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I always appreciate and pay attention when luthiers share "starting point" type thickness measurements with those of us who don't have the experience to optimize this for ourselves (and likely never will). Some such discussions have noted that with the variability inherent in wood, the high cost of warranty, and the great unknown of how much abuse their guitar will see, the factories generally have to tend toward the overbuilt. It's made me curious, with modern design concepts and statistical analysis, and long history of failure modes, etc., what thickness some of the factories choose to aim for on their higher end steel string models in recent years. I realize that different makes and models all have different bracing so to some degree this is a moot point... but has anyone got any measurements to share?
Gerard

I've seen a high end Martin that was about .110 (measured in the center) with finish....this one was a beautiful guitar and though it was moving around a bit, it appeared stable. Will it last 50 years?...I don't know..?! I believe Taylor and Collings both build thicker than this but I can't recall the actual numbers. I want to say collings starts at about .125. In general, the cheaper the instrument the thicker the top. You can only go thin if you really mind your manners and pay attention to the piece of wood at hand. Some manufacturers (higher end instruments) sand the edges of the top (as do many small builders) to give them a better tap tone.

When I was at PRS and we were starting up the acoustic line, the top thickness was a big point of discussion. Suffice it to say that they like thinner tops than I do. I've been out of there for a year or so now, so I don't know what they are building to now.

Trev

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Tony McManus has a Cocobolo PRS with a major upper bout top crack already, I am surprised that Paul hasn't made him another. It sounds fairly good so Tony might just want to keep playing it.

Fred

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Fred Tellier
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Trev, What top thickness were they building to when you were there? Sounds like it was too thin.

They varied and I would rather not get into specifics. I don't know what they are building to now, but I thought they were too thin at the time I left a year ago. They were basically seeing what they could get away with...the idea being that the thinner you build, the better the tone. Personally I don't think this is always true...I think on the "too thin" side you end up with something that I feel has excessive overtones and a diminished fundamental. With that said, the hand built prototypes I played sounded great (I left right after designing the CNC jigs...but before I got to see them in action!).

My judgment regarding the tops being too thin is mainly due to the degree of top deformation present soon after being strung up. I don't think this degree of deformation this early on bodes well for the life of the instrument. Over time, I would expect cracks and/or excessive bellying to occur on instruments that show this. In the long term, we're basically fighting against "creep". Based on what I have read, wood creeps or "stretches" to about 150% of the initial strain (aka deformation) value in the long term. This is assuming that the part is stable...and that you do not get into a vicious circle in which the deformation increases the stresses...which increases the deformation...which increases the stress....etc...

I report this all as humbly as possible, Steve Fischer is an excellent builder and his hybrid X/fan brace pattern makes a lot of sense. It's a really elegant and well thought out bracing system. We had many "discussions" about this while I was there....all the way up to the point where I was basically told "not to discuss it any more".

Trev

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Martin factory tours give out soundhole cutouts to the guests. Mine is 1/8".

Looks like they should have discussed it more if the tops are cracking. Thanks for the insight Trev.

There's an interesting picture in this post from a few years ago...
http://www.acousticguitarforum.com/foru ... hp?t=94923

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David Malicky

I'm still surprised that no factory has automated modulus testing. Seems like a simple way to get a jump on your competitors with basically a one time setup cost.

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I havent seen Tony's coco PRS , but I played his brw one a few months back ... you have to understand that looking at the average touring musicians guitar is not a true test of how well the guitar is built - is it built for the studio, or for touring ?? .. these guys travel all over, rarely take that good care of their instruments (ie temp and humidity) .. if they play a local gig and take a bunch of guitars in the back of a car in winter (Tony lives about an hour outside Toronto), they have no time to let them aclimatize ...if they fly, how long is it on the tarmac before its loaded ??? then when they arrive, its out of the case and played .. so it may go form 40-50 degrees (or colder) and dry to a 75 degree and humid room .. so be it .. when they break, they go get new ones.

A player I know in a hugely world famous band had multiple acoustic guitars literally fall apart on a european tour a couple years ago .... so they call the factory rep and ask for more ...

On a cruise I was on last year I checked out Ed Robertsons (bare naked ladies) taylor dread .. walnut/spruce .. the top had a 7-8 inch long crack on the top of the lower bout - still plays, still tunes .... the band calls there gigging stuff 'road lumber' .. the nice stuff they have stays at home or in their studio .... there was a guy in another band on the boat, his Taylor was literally covered over the top binding edge with duct tape .. looked gross, plugged in sounded like a guitar. As long as it works, its just a tool for these guys ....

tops usually dont exhibit cracks because they are too thin within reason (ie .100-.105) .. its from one type of abuse or another ...

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

From what i've seen .100 (measured in the center) is too thin. Tops that are this thin seem to be more prone to cracking due to humidity changes. I suspect - but can't prove in any logical way - that a drop in humidity causes the top to shrink and the box to "pull" the top apart. My personal opinion is that thicker tops are better able to resist this "pull". I've seen guitars split before they were even strung up...and I suspect it was due to them being too thin. Other's might not agree...

Trev

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(Good) classical guitars are in most cases thinner than 0.1 (spruce, Hauser clones excepted) A very common interval is 0.08-0.09 and often down to 0.07, even 06 at the edges.

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Build log

Top thickness depends also on the size of the soundbox. Smaller guitars can take thinner tops for obvious structural reasons.
D-18s and D-28s from the early '70s I've measured were .115". It doesn't mean much as the quality/stiffness of the sitka varied greatly on those guitars. I've seen some pretty spongy tops on D-18s, and those sounded great.
I think a few pre-war OMs measured .118", but bracing was relatively low and aggressively scalloped.
I've never gone under .100", even for a single O. For larger orchestra size guitars it varies between .105" and .115", depending on the top, the strings gauge that will be used and the sonic goal.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Before Taylor implemented their edge relief, their thicknesses were .110 for Grand Concerts and .115 for jumbos. This is before they changed their bracing and the edge reliefs.

Tony Karol is spot on -- top thickness has absolutely nothing to do with cracking and everything to do with abuse. It can also be resultant to bad design. Case in point the Martin pick guard crack. For years, Martin would apply the pick guard to the raw wood top and finish over the whole thing. As the guitar aged, the celluloid shrunk at a different rate from the top, causing the tops to split between the bottom of the guard and the bridge. Poorly stabilized wood and materials that aren't all at the same moisture content can cause stresses in the top making it more susceptible to cracking.

That's an interesting point regarding the Martin pickguards, I hadn't heard that before.

The question I always ask at this point is: "If the top was .250 thick, would it still have cracked?".

I don't know for sure all of the things that do and don't make them crack...but my gut feel is that thicker is better for crack resistance (although 1/4" is obviously unrealistic!). I realize that even logs crack and check...but I think seasoned wood is a different animal than a wet log.

Best,
Trev

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