottom out the strings, especially the bottom ones. But I can't really raise the action without having just way too much saddle sticking up and making the overturning moment on the bridge even more dangerous. Of course that's not really a reasonable goal anyway, I asked the builder if he had any super-light, super-responsive fingerstyle guitars that would sound good with the lightest possible touch and that's what he delivered. But I like the way it plays so much I'd hate to use a different guitar on the occasions where I want to use a pick and dig in a little.

P.S. It's a bolt-on neck so if I wanted to do something so extreme as reset the neck angle that should be reasonably risk-free. Still a job for a competent luthier but not major surgery.

[Greg Mirken]

FYI, that's not a shallow neck angle, just the reverse. Sounds like your builder delivered a super-light, super-responsive fingerstyle guitar that responds to the lightest possible touch. It's not reasonable to expect it to handle aggressive flatpicking too, just like it's unreasonable to expect a fast, super-handling sports car to be very good at hauling gravel.

I'm unsure what sort of response you are looking for. Resetting the neck for a shallower angle is trickier than setting it back further. How much relief is there? Maybe a teeny bit more would give you an acceptable compromise. Or use a different guitar for the flatpick stuff. Is that so terrible a problem?

[Brent Hutto]

I guess I had two questions buried in there...

One was does it makes any sense to try and get a little more action height. If it were just about making a saddle that's 0.020-0.030" higher, normally I'd do that. But as high as it is already and given the stress on this cedar top that's probably a no-go, right? And you're right Greg, now that I see it written out there's no way I'm screwing around with a fine guitar by trying to reset the neck!

The other question is a little more vague. Is setting up the geometry for such a tall saddle (strings 0.550" above the bridge) a usual way to get responsiveness and a full tone for light fingerstyle use? From a design standpoint I'm just trying to learn what goes into making a sensitive "sports car" guitar. Paper-thin soundboard, back and sides is surely the main thing but I thought maybe the geometry helps, too.

[Greg Mirken]

Brent, I don't think I would try to get higher action with a taller saddle, at least not without asking the builder. But I asked about relief because adding a bit if the neck is very flat might solve your problem. But of course, it's a compromise.

[Russell Vance]

This question got my interest - I often get confused when talking to acoustic builders because they have a dispensation from the laws of physics...just kidding, sorta!
The longer the lever acting on the sound board (the distance from the sound board to the top of the saddle) for any given tension (string gauge tension and soundboard wood tension) the more likely something (fill in the blanks here all you guys who know) is to happen - the logical extrapolation is that an extremely short lever (shallow bridge/saddle height) will need a bigger hit to get the top moving and so on........however, there are nuances here - the longer the lever (bridge/saddle height) the smaller the movement of the transfer of string energy for any given string deviation - but the probability of transducing complexity in the string frequencies is probably higher. Now, there are books about this stuff but I was feeling in an interested mood - maybe a bit of discussion to open up the subject some - I don't have a clue because I'm an electric guy but it's a good subject - tone generation. Rusty.

[Christian Johnson]

Rusty, here's what I know (not much!) and what I've experienced. The higher the saddle relative to the top of the bridge, the more break angle and string tension is produced, pushing the saddle downward into the top of the guitar and thus, more tone and volume. Bracing is a big factor here too. Customers want me to lower the action on an acoustic with bad neck angle by sanding off the bottom of the saddle, It will lower action, but I've had it completely change the tone too. I worn customers of this before proceeding.

[Bob Webster]

Okay, here's my two cents on this. First off, most of the folks in academia who've studied this stuff say that the forward and back rocking mode of the bridge (from the pull/push motion of the string) is not important in tone generation, by which they mean that there is no disproportionately high intensity of the first harmonic that would arise from the rocking motion. I've seen nothing in my own data that would contradict that. Basically all of the sound from the top is generated from the up and down motion of the string, and of course, the bridge.

Most classical builders work pretty hard to keep their saddle heights at or under 10mm from the top (~0.40") because, a) one can get too much break angle if it's taller and b) there's just too much torque on the top if they're taller. There is, however, another subtle point in all this and that concerns the angle of the string coming off the saddle relative to the plane of the top. It's kind of wierd, but if you have an angle at the saddle that is less than 90 degrees, one can see a lowering of the main body resonance of the box when you string it up, relative to having no strings on it. It's as if the tension of the strings is translated into extra mass on the top. If the strings are parallel to the top so that the angle at the saddle is 90 degrees, that doesn't happen. To take that further, the guys that build with elevated fingerboards end up with string to saddle angles of >90 degrees and these builder claim that it imparts a more harp-like tone to the instrument, whatever that sounds like coming from a guitar.

Finally, a tall piece of bone is more flexible that a shorter piece of bone, for whatever that means to efficiency of energy transfer.

Cheers,
Bob

[Russell Vance]

Neat - as I said I'm clueless on this transfer of string vibration to the top but intuitively know that there is more in play here than just one set of primary movers and shakers - a whole bunch of crossover stuff ensuring endless permutations and fodder for discussion - but, I'm taking it all in - next ! Rusty.

[Len Biglin]

Hi Bob , this is an interesting subject . are you saying that a string to saddle angle<90 will have the effect of putting weight on the bridge , like pressing down on the bridge with your hand ? that makes sense. I dont know much about harps (mostly see home made ones) but they have the strings coming out of the soundboard at 90deg.which must put a huge load/stress into play too.....Next! Len

[Bob Webster]

Exactly. Len. It's as if you'd installed a much heavier bridge. The main top resonance can drop as much as 3-4 Hz, which is substantial. Depending on how the top is braced, this can actually hurt or help because it changes the relationship between the air resonances and the top resonances. Voodoo, huh? It should be noted that there have been no formal rigorous studies on this issue, so the information regarding the impact on tone hovers somewhere between the hypothetical and the anecdotal. Real firm ground.

As far as setup goes, there are basically three camps. The first camp builds in a ~2.5-3mm forward lean of the fingerboard at the nut end relative to the plane of the top rim and uses a fingerboard of constant thickness. The string/saddle angle can end up very close to 90deg depending on the arch of the top.

The second camp, of which I am a member, tapers the fingerboard a specific amount and the plane of the fingerboard underside is contiguous with the plane of the top rim. The angle at the saddle is slightly less than 90deg.

The third camp are the folks that use an elevated fingerboard and setups vary among these builders, but the angle is always somewhat greater than 90deg.

What does this all mean? Beats me. We of the second camp always have to use a little windage when voicing the top. However, absolutely outstanding guitars have been built by members of all three camps. For example, Contreras in the first, Marin in the second, and Byers and Humphreys in the third. In the end, I think the choice of what to do boils down to how much hassle you want during the building process to establish your target action. The second method is vastly easier than the other two so long as you have a convenient way of tapering the fingerboard (like a hinged taper jig and a Performax sander).

Cheers,
Bob

[Ned Knepp]

I once had a conversation with a mechanical engineer who also happen to be a mandolin player. He was considering changing the tailpiece on his instrument and his major concern in making the decision was how changing the lateral angle of approach the string had to the bridge would change the downward pressure on the bridge. In essence, the tailpiece determines the lateral angles of the strings as they approach the bridge. The more lateral angle the strings have ( both left and right), the more the string forces are focused at an angle to the top and the more downward force is canceled by the opposing forces of the bass and treble sets of strings as part of their force is dissipated by one another .

Straightening out the lateral approach angle has the effect of rotating the forces more vertically thus lowering the amount of force that is canceled by opposing strings and bringing more of that force to bear on the top of the instrument. Now it seem that this adjustment to down force at the bridge could be used to ( and probably does) offset the heavier string pressure caused by a less that perfect angle in the string coming off of the bridge on an arch top instrument.

It's just a thought anyway.

Ned

[Rick Homan]

Great discussion, and a timely one for me! I have just acquired an old no-name, all-birch, 14" guitar. It is a flat-top, but has a tailpiece and shows signs of having had an elevated pickguard and loose bridge. The back-set on the neck (seemingly original) is such that string-height at the bridge would have had to be in excess of .75".

I am thinking it would be an ideal candidate for conversion to pin-bridge, but am wondering how to do it. I take seriously Greg's remark that, "Resetting the neck for a shallower angle is trickier than setting it back further." OTOH, if I leave the neck angle, the bridge would have to be thicker than a conventional bridge. This would seem to create a more-extreme example of what Brent described in the original post here.

Perhaps, I should just make a nice little moustache bridge for it rather than getting into serious R&D. Any thoughts?

[Tim Mace]

Also some fretboards are thicker which in turn raises saddle/bridge height.I only recently realized this.