Official Luthiers Forum!

Transferred from the Tap Tuning thread, here's a new thread on the topic of stiffness, strength, and creep in guitar tops. Hopefully that thread will be helped by fewer topics in it.

The main prior posts (from viewtopic.php?f=10101&t=33563&start=50)...

_________________
David Malicky

For some points of reference, and because creep takes a long time to test, are there any historical guitar models that are especially known for being prone to creep (sunken soundhole and/or bottom bellying)? And, any models that have been resistant to creep?

_________________
David Malicky

Gibson guitars with the pre-profiled X-braces that make the lap joint look like a butt joint. The tops cave in, but this is as likely to be failure of the so-called joint as much as creep. D35s (0.25" bracing) used with 13s, but the 13s don't seem to be totally necessary to precipitate a problem. Older guitars that still look good are the ones that still have a high top curvature(!), because they were built that way initially e.g. some of the Larson Bros. models, a very rare beast in these parts though, so hardly a statistical sample.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I don't wish to derail this thread but having access to Trevor Gore allows me to ask this question, Does falcate bracing have any acoustic properties or is it strictly structural? In building baritone ukuleles, the string tension is not a major problem but stiffer bracing might allow a thinner (lighter) soundboard.

Any opinions?

Bob

I thought that all of the acknowledged "killer" pre-war Martins had pronounced "bottom bellying" - what an evocative phrase . Is it necessarily a bad thing if the guitar remains with good playability and in fact may it be a good thing soundwise? Good point about the Larson brothers guitars Trevor with their top arching pre-built in without waiting for it to develop. Also the cylindrical top arched Howe-Orme guitars of the late 19th early 20th century.

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Carbon fiber reinforced bracing is quite interesting I think. It's also quite interesting how most guitar makers are using it.

The typical usage as I've seen it, uses CF sandwiched between to pieces of spruce so that the CF runs perpendicular to the top. This is actually the least effective use of CF in terms of adding stiffness to the structure. However, those that are using it say that's o.k., they're using the CF not to increase the stiffness of the structure but rather to eliminate the cold creep of wood.

From a stiffness standpoint, capping the brace would provide the most resistance to torque and the stiffest brace. This however has the effect of eliminating the ability to remove height from the brace to control stiffness (this also is the argument against I beaming braces as well). But, if you're not shaving your braces that becomes a non issue. It also means that unless you have a square brace that's only got a straight taper, it would be quite difficult to scallop braces if that's your thing.

edit: Just to put a fine point on it - generally speaking, the load on any beam is borne by the most proximal and most distal surfaces, the inner "stuff" just connects those two surfaces. The reason I mention it is because of my statement above about the stiffest brace being one capped rather than sandwiched. The capped brace will be stiffer in one plane, but close to the same as a solid wood brace in other planes.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

David82282 will be after you if you derail this thread!

Falcate bracing can be quite efficient structurally, so it can give more stiffness for less mass, which you can use to acoustical advantage.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks, Trevor and Dave, for the historical info. If not bottom bellying, what do you think about over-rotation of the bridge? Sounds like high curvature is quite effective.

No worries on derailing... threads naturally beget themselves.

Andy, yes, carbon fiber is interesting, and using as 'web' or 'flange' makes a huge difference to the moment of inertia. Also the shaving feasibility as you point out. That's also interesting that most use it as 'web' to prevent creep. I'm wondering, given the low shear strength of wood (~1/5 of longitudinal), whether a CF cap/'flange' (on top a wood brace as the 'web') might be so stiff as to cause a shear problem in the wood. Looks like E_cf_uniaxial is about 16x E_wood, which makes for a hefty effective flange. I rarely see a beam limited by shear strength, but this one has potential. To find the shear load (V)... maybe resolve the bridge moment into down and up forces (a couple) near the front and rear edges of the bridge, where it crosses over the braces.

Does anyone know how much wood cold-creeps in shear vs. longitudinal?

For those using CF as the 'web', how thick is that layer, typically? I wonder how much the adjoining wood contributes to the total E*I. I can see it helps the glue area to the top a lot, and probably helps customer acceptance?

_________________
David Malicky

Good point.
Not to hijack the thread again, but FWIW I was looking at the possibility of Balsa/CF for braces, and Googling "best glues for Balsa to CF" came up with aeromodelling forums indicating that CF capped spars are best spirally wrapped over the CF layer with some kind of thread - I found CF, polyester, even dental floss(unwaxed!) is suggested to help prevent delamination.
Spiral wound braces anyone?
(Probably steel strings are out for balsa braces because of the weakness of the wood to shear forces, although used by some classical builders with CF lattices, Greg Smallman for example)

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Why?

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Please excuse my ignorance on the subject but could someone define the terms ‘creep’ and ‘cold creep’ as they pertain to wood in guitars? Thanks.

Joe

Plastic deformation over time. i.e. a piece may deflect a certain amount under a certain load but over time, it will deflect more and will not return to its original configuration when the load is removed.

I think we use the term cold creep because as we all know, wood can be plastically deformed quite easily with the application of heat.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Thanks Andy… makes perfect sense.

Joe

Balsa will fail in shear on lattice braced classicals if the balsa substrate is not chosen appropriately. The ranges of stiffness, density and strength is huge in balsa, so be very careful. Spiral winding done properly is a potential solution. I do some analysis in the usual place on composite braces. For steel strung instruments I concluded that by the time you had enough balsa in there to take the shear you'd be better off (less mass) using something else, e.g. spruce.

Using CF obviously limits what you can do in terms of "after the event" brace shaving. That's why I came up will a bunch of alternatives so it is still possible to tune modes without having to hit the braces.

Over-rotation usually leads to an eventual failure, with the bridge or bridge plate or both detaching. I try to limit bridge rotation to ~2 degrees, which puts you in the right ballpark on both classical and SS guitars to hit the preferred modal frequencies. If you start with the right amount of curvature in the top, 2 degrees of rotation is possible without precipitating a reverse curvature, so you avoid the roller-coaster effect, which I've never found to be a great selling point! There's a chart in the book which shows how the natural frequency of a plate varies with curvature.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

FWIW, a reference I saw in some homebuilt airplane literature suggested that the initial deformation under the design load limit should be no more than 1/3 of the acceptible long term creep deformation for wooden aircraft structures. In other words; if your aircraft weighs 1500#, you're designing to +3.5 Gs, and you can't stand to see the wing tips come up more than, say, a foot, then you should see no more than four inches of delfection when you load the wing with 5250# of sandbags. On a guitar, you could put on 'heavies' and measure the deflection in front of the bridge: much less work. Over time, you might see as much as three times that much deflection in that spot with the same strings.

IMO, glue creep is a much bigger factor than wood creep in tops: PVA and AR glues don't stay put for long when subjected to shear loads.

Hmmm, interesting! I've never had a problem gluing on bridges (SS and CL) with Titebond (AR glue). Perhaps method is at least as important as materials here.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

The only guitar I have owned which experienced a bridge failure is a 1975 guild 12 string which I have have owned from new.
As far as I know it was never subjected to heat stress and shows no other damage.
The bridge didn't lift but slid forward about 1.5mm until the bridge pins stopped it.
That one example was enough for me to avoid AR type glues for bridges.

One former student, who worked for Guild, said that they used PVA because a 5 gallon bucket was $5 cheaper. I, too, have seen a few bridges on factory guitars with narrow line of bare wood behind, and a small ridge of finish in front. I never had any such problems when I used Titebond on my own instruments: as you say, probably technique and surface prep make a lot of difference.

I once had to replace a bass bar on a violin that had been glued in with PVA. The bass side of the top was nearly flat, but the ends of the bar were still glued down; it had just slid. I had quite a time getting some arch back in that top. Since then I've regarded PVA as the equivalent of chewing gum: it doesn't really hold anything, but you can't get it loose, either.

Titebond and that ilk are a decided improvement, but not 'creep free'. I have noticed that since I started using HHG for top bracing I've seen less deformation in my tops, and I'm making them a bit thinner now than I used to.

When doing stiffness and mass calculations for CF constructions, remember that many CF products are actually epoxy-impregnated CF composites. And it is not at all obvious to me that the various moduli (and densities) I've found quoted for "carbon fiber" are for actual pure carbon fiber, or specific epoxy+CF composite material(s).

_________________
The member formerly known as erikbojerik....

Erik raises a good point. Whichever way you choose to use CF, either as strands or preformed laminates, you're probably better off "dummying up" a brace and testing it rather than relying totally on calculations.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Sorry Filippo - as you rightly say my question and your answer was confusing myself. I don't think it's a bad thing and I do think it's a good thing soundwise.

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010