Nylon Fretboard Radius

[Tom2]

My newest custom crossover design spec insight is about fretboard radius.

The main challenge of designing a crossover is to make the neck as narrow as possible while still honoring the physics of nylon. My original experimentation revealed that the nut width needed to be at least 7mm wider than the string spacing, in order to prevent the first string from slipping off the edge of the fretboard. I also found that my preferred string spacing is 41.5mm, resulting in a nut width of 48.5mm.

After playing this configuration for about a year, and just playing freely without analyzing, I have discovered that the first string needs a little bit more space to the fretboard edge. So I reduced the string spacing to 41mm, which allowed me to keep the nut width at 48.5mm while adding a little more space for the first and 6th strings. This worked, but it also reminded me that I really do prefer a 41.5mm string spacing.

So I considered increasing the nut width to 49mm, but this is starting to get wide. I then had a flash of insight regarding fretboard radius. A radius causes the fret edges to slope away from the center of the fretboard, allowing the strings to slip away from the center of the fretboard, towards the edge. A sharper radius produces more slope and increases the potential for slippage. With steel strings this isn't an issue, because the higher tension keeps the strings in place with any radius, and the only issue is comfort for playing barre chords.

Nylon, however, is a slippery slope, and a flatter radius would reduce the steepness of that slope. Maybe this is why classical necks are traditionally flat. My current crossover has a 16" radius (400mm). So does a standard Emerald. Rainsong has a 20" radius (500mm), and I'm thinking this may be superior for nylon strings.

While I can simulate different neck widths by making offset nuts, I can't simulate a fretboard radius change, so my next step forward can only be a leap of faith (which I don't like, hence the experiments).

I will definitely go with a 20" radius, because basic geometry shows that it will provide some improvement, but will it be enough to keep the nut width at 48.5mm when I return to my preferred 41.5mm string spacing? Right now, my intuition is saying yes, and I can always make another offset nut if the first string still needs a little extra space.

YMMV

[tkoehler1]

I currently have a classical that I had modified with a 20" radius on the fretboard. I love it! For myself it is definitely easier to play. This guitar has a 52mm nut and 59mm saddle, so the board is fairly wide. But a 20" radius is so slight only your fingers will notice .

TK

Tom2;

To paraphrase a political operative, "I love it!" I really look forward to your first custom order. I could see Blackbird and Emerald working with you to the parameters that you are so well laying out. If and when you are ready, let me know, I'd probably buy into to your vision.

[Tom2]

We're getting there.

My guess is that once this design is dialed in and fleshed out, many people who do not yet know that they want one of these, will want one.

This fretboard idea is an example of why I work relatively slowly. We don't get to choose when insights occur, but we can set a place at the table for them as an invitation. Before this insight arrived, I had no answer to the question of flat, 20", or 16" radius, or why. Now I do.

Awareness of my ignorance provides the wisdom to leave space for inspiration and insights that are beyond my conscious control. I suppose that's the basis for creativity in general. And creative self expression is why we are all here.

Tom2;

You've set the table nicely. I think that sound holes and top surfacing are also worthy of place mats.

I don't remember whether you've discussed the placement of sound holes, but my intuition is that a center hole is traditional and probably for a very good reason. I am also a bit of a skeptic when it comes to sound ports--I like them from a players point of view but suspect that they dilute projection to the audience.

Top surfacing was an early discussion in the old MacNichol forum. The Cargo came in various finishes from raw to paint. There seemed to be some consensus that the thicker surfacing led to a darker, more mellow sound and a slightly diminished projection. The discussion is made more complex by the introduction of veneers as a top layer.

As you have pointed out in previous discussion, there are multiple parameters to consider and every one is, in its own way, significant. I believe that sound holes and surface treatments have significant effects on both tone and projection.

[Tom2]

Originally Posted by tkoehler1

I currently have a classical that I had modified with a 20" radius on the fretboard. I love it! For myself it is definitely easier to play.

Thanks for verifying the desirability of a 20" radius with nylon.

Originally Posted by EvanB

Tom2;

You've set the table nicely. I think that sound holes and top surfacing are also worthy of place mats.

I think they are worthy of their own threads. Maybe I'll start one now.

[rwtwguitar]

I am glad you are able to dial in exactly what you are looking for.

FWIW I have built a handful of nylon string crossover CF guitars. One thing people respond positively to is the 1-3/4" (45mm) nut. A 2" or 2-1/8" nut (51-54mm) was one big reason given for NOT getting a nylon string guitar.

I cut the nut to space the low string 1/8" from the fretboard edge, 5/32" for the high string. That is about 1/32" (1mm) more than on a nut cut for steel strings. That makes the string spacing about a mm less between each string than what you are specifying. So far, that seems to be a positive thing with the people who have played one.

Of course, my guitars generally go to jazz players looking for the feel and sound of nylon, not classical players (at least none yet). These are archtops, not flattops. My necks are thin compared to a classical guitar and have a "C" profile compared to a classical "D". So I suspect very different from what you are looking for.

Still, hopefully that is a useful data point for you.

RWT;

If I remember correctly you contributed to the thread dealing with a nylon string electric guitar--I think you offered a photo of an arch top nylon string made by you (a beautiful piece of work, by the way.)

I agree with the difficulty of selling a crossover with a 2" nut width. For a steel string player that width is excessive. I think the 2 3/4" (45mm) width is tight for nylon players.

I prefer the 1 7/8" (48mm) nut width and have found that both steel and nylon players have had no problems in adapting to my favored crossover width.

[rwtwguitar]

That seems a reasonable compromise. I have a testbed guitar. I'll make a new neck at 1-7/8" and see how it feels.

[Tom2]

Your design has a nut width that is 9/32" wider than the string spacing, which is 7.1mm. My experiments reveal that 7mm is the absolute minimum, and that after a year of playing, I'm inclined to bump that up to 7.5mm. I also build offset nuts that give the first string a little extra space, at the expense of the 6th string. So we are on the same page.

We both agree that a crossover neck needs to be as narrow as possible, and as Evan points out, a 1 7/8" nut is generally preferred over 1 3/4". I've gone into hyperdrive on this, and am now adjusting individual string spacing in 0.1mm increments, fretboard edge gaps in 0.25mm increments, and nut width in 0.5mm increments, all to get maximum string spacing with minimum neck width.

Right now, I'm looking at how the slope of fretboard radius affects first string slippage. If a 20" radius allows the first string to be 0.25mm closer to the edge than a 16" radius, then I can reduce nut width by 0.5mm if I go with a 20" radius.

What fretboard radius do you use?

[rwtwguitar]

I use a compound radius from 16" at the nut to 20" at the 20th fret.