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I know usualy plate thickness is determined by its stiffness, the question is , i general terms does a larger topped guitar , ie, D size or jumbo tend to call for a thicker top plate ?? or would you still use the players style and set up to determine plate thickness ? thanks Jody

I would think scale length and string tension would have the larger role as compaired to the body size. even thought most smaller body will often have shorter scale with less string tension, you also have biger body guitar with shorter scale and lower string tension. That said it is true the spands cross the the body have an affect on the flexability of the top. but a large percentage is most ofen compensated for via bracing. If you stop and think about it the vast majority of guitars are within an 1 1/2" or less in length and 1 1/4" or less in width of each other.

Generally speaking, the bigger the plate the thicker it has to be. The limiting factor is the stiffness; how much it will flex under load. The main factors will be string type, string tenson, the height of the saddle above the top, and the span of the top.

Classicals will have less tension string steels, and 12-strings have the most. I think the difference in tension between 'lights' and 'heavies' amounts to about 50%, which is a lot, but less than the difference between nylon and steel. Those skinny strings in thr 12-string set are pretty close to their braking point, and carry a lot of tension: the total load is just about double that on a 6-string, I think.

At a given span, the stiffness of the top will vary as the cube of the thickness. Thus adding 10% to the thickness makes the plate about 30% stiffer, and making it about 25% thicker doubles the stiffness.

I'm trying to remember the formula for the deflection of a beam: I think the deflection under a given load goes as the fourth power of the span: twice the span, 16 times the deflection. Don't take that as gospel, though!

Bridge torque is what kills guitars. The taller the bridge the more the torque, and the stiffer the top wil have to be to handle that load over the long term.

I'm talking here as if there was no bracing on the top. Obviously, there will be,. However, my experience suggests that I get the best sound when the top and the bracaing are more or less in balance,in some sense, so I don't like thin tops with heavy bracing. Maybe that works better for you.

The upshot, then, is that you do have to make the top thicker if you make it wider or longer. In fact, the added thickness goes up faster than the added span: if you make the top 10% wider, you will probably have to make the top more than 10% thicker. Small guitars can have amazingly thin tops, and this helps make them louder than you'd expect. By the same token, making a guitar much bigger doesn't always make it alot louder, since you have to beef it up so much to avoid self-destruction.

I loaned out my copy of Hurd's 'Left Brain Lutherie', which has a lot of info on all of this. There was also an article in 'American Lutherie' a few years back on how to figure the scaling factors.

I've seen formulas that suggest that deflection increases as the square of the span or the cube of the span but never the fourth power. But that's with a simple span which a guitar is not. Safe to say though that a 10% increase in span would equal something much larger than a 10% increase in deflection.

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http://www.chassonguitars.com

I'll have to double check but I'm pretty sure it is a linear relationship with length.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

I just did a very quick and dirty test on my deflection rig. The way my rig is set up, it is hard to change the span and keep all things equal but it gives a reasonable approximation. Same piece of wood, same weight, different spans. Simple span with a point load in the middle.

14.5" span defelcted .250"

16" span deflected .375"

So ballpark 10% increase in span resulted in a 50% increase in deflection.

If nothing else, it tells you that size matters (and that switching from a 14 to a 12 fret bridge position is huge).

Maybe some other folks could repeat it on their rigs.

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http://www.chassonguitars.com

Alan, I downloaded a copy of the Efferlink article "Forces on Archtop Guitars" (from American Lutherie) which indicates

Downward force at bridge = 2 x string tension x sin(breakover angle/2)

Not sure how/if torque component affects this formula (don't think it should). Once you get the downward force, it is a bit like calculating required joist thickness for a given column width.

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Dave
Milton, ON

Thanks to everybody for the input: seems like I'll have to go look some thins up.

Archtops and flat tops are quite different in the way they're loaded. With archtops it's all compression; downforce on the bridge and inward from both ends via the neck and tailpiece. With flat tops the in plane tension and compression forces onthe top are not nearly as much of a problem as the torque of the bridge, which is simply not a factor on archtops. Since the examples givenby tho OP were flattops, I was assuming that's what was interesting to him.

We could go on for weeks on this, I'm sure

I want to thank everyone for taking the time to weigh in with their sophisticated technical data, i realy appreciate it , I guess things are not always as clear cut as i would like to think they should be ! your answers definitaly improved on my idea of how a flat top functions, and gives me some parameters to apply to my building , thanks again ! Jody

I just checked - it is L^3 for both.


I checked here - http://www.engineersedge.com/beam_calc_menu.shtml

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.