Falcate pattern without CF?

[Francis]

Question about building with a Falcate brace pattern without carbon fibre.
I’ve just completed a Triple O multiscale guitar. Lutz spruce front, cherry back & sides, mahogany neck.
Although I’ve used a Falcate bracing pattern I’ve not used CF but rather laminated the braces: spruce/ sapele/spruce, having a central cross section of 9mm (H) and 7.5mm (W) each lamination being 2.5mm.
I’ve used a bridge pad shaped between the braces and tapering in thickness to the soundhole. The braces taper to 1.5mm at the ends - the ends are truncated at the sides. Sides are laminated, cherry (outer) wit 3 layers of tulipwood.
I would like to know if others have adopted this method and what results they’ve achieved.
Whilst this guitar has turned out well I don’t know if I’ve just been lucky or whether it’s a method worth continuing with. Others comments and thoughts much appreciated.
Cheers, Francis.
Image of the bracing and the unfinished guitar:

[kiwigeo]

Hard to make any meaningful comments without seeing some tap test results. The first thing I'd be looking at would be the main top frequency in light of the lack of CF tow under and on top of the falcate bracing. I suspect it may turn out to be low side.

I'm nearly finished on a multiscale falcate braced OM cutaway. I used King Billy Pine and CF for the first set of falcate braces but they werent stiff enough. The second top had spruce braces with CF tow and dimensions were as per Gore and Gilets book. Even then the main top frequency wasn't as high as I'd have liked it to be. As per comments from Trevor it's always best to go a bit stiffer on the bracing as its easier trimming the main top frequency down by adding side mass rather than trying to bring it up.

All the above aside..in the end if the guitar sounds good to you then its mission accomplished

[Francis]

Thanks for the comments.
I've done some Micky Mouse spectra using a phone on the closed box. I'm waiting for publication of the Gore/Gillet book, at the moment I'm working by feel and sound...
Francis

[jeffhigh]

Hi Francis It's looking good

I have only ever used CF capped top and bottom Falcate braces.
Looking at what you have done with laminating in a central strip of sapele, unfortunately it will NOT have increased the brace stiffness compared to just using spruce.
The published Modulus of elasticity for Sapele is 12 GPa which is exactly the same as for Spruce.

[Francis]

Agree with you. However the twin glue lines running through the laminations when I compared a spruce only and the spruce/sapele/spruce combination on a short pair of 150mm straight test pieces did show an improvement in bend resistance. It wasn't the most scientific of tests but kept tight enough to persuade me to try that route.
I will use CF on a future guitar, current build will again have laminated braces - this time probably rosewood as the central component. More testing first though.

Quick question do you channel the bottom of the braces to take the CF so the CF and the brace material contact the top? Does the CF completely cover the bottom of the brace negating a channel?

Cheers, Francis

[kiwigeo]

Francis,

A few comments:

1. Gore covers laminating braces in various ways. Running a stiffer element (CF or wood) vertically sandwiched between less stiff outer layers isnt as effective in increasng overal brace stiffness as running the stiffer element on top and bottom of the brace.
2. No channels used in the bottom or top of the brace although Ive at times been tempted to try this......just give the soundboard a light coat of resin and then lay in the CF tow carefully using a very thin paint brush. It takes a bit of practise. The trick is not to use to much resin which will cause the CF to "float" when you press in the brace. CF tow gets laid on top of the brace in similar fashion. Also tease out the CF tow fibres at the ends into a splay pattern.

Agree with you. However the twin glue lines running through the laminations when I compared a spruce only and the spruce/sapele/spruce combination on a short pair of 150mm straight test pieces did show an improvement in bend resistance. It wasn't the most scientific of tests but kept tight enough to persuade me to try that route.
I will use CF on a future guitar, current build will again have laminated braces - this time probably rosewood as the central component. More testing first though.

Quick question do you channel the bottom of the braces to take the CF so the CF and the brace material contact the top? Does the CF completely cover the bottom of the brace negating a channel?

Cheers, Francis

[lamanoditrento]

Quick question do you channel the bottom of the braces to take the CF so the CF and the brace material contact the top? Does the CF completely cover the bottom of the brace negating a channel?

I know the Brisbane AGMS rout about 0.5mm on the bottom of the braces to take the CF but not the top.

[jeffhigh]

It is also important to recognise that the CF capping is not just about increasing stiffness, it is providing resistance against long term creep whilst providing an appropriate stiffness.
So you don't get excess bridge rotation and belly behind it with advancing years.

And yes, there is no need to rebate the brace underside if correct technique is used.

[blackalex1952]

How thick are your sides?
The program that Trevor recommends is called Visual Analyser. It's free. There are posts on this forum re setting it up and using it.( viewtopic.php?f=1&t=3696&p=76370&hilit= ... ser#p76370 ). Chladni testing is good also for working out what the top is doing. When you get the books, go to the section about specific mobility and you will be on your way!

[Francis]

Thanks all - great info on the bracing and Visual Analyser.

I need to do some more research on CF and resins and where to find them.

Cheers

[kiwigeo]

Thanks all - great info on the bracing and Visual Analyser.

I need to do some more research on CF and resins and where to find them.

Cheers

1. You're after CF tow for the bracing and matting for the bridge.....discussed in threads on this forum: viewtopic.php?f=1&t=7323&p=80572&hilit=CF+tow#p80572 There are numerous Ebay suppliers around.
2. I'm using West Systems epoxy resin for laying in CF tow and matting..available at Carbatec. For laminating braces I use Techniglue epoxy.

[blackalex1952]

I know a luthier who uses "thixotropic" resin for the job. I don't believe that the west system is thixotropic. Maybe it's better, I haven't used it.-Ross

[jeffhigh]

I don't think I would use thixotropic for this, you would want to use the ordinary thin resin so that you saturate the CF Tow.

[Francis]

Thanks - found a UK supplier of CF tow and resin.
http://www.easycomposites.co.uk/#!/fabr ... ow-3k.html

Another question regarding applying the CF tow and resin to a curved Falcate brace - how critical is the tension the CF tow is applied under?

While I can see that to much will just pull the tow off the brace, too little will allow buckling of the fibres. I'm guessing applied tension will be just enough to keep the CF tow following the brace curve without distortion?

Thanks for the help guys

Francis

[jeffhigh]

No real tension, it won't follow the curve, you just sort of pad it into position with a small fairly dry stiff brush.
It soaks up the resin which prevents any "buckling of the fibres" once set.

[routout]

I was just curious ,do you guys use maple in your lamination's I just think it tenancy to bend is really superior to some other woods .I was thinking this because it might not introduce as much stress in the bends ,just a thought cheers john.

[kiwigeo]

I was just curious ,do you guys use maple in your lamination's I just think it tenancy to bend is really superior to some other woods .I was thinking this because it might not introduce as much stress in the bends ,just a thought cheers john.

I was using King Billy.....it bends like butter but the stiffness of the stuff I was using was low and I ended up having to replace a top on one instrument. Spruce is what I'm using now.....its not too much of a pain to bend. If you're having trouble bending the stuff then you can always make the pieces thinner and go for a 4 ply lamination.

Im not quite sure where youre going with the comment about stress in the bends. Once the layers are glued up the braces are rigid...theres no spring at all.

[routout]

I got now true once glued they are not going any where ,just me over thinking engineering again .