skivvie,

You wrote: "When I measure them (with expensive metal rules), I get 12-21/32" from nut to front of the crown on the 12th along the 3rd string." If you're trying to determine scale length, why would you not measure the distance from the center of fret twelve to the rear face of the nut?

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Originally posted by skivvie

Hello all! Long time reader first time poster. Question first then more about me:

I'm confused about the scale lengths of a few of the banjos I own. I'll leave builders out of this, but they are all reputable respected names. My biggest question is on the two 25.5" scale lengths. When I measure them (with expensive metal rules), I get 12-21/32" from nut to front of the crown on the 12th along the 3rd string. As I understand it, this should be 12-3/4". When I compare the spacing of the frets along the entire length of the fretboard to a 25.5" scale length fretboard I just CNC'd using measurements from multiple scale length fret spacing calculators (stewmac's being one of them) and confirmed with calipers to match... the confusing banjos fret spacing is overall tighter over the length of the fretboard.

So my question is... is there a reason for this discrepancy I don't understand? Does nearly a 10 thou (3/32") difference really matter?

I'm a very new builder. I've built exactly one guitar under a luthier and one banjo at a week long building class... so I know there is a LOT I don't know. Just trying to make sense of it all... any insights greatly appreciated!

-loren

PS - I hope to be posting more in the near future regarding my big project... building banjos out of used skateboards... the "why" of which I'll explain later at some point =]

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First, the measurement:  measure fom the front of the nut to the center of the 12th fret. 
 

Second, the existing banjos.  How do you know for certain that they are exactly 25.5?  If they are less, that would explain the tighter spacing in the high frets. 
 

Third, the arithmetic.  3/32 equals .09375.   "10 thou" doesn't seem like what you intended. 
 

May want to remeasure the supposed 25.5 banjos.  Whatever you get (x2) is the true scale.  Also can check the 5th fret which is 1/4 of the scale. 

Measurement: I measured along the 3rd because as I understand it, the nut and frets are perpendicular to the 3rd string. So I measured front of the nut at the 3rd to where it meets the front crown of the 12th fret. Regardless, when I measure the "aesthetic" centerline... the measurements are imperceptibly different.

Existing banjos: I only know that they were advertised as being 25.5"

Arithmetic: Yes,  You are correct I meant 100 thou.

Good call on the 5th... didn't think of that. That actually measures 6-3/8" or maybe 1/64" less than that. So that one seems correct?

Thanks for your input!

ANY forward bow in a neck will shorten the scale length measurement.

I learned this on a mint 2000 Martin D45 that intonated perfectly when new, but now 26 years later was a bear to tune to some sort of compromise that was satisfactory. Martin states their scale length is 25.4 inches. I consistently got 25 3/8" which is 25.375.

A luthier friend sighted down the neck and said it needed a good crank on the truss rod! (I've never cranked a truss rod on anything, ever.)

It alarmed me how much he cranked that truss rod, but he got the fingerboard nearly flat again, urging me to crank it if there were any problems after a week or so to settle. Basically he solved my problem, so I've left the truss rod alone.

Even a little bit of forward bow can shorten the scale.

Just throwing this out for ponderization.

What do you mean by "front of the crown", and why are the measurements taken there rather than to the center of the crown?

That is where the string hits the fret, so it's the point from which the length of the vibration length would want to be measured from? That's my assumption and what I've read elsewhere at some point. Regardless, that is a tiny tiny difference. Compared to the overall measurements I'm talking about, it seems like a rounding error.

skivvie,

I also don't understand your reference to "the front of the crown", but that is of no consequence anyway, because where the string contacts the fret has nothing to do with ascertaining the scale length. Measure the distance from the center of fret 12 to the face of the nut, and multiply that measurement by two; that will be the value of the scale length.

Okay, this obviously makes sense, I'm not sure where I got that from and why I was overthinking it (curse of my brain). You make the scale length from nut to bridge and center of 12th fret slot should be half the scale. Got it.

So... any ideas on why an advertised 25.5" scale would be off by 3/32" and the fret spacing doesn't match what a 25.5" scale length "should" be?

Point us to the advertisements for the two banjos and I'll take a look. 

And what is the difference between the front of the crown and the center of the crown?  I'm losing something in the translation. 

The neck needs to be perfectly straight to do the measurement. Truss rod neutral and no strings attached. Otherwise you have a desired neck bow which makes your measurement not comparable.

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Originally posted by Alex Z

Point us to the advertisements for the two banjos and I'll take a look. 

And what is the difference between the front of the crown and the center of the crown? I'm losing something in the translation.

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Not interested in "naming names" I just want to educate myself.

Let's please forget I said anything about crown's... I was ill-informed and didn't think that through. It's irrelevant to what I'm asking. Nut to fret center... got it. 

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Originally posted by pfalzgrass

The neck needs to be perfectly straight to do the measurement. Truss rod neutral and no strings attached. Otherwise you have a desired neck bow which makes your measurement not comparable.

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Understood. I adjusted truss rod to neutral and am still getting similar measurements. Having no strings attached shouldn't change length of nut to fret. Again that is all in the weeds... my general question is whether or not it's common practice, doesn't matter, or if there's some other reason I'm missing or don't understand about the way scale lengths work where a supposed 25.5" scale length instrument would have different fret spacing than what I'm told it should be by fret calculators and my own CNC'd fretboards based on those calculators.

The straight neck is theoretically correct.  

Practically, however, the differences are very small.  For example, if a neck that is  12.5 inches from nut to center of 12th fret is bent up 1/4 =.250 inch at the nut -- which is a lot of neck relief --  then the distance reduces from 12.5" to about 12.49", a difference of only .01 inch.

The poster is seeing and wanting to explain differences of 3/32 = .0935",  nine times as much.

A typical neck relief of .020" would result in a difference in the measured length of about .001", or reducing from 12.5" to 12.499".

"if there's some other reason I'm missing or don't understand about the way scale lengths work where a supposed 25.5" scale length instrument would have different fret spacing than what I'm told it should be by fret calculators and my own CNC'd fretboards based on those calculators."

"Supposed" is the key.  If there is not enough information to determine if the "supposed" scale length is the actual true scale length, then there will be no resolution.  It's not a matter of blame or "naming names," it's a matter of how to get to the source information -- not the hearsay marketing information -- for understanding.

For example, is the Martin guitar scale 25.4" the way it is advertised?  No.  Is it 25.375" rounded?  No.  Is it 25.34?  Well, for some guitars yes, for others no.   The differences have to do with the laying out of the fret spacing by the "rule of 18" versus logarithmic calculation.

All the evidence and measurement so far point to 25.5" not being the actual true scale length, even though it is advertised, and only the poster has seen the advertisement or knows the maker and model of the banjo.  So to try to explain the variances as if the variances are in fact stemming from a 25.5" scale length is not going to get far.

It is quite possible that the banjos don't have a 25.5" scale?  The question is, how to find that out?

Similarly, one can measure Martin guitars all day long and never come out with 25.4" as a measured scale length, and never be able to explain the difference simply by measuring various aspects.  We're not talking about dismissive "weeds."   We're talking about necessary precision. 

Hope this helps.

Just for example.  This appears on the "Banjo Craft" page:

"Scale length is the length of the string from the nut to the bridge. This is the active part of the string that vibrates when struck. Scale length measurements do not include string ends above the nut and below the bridge."

and

"A standard scale length for steel strung open back banjos is 25.5 inches."

And a photo of a banjo shows a tape measure from the bridge to the nut at 25.5 inches.

But the statement "Scale length is the length of the string from the nut to the bridge" is not true.  That's the string length, not the scale length.  And so the banjo in the photo -- despite the photo, despite the tape measure, despite the claim "advertised" -- may not in fact have a scale length of 25.5, but rather shorter if the bridge has been placed in the correct place for proper intonation.  Don't know what the heck the true scale length of the banjo pictured is.

So ya gotta get to the source information first, not assumptions or suppositions, if ya hope to have an answer to scale questions

The nominal scale length is always nut-12th x 2. This is the number from which the fret spacing is calculated. It's the only fixed measurement relating to the strings.

The saddle of the bridge is not at the nominal scale length, however, but a little further back. This is what is confusing the original poster.

This is because of compensation. When you press down a string you raise it's pitch. To compensate for that, the saddle is moved back a little. Thin strings require less compensation than thick, high action requires more than low. So the position of the saddle depends on the strings chosen and the action. If these change, the saddle needs to move.

This is why saddles are slanted and, on reentrant instruments like banjos, intonation is often a compromise.