Notes about designing a guitar.

[CAI Kabrell]

Moi luthiers

To design a guitar it essential to keep track on the basics.

To be able to improve the old build, some things needs to be documented.

The body size and scale length is related.
For our cutout design the weight distribution will be different.
COM. Center Of Mass.

A longer string will have more harmonics. But we also need to keep the string tension upp for high vibration. Strings matter.

Bridge placement matters also

The center of mass needs to be close to the waist. So it sits well in the lap. It is attempting to design it with a smaller upper bout.
Just measure it on some of the guitars you have and reflect how you like to play it.
Simply balancing the guitar on a dowel or rib.

Measure it and note the % of total body length. I use the distance from the top as reference. Sometime I also calculate the offset to the waist ( in procent).

Stick to it with glue

CAI


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[Marcus Daniels]

I had to bag out early last meeting so I missed the last bit from Cai on design. I have a couple questions, mainly for Cai, but also everyone else participating in the Kabrell Cutaway build. 

1)What scale length are you using? 

I will be doing 25". This is what I typically build with on my full-size acoustics and electrics. It is only 1/10 of an inch different than a Martin short scale. 

2) How many frets are you using? Or rather, how many frets are extending over the body? 

3) Is your goal to have the bridge dead center of the soundboard? 

On my baritone ukulele, piccolo classical and piccolo steel string acoustics, I do a 22" scale with a 14^th fret neck joint and this puts my bridge more or less dead center soundboard in the lower bout. This was by design. 

4) What kind of bracing will you be doing? 

I have a hybrid X/Fan bracing that I've been using on several guitars. I will probably do a modified version of that. 

Marc

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From: in...@googlegroups.com <in...@googlegroups.com> on behalf of CAI Kabrell <caika...@hotmail.com>
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Subject: {INLA}3919 Notes about designing a guitar.

 

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[Marcus Daniels]

Here is my version of the Kabrell Cutaway. 25" scale, 26 frets. The neck joint of the upper bout is just past the 12th fret, neck joint at the body(?) is at the 24th. Spent some time drawing and redrawing the snail. Used gallon, quart, and pint cans, French curves, and a flexible stainless steel ruler. Initially had the curve deeper into the body, but didn't look right. The extra lines by the treble side horn are an idea I had from Cai's "goof up" on his Kabrell Cutaway baritone ukulele. He said he didn't make the treble side horn big enough so has to add a piece on to simulate a horn so it'll sit on his knee. I figured why not make the lower soundboard actually round and add in the horn for playability. This could possibly  be done for the upper horn as well.

Marc

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From: in...@googlegroups.com <in...@googlegroups.com> on behalf of Marcus Daniels <marc_from_th...@hotmail.com>
Sent: Monday, December 22, 2025 1:15:44 PM
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Subject: {INLA}3920 Re: Notes about designing a guitar.

 

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[Rob McAllister]

Hello Mark,

 

From my perspective, your drawing looks good.

Here is my 6^th attempt at drawing a Kabrell Cutaway.

I had tried circles, ovals and a couple manually adjusted spline curves.

None of those looked any good.

Thank you, Cai, for suggesting the spiral (aka Snail) shape for the cutaway.  

The spiral is definitely the secret sauce.

 

Currently I am leaning towards building a classical guitar, but that could change.

This drawing has a 650mm scale length with 23 frets. The bass side horn connects at the 12^th fret.

 

For those interested in using a computer to draw up a guitar design, there are two free tools I used to create this drawing.

 

First, I use the online tool, by R.M. Mottola, called “G Thang”.  A very useful tool to draw out a complete guitar design.

You start by selecting a common size and shape and then make adjustments from there.

https://www.liutaiomottola.com/formulae/gthang.htm

 

From this web app, I save the GTHANG file and export it to SVG (aka vector graphics format)

 

I then used a free SVG editing application on my Windows PC called “Inkscape”.

This app is available for Windows, MacOS and Linux.

This tool requires some practice but it’s easier than using some of the other popular graphic editors.

There are tons of tutorials and online resources.

https://inkscape.org/

 

The SVG format is great because you can resize it as much as you want and it won’t get jagged or pixelated like JPG and other graphic formats.

SVG is also great for use in 2-D CNC applications.

 

Best Regards,

Rob

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[CAI Kabrell]

Hi marc.

The steel string Kabrell cutaway had a hybrid X brace with fans.

X brace works well.   That’s them reason we use it.

It does not stiffen up the top along the grain to much.

But it does reinforce cross the grain well.   And it allows the top to twist.

Read easy for some of the natural top modes.

Fan brace works also well.  It has been used for a long time.   It reinforce the top with the grain.   But nylon string has much lesser forces on the top.

Robert Buchet French luthiers used a cross brace about 7 mm tall under the bridge.

To shape the dome and reinforce the top cross grain.

I have built a couple guitars with this cross brace and I do like the low end that it gives.

So the question is how open or closed the X brace should be.

With Normal guitars the X brace have three tasks.    To Control the tone and structurally prevent the top to collapse into the hole.  Also to anchor down the bridge so it is not rolling (peeling off) the top.

We don’t have that problem with Kabrell cutout.   

No hole means we can get away with thinner top and lighter bracing.

No hole means we have more freedom to place the X were I belongs acoustically.

So were is that???  Martin changed their X.  How open the X will change the sound.

Decisions decision decisions.   

For example : a Smaller body may need some help in the bottom register.

The X is more open.

So the X can be placed closer to the bridge and better connect to the bridge.

The bridge patch may be smaller.

For me I try to avoid parallel surfaces.   So i gently fan the bottom braces and have the not parallel to the xbrace.   But that’s just me.

What wood to use for the braces matters.

Shape of braces matters.

I always tuck under my braces.   So it they come lose they don’t rattle.

Some ukes I have scalloped the braces to nothing.  Then I secured the fan brace with gloves on top of the end.  A glob.

And finally.  

I scallop the braces to meet a target deflection.   

I have used about 2 to 4 lbs.   were the bridge should be.

Just measure a good guitar and use that measure as a target.

Happy scalloping

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On Dec 22, 2025, at 1:15 PM, Marcus Daniels <marc_from_th...@hotmail.com> wrote:



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[CAI Kabrell]

Moi luthiers

Rob.   

Looking good…

Just my comments on my first reflections.   Note they may change with time.

The shape of the snail looks good.    

It could be a hair smaller to make space for the X if you are going to use one.

The mid neck connection I want to have it 30 to 40mm.

Your start of the snail makes the snail be wider at the neck to just be more narrow about 1.5” in on the snail.   I maybe would make that more dominant to make it a design feature.   But I usually try avoid that.  Read 90degrees (or just less).   Or Try to move the snail to the right.   We usually don’t need a lot of space for the thumb.

I would have 2 or 3 frets on the body.   

Some players don’t like to play (pick or strum) over the fretboard.  

 You are basically there.

Your drawing looks very similar to mine.    

I have also used a half ellips.   To match / fit to the upper bout shape.

400 grit sandpaper makes great things come alive.

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On Dec 22, 2025, at 21:38, CAI Kabrell <caika...@hotmail.com> wrote:

 Hi marc.

The steel string Kabrell cutaway had a hybrid X brace with fans.

X brace works well.   That’s them reason we use it.

It does not stiffen up the top along the grain to much.

But it does reinforce cross the grain well.   And it allows the top to twist.

Read easy for some of the natural top modes.

Fan brace works also well.  It has been used for a long time.   It reinforce the top with the grain.   But nylon string has much lesser forces on the top.

Robert Buchet French luthiers used a cross brace about 7 mm tall under the bridge.

To shape the dome and reinforce the top cross grain.

I have built a couple guitars with this cross brace and I do like the low end that it gives.

[CAI Kabrell]

Moi luthiers

Here is Taylor’s bracing patterns.

Some are looking real interesting.

https://www.taylorguitars.com/guitars/acoustic/features/bracing

Live to carve and tap

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On Dec 22, 2025, at 21:38, CAI Kabrell <caika...@hotmail.com> wrote:

 Hi marc.

The steel string Kabrell cutaway had a hybrid X brace with fans.

X brace works well.   That’s them reason we use it.

It does not stiffen up the top along the grain to much.

But it does reinforce cross the grain well.   And it allows the top to twist.

Read easy for some of the natural top modes.

Fan brace works also well.  It has been used for a long time.   It reinforce the top with the grain.   But nylon string has much lesser forces on the top.

Robert Buchet French luthiers used a cross brace about 7 mm tall under the bridge.

To shape the dome and reinforce the top cross grain.

I have built a couple guitars with this cross brace and I do like the low end that it gives.

[CAI Kabrell]

Marc

The 4 question reply

#1   What scale Cai guitar will be?

Reply.  I am mimicking a Gibson L00.   I found from my templates a 24.49”.   So that is what I will use.  For you 25”. Sounds like a good choice.  I assume a 12th body joint.

It’s also depend on body size.

#2. How many frets over the body.

For the T00SS.  That’s what a call it.   Modifier Torres body, Gibson L00. Steel string.

I am not chasing a number of frets.   As long it’s more.   I usually trying to keep it short.

Mening just 1.5 to 2.5 frets over the body.   My latest version of T00SS has 26.5 frets.

Why half frets.   Just a CAI thing.  I shape a curve to the end of the fretboard.   To add little more frets to the high 3 strings.  I have done more frets over the body.   But I do not see the reason.   I look for “visual harmony”.    As I have mentioned before.   

Some players might not like to play (picks on the fretboard.  I usually expect to have 28 to 35 frets (that’s what I have ended up with.

#3. Bridge placement.

It depends also what the sonic performance I aim for.   Still for me, I am learning the impact.   In general I want the bridge to sit just behind the top of the dome.  To resist dome distortion.  So yes somewhere exactly in the middle (sonically the most efficient , read loudest) place.   +/- 1”. 

#4 what kind of fan brazing will Cai do.

My design is still alive.   The changes I will try out is to move the X closer to the bridge than I ever have done before.   So a X brace to only support the bridge.   Since I am having a long small body I will try to help the base more by an open X.  Kind of 94 to 98 degrees.   And two non parallel bout braces and single side braces.   But I need to draw it to see how the vibrating surfaces look in size and shape.    I also try to gain sound board area around the upper treble X brace leg.   The “horn” does not have a lot of soundboard around it so that brace will be very stiff.   

I am smoking ham, boiling another ham, sausage, making Xmas porridge, gravlax and dinner (Donner kebabs).  I will do my best to quickly answer any question.

We celebrate Xmas eve.   Anything to make Xmas lasting longer.   And share the packages under the tree at 5 in the evening tomorrow.   And yes we also celebrate second day Xmas.   

Happy Xmas 

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On Dec 23, 2025, at 4:34 PM, CAI Kabrell <caika...@hotmail.com> wrote:

 Moi luthiers

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[CAI Kabrell]

More about bridge placement.

Altered Bridge placement will change the tone.

It’s done a lot.   12th fret or 15th fret.

 If 12 th fret bridge has the bridge in the middle or very close to it.   14th fret will move the bridge 2 frets.   Yes we have kind of a number.

Remember my first Kabrell cutout the body joint was at 11th fret.   I had a feeling that to do a 12th fret would make it harder to play.   The fingering hand has to twist more.   So I went for a longer scale (more harmonics),   Bridge in the center and playability.  

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On Dec 23, 2025, at 18:42, CAI Kabrell <caika...@hotmail.com> wrote:

 Marc

[Abel Su'a]

Hey Cai,
I still have my snail but don't know if it will still work according
to all you have done to develop it. I have my Hauzer 37 body I was
going to use but I think I want to use what you are using. But I am
open to other body forms as well as a Baritone Ukulele. So look
forward to your classes.

Abe

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[CAI Kabrell]

Morning Abe.

I made a Kabrell cutout on a hauser body.  

So the body is right choice.    I used a large snail.   But I do not know what shape and size your snail is.    

What I learned is the mid neck joint should be straight or curved in the other direction.

That design hits the hand.  4 holes work great.   The ability to add holes is a great tool to tune the body resonance.   To raise the body resonance.

Rob. Here you can se I have bindings on all surfaces. 

And here you can see I did a checker rosette around the cutout.   Should have done it wider and more contrasting.

The top is port Oxford cedar.  The top is to stiff.  But I should go in and carve some away of the braces.   That’s nice feature to have a side hole to reach in and also to see what you are doing in the cutout holes.

Abe.  Do you still have the tenor guitar mold.   That could be a candidate for a baritone uke with Kabrell cutout.   I have not attempted to do a cutout on a uke.   I expect a about 100 mm wide snail may work.   Or a U shaped.    Just cut a paper U and see how it looks.  

About the next class.   It will be first Wednesday day in January at 5.

Merry Xmas to you all.    I have to go now.   Packages to open.

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On Dec 24, 2025, at 19:04, Abel Su'a <kuha...@gmail.com> wrote:

Hey Cai,

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[Abel Su'a]

Thank You Cai for that info and yes still have the Bari/Tenor guitar mold and will try your suggestion.

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