Measuring Top Radius

[rgogo65]

I think this has been discussed before, and I was sure I archived the info, but I can't seem to find it anywhere and my math skills are lacking :)
What is the best way to actually measure the top radius on a Dread, or any guitar for that matter..finished and strung to tune it should be about 40' right? in a 15' portion of a 40' radius the vertical curve or actual measurement from a level source to the top at the edge...is pretty darn small in my minds eye?
Thanks,
Ray

[Freeman]

I think it is usually 20-28 foot, 16 or so for the back. You could put a straightedge across the widest part of the lower bout and measure from the edge of the rim to the strightedge on each side (make sure they are equal or average the measurement). Because most formulas for chord calculations do not use that little segment, what I would do is simply plot the chord length (distance across the lower bout between edges, and that height (called sagitta) on an autocad drawing, then strike a 3 point arc between them. Listing the information about the arc will give you its radius.

Another way to at least approximate it would be to make a couple of cardboard templates of, say, 15, 20, 25 foot radius that are at least as wide as the lower bout. Autocad makes that easy, but you can also use a string and pencil. Cut them out and trial fit on your guitar - see which is closest and extrapolate the rest.

Don't do it when the strings are under tension, that distorts the top but a recent discussion at MIMF indicated probably not too much.

[tippie53]

Radius can be a trick thing to measure. when a radius is built in ,the string tension and humidity will make things change to a degree. Often a 28 foot radius is discussed but it is more than that. You have to think about the neck angle. A 1 1/2 degree neck angle will work on a 28 foot top 14 freter and a 30 of a 12 fretter.

[rgogo65]

OK...I guess I'm having a problem explaining what I'm looking for. I'll try another approach..maybe I can do better...I'm afraid I'm not being very articulate, sorry...
I have an older guitar and I THINK the top is pulling up, but I can't be sure...it isn't much if it is..how can I measure it side to side and calculate the radius..I saw demo or something on the site a year of so back bur I can't find it...
So...can I lay a straightedge (Level) across the lower bout and get a pretty close idea by measuring down to the top at the edge what the radius is? If so, what would the measurement be if it were 28'? It's 13" across the lower bout...so I'm I correct in assuming that that would be 13" of a 28'radius? To me..it would be pretty flat but "Pretty Flat" is not a very accurate measurement :):)
Another was to do it I guess, would be to lay out a 28' radius on cardboard,cut it out and use it for a "Gauge" How do I accurately mark out such a radius?
Guess I'm being pretty thick headed here...thanks

[tippie53]

I don't know what the manufacturers spec would be , but Martin spec is , lay a straight edge across the top behind the bridge and measure the gap at the sides. If the total adds to more the 1/2 inch , things are not good. Often this can be corrected but changing the bridge plate and also check for the braces working loose.
Humidity can cause the top to rise also . there are more than one thing that can cause things so try and nail down your issue. Without a hands on inspection the best we can do is guess at what your guitar problem may be.

[kencierp]

You really need a gage to get a "close estimate" --- the bridge and bridge plate have flattened the sound board to some degree and the string tension causes doming in the other direction. Freeman has provided the math solution above, its exactly how the measurement is calculated -- math is science and no way to get around it. Its your project and perhaps none of my business -- but if you could share, why would it be necessary to take a contour measuremnt when you seem to know the top is problematic and already distorted from its original shape?

[Darryl Young]

Ray,

I'm not sure if this helps, but across a 13" bout with a 28ft radius, the rise would be 0.063". I have a spreadsheet that calculates either the rise or the radius if you're interested.

[rgogo65]

You really need a gage to get a "close estimate" --- the bridge and bridge plate have flattened the sound board to some degree and the string tension causes doming in the other direction. Freeman has provided the math solution above, its exactly how the measurement is calculated -- math is science and no way to get around it. Its your project and perhaps none of my business -- but if you could share, why would it be necessary to take a contour measuremnt when you seem to know the top is problematic and already distorted from its original shape?

Ken,
I'm sorry. I didn't realize Freeman had provided the math solution above. I don't own an Auto Cad program...wouldn't know how to use it if I did.

My math skills are lacking....and I didn't understand his response. The truth is, I don't know HOW to lay out 13" of a 28' round circle radius and determine the height of the rise at 6.5".

I've already admitted that I'm stupid, I guess that pretty much covers it, doesn't it? :)

I was trying to find a way to "CHECK" or determine if I had a problem coming on..or not, in an effort to avoid something serious.

Usually, with most things, there is a tolerance of sorts, "If it falls between .033 and say .078" as an example, you're OK? See? I don't KNOW if I have a problem or not...that was the whole point.

I'm sorry if my question irritated you, I don't know everything, I'm NEW at this...but I'll try really hard to do better and avoid that mistake in the future...:)

[rgogo65]

Ray,

I'm not sure if this helps, but across a 13" bout with a 28ft radius, the rise would be 0.063". I have a spreadsheet that calculates either the rise or the radius if you're interested.

Thanks a lot Darryl, that's what I was looking for, I would REALLY appreciate the spreadsheet info..very cool! Thanks again. :)

Ray

[kencierp]

Your question certainly did not irratate me in the least (why would that be the case?) I don't think you or any one else is stupid -- lighten up on yourself -- this is an information and problem solving venue and no one knows it all --- my point would be, if you don't know (as John mentioned) what the production specs are how would know if the current contour is a problem (out of speification?). It would seem that you have some sort of set-up issue or maybe not -- how's the action of the guitar? Does it play well/easy? In tune along the fret board? Acoustic guitars change over time and many would tell you that all guitars at some point will need a neck set because the soundboard shape has changed --- no matter how it was originally built. Heck Bob Taylor built a business on this premise by creating an easy neck re-set adjustment system. Playability is the concern not the look of the top -- I can tell you that I've seen some pretty wierd looking wavey soundboards on some very,very expensive instruments -- we have Elderly's here in Michigan they have a huge selection.