Over the past 40 years, I've written several articles and columns on the importance of string-break angles. It is an important topic that I explore in my books The Luthier's Handbook
and in How To Set Up The Best Sounding Banjo
. I believe string-break angle is not just important, it is critical.
String-break angle is the angle the strings make as they pass over the bridge. Too little string-break angle means little or no down pressure on the soundboard (or head in the case of the banjo), and thus, little or no transmission of energy from the strings to the soundboard or head. Too great a string-break angle means excessive down pressure on the soundboard or head, and this can result in the soundboard or head being restricted in its ability to vibrate, which results in little or no energy being generated by the soundboard or head. Somewhere in the middle, there is a sweet spot, where the down pressure from the bridge can properly drive the soundboard or head and make the instrument work efficiently and effectively.
Here is an example of how we evaluated and measured an instrument with a 7° neck pitch and an excessively-high 20° string break angle (the yellow lines indicate the neck pitch and the red lines indicate the string break angle):
It is important to realize that the bridge pushes down with a certain amount of pressure dictated by the string break angle and the gauge of strings. That pressure is met with an equal pressure or force from the soundboard pushing up and a stasis is reached. If that were not the case, the down pressure from the bridge would continue to push in on the soundboard until it imploded. So, there is a relationship between the stiffness of the soundboard and the load imposed on it by the bridge.
String break angle controls the amount of the strings' lateral force that is directed towards the soundboard or head. Banjos are fitted with tailpieces that can be adjusted up or down which, in turn, alter the string-break angle.
I receive at least one email or letter a week in which a luthier describes a mandolin that "sounds dead" or "weak" or one that "lacks tone." Typically, my first response is for them to send me a straight-on side-view photo of their mandolin. Some folks ask "why?" but most respond quickly with a photo or two. Once I receive the photo, I bring it into Adobe Illustrator® so that I can accurately calculate both the string-break angle and the neck pitch. In 95% of the cases where the luthier describes a problem, my intuition turns out to be correct, and I discover that many of the problem instruments have 18° or 19° string-break angles, and when I check the neck pitch, I find that necks are set at 7°, 7.5°, or even 8°.
String break angle can be easily measured with a protractor like this:
The ideal string-break angle for a mandolin is 16°. At 16° a set of D'Addario J73 mandolin strings exerts a down pressure of 45.5 pounds at the bridge. At 17° the load increases to 50 pounds. At 18° the load is 54 pounds, and at 19° the load is just under 60 pounds. Above 19° the down pressure increases exponentially and not only does the amplitude and tonal qualities diminish quickly but severe damage to soundboard, bridge, or both is sure to occur.
A red spruce soundboard carved to the proper graduations and with tone bars attached can withstand a load of about 75-80 pounds before it implodes. So the ratio of down pressure to the failure point is about 1:2. A violin soundboard carved to proper graduations and with bass bar attached can withstand a load of about 100-110 pounds before it implodes (usually on the bass side because the soundpost supports the treble side). The violin bridge - which, by the way, typically has a 15.5° to 16° string-break angle - exerts a down pressure of about 25 pounds that gives the violin a 1:4 ratio.
String-break angle is the result of neck pitch. The higher the neck pitch, the taller the bridge must be and the greater the string break angle. In 99% of the cases where we study tonal or broken bridge-saddle problems that luthiers present to us, the problem is inevitably excessively high neck pitch settings. Original F5 mandolins were set to 5.5° with some measured at 5°. A setting of 5.5° to 6° (which we prefer) is ideal to deliver the timbre and "chop" for a bluegrass mandolin. Anything above this presents an array of problems to the instrument and owner.
We have also seen cases were the neck pitch was correct at 6°, but the neck was set too high in the rim which lowered the body of the instrument and the location of the tailpiece relative to the neck. This lower position of the tailpiece caused the string-break angle to exceed 16°.
One reason we like our V-joint neck connection system is that the neck is aligned and secured in place relative to the crown of the soundboard which, in turn, guarantees the correct string-break angle, instrument after instrument.
In addition to constraining the movement of the soundboard, an excessive string-break angle imposes too great a load on the bridge. The typical two-post Gibson-style bridge saddle is close to its failure point with J74 strings and a 16° string break angle (which impose a load of 49 pounds). String break angles in excess of 16° with these strings presents the bridge saddle with loads it was not designed to handle and often leads to saddle failure.
If you are interested in more about string-break angles, please visit our Addendum webpage
for the previously mentioned books. Here you will find photos of how we measure the string-break angle and how we measure the loads along with a chart that shows the loads for a set of J73 strings at strings break angles from 0° to 22°.