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Controlling weight of guitar via back and side thickness?
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Author:  Paul Burner [ Mon Aug 24, 2015 8:54 pm ]
Post subject:  Controlling weight of guitar via back and side thickness?
Example: 3 sets of wood - Claro Walnut, East Indian Rosewood, African Blackwood.

All of them thicknessed to the same .080-.085 range for back and sides.....

This produces guitars of what feels like drastically different weights.

Do YOU thin stiffer and heavier woods a bit more than those that weigh less?

Just looking for an overall conversation on wood/guitar weight in regards to thicknessing wood for different species.

Looking for some more knowledge on how to get my Blackwood guitars to weigh a bit less.

Thanks
Author:  Terence Kennedy [ Mon Aug 24, 2015 9:20 pm ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
One thing you can do is double sides with a lighter wood for the inner side. I like what double sides do for the sound of a guitar.
Author:  profchris [ Tue Aug 25, 2015 4:20 am ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
I'm a uke builder so this may not translate, but ....

I thickness until I get the stiffness I'm after - thickness is irrelevant (up to the point that I have structural concerns). If the wood were heavy and not stiff, I wouldn't use it. So far as I can tell, weight (or more accurately density) doesn't correlate with stiffness - so I'll try to start with wood that seems light for its species, hoping to get a higher stiffness to weight ratio.
Author:  DennisK [ Tue Aug 25, 2015 5:24 am ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
The added side mass of dense woods at standard thickness may be good for tone, like Trevor's mass-loaded sides. But I'm obsessed with low overall weight, so I do vary the side thickness depending on the density/stiffness of the wood and depth of the soundbox. Not a particularly scientific process. I mostly just flex it by hand until I like the feel of it.

Honduran rosewood can be super thin because of it's extreme stiffness. .065" is the same stiffness as .085" Honduran mahogany, going by Young's modulus from wood-database.com, and thickness cubed. African blackwood is not very stiff for its weight, but still can be .070" by the same calculation. And can be thinner still if you use full height side braces to stiffen it back up.

As for backs, the .080-.085" range is around where I tend to work. Perhaps a bit thinner for the really dense woods, and thicker for walnut and mahogany, depending on the sound you're going for. You can get away with thinner backs on smaller guitars as far as stiffness goes, but it's the large guitars where weight is really an issue. And in my not-so-experienced opinion, weight is the more important parameter for back tone, since you can adjust stiffness with the braces. But with something as dense as blackwood, even a small guitar could have a very thin back and be heavy.

One of my current projects is a 16" lower bout with cocobolo back, which I currently have about .080" thick, and 320 grams. That seems pretty heavy, and it still feels reasonably stiff, so I'll probably shave it down to 300g or lower when it's time to glue the braces. Lattice bracing wouldn't be a bad idea for durability's sake, but I'll probably just do 4 ladders like usual since I don't have a go-bar deck or radius dish, and lattice is hard to glue without them.
Author:  Hesh [ Tue Aug 25, 2015 6:22 am ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
I'll second what Dennis is saying and that side thickness also depends on what wood you are using and kind of sort of locks you in. I would suspect that your African Blackwood guitar is going to have to weigh a bit more than say a Hog instrument because of the density of the back and sides.

You can go thinner but since you are not using double sides the box may deform it's upper bout sooner resulting in a premature need for a neck reset.

When I built two identical guitars at the same time with the only difference being one was BRW and the other was Koa the BRW instrument weighs 4 - 5 ounces more than the Koa guitar.

I wasn't willing to thin additionally the BRW knowing as I do that the back and sides "color" for lack of a better term the resulting tone and that was a tone that I wanted.

Regardless though my OM's always came out flirting with 4 pounds either a tad under or a tad over and that was light enough for me.

Pre-WWIMartins can often weigh 3 pounds and sometimes a bit less, this is for OO sizes. One of them that we are working on at preset has sides (BRW) that are less than .060" thick.... It also has lots of cracks on the sides too.... :)
Author:  mcgr40 [ Tue Aug 25, 2015 8:46 am ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
I think the modulus of elasticity of wood/density relationship is one that does not vary much with species. So I believe the denser the wood the thinner you can go. But, you have to remember the species characteristics for movement. (my notion)You want a dense wood you can thin out, but you want one the species to move as little as possible for safety/crack protection.
Author:  Tom West [ Tue Aug 25, 2015 9:13 am ]
Post subject:  Re: Controlling weight of guitar via back and side thickness
Only so much can be achieved by thinning the sides and back, with the exception of the lamination of light wood as Terence suggested. Open back tuners are a big help, but if you must use closed at least use wooden handles. Neck and tail blocks can be made of lighter wood such as spruce. Holes can also be vertically placed in the heel block. Careful selection of neck wood helps. Some Mahogany is as heavy as stone( my favorite) some as light as a feather. Lighter fret board wood, EIR as compared to Ebony. EIR now for the head plate etc, etc. Each item separately does not seem like much but they do add up.

Tom
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