Ibanez neck radius

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Author Message   Biggins Username: Biggins Registered: 02-2006 Posted on Monday, February 27, 2006 - 7:01 pm:    I have a '79 Roadster guitar and I'm trying nto find out the neck radius for accurate setup purposes. Do all Ibanez guitars have the same radius? I read somewhere that they are commonly 12" but surely it would vary from one model to the other.   Craigjc Username: Craigjc Registered: 11-2002 Posted on Tuesday, February 28, 2006 - 7:12 am:    I own 4 RoadstarII guitars and they do vary based on certain years. The earliest RoadstarIIs featured a vintage radius, approx. 7 1/4 inches, very much like old Fender guitars. Assuming that these are closest to Roadsters, I would guess the Roadster would be similar. I don't have the exact spec. on the Roadsters - maybe Mr. Roadstar can help...   Biggins Username: Biggins Registered: 02-2006 Posted on Tuesday, February 28, 2006 - 1:55 pm:    Yes, I have my suspicions that it may be the vintage 7 1/4" radius but wondering if someone can confirm this.   Jeffsailor Username: Jeffsailor Registered: 01-2004 Posted on Wednesday, March 01, 2006 - 12:48 am:    Here's how you can calculate the neck radius: Consider the diagram above. The shaded area within the circle represents a cross section of the neck. To calulated the radius of the circle/neck, use a pair of calipers or a contour gauge to measure the maximum height (h2) and width of the neck. A countour gauge will actually give a more accurate measurment since it will allow you to eliminate the fretboard radius. Half of the neck width is represented by w in the diagram. From these two measurements, the radius r can be calculated. The Pythagorean Theorem defines the relationship between r, h1 and w as: r2= h12+ w2 Although h1 is not known, it is equal to the radius of the circle minus the height of the neck h2, or h1 = r - h2 Replacing h1 in the first equation with r - h2 results in r2= (r - h2)2+ w2 The radius r can be found by solving the above equation. First, expand (r - h2)2 to yield r2 = r2-2rh2 + h22 + w2 Subtracting r2 from both sides of the equation results in 0 = -2rh2 + h22 + w2 Adding 2rh2 to both sides of the equation results in 2rh2 = h22 + w2 Dividing both sides by h2 results in 2r = h2 + w2 / h2 Finally, dividing both sides by 2 results in an equation that defines the radius in terms of the known measurements: h2 and w. r = (h2 + w2 /h2) / 2 or in english: Measure the width of the neck. Divide it by two, then square it. Divide the resulting number by the height. Add the height to the result, then divide by 2.   Craigjc Username: Craigjc Registered: 11-2002 Posted on Wednesday, March 01, 2006 - 8:26 am:    Jeffsailor, in your formula, the actual thickness of a neck will influence the outcome. This is absolutely not true in reality. For example, I can take a guitar with a 10" radius fingerboard, shave the back of the neck until the inner truss rod channel is visible, and the fingerboard radius is still 10". "Measure the width of the neck. Divide it by two, then square it. Divide the resulting number by the height. Add the height to the result, then divide by 2." 2" wide, 1.5" height. r = ((1.5 + 4) /1.5) /2 = 1.833 ??? ======================================== The easiest way to measure a fingerboard radius is to use a radius gauge. radius gauges Assuming you're not going to actually need these nice gauges in the future, you can make disposable ones : Draw a circle with a drafting compass (drawing compass, divider, etc.) with 7 1/4 inch radius on some firm paper or cardboard. Cut out the circle. Throw the circle away. The remaining piece can be cut (in half, whatever). The arc on this remaining piece can be placed on the fingerboard to match (or not match) the fingerboard radius. Repeat as necessary with the following common fingerboard radius: 9 inch 10 inch 12 inch 15 inch 20 inch By the way, I just use the radius sanding blocks I have to match the radius.   Jeffsailor Username: Jeffsailor Registered: 01-2004 Posted on Wednesday, March 01, 2006 - 9:02 am:    Craig, This calculation is used to determine the radius of a circle when all you know is a the width and height of a bisecting secant of that circle. It can be used to calculate the radius of the back of the neck as long as you eliminate the fingerboard from the measurments (which is easy to do with a contour gauge). And In fact, can calculate the radius of the fingerboard if you do not take into account the thickness of the board. The hardest part of this calculation is determining the height measurement. This should only take into account the amount of deflection the circlular part of the board/neck makes, not the whole board/neck itself. So for a fingerboard, it would be equal to the difference between its height at the very edge and its height in the middle. I agree, the easiest way to calculate the radius of the fingerboard is to use a radius gauge. But for those that don't have them, this calculation works, as long as you put in the correct measurments... Good tip for the disposable gauges. js   Craigjc Username: Craigjc Registered: 11-2002 Posted on Wednesday, March 01, 2006 - 10:44 am:    I see what you're attempting now. Yeah, fingerboard height is tough to measure. Sorry, the appearance of your diagram suggested accouting for actual thickness of the neck. If manufacturers always published these specs, we wouldn't be forced to figure them out ourselves.   Biggins Username: Biggins Registered: 02-2006 Posted on Thursday, March 02, 2006 - 1:03 pm:    Brilliant!! thanks guys, I will calculate the radius according to Jeffs formula and use Craigs more hands on approach of a cut-out radius gauge to confirm.   Guitartim Username: Guitartim Registered: 04-2001 Posted on Friday, March 03, 2006 - 12:20 pm:    Interesting stuff indeed! For years I just picked up the guitar and viewed it from the butt end, looked up the fret board like a gun sight to see the degree of curvature (radius). Then decided if it was either: A: Flat as a fryin' pan; B: Slightly curved like a twelve year old girl; or C: Arched like the earth's horizon. Not too scientific, but it works for me. Tim

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