Carbon Fibre Threads

[TallDad71]

I have Trevor and Gerard's amazing books and thought it's time to build a Falcate braced steel string guitar. The process of adding a single tow of carbon fibre sounded exciting so I thought I would experiment to see exactly what its addition does.

So I built a brace 500m long 5mm thick and 13mm tall out of three pieces of spruce 1.7mm thick, laminated together. The brace was held in a jig such that the one end was held firm and the other had room for a 100g weight to hang off it in order to measure how much it bent or deflected.

Once the weight was added I measured the amount of deflection in mm from its start point. I then removed 1mm from the brace's height and repeated the experiment until the baton was 7mm tall, the target maximum height.


Height (mm)-------Deflection (mm)-----mass (g)
---------13----------------6----------------13
---------12----------------7----------------12
---------11----------------9----------------11
---------10----------------11----------------10
---------9----------------14----------------10
--------- 8----------------20----------------9
---------7----------------27----------------8

As you would expect, the thinner the brace the more deflection it would undergo and the lighter it would be. I only have kitchen scales for the weights, hence two 10g measurements!

The brace loss the majority of its stiffness after its depth dropped below 9mm.

I then added a single tow with epoxy to the top of the original 7mm high baton. At the same time I had a second baton which I had used as a control, to this I added a single tow to the top and another to the bottom. I re-measured.

Height (mm)----------------Deflection (mm)---------mass (g)
Single Tow Top-------------------------------23----------------8
Single Tow Top and Bottom----------------20----------------9

The results indicate that adding the carbon fibre to the top and the bottom gives the same overall effect as leaving the baton at 8mm tall. In which case the carbon fibre thing seems to be an extra step for no extra gain in stiffness.

One thing I can't quite get right in my head is what would the results look like if my brace was tapered at both ends. Would the carbon fibre really come into its own then?

This is a bit of a conundrum to me as I am certain that Trevor and Gerard are right to recommend adding Carbon Fibre but I can't work out why. Any guidance is gratefully received.

[Trevor Gore]

...I am certain that Trevor and Gerard are right to recommend adding Carbon Fibre but I can't work out why. Any guidance is gratefully received.

Design: Section 4.4.6, Paragraph 1.

[TallDad71]

Thanks for getting back to me Trevor.

Despite a degree in Maths and a lifetime of computer coding I have a bent for understanding things holistically rather than technically. So I read that para and got confused at the point where you wrote "virtually eliminates Cold Creep"

If CC is caused by constant stress on a material, adding the CF strip can't remove all of that bending stress, it lowers it, hence it can extend the lifetime of the brace, I get that.

In my head I can't quite grasp how replacing 1g of wood, in my experiment above, with 1g of Carbon Fibre / Epoxy, has enough effect that it will shield the spruce from the effects of Cold Creep for a 'significant' amount of time, virtually eliminating it altogether.

Love your Gore/Gilet books Trevor. It does exactly what great books should do, they answer lots of questions about design, oh then they pose a whole lot more for to ponder on!.

Best Wishes

[johnparchem]

Just looking at your numbers, and I think you get this, Looking at the addition of mass replacing 1g of wood, in my experiment above, with 1g of Carbon Fibre / Epoxy is a red herring, rather look at the deflection 27 to 20. a giant change in the deflection thus stiffness using a material that does not cold creep in this application at no cost in mass (relative to the wood it replaced).

The change in deflection is certainly significant, thus the increased stiffness due to the CF is also significant thus I suspect using a material that has "virtually" no cold creep providing a significant proportion of the stiffness could significantly protect the brace from cold creep.
Just running a thought experiment if carbon fiber was many times more resistant to cold creep than spruce, having 15 to 20% of the stiffness of the brace provided by the carbon fiber, one can easily see how cold creep can be virtually eliminated in the life time of a guitar. My first falcate SS is 10 years old with no noticeable cold creep.

[TallDad71]

Thanks for your input John.

I will be using the CF on my current build. Despite the messiness of epoxy and fibre threads, my main reservation, I’m curious as to its overall effect.

Another logical conclusion is that the spruce will deteriorate due to cold creep, at which point the CF will take over and prevent bending and bulging for some extra time. So why not make the whole brace structure out of CF? Build a mould, ram it full of CF and epoxy, sand it back and hey presto a repeatable, measurable solution

Another quick question

Your eleven year old guitars, do they show signs of cold creep? My first build is on its 17th birthday and it i seems ok to me. That said I never took bridge rotation and sound board deflection measurements back then.

[peter.coombe]

I will butt in here since I have been using carbon fiber in my flat top mandolins for a while now. In the mandolin world there has been even more misunderstandings about carbon fiber. This is all covered in the Gore/Gilet book but bears repeating here. The advantage of Carbon fiber composite is resistance to cold creep and a very high stiffness to weight ratio. However, carbon fiber composite is not lightweight compared to wood. It is much lighter then steel, but with a density of 1750kg/m3 is denser than Ebony. So you need to be careful how you use it or else you can end up adding mass for a given stiffness. Carbon fiber composite is most effective when pulled, when compressed it is less effective, and it is more effective the further away from the axis of bending it is. On or close to the axis of bending you are just adding mass. That is why laminating carbon fiber composite in braces does not work. Too much added mass because much of it is close to the axis of bending and thus is ineffective. For guitars the forces are rotation around the bridge so carbon fiber tow above and below the braces, as described in the book, makes a lot of sense. With mandolins the forces are compression only because there is a tailpiece and floating bridge. So adding carbon fiber tow between the brace and the top is mostly adding mass because that is very close to the axis of bending so I don't do it. The braces are shaved at the ends so are arched with the apex of the arch directly under the bridge. With the compression force of the bridge, the carbon fiber tow is being pulled so is at it's most effective. In fact so effective I can thin the top down so the overall top mass is about 30% lighter. With a thin top, most of the stiffness is in the carbon fiber so there is more consistency from top to top. Add to that the top is not going to slowly sink, as many of them tend to do, and it is a big win win.

There are always disadvantages that go with the advantages. The main disadvantage is that once the carbon fiber tow is installed there is no way of making any adjustments. So you need to get it right the first time and the book covers how to do just that with guitars. I don't have that luxury because mandolins are different, but I can measure the free plate modal frequencies of a top with the braces on before installing the carbon fiber tow and make adjustments then. From previous measurements I know what affect the carbon fiber tow will have on the modal frequencies so can adjust the bracing so I will nail what I want to measure after the carbon fiber tow has been installed. The other disadvantage of carbon fiber is that if it fails it breaks, and you can't glue it back together. it has to be replaced. That can create a problem with repairs, but the only reason why carbon fiber composite is likely to fail in a guitar or mandolin is gross abuse. Sadly, airlines are quite good at that.