Classical build - thinner plates?

[hat]

Are classical guitar backs, ( and sides) typically a bit thinner than steel string builds? what about the top thickness? Does it matter much as to species re thickness? (spruce vs Cedar vs redwood) ?

[redir]

It all matters yes. Backs and sides are often built to spec but a top should always be built to the material properties of the given piece of wood. There are many ways to determine that from a more scientific acoustic testing to a luthiers experience. Generally sides and backs are a bit thinner especially for Flamenco guitars but there are all sorts of design philosophies too. Thicker stiffer sides for example can make the vibrating surface area of the top extend out more towards the sides and couple the top with the back. A thicker back can make a more reflective guitar, and so on.

Do you have any books for this construction project?

[Alan Carruth]

When thinking about the top thickness what matters most structurally is it's bending stiffness along the grain. You want it to be just stiff enough to hold up over the long term under the torque of the string pull. Bracing is used to add some stiffness without adding too much mass, but the top thickness is key to getting a good tone, since most of the weight is in the top and you're trying to get the weight down, particularly on a Classical.

The torque on the bridge is a product of the tension of the strings and how high they are off the top. Nylon strings typically pull about half the tension of steel strings, and are normally not quite so high off the top, so the torque is lower. OTOH, the bracing on nylon string guitars is not so stiff, so more of the load is taken by the top.

The actual stiffness of the top varies as the cube of the thickness: making a nylon string top half as thick as the same wood for a steel string would leave it only 1/8 as stiff. To make a top half as stiff, you'd make it about 80% of the original thickness, all else equal. Taking into account the difference in bridge height and bracing, I tend to make tops for nylon string guitars about 85% as thick as I would if I were using the same piece of wood for a steel string.

I could probably go on for the better part of an hour on this, but that should be enough to get you started if you already have some experience making steel strings.

[hat]

Thanks both for your input. I am just starting to think about this project. I have done some steel string guitars, but never a classical. I have a few nice sets of native quartersawn Walnut, and want to use that for the back/sides, and Cedar for the top I think. at this point I'm just gathering ideas and thoughts...