Top arch radius measurements

[Lawrence]

Sorry if this has been hashed out already, but my search of the archives found nothing. What is the radius of the arch on carved mandolin tops, I wonder? Of course there would be huge variability, but would it be, say, 1' ish or 12' ish or what? Relatedly, is there a good way to measure the arching of a top? It's been too long since geometry class, I'm afraid.

Thanks!

-Lawrence

[Antlurz]

Mostly, it is shaped more like a Bell Curve. You could propbably come up with an "average" arch, but for practical purposes, it wouldn't do you any good. It constantly changes as it goes from the center to the edges...

Some builders undoubtably shape them more like an arch, but I think most are more of a progressive curve.

Ron

[sunburst]

I'm actually kind of surprised that I found this, but here is a link to a thread containing a discussion of a curtate cycloid.

FWIW, I find that a curtate cycloid doesn't please my eye when carved into a mandolin top. I "measure" my arch by "rolling" the top and back while looking across them, and by side-lighting.

[Antlurz]

I know it's commonly referred to as a curtate cycloid, but unless I'm mistaken, I can't see how it could correctly be called that, even though a lot of folks do when referring to a mandolin or violin top. The reason I say that is there is no way that I can see, regardless of how one adjusts the defining mechanism, to get the right height and width of the mandolin shape.

Setting the "crank" arm very close to the center would approximate it, but still in reality, the outer quarters of the curve wouldn't follow very close two what is normally done, would it?

Or have I got my hat on backwards?

Ron

[Michael Lewis]

There are as many possibilities as there are people making mandolins. Since the arching effects the tonal response you need to make several variations to find the one that suits your ear. Also to consider, the arching is the structure. Older Gibsons were carved with a dramatic swoop from the recurve area up to the "table" of the top, and then more or less flattened out. I call this style of arching "traditional" because you can see it on most arched instruments through the years. My preference is to use a smoother and flatter arching style, but maintaining the same height at the bridge.

Good tone and volume can be obtained with just about any arching if you are lucky. Great tone and volume come from a much narrower range of arching.

[sunburst]

Ron, To convince myself, I once used the "cave man" method of deciding the width of the plate at 10", putting the recurve at 7/8" in from the edge (subtract 1 3/4" = 8 1/4"), figured the diameter of the wheel (8.25 / pi = 2.626"), decided the height of the arch at 1/2", figured the distance from the center of the wheel to get that arch at 1/4", put a pencil through a hole 1/4" from the center of a 2.63" wheel and rolled it down a straight edge. The resuluting drawn curve is just about like a typical mandolin arch. The part from the recurve in toward the center for the first couple of inches is a little different than what I generally carve.

[Antlurz]

The part from the recurve in toward the center for the first couple of inches is a little different than what I generally carve.

Yup. That's the area I meant when I said the outside quarters of the profile.

(That's not bad for self described "cave man" method's BTW

My methods usually tend towards pre-Cro Magnon... )

Ron