Warning: This is a bit of a rant.
This thread will only make sense to those of you who have read Gore & Gilet Guitar Design/Build books.
Sorry, but I’m not going to attempt to explain anything to those who haven’t read the books. (That’d be far too difficult a task to do here).
That said, in arriving at some target data from guitar data, I have made some assumptions.
If you don’t agree with them then say so – it can only help us all.
Ok then.
First up, is it really worth doing??
Professional builders have proven that the standard fan bracing with bridge plate works well.
Actually, some have even proven the value of no fan braces when a large carbon fibre bridge plate is installed.
Installing a single piece of carbon fibre weave is a hell of a lot easier than making multiple bent braces & gluing them in with carbon fibre strands.
Is it a case of just re-inventing the wheel?
Three things swayed me to go ahead.
1: I know that Allen spent a considerable amount of time and effort creating his lattice braced ukes.
2: The concepts of the bracing system, whilst intended for the guitar, should translate well to a ukulele – it’s the same physics right!?!
3: As Rick Turner is so fond of saying – Stop thinking about it and JUST DO IT.
So after stewing for quite a long time on the info presented in these books, I decided it was finally time for action.
The bracing system is just one of the concepts presented by Gore/Gilet.
An important aspect is being able to repeatedly obtain the same stiffness in a plate regardless of its individual characteristics.
This is achieved by varying the final thickness of different plates so that they all have the same stiffness.
I first set up the Plate Thickness Calculation (4.5-7) in Xcel.
The authors kindly include a table (4.5-3) of their own test data on various tonewoods which allows you to check that your spread sheet calculations are entered correctly & your answers are correct. (I found that there was a small margin of error between my determined values and the table’s)
Righto, time to apply all this to a concert uke test build – let’s call her Prototype 1.
Again, the author’s kindly include their target stiffness values (f) for both steel string and classical guitars.
So what is the target stiffness of a concert uke plate?
I don’t know either
So here’s what I did.
Most of the experienced uke builders here have suggested using a top plate thickness of between 1.8mm (for hardwoods) and 2.0mm (for spruce).
Obviously this is very general, but it served to give me a starting target.
I inverted the equation so that I could enter in a design thickness value of 2mm and get an output value for the target stiffness.
As my top plate is englemann spruce , I entered in the 10 different sets of values for englemann spruce given in table 4.5-3, using the body length & lower bout dimensions of my concert ukes.
This gave me a stiffness value average of f=175 (with a range of 159 to 189)
But this is giving the f value for a standard braced top.
By using a falcate bracing design I want to improve on the concept of top weight reduction without loss of stiffness (as Allen does with his lattice braced tops).
So I’ve arbitrarily reduced the stiffness value to f=150.
Out to the wood stash and grab the pre-prepped bit of spruce I intend to use for the top.
I used Strobosoft to find the Long, Cross and Twisting vibration mode frequencies.
Then weighed it & measured it.
Plugging all this back into the equation gives a target thickness for my intended top of 1.7mm
Seems to me like a reasonable place to start the build.
Now came the brace layout design.
This wasn’t simply a case of reducing the plan for a falcate braced guitar.
I drew up the concert uke top plate, marking in where the saddle will be, where the strings will anchor, the bridge itself (using David Hurd’s website info), etc.
Using the rules as outlined in the Design book, I laid out the initial brace positions then used a spline to form some relaxed curves. Checked things, revised, checked revised. You get it.
One of the things you will notice about this bracing design is the lack of a lower transverse brace. All string tension has to be sustained by the main falcate braces or I’m going to have a nice failure when strung up (which in itself isn’t a bad thing when you’re trying to gain new information).
How thick the braces be to stop failure? How high? How often do I see these questions on forums……..
Drawing yet another unsupported conclusion from the correlation between guitar & uke, I am going to use the book’s suggested 2 degree of bridge deflection under string tension…..and say it’s the same for a uke.
Even having decided this, I can’t measure the actual deflection until it’s built.
So I’ve arbitrarily decided to use 3mm wide & 5mm high braces with carbon fibre rovings top & bottom.
This will be my starting point for future efforts (assuming the thing doesn’t just explode).
Included in my design is a carbon fibre bridge plate.
Unlike a more traditional design, the bridge plate does not extend past the ends of the bridge.
Instead, the secondary falcate brace ‘peaks’ are under the bridge ends.
The braces were made from 1.5mm strips of sitka spruce.
The main falcate braces were simply bent around a form and glued.
The secondary falcate braces were bent around the form with heat.
I glued them with Titebond and found no ill effects due to swelling or cupping.
And that’s where I’m currently at.
The rest of the body is built waiting on the top.
The neck is well underway.
I start a new day job on Monday so I’m probably going to lose a lot of workshop time initially, but I’m doing a 3 day course with Trevor Gore on June 9-11 & hope to have this knocked up for testing by then.
Remember, comments & criticism are encouraged.
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