View Full Version : radius basics

Izaac Walton
Mar-05-2004, 11:23am
Can someone give me some basic information on radius fretboards? I see numbers like 9.5", 11", 20", and terms such as compound radius. What do these numbers and terms mean?

Also, how are the nut and bridge typically set up on a radius fretboard? I've read that grooves can be cut at certain depths to create this effect. I've also read that strings need to be at "proper" depths to get good sustain.

Thanks for your help!

John Flynn
Mar-05-2004, 11:37am
I am not a builder, but I am a big radius fan. Think of the radius number as describing the radius of a hypothetical circle. The higher the number, the bigger the circle. The side to side curve of the fretboard and frets is just a small arc, or part of that circle. So if you have a 10" radius fretboard, if you could continue the arc of a fret until it became a full circle, that circle would measure 10" from its center to any point on the circumfrence.

A compound radius changes as your go down the fretboard. So it may be 9.5" at the nut and flatten out to 16" at the end of the fretboard, or whatever. This is very common on electric guitars, becuase of the need to bend strings and not have them "mute out" on a higher fret. But it is also used on some mandos, notably the Rigels and Red Diamonds.

IMHO, nuts and bridges should be radiused appropriately to the fretboard. It would be possible to cut slots deeper, but that would seem to violate rules of good set up that I am familiar with. I am no expert, but as a player and luthiery customer, that is what I would demand and expect.

Mar-05-2004, 12:50pm
This is the same discussion I am having on another thread and my question is ,Should the string slots be cut at descending depths from center or will that through the intonation out ?

Mar-05-2004, 4:00pm
The nut should hold each string so that you get a certain amount of clearance above the first fret. Each player has his/her preference for nut height, but generally the first fret clearance will be slightly higher than the clearance at the 2nd fret while fretted at the 1st (i.e, the nut acts like another fret, but with a little bit more height to avoid string buzzes behind the fretting hand). Thus the string height at the nut will follow the curvature of the fingerboard at the 1st fret for a radiused fingerboard. One of the most common setup problems for mandolins is too high of a nut, which makes the notes hard to play and a little sharp in tuning when playing on the 1st fret. Does that help answer anything?

Mar-05-2004, 5:37pm
Just to put things in perspective, if your fingerboard has a 9.5" radius at the nut, the center of the board is only 0.004" higher than either edge - assuming a width of 1.0625"

Mar-05-2004, 6:47pm
Thank's for helping me to understand guy's.
I hope I dont wear you out on this subject but I'm trying to get the most out of my mando [are'nt we all]
I know my fretboard came from stew -mac so I called .
They told me it is a 12" radius ,So if that's the case where on the fretboard is that measurement true and what should the bridge radius be?
And I'm sorry to ask again but I'm concerned about intonation
so do I have to cut the string pairs at the same level or can they step down with the radius?
And I'm speaking at the bridge.
I'm going to sand a radius into an extra bridge I have to see if I like it because the only one I found cost forty dollars.

Mar-05-2004, 9:47pm
Unless it is a compound radius, the 12" radius would be constant and measured anywhere on the fingerboard. You would want the bridge to be 12" also. Notching a bridge that had a matching 12" radius would require you to make each string notch the same relative depth from the top of the bridge.

Mar-05-2004, 10:59pm
"Just to put things in perspective, if your fingerboard has a 9.5" radius at the nut, the center of the board is only 0.004" higher than either edge - assuming a width of 1.0625"

I calculate 0.0148" down at either end from the center. But it still isn't much.

Mar-05-2004, 11:52pm
Thank's to all
The Cafe is one cool place.
all this knowledge right at our fingertips.
I'm going to experiment with the bridge.
I'll be back if I have any problems.

Mar-06-2004, 1:12am
How embarrassing!

Simple Pythagorean Thereom: a^2 + b^2 = c^2

We know c = 9.5, so c^2 = 90.25
We know a = 0.53125, so a^2 = 0.2822

Subtract, gives us 89.9678, the SQRT of which is 9.4851

Subtracted from 9.5 gives exactly what Flowerpot said: 0.01487

When the value I got didn't seem right, I should have double checked my layout (which turned out to be incorrect).

Sorry gang.

Mar-06-2004, 1:19am
Yep, that's how I did the math, Gary. That'll teach you to trust a computer -- (spoken by somebody who relies on computer sims for all their work and has a healthy disrespect for the results).

Mar-06-2004, 1:28am
I just went back and re-did my layout and got the correct answer. For some reason, when I did it this afternoon, I entered 9/32 as being 1/2 the width at the nut - don't ask me why. Using the correct values in my layout, I actually did get the right answer 0.01487

The good news is, if the width at the nut was only 9/16 ths of an inch, the crown of the radius would only measure 0.004" - of course, I think that even the width of Jamie Weins' piccolo mandolin nut is (substantially) more than 9/16 ths...