Tailpiece changes string tension ?

[bluesmandolinman]

I already posted this questions by error in the classcial section .... but wanted to post here. So one more time my question :

I just found the following information on the Compton website :
GIBSON AJr (#11044(?)): 1927 Spruce and bird's eye maple.

The clamshell tailpiece has been replaced with the standard tailpiece found on the more expensive models to change the string tension.


This is new to me....

Can anybody explain how this works ? Does it increase or decrease the tension ? What is the advantage ?

Thanks !

[Dave Cohen]

String tension depends on the length of the string, its mass, and the pitch (frequency) of its first harmonic ("fundamental") mode of motion. The relationship was first derived by Bernoulli in the 18th century. When you tune a string to a given pitch, you are establishing the tension for that string between the nut and the bridge saddle; the tension of that string is the same in the "afterlength", i.e., the length of string between the bridge saddle and the tailpiece connection. What changes is the pitch or frequency. So, other things being equal, a shorter string afterlength will have a different pitch, but the tension will be the same. The tension is constant throught the entire length of the string.

[Bob A]

Is there an effect from changing the "break angle" of the strings over the bridge? (I wonder what effect the De Meglio system might have. It is a bar crossing the strings between the bridge and tailpiece, which bar is screwed to the soundboard of the instrument. It seems like it would not only change the break angle, but also exert an upward-pulling force on the top.)

[G. Fisher]

I understand that the string tension is the same. But, can't the downward pressure on the top change with the angle the strings cross the bridge?

[mandolooter]

G. Fisher...yes that does change it. More angle = more pressure.

[bluesmandolinman]

Dave , thank you very interesting !
If I understand your explanation correctly the information said about the Ajr still doesn´t make sence from the "afterlength-point of view"...

Concerning the neck angle :
I know that Gibson changed the neck body angle for that reason in the early years.But if I look at the clamshell tailpiece (on Ajr s ) and compare it to the "The Gibson" tailpiece I don´t see that it changes the angle that much... Or ?

oh and .... more pressure = more volume , right ? Other advantages ?

I am asking to understand if it would make sence to upgrade my Ajr tailpiece ... http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif

Thanks again !

[Dave Cohen]

The downforce from the strings through the bridge on an archtop instrument is given by

F = 2Tsin(alpha/2)

where alpha is the string breakover angle, and T is the total string tension. I just measured the breakover angle on one of my mandolins to be approximately 14 degrees. At that angle, the downforce will be about 25% of the total string tension. If the breakover angle were increased to a whopping 20 degrees, the downforce would increase to about 35% of the total string tension.

Early on, I made a few mandolins with neck angles of around 7 - 8 degrees instead of the usual 4.6 degrees. They had very tall bridges and correspondingly high breakover angles, but were not any louder than the mandolins with more normal neck angles. In fact, I removed the neck from one of them and replaced it with a neck at the usual 4.6 degree angle. I didn't notice a loss of volume; the mandolin had lots of volume with the replacement neck.

Now, from an experimental point of view, there are some huge cautions about my observations. One is that where memory is concerned, huuman hearing is extremely unreliable over longer time intervals than a few minutes, maybe even a few seconds. Another is that I am only presenting one data point, and am not accounting for the many other differences in any two mandolins. But what I CAN safely infer from my experience is that whatever the sonic differences from changing the breakover angle may be, they are nevertheless well within the range of differences resulting from all of the many other structural differences in any two mandolins.

Re "More pressure (downforce?) = more volume?": The downforce from the strings is a static force. The force(s) which set the top and back plates in motion (which in turn set the air inside and outside the cavity in motion) are dynamic forces resulting from motion of the bridge when the strings are set in motion (i.e., "plucked" or "picked"). I'm pretty sure that the magnitude of the dynamic forces is small compared to the magnitude of the static downforce. Further, the relationship between the dynamic forces and the static downforce may not be a simple one. Certainly the dynamic forces are related to the strength of the 'pluck'. Somewhere in an earlier thread, you will find that some of us agreed that too much downforce seems to 'bury' the dynamic forces, with a detrimental effect on volume.

Iirc, Gibson used a 3 degree neck angle on oval hole mandolins, and a 4.6 degree angle on f-hole mandolins (at least, on the early F5s they did). I attribute that to differences in the way the oval and f-hole tops were carved.

I think that changing the tailpiece on your Ajr would have a minimal effect. Still, there are lots of people who have convinced themselves otherwise, and some of them are willing to give testimony to that on the www. I can't refute what they claim without seeing some objective data, and they usually only provide subjective impressions. All I can do is share my experinces.

[Paul Hostetter]

Perhaps a simpler way of explaining this is to point out graphically that whether using a clamshell or the more elaborate Gibson tailpiece, the string still rests on the same plane from the edge of the mandolin to the bridge top:

http://www.lutherie.net/gibson.mandolin.tpc.angle.jpg

Even doing something extreme to significantly alter the downward pressure of the strings on the bridge top rarely results in a substantive change in the instrument's output.

[bluesmandolinman]

Dave and Paul , THANK YOU !!!

I apreciate your detailed and educating informations to my question
... and I am glad that I can keep my Ajr in it´s original condition http://www.mandolincafe.net/iB_html/non-cgi/emoticons/smile.gif