Back and side thickness

[rgogo65]

I notice that this seems to be all over the place...so my question is why? The type of wood? Personal preference? LMI for instance declare that their back thickness is .140 and their sides are at .120 no matter what type of wood.
I see others at .125 for both. and still others at .115. To me it's quite confusing and I'm sure it is for others as well.
StewMac thickness' are at .095-.100 for the sides and .115-.120 for the backs...so I don't get it.. :(
I've been TOLD that the sides should be in the .095-.100 range and .100-.110 for the backs and GOOD tops will sand out at about .100, so what is the REAL deal here Guys and why?

Thanks, Ray (:>)8>

[David L]

That's a loaded question!

David L

[tippie53]

I am using Martin Specs . I am on #145 and found that heavier guitars tend to be less responsive . There is a balance. Older Martins I have seen as thin as .065 on the Mahogany and .070 on Rosewood . Backs from .085 to .100 . The tops where a bit heavier on the tucked pre war .105 to .115 and after 1940 .100 seems the norm . You may use what you think is right for you .

[kencierp]

There is simply no hard and fast rule regarding side and back thickness. When we are thinning backs for kits they are worked to a dimension that allows some flex along the length so it is able to conform to the contours of the rim. This is "usually" about .100 +/- Side are about .080 leaning toward a little thinner. Way back -- when I was purchasing sides from Martin, their average was just over .075" -- even thinner on the cutaways.

The better drawings out there (GAL) have measurements taken off actual instruments, but even then I've seen mistakes. Don't forget that many plans are just somebody's rendering with dimensions pulled out of the air, the design can be flawed in many ways. I have a couple of Gibson drawings that are really bad -- one shows the sides to be .125" thick -- that's not going to happen. Don't over think this -- and as John has pointed out a couple of times there is no "magic process" for consistently bending every species or thickness of wood available to the small shop -- even LMII makes you sign a damage waiver if asked to bend a personal set of sides.

[kencierp]

Oh -- as for sound boards -- the actual piece material determines how thick "it wants to be" generally thinner (lighter) is better -- but sometimes to keep the cross grain stiffness from becoming flabby a top may be a bit thicker. Most times kits, come with tops that are .125" - 150" the expectation is that the builder will thin these tops to some degree. KMG sound-boards are thinned and ready to go with the exception of final sanding. While there are formula guitars -- there is no magic formula to calculate best thickness.

[David L]

Now you see why I said That's a loaded question. I've read various articles (don't ask me where or who the authors were) that go into all kinds of stuff like,what type of wood is it, what body style are you building, what type of sound/tone are you looking for, what type of top will be mated to what type of B&S and then up jumps the devil and that whole crock of thinner wood/heavier bracing-thicker wood/lighter bracing thing tries to get into the act.
I think it was John that said "put three luthiers in a room and ask them a question about something and you're liable to four or five opinions"
A whole lot of people make a whole lot of claims most of which there is no evidence based data to back it up, usually it's just an opinion.
We could over-analyze this til the moon turns blue and probably be worse off than before we asked the question.
What I do is smply ask Ken or John how thick it should be (more likely than not they are going to be the ones who will be thicknessing for me anyway) and I believe that they are going to put me in a "happy medium" range and will perhaps allow a little wiggle room for final sanding.
I believe that the most important thing (assuming that quality material is used) of all the many different things that play into how well a guitar will sound is quality of workmanship, bar none. A guitar is an intricate assembly consisting of many interactive components and if one just throws one together the end result will most likely be a crappy guitar. I believe that if I take my time, double check everything, make sure that there are no built in stresses, my flats are flat, my radii are correct as well as angles, proper glueing, etc... and most importantly, when I'm not sure, post on the forum and ask no matter how embarrasing it may seem. I believe that if I follow this procedure that more likely than not I will end up with an acceptable instrument, but hey, that's just my OPINION!

David L

[Darryl Young]

My opinion is that the soundboard thickness and the stiffness of the top used (along the grain and across the grain) is one of the largest influences on the tone of the guitar. Maybe choosing a bracing pattern has as much affect, not sure. You can definitely kill the great tone of a guitar by leaving the top too thick/stiff.

BTW, the stiffness of the top should affect what thickness you choose for the top as these two variables affct the overall stiffness to resist the pull of the strings so the guitar is stable over time.

The back stiffness/thickness is a another whole can of worms. How thick/stiff you choose to leave the back likely is determined by how you view the function/purpose of the back (stiff and reflective or thinner and responsive). I must admit I don't have my mind around this. One has to keep in mind that the back serves a structural purpose to help keep the neck block from rotating. I'm guessing each luthier's thoughts on the function evolve over time.

I'm glad John shared the info he has on old Martin back thicknesses. More data is a big help! I just discussed this with Mario Proulx a bit. His thoughts were......don't be afraid to let Mahogany sound like Mahogany. In other words, don't try to make a Mahogany back as stiff as rosewood. His and John's comments help a lot. Of course, one has to look at the piece of wood in-hand and evaluate how well quartered and how stable it is. That has to go into the equation as well. One might have to err on the side of caution with a wild figured wood.

[rgogo65]

Thanks everyone..I truly DIDN'T plan to induce an "Loaded Question" or create conflict...it was a genuine question I felt those new at this would ask (Including ME) and I appreciate your reasonable, intelligent, remarks.
It shows that there is a LOT of (Seat of the pants) stuff here you only learn from experience and by and large "Your way".
It appears to me that there IS no pat answer.
I remember I had an Uncle who used to say "If you don't know your jewels, know your jeweler" many of us don't have a life time to compile large quantities of experience and we have to rely on those who are knowledgeable and willing to share, or supply the rest of us with a "Workable" combination with which to start with. That's why folks like you are so important to the average hobby crafter.
Thanks again Fellas!

Ray (:>)8>

[Darryl Young]

Ray, also consider that tonewood supplier leave the back/sides overly thick as the expectation is that the luthier wants to control and set the thickness of the back/sides/top. Kit providers do a little more and usually thickness the wood so it can be used as-is (not requiring additional thicknessing). Now you have the option to thin the back/top more if you want to......but it isn't required. Typically the kit supplier has already bent the sides so they thicnessed them per their own specs before bending. Probably no purpose to thin the sides more after bending (and it would be very difficult) so this is practically never done.

Hopefully that explains why you see much larger thicknesses from tonewood dealers........they don't build the guitar for you, just supply the wood for you to make the appropriate choices.

[Darryl Young]

I am using Martin Specs . I am on #145 and found that heavier guitars tend to be less responsive . There is a balance. Older Martins I have seen as thin as .065 on the Mahogany and .070 on Rosewood . Backs from .085 to .100 . The tops where a bit heavier on the tucked pre war .105 to .115 and after 1940 .100 seems the norm . You may use what you think is right for you .

John, just for clarification. Are you saying on the older Martins you've seen side thicknesses of .065 on Mahogany and .070 on Rosewood? And then back thicknesses of .085 - .100 on those same guitars? Till I re-read the post, I thought you were saying back thicknesses as thin as 0.065" on old Mahognay guitars.......now I'm unsure if you are referring to sides or backs after re-reading.

Thanks!