Official Luthiers Forum!

I suppose like many folks, I've always located compensated bridges using a rule-of-thumb that I learned way back. In my case, 3 mm saddle slots are routed in a jig with an angle formed by 1/4" slope over 6", then locate the bridge square to the centre line and with the middle of the slot at the high E string position 2 mm back from the true scale length (2 times the nut to 12th fret, regardless of whatever nut compensation I might have created by shortening the nut to 1st-fret interval). Sorry about the mix of Imperial and metric measurements, but as a baby-boomer that's the way I tend to build.

Now, that rule has always allowed me to do a pretty good job with final intonation, even on a 3 mm saddle, but being a scientifically trained person, I thought I could incorporate proper compensation specs into my fingerboard calculator http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=30835, and I thought I would specify a compensated measurement to the saddle (either leading edge or mid-point) at both the high and low E string positions. So I've started some research on how various sources specify saddle compensation, and I've been a little surprised how much they seem to vary!

Take, for example, the often-cited Stewmac calculator http://www.stewmac.com/freeinfo/Reference/Calculators/i-fretcalc.html. Let's consider a hypothetical acoustic guitar with a 25" scale. The Stewmac calculator gives the measurement to the high and low E-string break points as 25.087" and 25.210", respectively. Now have a look at the instructions for the Stewmac saddlematic http://www.stewmac.com/shop/Tools/Special_tools_for_Bridges/Saddlematic.html?tab=Instructions#details. If we use that device as decribed in the instructions, the middle of the saddle slot will be located at 25.109" and 25.203", respectively. Break point and middle of the saddle slot are, of course, not necessarily the same thing, but I expect most builders would interpret them the same way. The calculator would locate the saddle at the high E about 0.5 mm closer to the nut and the low E about 0.2 mm further back -- i.e., a steeper angle.

Ideally, the optimum compensation angle is likely to vary with the saddle thickness, to maximise room for individual string intonation. I'm likely to start putting much thicker saddles in future guitars, as 3 mm doesn't give that much room for intonation and some strings can be a problem (particularly on 12-string saddles), even if most are just fine and I don't get complaints. If one could calculate the correct compensation for ALL strings individually, one could locate the saddle at the best possible position, regardless of its thickness.

Am I being a bit anal? Probably, but a spreadsheet can make complicated calculations dead easy, assuming you can find reasonable estimates of the various string parameters. I've not seen any formulae for calculating "theoretical" compensation -- perhaps these exist in the Gore and Gilet books, which is great, 'cause I'm expecting to order those in a month or so anyway. But perhaps someone knows a web source that has a proper mathematical treament of the subject, with formulae, rather than "rules"? Frankly, I've not yet found what I'm looking for with Google. Let's gather up relevant sources in this thread, as I'm sure there are many who'd like to sort through this issue on their own.

Yes, the Gore/Gilet books cover that.


Yes, here:

• Greg Byer's approach + the Article Classic Guitar Intonation in AL #47, Fall 1996, The Big Red Book of American Lutherie, Vol. 4
• Mike Doolin's approach (eight pages) Edit: actually more about general intonation issues than a mathematical approach to saddle compensation
• John Gilbert's approach + spreadsheet for Gilbert's approach (spreadsheet is my creation from some years ago)

I hope that helps until you get the Gore/Gilet books

Cheers,

This is great, Markus. Exactly the kind of treatment I was looking for! Hopefully these will also be useful to other members looking to calculate compensation specs for less common instruments and string gauges.

If folks have other hardcore references, please add to the thread.

Bill Cumpiano's latest blog entry deals solely with saddle compensation, and is very much required reading IMO .

Saddle compensation is not a simple thing as the variables of a particular instrument are often different from another. The things that will influence the compensation are string gauge , the alloy used ,age of the strings and the placement and angle of compensation used on the bridge.
It is a fairy tale to think one compensated saddle design will work on all guitars. Lets look at the variables and what they do to influence things.
Action height :
The higher the action the more the strings will pull sharp
String gauge:
Light gauge and med gauge will have different tensions and working length of the string
Alloy
this will also influence to a degree the work hardening of the string
Compensation angle
Most common used is about 3 degrees
Compensation length
This is the extra length a build will use to place the saddle position on the guitar.
Zero fret position
The point where the string is actually in the playing line or point .0 of the string length

As a string gets played and ages the properties of the strings will change. This phenomenon is called work hardening. The strings will loose some flexibility and will harden. The string is also being corroded . This changes the intonation. Often the B or G strings will be hard to keep tuned.

So as you look at the needs for compensation you have to look at the playing style that the guitar will be used in. I make my own saddles to the individual guitar. Martin used to use a 3/32 saddle. I like to make my saddles .110. That gives me a little more adjustment room.
When I do a guitar I like to make the set up first then do the compensation last. With fretted instruments you may not get every fret to note true . I like to use the different positions to check intonation so there is a better cents placement up and down the neck. The higher the action the more to compensate to the flat side . The farther up the neck the more accurate your spacing needs to be. It won't take you long to learn to make a good compensated saddle.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it