Octave mandolin fretboard taper measurements wanted

[Tim Collins]

I wonder if anyone can measure their octave mandolin fingerboard width both the nut and 12th fret (or 20th fret)? I'm trying to get a feel for both the width and taper (the bridge-end is wider than the nut end) that folks use for octave mandolins.

All the luthier web sites I looked at do not give any information I can use to compute the taper. They only give the nut width and scale length and that isn't enough. I need another width at some measured distance from the nut. I can compute distances from fret numbers if I have the scale length so that's good too.

Do octave mandolins just use a standard mandolin bridge? I cannot tell. Once I have the taper I can compute the bridge width easily enough.

I'm an engineer at heart (not by training!) and I crave numbers.

I am just starting out with an idea for an octave mandolin (22" scale length) and I want to make some prototype fretboards with different nut widths. Talking with some friends I realized that we'd all like to try a wider nut, or see what it is like to use a standard mandolin geometry, pretend it is a four string acoustic guitar, or make something to suit small players. I built a CNC (MPCNC, google it) and I am hoping it will let me run off a half dozen or so prototypes that I can string up and mess with.

[Peter Wilcox]

If you know the string spacing at the nut and the bridge, and the scale length, you can find the string spacing anywhere along the fret board, and add the amount you want to that for the board overhang, even if you need to draw it out.

I googled "octave mandolin string spacing bridge" and got lots of hits. My first acoustic instrument many years ago was an Irish bouzouki, and I just found a photo of one on the web, measured everything on the photo, and scaled it up.

There is also a very useful fret board design tool for guitars, https://www.ekips.org/tools/guitar/fretfind2d/ , that you could probably modify using 4 strings, and then double the strings with appropriate spacing.

[Tim Collins]

Hi Peter. I may resort to measuring from a photograph or making a visit to a very expensive music store nearby with a plastic ruler hidden in my coat pocket. I'd rather someone else measure something they have in hand to save me the potential embarrassment.

I will also measure off a paper plan. I found a couple plans for free and bought a few full size plans (including MIMF) and found some books with plans. I have an Irish Bouzouki book on order which looks promising. The Irish Bouzouki seems like a legit starting point.

Returning to my original question, I can't find any other widths at any length besides the nut for a Northfield or a Brock or a Weber or a Trinity or what have you. Anybody out there go through the process of reverse engineering an existing instrument that you like? I'm really only doing acoustic not electric at least for the next year or so.

For a normal size mandolin you can get measurements out the wazoo as the original Bill Monroe-era instruments have been lovingly analyzed for decades. I got the width and radius for both the nut and 12th fret for my Collings MT which I also want to replicate. They put it on their web site. No one else does that apparently though I could be wrong.

[Jim McConkey]

I made mine 35 mm (1 3/8") at the nut. 45 mm (1 11/16") at the 12th fret, and 48 mm (1 7/8") at the 20th, 585 mm scale length (about 23", if I remember correctly). If anything, I wouldn't mind the fretboard being slightly wider. I made my own custom compensated bridge, so I can't tell you for sure about whether a regular mando bridge would work. The highest and lowest strings are 38 mm apart at the nut.

[Tim Collins]

Thank you for your measurements Jim. Those numbers look sensible. Do you recall your reasoning for choosing that size?

Regular mando bridges are compensated and the strings are more or less standardized. The strings for an octave would be very different. Folks often mix gauges, even mixing wound and not-wound strings for one pair. I think I would want to build an electric guitar style adjustable bridge just to get the string lengths right and use that to make the real bridge. Stewmac sells such a gadget for this exact purpose, I am pretty sure, but I don't think I will be using one that often.

[Jim McConkey]

No, sorry, it's been 20 years and I have no clue why. I did not have access to many plans back then, and Graham McDonald's Octave Mandolin book wasn't out yet. I remember carving the neck more or less by feel, and found that a rounded V fit my hand much better than an even curve. Maybe I determined the width by feel, too. I thought I have posted a discussion on its construction, but even I couldn't find it in our Library. But the Library does have plenty of discussions on bracing and other topics. I should point out that we have plans for a "Traditionally-Braced Cittern/Octave Mandola" that might be of some use. McDonald's Octave Mandolin book is also an excellent reference, but, of course, I cannot seem to find my copy right now. Agreed that the compensation will be very different between a normal and octave mandolin, but I'd stay away from an adjustable. The added weight will kill the top. It is easy enough to make a custom hardwood bridge once you have chosen your strings.

[Tim Collins]

Right, the adjustable is a temporary jig to capture the length for the four string pairs.

[Jim McConkey]

Gotcha! I think I basically did the same thing with 4 wedges, got the intonation correct, then made a single compensated bridge.

[Clay Schaeffer]

In Graham McDonald's bouzouki book he shows the nut for a four course instrument as 34mm with the distances being (from the left to right in mm) edge to first string 3.5 -3.5-6.5-3-6-2-5.5-2-2 to edge of nut. Total distance of string band 28.5mm.
At the bridge he has a width of the string band of 44mm with distances (left to right in mm) of 4-10.5-3.5-10-3-9.5-2.5.
Although this is for a bouzouki it should work for almost any string length. The width of the nut needed for fretting and the width at the bridge for strumming the strings will be fairly constant for most players (although some will want more and some less). If the player is coming from playing a guitar you may want to use the spacing of four courses of a 12 string guitar. On the other hand, if a mandolin player they may be comfortable with what Graham has given for the bouzouki.

[Tim Collins]

Thanks Clay. My copy of the book has not arrived yet and I was eager to get the numbers. Thanks also for updating the thread.

I would guess (and I suppose I could experiment) that the same ratios would work to scale up and down both the scale length and also to add a fifth course of strings. I just learned that a Cittern has 10 strings and the width is very close to a normal guitar and that at least one person made a Cittern out of a 3/4 size guitar.

Slight digression from the main topic. The extra lower strings are not uncommon for electric mandolins. I would guess emandos use same proportions used for Stratocasters. I'll have to dust mine off and see.

I'll post my notes on the measurements here once I'm done doing all my homework.

[Jim McConkey]

You might want to mock up the top of the neck out of a 2x4 or whatever to see if it works for you. Mine neck was ever so slightly wider than McDonald's dimensions, but, as I said, I would probably make it wider if I were to make another. It all depends on your preferences. There are no real standards across these instruments, be it cittern, bouzouki, or octave mandolin, either for size or number of strings. 4 doubled courses is the most common, but 5 is not unheard of.

[Clay Schaeffer]

Hi Tim,
I have made some short scale (22 1/2 inch) double strung tenors (as I call them) on an LG 3/4 pattern body. The are strung with octave pairs like a 12 string and have worked quite well. One of them did have 5 courses (10 strings) and also worked fine.
On the Octave mandolins I found that using cylindrical arching on the soundboard and doing a reverse taper on the body (like the old citterns) allowed me to add some additional stiffness to the top and move away a bit from the "guitar sound" and give it a stronger attack.
For the 4 course instruments I found good quality mandolin tuners worked well and for 5 course individual mini tuners worked fine. Ebay had some 5 on a plate tuners, but they were somewhat marginal quality.

[Clay Schaeffer]

As Jim mentioned I also prefer a slightly wider string spacing than what McDonald uses, but again - it is player preference.
On the shellback instrument the player eventually wanted to explore 5 course possibilities, so with the wider neck I separated the strings of the lower course (and changed the string gauges) which actually worked fairly well. Later I did build her a 5 course,10 string cittern (Sobel's usage - actually, organologically most mandolin type instruments could be classified as citterns)

[Tim Collins]

Hi Jim and Clay,

I built a CNC machine (MPCNC) and for my first project I thought I'd use it to make experimental fingerboards. Fingers crossed, still learning.

Thanks for the pictures. It looks like a 3/4 size donor guitar and 5 courses of strings would make a pretty cool instrument tuned like an octave but with a low C perhaps. Citterns have different tuning apparently so as a mandolin player I would not know how to play it. The good thing about strings is you can change them around and try things out.

The back pattern on the cittern is pretty cool. I don't have a big bandsaw but I am happy to plane/sand something thinner so the use of smaller wood sections is very interesting to me. Gorgeous! My sense is that the glue and inside bracing make it plenty strong enough.

It never occurred to me how the taper of the body would affect the attack. Very interesting.

I've run into cylindrical arching for various acoustic and classical guitars. Putting a flat soundboard under tension by partially forming it around curved braces and/or body is sort of how violin soundboards are normally put under tension. I've encountered the 15-foot radius dome shape front and back, and the "Ribbecke Halfling Pin Bridge Archtop" which combines a dome (catenary arch) and an offset round sound hole.

My brother's classical guitar uses the offset sound hole, and the same luthier has supplied many of these for other professional musicians.

I also ran into a discussion that recommends against F-holes as they cut short the usable surface area of the soundboard leading to oversize (and hard to hold) bodies. This is a plus for a violin (sustain is your enemy) and mandolins (can't have those strings ringing all over the place.)

[Bryan Bear]

Clay, that clamshell back looks awesome!

[Jim McConkey]

Awesome chatoyance on that one! Like Clay's, the body on mine is also tapered, 75 mm sides at the neck, 84 mm at the tail. Not sure where I picked that idea up from.

[Clay Schaeffer]

Hi Tim,
Just to add some clarification - the reverse taper on the body allowed me to use a taller bridge with greater downbearing and the cylindrical arching added some stiffness to the soundboard which the combination of I think affected the attack of the instrument. Tilting the neck back to make the bridge taller may be equally effective.
My preferred tuning for a 5 course cittern was GDAea (Irish music). keeping the number of different notes limited I found helpful for open chords. I have known people to tune them CGDAe to good effect (I'm just not that good of a player).

[Tim Collins]

Hi Clay,

I never thought much about bridge design, other than wondering about a formula for deciding how wide, or maybe why or why not to use a full contact bridge. Height is a new thing for me. I guess the higher the bridge the more force gets transmitted to the soundboard. A double bass certainly has a high bridge, so must be related.

A cittern probably does not need to go lower, so going up instead to GDAea makes sense. On mandolin I often go far up the neck but just on the E string, This is awkward, so having a higher string would help me out. I suppose with CGDae you would more easily overlap a bass guitar and instead of jamming up into vocal range you would be able to get into a bass groove. Both seem valid.

[Clay Schaeffer]

On the scallop shell backed instrument pictured above the owner wanted to go lower (CGDae) so we split the bass course as I mentioned. She was a lifelong mandolin player and an excellent musician with a beautiful voice. After her passing her son brought it by for a tune-up and new strings. She will be missed.
The angle formed by the strings as they pass over the bridge and the tension on the strings determines the down bearing (down pressure)on the bridge and can have a great affect on the sound of the instrument. It is not so much the height of the bridge, but rather the the way it affects the angle of the strings over the bridge. As you mentioned with violin family instruments this can be a critical factor in the way they sound.

[Tim Collins]

Latest URL for FretFind2D is now at

https://acspike.github.io/FretFind2D/src/fretfind.html